A Comparative Review of IG-541 System Use in Total Flooding Application for Energized Electrical Fire

Abstract

Clean agent fire suppression systems are commonly used to protect areas containing valuable or critical equipment, especially in data centers and electrical substations, where traditional fire suppression methods are less effective or pose additional risks. This review evaluates the IG-541 fire suppression system as an alternative to halocarbon-based agents like HFC-227ea and FK-5-1-12, which are being phased out under environmental regulations, focusing on their application in energized electrical fires. IG-541 offers environmental advantages, including zero ozone depletion potential, no global warming potential, and negligible atmospheric lifetime, making it compliant with stringent environmental regulations. This review compares IG-541 with halocarbon agents across parameters such as extinguishing efficacy, safety considerations, environmental impacts, cost impacts, and system design considerations. Key findings underscore IG-541’s effectiveness in reducing fire damage without producing harmful by-products or exacerbating climate change. Furthermore, the study highlights the regulatory frameworks influencing the phase-out of halocarbon agents and the transition toward environmentally sustainable alternatives. While IG-541 emerges as a promising replacement for halocarbon agents, further exploration into its application in varied fire scenarios and energy-intensive environments is recommended to optimize its deployment.

1. Introduction

Clean agent fire suppression systems are commonly used in the building industry to safeguard facilities, including computer rooms, data centers, switch rooms, motor control centers, process equipment, and other critical assets. These systems are typically implemented to secure high-value assets, ensure company continuity, and safeguard people and processes. These systems are ideal for protecting delicate equipment, documents, or artefacts since they leave no residue after discharge. Clean agent fire suppression systems consist of extinguishing agent storage cylinders connected to a network of discharge pipework and nozzles that deliver the extinguishing agent to the room being protected. They are often equipped with dedicated fire detection and control systems that provide an automatic discharge if a fire incident happens [1]. The tremendous development in computing demand for artificial intelligence has accelerated the global expansion of data centers. According to predictions by the International Energy Agency (IEA), energy consumption in data centers could double by 2026 due to the rise in power-intensive workloads, such as AI and cryptocurrency mining [2]. Therefore, selecting clean agent fire extinguishing systems to safeguard the essential and costly equipment in data centers is of utmost importance to the owners. This is being done to prevent any potential harm to the assets and to avoid any interruptions in operations. In addition to data centers, most of the industrial and manufacturing facilities, as well as the oil and gas industry, in use today run on electricity through an electrical substation. A high-voltage electric power system facility called a substation is an essential element of any industry. These are the places in the power network where busbars and transformers are linked for the purpose of power distribution and transmission via switchgears and circuit breakers. Arcs from motors, switches, and electric igniters are just a few of the sources of ignition that can occur in an electrical substation. The associated industry’s frequent electrical fire events in substations, which cause injuries, fatalities, and property damage, must be prevented to protect people and property [3]. The advantages of clean agent fire suppression systems, particularly their effectiveness in suppressing energized electrical fires, have made them a preferred choice for installation in substations, ensuring the safety of both people and property.

Automatic sprinkler systems are the most common type of fixed fire protection system and have a long history of proven reliability. However, they are not suitable for certain hazards, such as live electrical equipment, flammable liquid fires, and any risks that would react violently with water. Dry chemical systems are effective against flammable liquid fires, including spray fires, but they provide minimal cooling and become ineffective once the powder settles. Additionally, powders are not recommended for use in occupied spaces due to post-fire cleanup issues. Low and medium-expansion foam systems are particularly effective for liquid pool fires. The foam forms a barrier between the fire and its oxygen supply while also cooling the fuel. However, since foams are aqueous solutions, they should not be used for risks that would react violently with water. While these traditional fire suppression systems perform well in specific applications, they are not sufficiently effective in some scenarios to replace the use of halon [4]. The purpose of this review is to assess the impact and effectiveness of the IG-541 agent fire suppression system as an alternative to the halocarbon agent-based fire suppression system, given the possibility that end users will eventually phase out the halocarbon agent entirely as part of the ESG (Environmental and Social Governance) approach to comply with the local standards and regulations. This review was conducted over various aspects, including performance in an electrical fire, safety considerations, environment impacts, economic considerations, and system design comparison.

2. Review Methodologies

This review systematically investigates IG-541 system use in total flooding application focusing on suppressing the energized electrical fire, providing a comprehensive comparison and analysis of the literature published between 1994 and 2024. The methodology employed was designed to ensure scientific rigor, comprehensive coverage, and relevance of the reviewed material.

Database selection: The review utilizes well-established academic databases to collect relevant literature. Major reference databases selected for the research include the National Institute of Standards and Technology (NIST), the National Fire Protection Association (NFPA), the International Organization for Standardization (ISO), the United Nations Environment Programme (UNEP), and Scopus.

Search strategy: Specific and comprehensive search terms were developed, including keywords such as “IG-541”, “FM200”, “Novec 1230”, “FK-5-1-12”, “Inergen”, “hfc-227ea”, “total flooding application”, “energized electrical fire”, “halocarbon agent”, “inert gas”. The iterative search process ensured comprehensive coverage of the relevant literature.

Inclusion criteria: Studies were included based on their publication in peer-reviewed journals, conference proceedings, research reports, and book chapters. The selection criteria also required that these studies be published within the last 30 years, written in English, and directly related to the research topic.

Article selection: The initial search resulted in 418 articles, which were then evaluated for methodological quality and content relevance. During this process, duplicates and studies not related to the topic were excluded.

The selected articles were thoroughly analyzed and synthesized to offer a comprehensive and up-to-date overview of the state of the art on the topic of clean agent fire suppression systems in total flooding applications for energized electrical fires. This review paper makes significant contributions to the field of sustainable fire safety by providing an unparalleled breadth and depth of analysis.

3. Energized Electrical Fires Overview

Understanding the chemistry between fire and electricity is essential, especially when examining how electrical fires form. Electrical fire is a fire that is directly caused by the flow of electric current or a discharge of static electricity if it is not the result of normal and purposeful heat produced by an electrical device. Electric current is a form of energy resulting from the movement of electrons, while static electricity involves electric charges that accumulate on the surface and remain stationary rather than flowing as a continuous electric current [5]. Heat energy may be produced from electrical energy in several ways, including heat generated by lightning, heat from arcing, resistance heating, dielectric heating, induction heating, static electricity heating, and leakage current heating. Arc/spark and hot components are two broad categories of potential sources of ignition in electrical systems. An arc or spark is an electric discharge that typically produces a popping sound. This phenomenon occurs when the air or gas between two charged conductors becomes highly ionized, leading to a breakdown that allows current to flow through a visible, luminous channel. On the other hand, a hot component refers to an element that releases enough energy to ignite a surrounding combustible mixture when exposed to any ignition sources. The discharge of capacitive circuits, the interrupting (opening) of inductive circuits, the opening or closing of resistive circuits with slow, intermittent interruption increasing the igniting capacity (hazard), and hot temperature sources can all act as ignition sources due to the electrical system. The mechanisms for ignition can be found in fuses, relay contacts, switch contacts, short circuits caused by damage or component failure, and arcing over between conductors or components [6]. According to NFPA 921, for ignition to occur from an electrical source, the electrical wiring, equipment, or component must be energized by the building’s wiring, an emergency system, a battery, or some other source. Additionally, sufficient heat and temperature to ignite a close combustible material must have been produced by electrical energy at the point of origin by the electrical source. The ignition by electrical energy involves transferring sufficient heat to a fuel (i.e., competent ignition source) by passage of electrical current to ignite material that is close [7]. Electrical fires are a significant type of structure fire, but the mechanisms that lead to their ignition are often uniquely specialized and require specific research. Key areas needing investigation include the physical mechanisms of ignition from electrical current, the minimum voltage, current, or power levels necessary for an electrical fire to occur, the time frame for ignition, electrical fires related to medium- and high-voltage circuits, and the fire hazard or risk associated with code-compliant electrical installations [8].

Some electrical faults associated with wiring or wiring devices lead to fire situations that can involve a two-step sequence: first, overheating, then arcing and igniting. For example, a wire may get heated due to excessive current or a poor connection. This degradation can result in a short circuit, especially when the wire bends or passes over a metal edge [9]. The thermal conductivity of the wire core significantly influences the spread rate, potentially having an impact comparable to that of the wire’s dimensions. Generally, a wire with a smaller diameter and higher conductance (due to a larger wire core and greater thermal conductivity) will experience faster flame spread, indicating a higher fire hazard, although it may be more challenging to ignite. During flame spread, molten polymer dripping is observed in thicker-coated wires. This dripping occurs more frequently in wires with higher conductivity and at lower pressures [10]. Solid polymers do not burn directly; their volatile decomposition products ignite in the gas phase when mixed with oxygen. Ignition occurs when these gases reach a lower flammability limit for the particular fuel–air mixture. When the surface temperature of plastics at ignition is close to the thermal degradation temperature, the thermal degradation reactions on the plastic surface occur more quickly than the rate at which heat is absorbed. As a result, the heat transfer process primarily controls the burning rate, heat release rate, and smoke production during flaming combustion. The three coupled processes needed for flaming combustion are shown in Figure 1: (1) heating the polymer; (2) thermal decomposition of the solid polymer to gaseous fuel; and (3) ignition and combustion of the fuel gases in the air. Heat is applied to the polymer surface via an ignition source (such as arc/spark and hot component) or thermal feedback of radiant energy from the flame, which results in the thermolysis of the polymer molecules’ primary chemical bonds. Low-molar-mass degradation products evaporate, and when they react with air (oxygen) in the combustion zone above the surface, heat is released along with carbon dioxide, water, and incomplete combustion products such as soot, mineral acids, carbon monoxide, and unburned hydrocarbons [11]. The clean agent fire suppression system is introduced to break the fire triangle at one or more places to prevent burning. Halocarbon agents disrupt the uninhibited chain reaction of combustion by absorbing the thermal energy of a fire, preventing the combustion reaction from continuing. Inert agents reduce the oxygen level to a point where the fire can no longer sustain itself.

Compartment fire development can be divided into four stages: incipient, growth, fully developed, and decay. Flashover is not a stage itself but a rapid transition between the growth and fully developed stages. The progression of an incipient fire is largely influenced by the fuel’s characteristics and configuration, making it a fuel-controlled fire, as shown in Figure 2a. The air in the compartment provides sufficient oxygen for the fire to continue developing. If there is enough oxygen, additional fuel will ignite, increasing the fire’s heat release rate. As the gases in the compartment heat up, they expand and increase pressure within the confined space. The fire can grow further through flame spread or by igniting other fuels in the compartment. Flashover is the sudden transition from the growth stage to a fully developed fire, resulting in total surface involvement of all combustible materials in the compartment. Flashover conditions are defined in various ways, and it is important to note that flashover does not always occur; sufficient fuel and oxygen are required for it to happen. In the post-flashover stage, energy release is at its peak but is generally limited by ventilation. A compartment fire may enter the decay stage as the available fuel is consumed or due to limited oxygen [12]. Although some oxygen remains, visible flames diminish, and the fire continues to smolder. High heat and smoke conditions persist, creating the potential for a backdraft. A backdraft is a smoke explosion that can occur when additional oxygen is introduced into a smoldering fire, causing the heated gases to ignite explosively. Warning signs of a possible backdraft include heavy, dense smoke with no visible flames in a tightly closed space, black smoke pushing out around closed doors or window frames, smoke-stained glass, and pulsating pressure from the fire, as well as the reversal of air movement pulling smoke back into the building through a doorway [13]. An idealized curve of a ventilation-controlled fire is depicted in Figure 2b. Similar to a fuel-controlled fire, it begins with ignition and progresses through the growth stage. However, as high heat-of-combustion fuels burn within the nearly airtight compartment, the fire enters a decay stage due to the limited availability of oxygen necessary for combustion. As the oxygen supply diminishes, the fire’s heat-release rate and the gas temperatures within the compartment decrease. If a door or window is opened while the fire is still burning, albeit at a reduced intensity, and additional fuel is present, the influx of outside air can cause a rapid increase in the fire’s heat-release rate. This can generate enough energy to cause a flashover in the room, a phenomenon known as a ventilation-induced flashover. Once sufficient oxygen is available, the fire can reach the fully developed stage and may become fuel-controlled again until it eventually decays or until suppression by the fire department [14].

The typical four stages of fire growth for an electrical fire in a data center, along with the possible responses, are listed below [15]:

(1)

Incipient stage—Overheating of equipment/circuits and trace levels of combustion gasses equivalent to the lowest amount detectable by an aspirating system without other detection. The possible response will involve alertness and action by the occupant, accompanied by the activation of a pre-alarm signal.

(2)

Smoldering (visible smoke) stage—Increased burning detectable by human smell triggers spot-type smoke detection and activates the highest alert level for aspirating systems. The possible response will involve action by the occupant and pre-alarm signal activation.

(3)

Flaming stage—Pyrolysis and flaming combustion activate multiple spot-type detectors, leading to increased room temperature and the development of an upper gas layer. The possible responses include activating the fire alarm, shutting down the HVAC system, and initiating either the clean agent system countdown or releasing the solenoid valve in the pre-action system.

(4)

Fire growth/spread stage—The rapid production of large quantities of smoke quickly activates multiple spot detectors, leading to a rapid acceleration in heat release and the activation of the nearest sprinkler. The possible responses include activating the fire alarm, triggering the sprinkler or gaseous system discharge, and closing the fire/smoke dampers.

The four stages of fire growth for an electrical fire in a data center can also be applied to other electrical-related facilities, such as substations and telecommunications, as these environments primarily contain cables and electrical equipment. Clean agent fire suppression systems can be designed to activate early, during the flaming stage of a fire, ensuring minimal fire damage since the clean agent does not harm assets or occupants. In contrast, pre-action sprinkler systems only activate when the sprinkler heads reach a certain temperature, resulting in more downtime compared to clean agent systems and causing greater fire damage to the building.

While it is generally recommended to disconnect electrical equipment from its power source before attempting to extinguish a fire, equipment owners may opt to keep the equipment running due to considerations such as the risk of casualties from power shutdown, hazards to people or property posed by the fire, economic losses from interrupted functionality, or financial losses from facility damage. Therefore, there are situations in which the design of fire suppression systems is predicated on the presence of electrical equipment that has been activated. Unfortunately, there is currently no accepted test procedure to determine the exact quantity of agent required to put out flames in situations where the input of energy from an electrical source might increase combustion. Various test methods have been employed to simulate the effects on energized electrical equipment. These methods include tests designed to suppress flames during realistic electrical failure events involving representative polymeric materials, as well as tests that aim to control key parameters such as external heat flux (EHF) [16]. Several studies have been conducted to simulate the impact of different clean agent gas concentrations on energized polymeric materials and metal surfaces. The test findings typically demonstrate that higher agent concentrations are necessary in the presence of energy input from various sources [17,18,19,20,21]. Unlike NFPA 2001 [22], which clearly defines energized electrical fire as a Class C fire and specifies the minimum design concentration for each clean agent gas, ISO 14520-1 [23] provides a guideline for applications using energizing electrical equipment because it recognizes the more challenging nature of energized electrical fires. Under NFPA 2001’s Section 5.4.2.5, the Class A minimum flame extinguishing concentration gives a safety factor of 1.35, which yields the minimum design concentration for a Class C hazard. In contrast, energized electrical fires have been designated as higher-hazard class A hazards because they exhibit the characteristics mentioned in ISO 14520-1’s caution statement in 7.5.1.3. As a result, the minimum design concentration for an energized electrical fire shall be higher than that of surface class A or 95% of the class B minimum design concentrations. As shown in

Table 1. Comparison of NFPA 2001 and ISO 14520 minimum design concentrations for each clean agent gas on class C fire.

Agent	Minimum Design Concentration (vol%)
	NFPA 2001 [22]	ISO 14520
FK-5-1-12	4.5	5.6 [24]
HFC-125	9.0	11.5 [25]
HFC-227ea	7.0	8.5 [26]
HFC-23	20.3	16.3 [27]
IG-541	38.5	41.7 [28]
IG-55	42.7	45.1 [29]
IG-100	41.9	41.5 [30]

, ISO 14520 specifies a more cautious and higher minimum design concentration for each clean agent gas than NFPA 2001. It was explicitly stated in ISO-14520 that the minimum design concentration of higher hazard class A should be applied to the following conditions: cable bundles greater than 100 mm in diameter; cable trays with a fill density greater than 20% of the tray cross-section; horizontal or vertical stacks of cable trays (closer than 250 mm) apart; and equipment energized during extinguishment period where the collective power consumption exceeds 5 kW. In contrast, NFPA 2001 merely indicates that a higher extinguishing concentration may be necessary for these conditions.

4. Clean Agent Gaseous Fire Suppression System Overview

Fire protection applications can generally be classified into two main categories: (1) those that permit the use of water-based sprinklers and (2) special hazards that necessitate the use of alternative fire extinguishing agents, such as carbon dioxide, halon, halon replacements, dry chemicals, wet chemicals, or foams. From the 1920s to the 1960s, carbon dioxide was the predominant gaseous fire suppression agent. However, beginning in the 1960s, halon-based systems became extensively utilized. The application of carbon dioxide is now largely restricted due to factors that limit its effectiveness and the health risks it poses [31]. The extinguishing mechanism of carbon dioxide involves displacing oxygen to a level below that required for combustion. Consequently, carbon dioxide is not recommended for use as a fire suppressant in occupied spaces, as the concentration needed for fire extinguishing is lethal to humans. Halons 1301, 1211, and 2402 are highly effective and important fire-extinguishing chemicals, but they are also potent ozone-depleting substances (ODSs). Currently, the availability of halons depends entirely on recovered and recycled materials. Defined in Group II of Annex A of the Montreal Protocol, halons are a class of bromine-containing halogenated chemicals that have been, and in some cases continue to be, used as gaseous extinguishing agents in various fire and explosion protection applications [32]. In 1987, the Montreal Protocol began restricting the production and use of halons due to their significant environmental impact, particularly their role in ozone depletion and global warming. Consequently, halon production was phased out in Montreal Protocol countries by 1994, though it was allowed in Article 5(1) countries until 2009 [33]. The discontinuation of halon production has significantly affected the fire and explosion protection industry. Halons were known for being clean, non-conductive, safe for humans, and highly effective. However, identifying appropriate substitutes for their various applications continues to be a challenge for fire protection experts. To reduce reliance on halons, the use of conventional non-halon fire protection materials has been encouraged. Additionally, new clean agent replacement chemicals and alternative technologies have been rapidly introduced [34]. According to the Halons Technical Options Committee, several gaseous fire extinguishing agent technologies have been commercialized as alternatives to Halon 1301 for use in total flooding applications that are suitable for occupied spaces, as shown in

Table 2. Alternative gaseous fire extinguishing agent to halons for suitable use in total flooding applications [39].

Agent	Composition
Inert Gases
IG-01	Argon, Ar
IG-100	Nitrogen, N2
IG-541	Nitrogen, 52 vol%; Argon, 40 vol%; Carbon dioxide, 8 vol%
IG-55	Nitrogen, 50 vol%; Argon, 50 vol%
Hydrofluorocarbons
HFC-125	C2HF5—Pentafluoroethane
HFC-23	CHF3—Trifluoromethane
HFC-227ea	CF3CHFCF3—1,1,1,2,3,3,3-heptafluoropropane
HFC-236fa	CF3CHFCF3—1,1,1,2,3,3,3-heptafluoropropane
HFC Blend B	HFC-143a, CH2FCF3, 1,1,1,2-tetrafluoroethane, 86 wt%, HFC-125, C2HF5, pentafluoroethane, 9 wt%, Carbon dioxide, CO2, 5 wt%
Perfluoroketone
FK-5-1-12	CF3CF2C(O)CF(CF3)2—Dodecafluoro-2-methylpentan-3-one
Hydrochlorofluorocarbons
HCFC Blend A	HCFC-22, CHC1F2—Chlorodifluoromethane, 82 wt%, HCFC-124, CHC1F-CF3, 1-Chloro-tetrafluoroethane, 9.5 wt%, HCFC-123, CHC12-CF3, 1,1-dichloro-trifluoroethane, 4.75 wt%, Isopropenyl-1-methylcyclohexane, 3.75 wt%

. Many studies have been conducted to compare the performance of alternative agents to Halon 1301 in various contexts [35,36,37,38]. As shown in Table 2, the clean agents can be categorized into two main types: halocarbons and inert gases. Both types are used in total flooding extinguishing systems, where the agent is discharged throughout an entire protected area to quickly reach the minimum design concentration needed for fire extinguishment. The minimum design concentrations vary depending on the international standards selected, the type of agent, and the specific hazard to be protected. HFC-227ea and FK-5-1-12 are the most commonly used halocarbon agents in the market for combating energized electrical fires in total flooding applications. Therefore, this review will compare these agents with IG-541, a widely used inert gas, from various aspects.

A clean agent gaseous fire suppression system is an engineered set of components designed to detect and extinguish fire quickly while alerting occupants. It works by filling the protected area with a gas or chemical extinguishing agent to prevent extensive damage [40]. As illustrated in Figure 3, the typical elements and components are as follows:

(1)

Discharge nozzle—It is used to distribute the extinguishing agent into the protected area.

(2)

Piping system—It is used to transport the extinguishing agent (inert gas or halocarbon agent) from its storage container to the discharge nozzle.

(3)

Control Panel—It integrates all devices and displays their operational status and condition.

(4)

Discharge or warning alarms—Electronic devices that provide an audible or visual alarm when detected.

(5)

Hazard warning or caution signs—It must be posted at the entrance to, and inside, areas protected by fixed extinguishing systems.

(6)

Automatic fire detection devices—A device that detects fire and generates an alarm signal.

(7)

Manual discharge station—A device that provides a way to manually discharge the fire extinguishing system.

(8)

Storage containers and extinguishing agent—The storage system discharges agent into the piping and through the discharge nozzles when activated by a manual or automatic device.

Figure 3. Typical arrangement of the clean agent gaseous fire suppression system and the components are as follows: (1) discharge nozzles; (2) piping system; (3) control panel; (4) discharge or warning alarms; (5) hazard warning or caution signs; (6) automatic fire detection devices; (7) manual discharge station; (8) storage containers and extinguishing agent [40].

Figure 3. Typical arrangement of the clean agent gaseous fire suppression system and the components are as follows: (1) discharge nozzles; (2) piping system; (3) control panel; (4) discharge or warning alarms; (5) hazard warning or caution signs; (6) automatic fire detection devices; (7) manual discharge station; (8) storage containers and extinguishing agent [40].

In the event of a fire, clean agent systems are activated by a suppression releasing panel that detects the fire using an automatic detection system. Once a fire has been detected, a releasing sequence starts, often with a delay to allow occupants to evacuate. Notification appliances in the protected area sound an alarm for a predetermined duration before the system is activated. The gas is released from the cylinders by the releasing panel via an electronic signal to a solenoid valve on the storage cylinders. The gas flows through the piping system and is discharged through open nozzles to flood the protected enclosure [41]. The clean agent gaseous fire extinguishing system can also be configured with a selector valve system to protect multiple hazard areas using a shared cylinder bank. In this setup, the manifold is equipped with normally closed selector valves, each featuring its own actuator and solenoid valve assembly. When activated, these valves release the clean agent into the designated hazard area. A portion of the total clean agent cylinders can be allocated to a specific hazard, facilitated by the use of non-return (check) valves in the cylinder pilot line. To manage pressure, a restrictor is installed at the outlet of each selector valve, ensuring controlled flow into the distribution piping. Optional isolation (lockout) valves may also be installed upstream of the selector valves. These valves must remain supervised in an open position and should only be closed during system servicing or routine maintenance [42].

4.1. Halocarbon Agent

These chemical agents contain chlorine, fluorine, or iodine, either individually or in combination. The categories of these agents include hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and fluoroidocarbons (FICs). They share several common characteristics with halons: they are all electrically nonconductive, clean agents and exist as liquefied gases or compressed liquids. However, these agents vary significantly in terms of ozone depletion potential, toxicity, volume requirements, cost, environmental impact, and availability. For instance, PFCs are recommended only for use in situations where no other agent meets the technical performance or safety requirements. All of these systems require precise design to avoid decomposition and prevent the production of HF. It is important to note that these agents can be stored and discharged using systems similar to those used for Halon 1301 [43]. Halocarbon systems are effective in protecting against Class A, Class B, and Class C fires. Generally, all halocarbons extinguish fires through a similar mechanism. As a halocarbon gas approaches a fire, it absorbs significant amounts of heat energy from the combustion process, thereby lowering the surface temperatures of combustibles below their ignition point. Additionally, halocarbons release highly reactive halogen ions that combine with free hydrogen, which is a secondary fuel source released during a hydrocarbon fire. This elimination of free hydrogen helps to prevent the fire from spreading while also cooling the temperatures at the heat source. The suitability of a halocarbon fire extinguishing agent for use in occupied areas (i.e., areas with people) or solely in unoccupied areas (i.e., areas without people) is determined by its toxicity [44].

Hydrofluorocarbons (HFCs) are second-generation substitutes for compounds that contribute to human-caused stratospheric ozone depletion. Since they do not contain chlorine, bromine, or iodine, their ozone depletion potentials are nearly zero. With the phase-out of chlorofluorocarbons (CFCs) and their first-generation replacements of hydrochlorofluorocarbons (HCFCs), HFCs have gained increasing significance [45]. In the early 1990s, Great Lakes Chemical Corporation filed a patent for HFC-227ea to serve as a replacement for Halon 1301 in special hazards fire suppression systems [46]. Then, DuPont Fluoroproducts reached a patent cross-licensing agreement with Great Lakes Chemical Corporation (GLCC) regarding the use of HFC-227ea in fire protection products, marketed under the trademarks FE-227 and FM200, respectively. HFC-227ea, also known as 1,1,1,2,3,3,3-heptafluoropropane with the structural formula CF3CHFCF3, has been listed as an acceptable substitute for Halon 1301 and for new applications in the US EPA’s SNAP program for both total flooding and streaming systems [47]. While HFC-227ea contains fluoride, there are currently no outright bans on its use. However, some countries have implemented restrictions that limit the sale of HFC-227ea fire protection systems. Due to its global warming potential being greater than 1, HFC-227ea is subject to F-Gas regulations. The primary extinguishing mechanism of HFC-227ea involves heat removal as the agent decomposes in the presence of flames, with some additional chemical activity potentially occurring. Since HFC-227ea is heavier than air, uncontrolled leakage at low levels can negatively impact the maintenance of agent concentration after discharge [48].

Table 3. Chemical and physical properties of HFC-227ea [49].

Properties
Chemical name	1,1,1,2,3,3,3-Heptafluoropropane
Chemical formula	C3HF7
Molecular structure
PubChem CID	67940
CAS number	431-89-0
Molecular weight	170.03 g/mol
Color/form	Colorless gas
Odor	Light, ethereal odor
Boiling point	−16.34 °C
Melting point	−126.8 °C
Density	1.394 g/cu cm at 25 °C
Critical density	0.582 g/cu cm
Vapor density	4.2 at 20 °C (Air = 1)
Vapor pressure	390 kPa at 20 °C
Viscosity	0.0132 mPa.s at 25 °C (gas); 0.184 mPa.s (liquid)
Latent heat of vaporization	132.7 kJ/kg
Refractive index	1.407 at 25 °C/D

presents the chemical and physical properties of HFC-227ea, as documented by PubChem.

In 2004, the 3M Company introduced Novec 1230 fluid as a next-generation halon replacement with a low environmental impact and a high margin of safety [50]. The complete chemical name for Novec 1230 fluid is dodecafluoro-2-methylpentan-3-one with the structural formula CF₃CF₂C(O)CF(CF₃)₂. In the ASHRAE nomenclature, which is used in the NFPA and ISO 14520 clean agent standards, it is designated as FK-5-1-12. This substance is also referred to as “waterless water” or “dry water”. FK-5-1-12 was introduced as an environmentally friendly alternative to HFC-based gaseous agents like FM200 (HFC-227ea), FE13 (HFC-23), and ECARO (HFC-125), which are being phased out under the EU’s F-gas Regulation. FK-5-1-12 extinguishes fires primarily through cooling. Due to its high density, it is important to consider the design of the compartment where it will be used, particularly with regard to low-level uncontrolled ventilation paths [51]. It has proven to be an excellent alternative to ozone-depleting compounds, offering an ideal combination of performance, safety, environmental benefits, and commercial acceptance [52]. As a fluoroketone agent, it represents a valuable class of compounds capable of producing nonflammable molecules with short atmospheric lifetimes, making it a sustainable alternative to halons in fire protection. The brief atmospheric lifetimes of these compounds lead to low global warming potentials (GWPs). These environmental characteristics are significantly different from those of halons and their first-generation replacements, such as HFCs and PFCs [53]. It combines with air to form a gaseous mixture upon discharge. The heat capacity of this agent/air combination is significantly higher than that of air by itself. This gas combination will absorb more energy or heat for every degree that its temperature changes because of its increased heat capacity. The agent/air combination absorbs enough heat at the system design concentration to disturb the circumstances necessary for combustion to take place. The presence of the agent increases the heat loss from the fire to its surroundings. This causes the combustion zone to cool to the point where the fire is extinguished [54]. FK-5-1-12 has sufficient vapor pressure and a low heat of vaporization, allowing it to achieve and maintain gas concentrations well above the levels needed for effective fire extinguishing. Adiabatic calculations indicate that the room air and incoming liquid agent provide enough sensible heat to vaporize the agent sufficiently for extinguishment. This is similar to HFC-227ea, with comparable energy requirements for evaporation [55].

Table 4. Chemical and physical properties of FK-5-1-12 [54,56].

Properties
Chemical name	1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)pentan-3-one
Chemical formula	C6F12O, CF3CF2C(O)CF(CF3)2
Molecular structure
PubChem CID	2782408
CAS number	756-13-8
Molecular weight	316.04 g/mol
Physical state	Liquid
Color	Colorless
Boiling point	49.2 °C
Melting point	−108.0 °C
Density, saturated liquid	1.60 g/mol
Density, gas @ 1 atm	0.0136 g/mol
Heat of vaporization @ boiling point	88.0 kJ/kg
Critical density	639.1 kg.m3
Liquid viscosity @ 0 °C/25 °C	0.56/0.39 centistokes
Vapor pressure	0.404 bar at 25 °C
Vapor density	11.6 (Air = 1)

presents the chemical and physical properties of FK-5-1-12 as documented by PubChem and the 3M company. Among halocarbon agents, FK-5-1-12 is undoubtedly the best choice for use in total flooding applications due to its low global warming potential (GWP) and zero ozone depletion potential (ODP), as indicated in

Table 5. Potential environmental impacts for the halocarbon agents used in total flooding applications [22].

Agent	Global Warming Potential (GWP) 1	Ozone Depletion Potential (ODP)
HFC-125	3170	0
HFC-23	12,400	0
HFC-227ea	3350	0
HFC-236fa	8060	0
HFC Blend B	1400	0
FK-5-1-12	<1	0
HCFC Blend A	1500	0.048

¹ GWP is reported over a 100-year integrated time horizon.

. Additionally, there is no international mandate for its phase-out. However, 3M has proactively decided to cease production because the PFAS substances present in the agents could potentially affect human health and the environment, as proposed by the European Chemicals Agency (ECHA).

4.2. Inert Gas Agent

Most halon replacements, such as halocarbon agents, have a global warming potential and may be subject to future regulations under the Kyoto Protocol. Additionally, the use of these replacements for fire suppression often results in the production of toxic or corrosive by-products, like hydrogen fluoride and carbonyl fluoride, raising concerns about the safety of humans and property [57]. Therefore, it is highly desirable to find alternatives with performance characteristics similar to those of halocarbons. The latest commercially available inert gas systems use nitrogen gas, which serves both as the fire suppression agent and the pressurization agent. Nitrogen gas has been a staple in the fire suppression industry, primarily used as a pressurization gas in total flood–fire suppression systems and in portable fire extinguishers [58]. Inert gases, often known as noble gases or rare gases, include helium, neon, argon, krypton, xenon, and radon. These gases are chemically inert, which means they do not react with other elements or compounds. The term “inert gaseous fire extinguishing agents” refers to pure naturally occurring gases present in the earth’s atmosphere (e.g., argon, neon, or helium) or a blend of inert gases with others, such as an argon–nitrogen combination. These substances are kept as non-liquefied gases. The term “passive agent” is sometimes used to describe inert gas extinguishing agents due to their method of fire suppression. These agents reduce the amount of oxygen available, making the fire insufficient to sustain combustion. Although CO₂ is not technically an inert gas, it is used in fire safety and has comparable qualities [59]. Unlike halocarbon agents, inert gas agents do not disrupt the chemical chain reaction or the cooling of a fire. According to the study by Rattananon and Patvicaichod, increasing the concentration of inert agents does not contribute to lowering the temperature inside the room [60]. There are four types of inert gas agents approved for use in total flooding applications in accordance with NFPA 2001. IG-541 is a mixture of three inert gases: nitrogen (52%), argon (40%), and carbon dioxide (8%). IG-01 consists solely of argon, an unblended inert gas. IG-100 represents nitrogen, another unblended inert gas. IG-55 is a 50/50 blend of two inert gases, nitrogen and argon.

Table 6. Chemical and physical properties of IG-541 [64].

Properties
Trade name	Inergen
Chemical formula	52% N2, 40% Ar, 8% CO2
CAS number	7727-37-9, 7440-37-1, 124-38-9
Molecular weight	34.08 g/mol
Physical state	Gas
Color	Colorless
Odor	Odorless
Vapor density at 1 atm	1.416 kg/m3 at 20 °C
Relative density at 1 atm	1.18 at 20 °C

summarizes the chemical and physical properties of IG-541. IG-541 is an abundant, non-corrosive gas that neither supports combustion nor reacts with most substances. It consists solely of naturally occurring gases that have no adverse effects on the ozone layer or the environment as a whole. Unlike halons and halocarbon agents, IG-541 does not contain halogen components such as chlorine, fluorine, bromine, or iodine, resulting in zero ozone depletion potential. Furthermore, IG-541 is not classified as a chemical toxin and poses no mutagenic, teratogenic, or carcinogenic risks. It also does not affect the central nervous system or sensitize the heart, unlike the existing halons and halocarbon agents [61]. IG-541 offers a significant advantage over other types of inert gases due to its inclusion of carbon dioxide, which enhances respiratory characteristics and allows humans to survive in oxygen-deficient environments. The carbon dioxide in IG-541 increases blood flow and oxygen delivery to the brain, counteracting the effects of carbon monoxide, which reduces the amount of oxygen available to the brain by binding with hemoglobin. Typical individuals will not experience adverse effects from exposure to IG-541, even over extended periods. Additionally, individuals with pre-existing cardiac or pulmonary conditions are not negatively affected by IG-541 during evacuation and can safely exit the environment without impairment [62]. IG-541 extinguishes fires by reducing the oxygen content to a level that no longer supports combustion. When discharged into a room, IG-541 introduces a specific mixture of gases that enable a person to breathe in a low-oxygen environment. This mixture actually enhances the body’s ability to absorb oxygen. Normally, a room’s atmosphere contains about 21% oxygen and less than 1% carbon dioxide. If the oxygen content drops below 15%, most common materials will not burn. IG-541 reduces the oxygen content to around 12.5% while increasing the carbon dioxide content to approximately 3%. This rise in carbon dioxide boosts a person’s respiration rate and the body’s capacity to absorb oxygen [63].

4.3. Regulations and Standards

Halocarbon emissions contribute to ozone layer depletion and climate change. Therefore, preventing these emissions through the halocarbon agent regulatory framework shown in Figure 4 offers the dual benefits of protecting the ozone layer and mitigating climate change. International regulations and national government actions may affect the support for existing fire protection systems that use halocarbons and their future availability. Most countries have incorporated the requirements of ISO 14520 and NFPA 2001 standards into their local regulations to govern the type and usage of halocarbon agents. Both ISO 14520 and NFPA 2001 outline the minimum requirements for the design, installation, approval, and maintenance of total flooding and local application fire extinguishing systems that use halocarbon and inert gas agents. Any halocarbon agent intended for recognition or inclusion in this standard must first undergo an evaluation process equivalent to that used by the U.S. Environmental Protection Agency (EPA) Significant New Alternatives Policy (SNAP) Program for total flooding agents [22,23]. EPA’s Significant New Alternatives Policy (SNAP) identifies and evaluates substitutes for end-uses that have traditionally relied on ozone-depleting substances (ODSs). These substitutes are assessed based on environmental and health risks, considering factors such as ozone depletion potential, global warming potential, toxicity, flammability, and exposure potential. The lists of acceptable and unacceptable substitutes for total flooding applications are updated multiple times each year [65]. The Montreal Protocol mandates a gradual reduction in the consumption and production of various ozone-depleting substances (ODSs), with distinct timelines for developed and developing countries. Under this protocol, hydrochlorofluorocarbons (HCFCs) used in fire protection are set to be phased out by 2020 in developed countries and by 2030 in developing countries. Hydrofluorocarbons (HFCs), introduced as non-ozone-depleting alternatives to facilitate the phase-out of HCFCs, are also subject to phase-down. Developed countries are expected to begin reductions in 2019, while developing countries will start by freezing HFC consumption levels in 2024, with some nations following in 2028 [33]. The Kyoto Protocol identifies carbon dioxide, methane, nitrous oxide, and the fluorochemicals HFCs, PFCs, and SF6 as the main gases contributing to the greenhouse effect and potentially subject to emission controls. It mandates that developed countries first halt the growth of their greenhouse gas emissions and then reduce them to 5.2% below 1990 levels. The Doha Amendment to the Kyoto Protocol established a second commitment period from 2013 to 2020. Initially entering into force in 2005 and ratified by 192 Parties, the Kyoto Protocol has since been superseded by the Paris Agreement. Unlike the Kyoto Protocol, which required only developed countries to reduce emissions, the Paris Agreement acknowledges that climate change is a global issue and calls on all countries to set their own emissions targets [66]. In the European Union, Regulation (EU) No. 517/2014 (known as the F-Gas Regulation) establishes rules to protect the environment by reducing emissions of fluorinated greenhouse gases. To achieve this, it establishes rules for the containment, use, recovery, and destruction of these gases, along with related ancillary measures. It also sets conditions for the market placement of specific products and equipment that contain or rely on fluorinated greenhouse gases, imposes conditions on certain uses of these gases, and establishes quantitative limits for the market placement of hydrofluorocarbons [67]. On 7 February 2023, the European Chemicals Agency (ECHA) proposed a ban on an entire class of per- and polyfluoroalkyl compounds. This restriction aims to reduce emissions of PFASs and their degradation products, which can persist in the environment longer than any other human-made chemicals. Additional concerns include their bioaccumulation, mobility, long-range transport potential (LRTP), accumulation in plants, global warming potential, and (eco)toxicological effects [68]. As part of this shift, 3M announced on 20 December 2022 that it would cease all PFAS manufacturing by the end of 2025, including the production of Novec 1230 (FK-5-1-12). While NOVEC 1230 does not deplete the ozone layer and has a low global warming potential (GWP), it contains PFAS substances that may pose risks to human health and the environment [69]. Despite 3M’s exit, other manufacturers may continue to produce the FK-5-1-12 agent, which is identical to NOVEC 1230, as the 3M patent expired in July 2020.

Figure 5 illustrates the regulatory framework of clean agent suppression systems since the regulation of these systems is required to ensure their effectiveness, safety, and environmental protection. Third-party certification is a conformity assessment process conducted by an independent body, separate from both the supplier and customer organizations. This process ensures that products and services comply with specified standards and other normative documents. The advantages of third-party certification include risk reduction, prevention of costly errors, and time savings for specifiers and regulators. For manufacturers and installers, the benefits include increased global sales, enhanced product or service value, and reduced liability [70]. Under Underwriters Laboratories, UL certification includes UL 2166, the Standard for Halocarbon Clean Agent Extinguishing System Units [71], which details the requirements for the construction and operation of halocarbon clean agent fire extinguishing system units. Additionally, UL 2127, the Standard for Inert Gas Clean Agent Extinguishing System Units [72], specifies the requirements for the construction and operation of inert gas clean agent fire extinguishing system units. Both standards of systems must be installed, inspected, tested, and maintained in accordance with NFPA 2001 and the National Fire Code of Canada. Under Factory Mutual Approval, approval is conducted according to FM 5600, the Examination Standard for Clean Agent Extinguishing Systems [73]. This standard outlines the testing and certification requirements for fixed fire extinguishing systems that use vaporizing liquid or inert gas clean agents for total flooding protection. During testing, the full systems, including their design, installation, operation, and maintenance instructions, are evaluated to ensure they perform effectively under worst-case conditions. Under the Loss Prevention Certification Board, the requirements for fire testing of fixed gaseous fire extinguishing systems are outlined in the LPS 1230 standard [74]. This standard evaluates the performance of halon alternative fixed extinguishing systems, including non-liquefied inert and halocarbon gas types, in accordance with the ISO 14520 standard.

In marine systems, various regulatory bodies and standards govern the requirements for clean agent fire extinguishing systems to ensure vessel health and safety. These regulations provide guidelines for the design, installation, and maintenance of these systems. Most marine regulatory bodies follow the International Maritime Organization’s standards, specifically the International Convention for the Safety of Life at Sea (SOLAS) [75], which outlines minimum safety standards to be adhered to for constructing and operating merchant ships. Since its inception in 1974, the SOLAS Convention has undergone several revisions and amendments. In 1998, the Maritime Safety Committee revised the guidelines for fixed gas fire-extinguishing systems used in category A machinery spaces and cargo pump rooms, requiring them to demonstrate the same reliability as specified in SOLAS regulations II-2/7 and II-2/63. These systems must also be tested to ensure they can extinguish various types of fires that may occur in a ship’s engine room [76]. Further amendments were made to MSC/Circ.848 with the issuance of MSC.1/Circ.1267, which outlines the potential risks and mitigation measures associated with fluorocarbon fire extinguishing agents. During fires, hydrogen fluoride (HF) vapor can be produced from these agents, leading to health issues such as respiratory and eye irritation. Control stations and manned locations during a fire should ensure that HF and HCl levels remain below 5 ppm, and other hazardous products should be kept below harmful concentrations for the duration of exposure. For all ships, the fire-extinguishing system design manual should include procedures to control decomposition products like HF vapor generated from fluorocarbon agents, as longer exposure of the agent to high temperatures can produce higher concentrations of these gases, which can impair escape [77]. In 2000, the Maritime Safety Committee amended Regulation 10 4.1.3 of SOLAS Chapter II-2 under Resolution MSC.99(73) to prohibit the use of fire extinguishing systems employing Halon 1211, Halon 1301, Halon 2402, and perfluorocarbons on all new constructions and new installations on existing vessels [78].

5. Performance in Energized Electrical Fire

As mentioned earlier, there is no established test method to accurately determine the exact amount of agent required to extinguish an energized electrical fire. This section will focus on the extinguishing performance of IG-541, FK-5-1-12, and HFC227-ea in energized electrical fires, using various types of testing. Generally, for protecting electrical equipment, such as servers and cables, the design concentration for an ordinary combustible (Class A) fire should be used according to the relevant standard when the equipment and cables are de-energized. However, if the electrical equipment and cables are not immediately de-energized and there is a delayed power disconnect, the design concentration for an energized electrical fire (Class C) should be applied as specified by the relevant standard [79]. FM Global conducted research to determine the minimum extinguishing concentrations required for various clean agents when dealing with energized electrical equipment. This research involved a series of tests using FM Global’s fire propagation apparatus (FPA), as shown in Figure 6. In these tests, a sample of polymethyl methacrylate (PMMA) was placed in the FPA and exposed to a specific concentration of a clean agent. An 1100 W electrical arc was then introduced to see if the PMMA would ignite. If ignition occurred, the concentration of the clean agent increased in subsequent tests until the PMMA no longer ignited. The concentration at which ignition could no longer be achieved was identified as the minimum design concentration necessary for extinguishing fires in high-energy electrical environments.

Table 7. Minimum design concentration for IG-541 and halocarbon agents in Class C fires across different test methods.

Agent	Minimum Design Concentration on Class C Fire (vol%)	Reference
IG-541	38.5 (≤480 V), 57.0 (>480 V)	FM Global [79]
HFC-227ea	7.0 (≤480 V), 12.0 (>480 V)
FK-5-1-12	4.7 (≤480 V), 10.0 (>480 V)
IG-541	49.0 (48 watts), 56.1 (192 watts)	G. Bengston and R. Niemann [82]
HFC-227ea	8.0 (48 watts), 9.3 (192 watts)
FK-5-1-12	4.5 (48 watts), 5.1 (192 watts)

presents the minimum design concentration of the IG-541 and halocarbon agents needed to suppress the energized electrical fire. If the electrical equipment is not de-energized before the clean agent concentration drops below these minimum design concentrations, reignition is anticipated [80]. FM Global’s testing indicates that higher agent concentrations are necessary for high-energy arcing faults exceeding 480 V. However, it is crucial to assess and limit the use of these higher concentrations in typically occupied areas to minimize exposure duration, especially when these concentrations exceed the agent’s NOAEL (no observed adverse effect level). When evaluating the effectiveness of suppressing energized electrical fires exceeding 480 V, G-541 demonstrates greater efficiency compared to halocarbon agents. IG-541 requires approximately a 50% increase in concentration, whereas FK-5-1-12 requires about a 110% increase, and HFC-227ea requires roughly a 70% increase to achieve the same suppression results.

Nieman et al. [82] conducted experiments using nickel–chromium resistance wire powered by 48 watts and 192 watts of DC electricity to ignite PMMA samples, as shown in Figure 7, testing the extinguishing effectiveness of agents such as FC-3-1-10, HFC-227ea, and HFC-23, with Halon 1301 serving as a baseline. They concluded that clean agent extinguishing and inerting concentration values for Class “C” energized fires should be based on the energy levels to be protected. Higher energy circuits necessitate higher agent concentrations to ensure adequate fire suppression. To prevent reflash or reignition of Class “C” energized fires, clean agent concentration values should be designed to include the test inerting value plus a minimum 10% safety factor. The increased levels of HF generated when using clean agent inerting concentrations to prevent reignition or reflash may necessitate limiting the time that electronic circuits can remain energized to prevent corrosion damage to sensitive electronic equipment. Subsequently, Bengston and Niemann [83] extended these experiments to include other agents such as IG-541, FK-5-1-12, HFC-125, and HFC-236ea. Their findings were consistent with previous experiments, and they additionally found that the design concentration standards of ISO-14520 for Class “A” fires are insufficient to ensure suppression, as they do not adequately prevent reflash and reignition. Bengston and colleagues [84] discovered that FK-5-1-12 shows significant potential as an alternative to the replacement of Halon 1301, such as HFC agents for use in equivalent concentrations. Its performance profile suggests that FK-5-1-12 could be a major advancement in efforts to eliminate or replace global warming substances (PFCs and HFCs) in total flooding applications.

Smith et al. [85] evaluated the effectiveness of HFC-227ea and L-15566 (the experimental product number for FK-5-1-12) in extinguishing continuously energized fires using both the conductive heating test protocol and a modified version of this protocol. The conductive heating test protocol involved testing with two different cable types, each heated with 1000 watts of energy while operating on 240 VAC. Upon sustained ignition, the extinguishing agent was discharged at the desired design concentration and monitored for 10 min post-discharge. In contrast, the modified conductive heating test protocol introduced a continuous ignition source throughout the test. This was achieved by placing a 6000 V, 20 mA electrical arc above the cable specimens, simulating conditions where conductive heating of cable connections occurs near an electrical arc. This setup allowed for the evaluation of scenarios where an arc develops between electrical equipment in close proximity to heated cables. The results from the conductive heating test indicated that agent concentrations lower than standard design values could extinguish a specific fire scenario. However, under the modified protocol with an electric arc, these concentrations were less effective in extinguishing the fire and preventing reignition. This modified protocol highlights the need for higher design concentrations in situations where internal heating may occur, and there is a possibility of an external ignition source being nearby. Previous testing of L-15566 (FK-5-1-12) at a 3.5 vol% concentration resulted in reignition during the modified test. Bengston et al. [86] updated the examination by increasing the concentrations to 3.9%, 4.5%, and 5.1%, which successfully prevented reignition. They concluded that higher agent concentrations were necessary to prevent reignition/reflash with both cables. Nonetheless, FK-5-1-12 was able to extinguish and prevent reignition/reflash at much lower concentrations compared to HFC-227ea. McKenna et al. [87] conducted a study on the effectiveness of HFC-227ea in achieving fire control using the ohmic heating test, conductive heating test, and printed wiring board (PWB) failure test. The test samples, including cables and PWBs, were selected based on their common presence in the telecommunications industry. The study found that fires initiated by and involving energized electrical circuits could be controlled by HFC-227ea at concentrations below 7%. Most of the test fires were completely extinguished within 15 s of agent discharge. In all tests, the circuits remained energized for 5 min after agent discharge, and no re-ignition was observed. In the PWB tests, the initiating arc continued to propagate after agent discharge, with intermittent flickering flames observed attached to the arc. These intermittent flames were immediately extinguished.

Braun and colleagues [88] conducted tests using a short-duration auto-ignition temperature method with six clean agents: N2, IG-541, HFC-23, HFC-227ea, FC-2-1-8, and FC-3-1-10, as shown in Figure 8. The nickel metal foil used in the experiments was cut, notched, and folded, with the shiny side exposed to the fuel–air–agent mixture and the reverse side in contact with two thermocouples. The foil was heated using two 20-volt power supplies connected in parallel, providing a maximum current of 40 amps, capable of reaching temperatures up to 1000 °C. Initial experiments were conducted with each new foil using only an ethene–air mixture, without any agent. Subsequent experiments introduced increasing concentrations of fire suppressant agents. The study found that the introduction of clean agents generally required higher foil surface temperatures to prevent ignition than when no agents were present. The experimental data indicate a correlation between the number of fluorine atoms in the agent and its effectiveness in inhibiting hot surface ignition. Notably, in the presence of an electrically heated metal surface, a total flooding suppressant design concentration of 120% of the cup burner value was often insufficient to prevent the reignition of flammable gases from nearby pyrolyzing polymers. The volume fraction of agent needed to prevent ignition of a stoichiometric ethene–air mixture was found to be 1.1 to 5.9 times greater than the amount required in the cup burner test apparatus [89]. Smith and Rivers [90] used the same six clean agents to test the radiant-enhanced extinguishment device (REED), as shown in Figure 9 to determine the minimum extinguishing concentration of agents at specified radiation levels. The main components of the REED include a circular brass base, two concentric Pyrex tubes, a brass shield, a glass bead mixing chamber, a fuel platform, a conical heater, and three calibrated rotameters for monitoring gas flow. The tests were conducted at energy levels of 0, 20, 40, and 60 kW/m². The fuels chosen for the study were PMMA, grey PVC, natural ABS, and polypropylene. The specimens, which were in rod form, had aluminum foil wrapped around their bottom and curved surfaces to minimize edge effects and contain dripping. The extinguishing volume fractions were determined by gradually increasing the amount of extinguishing media until the flames from the samples were extinguished. The requirement for higher concentrations for fuels exposed to continuously energized electrical sources was reaffirmed. For energy levels above 20 kW/m², the design concentrations for N2, IG-541, and HFC-227ea would exceed their respective NOAEL thresholds.

In conclusion, IG-541 and halocarbon agents demonstrated comparable effectiveness in extinguishing fires, although higher agent concentrations were required to extinguish fires caused by higher energy levels in energized electrical equipment. Therefore, IG-541 is a viable replacement for halocarbon agents if they are completely phased out from the market. Further research is needed to examine the impact of higher energy levels and various types of materials on the extinguishing concentration value of IG-541. This information is essential for determining the maximum allowable exposure time in occupied spaces.

6. Environment Factor

Three key environmental characteristics are of particular interest in evaluating IG-541 and halocarbon agents as potential halon replacements are shown below:

(1)

Ozone Depletion Potential (ODP): ODP quantifies a chemical’s capacity to destroy stratospheric ozone. It is expressed as ozone depletion per unit mass of the substance emitted relative to a standard of CFC-11.

(2)

Global Warming Potential (GWP): GWP measures the impact of a chemical’s emission on global warming relative to a reference gas, typically carbon dioxide (CO₂).

(3)

Atmospheric Lifetime: This metric reflects the persistence of a chemical in the atmosphere. Chemicals with longer atmospheric lifetimes tend to contribute more significantly to global warming and may pose concerns regarding unexpected long-term environmental effects.

ODPs, GWPs, and atmospheric lifetimes are calculated but cannot be measured. HFCs are gaining increased attention as substitutes for ozone-depleting substances because, unlike HCFCs, they do not deplete the ozone layer and have shorter atmospheric lifetimes compared to PFCs. However, there remains significant concern about the role of HFCs in contributing to global warming [91]. As demonstrated in

Table 8. Environment characteristics of IG-541 and halocarbon agents: ODP, radiative efficiency, GWPs, and atmospheric lifetime.

Agent	ODP [22]	Radiative Efficiency (W m−2 ppb−1) [92]	GWP-20 1 [92]	GWP-100 1 [92]	GWP-500 1 [92]	Atmospheric Lifetime (Years) [92]
HFC-227ea	0	0.273	5850	3600	1100	36
FK-5-1-12	0	0.028	0.411	0.114	0.033	0.019
IG-541	0	0	0	0	0	0

¹ GWP is reported over a 20-year, 100-year, and 500-year time horizon.

, IG-541 and halocarbon agents do not have the potential to deplete the ozone layer because they do not contain bromine or chlorine, which are responsible for the destruction of stratospheric ozone by other ozone-depleting substances. However, it is important to understand how ozone-depleting agents that contain bromine or chlorine, such as halons and HCFCs, can damage the stratospheric ozone.

Ozone, a naturally occurring gas in our atmosphere with the chemical formula O₃, exists predominantly in two regions of the atmosphere. Approximately 10% of Earth’s ozone is found in the troposphere, which extends from the surface up to about 10–15 km in altitude. The remaining 90% is situated in the stratosphere, the atmospheric layer between the top of the troposphere and an altitude of about 50 km. Stratospheric ozone is naturally produced through chemical reactions involving solar ultraviolet (UV) radiation and oxygen molecules, which constitute about 21% of the atmosphere. Initially, solar UV radiation breaks apart an oxygen molecule (O₂) into two separate oxygen atoms (2O). Subsequently, each of these highly reactive oxygen atoms combines with an oxygen molecule to form an ozone molecule (O₃) in a binding reaction. These reactions continuously take place whenever solar UV radiation is present in the stratosphere. Consequently, the highest ozone production occurs in the tropical stratosphere. In the overall process, three oxygen molecules plus sunlight react to form two ozone molecules [93]

O₂ + UV→O + O

(1)

O + O₂→O₃

(2)

Overall reaction: 3 O₂ + UV→2O₃

(3)

Although the UV radiation dissociates the ozone molecule, ozone can reform through the following reactions, resulting in no net loss of ozone:

O₃ + UV→O₂ + O

(4)

O + O₂→O₃

(5)

Ozone is also destroyed by the following reaction:

O + O₃→O₂ + O₂

(6)

These reactions are referred to as the Chapman reactions. As altitude increases, the reaction between oxygen molecules and oxygen atoms slows down, whereas the reaction between ozone molecules and UV radiation accelerates [94]. Halons, HCFCs, and other halocarbons that contain chlorine or bromine have been demonstrated to contribute to the depletion of stratospheric ozone. While natural phenomena can lead to temporary ozone loss, chlorine and bromine released from human-made compounds like CFCs are now recognized as the primary cause of this depletion. These compounds are highly stable and do not readily break down in the lower atmosphere, allowing them to persist for extended periods. Due to their stability, these halogenated compounds eventually reach the stratosphere, where ultraviolet (UV) radiation breaks them down, releasing free chlorine atoms [95]. The chlorine atoms released from these compounds by photolysis and oxidation react rapidly with ozone, initiating a catalytic cycle through the following reaction:

Cl + O₃→ClO + O₂

(7)

ClO + O→Cl + O₂

(8)

O₃ + O→2 O₂

(9)

Stratospheric bromine is generated through the photolysis and oxidation by OH of brominated compounds released at the surface. Brominated hydrocarbon compounds, such as halons, are produced solely by industrial activities. The most efficient catalytic cycle involving bromine atoms is one that couples bromine and chlorine radicals.

BrO + ClO→Br + ClO₂

(10)

ClO₂ + M→Cl + O₂

(11)

Br + O₃→BrO + O₂

(12)

Cl + O₃→ClO + O₂

(13)

2 O₃→3 O₂

(14)

In conclusion, the relative significance of chlorine and bromine in the stratosphere is determined by their concentrations and the reaction rates of their specific catalytic cycles. Consequently, this process is effective only at altitudes above 30 km, where the concentration of atomic oxygen (O) is sufficient. At lower altitudes, the destruction of odd oxygen occurs through different catalytic cycles that do not require atomic oxygen [96].

The quantity present in the atmosphere must be monitored annually over a specific time period, as the global warming potential (GWP) is a cumulative effect calculated over that duration. These calculations account for the atmospheric lifetime of the chemical, as well as its infrared-absorbing properties, which also influence its GWP. GWPs are used to estimate the future global warming impact of a substance over a given time frame by multiplying the weight of the greenhouse gas by its GWP for that specific time horizon. The resulting value can be compared with others to determine which substance will have the least or greatest impact over that period. Emitting a large amount of low-GWP greenhouse gas may result in less global warming than a small release of high-GWP greenhouse gas over a certain time horizon. Different time horizons can yield different outcomes [97]. The global warming potential (GWP) estimates the cumulative warming effect of a unit mass of a specific greenhouse gas in the current atmosphere compared to that of carbon dioxide (CO₂). Carbon dioxide, known as reference gas, is given a GWP value of 1, regardless of the time horizon or other factors that may affect the GWPs of other greenhouse gases. GWPs are formulated based on the concept of radiative forcing, which links radiative forcing to climate response. This metric allows for the comparison and aggregation of the climate effects of different greenhouse gases (GHGs). A higher GWP indicates a greater potential for a GHG to cause global warming. The GWP value is influenced by two main factors: the ability of the GHG to trap heat in the atmosphere, described by its radiative forcing or radiative efficiency, and the atmospheric lifetime of the GHG, described by its decay rate. The radiative efficiency of a gas is not constant, so changes in the concentration of the reference gas CO₂ affect the GWPs of all other GHGs. GWPs are calculated for specific time horizons, with the Intergovernmental Panel on Climate Change (IPCC) providing values for 20, 100, and 500 years. Therefore, the choice of time horizon affects the relative importance of different gases when using the GWP metric [98]. The most commonly used time periods are either 20 years (GWP-20) or 100 years (GWP-100), being the norm used to report aggregated GHG emissions, including some IPCC communications [99]. Hydrofluorocarbons (HFCs) are being used as replacements for many chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) that are being phased out under the Montreal Protocol on Substances that Deplete the Ozone Layer. Unlike these ozone-depleting substances (ODS), HFCs do not harm the ozone layer. However, they are extremely potent greenhouse gases (GHGs), with a global warming potential up to thousands of times greater than that of carbon dioxide. The use of HFCs is currently increasing more rapidly than any other category of GHGs. Projections indicate that HFC use could increase by as much as 30-fold by 2050, potentially contributing up to 0.1 °C to the global average temperature rise by mid-century and increasing up to fivefold, potentially contributing up to 0.5 °C by 2100 [100]. Although HFCs are harmless to the stratospheric ozone layer and generally have lower radiative efficiencies compared to the most common ozone-depleting substances (ODSs), they are potent greenhouse gases. As a result, HFCs were included in the list of substances regulated by the 1997 Kyoto Protocol under the United Nations Framework Convention on Climate Change (UNFCCC). Later, specific HFCs were incorporated into the Montreal Protocol framework through the Kigali Amendment in 2016. HFC-227ea is present in the atmosphere at lower abundances and contributes relatively little to the overall radiative forcing of HFCs. It is utilized as a fire suppressant, mainly serving as a replacement for halons in both streaming and total flooding applications [101]. It is crucial to understand the naturally occurring greenhouse effect and identify the greenhouse gases, such as HFCs, that contribute to it. Solar radiation enters the Earth’s atmosphere from space, with a portion being absorbed by the Earth’s surface. The Earth then re-emits this radiation back toward space, but its properties change from high-frequency solar radiation to lower-frequency infrared radiation. Greenhouse gases, which are transparent to solar radiation, effectively absorb infrared radiation. Consequently, this radiation, which would have otherwise escaped into space, is retained, leading to a warming of the atmosphere [102]. Therefore, the high radiative efficiency of HFC-227ea leads to a higher global warming potential compared to FK-5-1-12 and IG-541, as shown in Table 8.

With the phase-out of halons, HCFCs, and HFCs, FK-5-1-12 is becoming a popular alternative as an environmentally friendly fire suppression agent due to its negligible global warming potential (GWP). A study by Taniguchi et al. found that FK-5-1-12 is removed from the environment through photolysis, which occurs over approximately 1–2 weeks. Its global warming potential is negligible due to its short atmospheric lifetime. When FK-5-1-12 undergoes photolysis in air, it produces CF₃C(O)F and COF₂. CF₃C(O)F is absorbed into rain, clouds, or seawater, where it hydrolyzes to form trifluoroacetic acid, while COF2 hydrolyzes to produce CO₂ and HF [103]. A study conducted by MIT explored the atmospheric loss mechanisms of FK-5-1-12, demonstrating that while the chemical does not react with hydroxyl radicals (OH), it undergoes significant degradation when exposed to ultraviolet (UV) light. The authors evaluated the UV cross-section of FK-5-1-12 and found a maximum absorption wavelength of 306 nm. Photolysis in the lower atmosphere will act as a substantial sink for this chemical due to its high absorbance at wavelengths over 300 nm [104]. The scientists came to the following conclusion: “In fact, the absorption spectrum is comparable to that of acetaldehyde” [105], a species with a roughly 5-day lifespan against solar photolysis [106]. Hence, they anticipate that FK-5-1-12’s atmospheric lifetime against solar radiation will be between three and five days because of its bigger absorption cross-sections. According to recent laboratory observations, FK-5-1-12’s photodissociation rate is comparable to acetaldehyde’s, within an experimental error [107]. Jackson and colleagues [108] examined the potential roles of photolysis, hydrolysis, and hydration in determining the environmental fate of FK-5-1-12. They found that hydration is not a significant pathway for FK-5-1-12. Although the hydrolysis rate constant is much higher than that for photolysis, the typically low levels of liquid water in the atmosphere make hydrolysis less impactful. Even during nighttime, the production of PFPrA and HFC-227ea would be minimal, indicating that photolysis is the predominant atmospheric fate of FK-5-1-12. Therefore, photolysis will always be more dominant than hydrolysis due to the low amount of liquid water in the atmosphere, which outweighs the higher hydrolysis rate constant. Even during heavy rain at night, significant hydrolysis is unlikely to occur. Nevertheless, the FK-5-1-12 underwent a re-assessment using the IPCC 2021 evaluation method and was subsequently reported in the IPCC WGI Sixth Assessment Report. By applying the IPCC 2021 evaluation method with a 100-year integration time horizon and a radiative forcing of 0.028 Wm-2ppb-1, a global warming potential (GWP) value of 0.114 was determined [92]. As a result, its relatively low GWP value makes it significantly more environmentally friendly than HFC-227ea, although it still cannot compare to IG-541, which has a GWP value of zero.

The atmospheric lifetime of HFC-227ea, a hydrogen-containing fluorocarbon, is predominantly determined by its reaction with hydroxyl (OH) radicals, while other degradation pathways, such as photolysis and reactions with O₃, HO₂, or NO₃, are negligible. The rate coefficient used to calculate the tropospheric lifetime of HFC-227 suggests it is approximately 40 years. The main products of HFC-227ea’s tropospheric oxidation are identified as trifluoroacetylfluoride (CF₃COF) and, by implication, carbonylfluoride (COF₂). Both products are expected to undergo rapid hydrolysis in the tropospheric cloud environment, resulting in trifluoroacetic acid (CF₃COOH), CO₂, and HF [109]. CF₃COOH is not toxic to animals but does exhibit a mild herbicidal effect. The concentration of CF₃COOH anticipated in rainfall due to the atmospheric degradation of HFCs is significantly lower than levels known to affect plant systems. The hydrolysis of COF₂ and CF₃-OH produces HF and CO₂, which do not pose environmental concerns [110]. Prior to the research conducted by Zellner et al., Nelson et al. [111] determined that the atmospheric lifetime of HFC-227ea is 42 years, using a similar approach that evaluated its reaction with OH radicals as the dominant tropospheric sink. In contrast, oxidation by hydroxyl radicals and wet deposition/rain out are ineffective at removing FK-5-1-12 from the atmosphere. The primary atmospheric sink for FK-5-1-12 is photolysis. FK-5-1-12 has been shown to photolyze rapidly in the atmosphere, resulting in a short atmospheric lifetime. The degradation products from the atmospheric decomposition of FK-5-1-12 already exist in the environment and are not expected to be harmful. Despite its rapid degradation in the environment, FK-5-1-12 remains sufficiently stable for use as a clean extinguishing agent [112]. D’Anna et al. [113] conducted a photolysis study under natural sunlight conditions in a simulation chamber and estimated that the atmospheric lifetime of FK-5-1-12 is approximately one week. Despite FK-5-1-12 having a large integrated absorption cross-section, its short atmospheric lifetime results in a negligible global warming potential. In conclusion, according to the IPCC documented values, the atmospheric lifetimes for HFC-227ea and FK-5-1-12 are 36 years and 0.019 years, respectively, which align with the researchers’ evaluations. HFC-227ea has a significantly longer atmospheric lifetime compared to FK-5-1-12, indicating that HFC-227ea is more persistent in the atmosphere. This persistence contributes to the growing concern regarding its impact on global warming. In contrast, because of its mixture of naturally occurring gases, IG-541 has excellent environmental credentials and contains no harmful hydrogen fluoride-producing chlorofluorocarbons (CFCs). It is also considered an inert gas with no adverse effect on the ozone layer, zero global warming potential, and no atmospheric lifetime. The suppression agent is not regarded to be harmful to the environment and does not come under the ’F-gas Regulations’ because it contains no fluorinated greenhouse gases [114]. Although IG-541 contains 8% CO₂, which is known to contribute to global warming, this CO₂ is not artificially produced like in halocarbon agents; instead, it is directly sourced from the environment. As such, IG-541 is a highly environmentally friendly option, especially for total flooding applications and suppressing energized electrical fires, compared to halocarbon agents.

It is worth noting that 3M discontinued manufacturing of FK-5-1-12 owing to the presence of PFAS substance, even though the PFAS contained in FK-5-1-12 is not on the EU regulation’s banned list. Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic organic compounds characterized by a wide range of structures, properties, uses, bioaccumulation potentials, and toxicities. Despite their diversity, all PFAS share the common feature of containing perfluoroalkyl groups that are highly resistant to environmental and metabolic degradation. Consequently, most PFAS are either non-degradable or eventually transform into stable terminal products, which are still classified as PFAS [115]. The European Chemicals Agency (ECHA) [68] has proposed an unprecedented ban on the entire class of per- and polyfluoroalkyl compounds. If approved, this prohibition will impact over 7 million such substances, as reported by PubChem [116]. This will undoubtedly affect the FK-5-1-12 agent if regulatory action is taken because it is a fully fluorinated ketone and fits under the previously stated broad definition of “PFAS.” Fire Suppression Systems Association (FSSA) expressed a divergent perspective and strongly urged ECHA to exclude FK-5-1-12 agents from the ban list. FK-5-1-12, in contrast to aqueous film-forming foam (AFFF), has the potential to partition into the groundwater during end use. Upon discharge from the fire suppression system, it rapidly transforms into gas and completely breaks down in the atmosphere within approximately one week. Thus, FK-5-1-12 is not considered persistent in the environment [117]. Furthermore, the Environment Protection Agency (EPA) has classified FK-5-1-12 as an acceptable use under the Significant New Alternatives Policy (SNAP) program for total flooding applications. This is because it does not present a higher overall environmental and human health risk compared to other approved alternatives for this specific purpose [118].

7. Safety Consideration

The toxicity of a clean agent fire suppression system in a total flooding application is a major safety factor, particularly in the normally occupied space. Table 6 presents data regarding the toxicological and physiological impacts of IG-541 and halocarbon agents, as defined by NFPA 2001. It also includes the safety margin of each agent in comparison to the minimum design concentration specified by NFPA 2001 and ISO 14520 standards. The no observable adverse effect level (NOAEL) is the highest concentration at which no adverse physiological or toxicological effect has been observed. The lowest observable adverse effect level (LOAEL) is the lowest concentration at which an adverse physiological or toxicological effect has been observed. Cardiac sensitization and hydrogen fluoride (HF) formation from thermal decomposition products are the primary acute toxicity effects of the halocarbon agents. LC50 is the concentration lethal to 50 percent of the rat population during a 4 h exposure, specifically for halocarbon agents. It is a statistically derived concentration that, over a specified exposure period, is expected to result in the death of 50% of the test animals within a given timeframe. This term is relevant to experiments involving inhalation exposure and aquatic toxicity testing [119]. Test Guideline No. 403 (TG 403) is designed to gather sufficient information on the acute toxicity of a test substance to enable its classification and provide lethality data for one or both sexes, as needed for quantitative risk assessments. This guideline presents two testing methods. The first method is a traditional protocol where groups of animals are exposed to either a limit concentration (limit test) or a series of concentrations in a stepwise manner for a predetermined duration, typically 4 h, although other exposure durations may be used to meet specific regulatory requirements. The second method is a (C × t) protocol, where groups of animals are exposed to either a single limit concentration or a series of multiple concentrations over various durations [120]. The study, conducted in accordance with TG 403 guidelines, involved exposing rats to a concentration of 788,696 ppm of the test material. The substance was continuously administered in oxygen to the test chamber for a duration of 4 h. Following exposure, the rats were monitored for a period of 14 days and then subjected to terminal necropsy. The results indicate that HFC-227ea has an acute inhalation LC50 greater than 79 vol% in rats [121]. This finding is consistent with the NFPA documented value of a concentration greater than 80 vol%. As shown in

Table 9. Toxicity information and safety margin for IG-541 and halocarbon agents use in occupied space for total flooding application.

Agent	LC50 (vol%) [22]	NOAEL (vol%) [22]	LOAEL (vol%) [22]	Class C Fire Minimum Design Concentration (vol%) 1	Safety Margin (%) Compared to NOAEL 2	Safety Margin (%) Compared to LOAEL 3
HFC-227ea	>80.0	9.0	10.5	7.0–8.5	6–29	24–50
FK-5-1-12	>10.0	10.0	>10.0	4.5–5.6	79–122	79–122
IG-541	-	43.0	52.0	38.5–41.7	3–12	25–35

¹ The range of minimum design concentrations for Class C fires is determined by using the NFPA standard as the lower limit and the ISO 14520 standard as the upper limit. ² The range of the safety margin is determined by comparing the Class C fire minimum design concentration with the NOAEL, ranging from the lower percentage of the ISO 14520 standard to the higher percentage of the NFPA standard. ³ The range of the safety margin is determined by comparing the Class C fire minimum design concentration with the LOAEL, ranging from the lower percentage of the ISO 14520 standard to the higher percentage of the NFPA standard.

, the acute inhalation LC50 of FK-5-1-12 is greater than 10 vol%, whereas the acute inhalation LC50 of HFC-227ea is greater than 80 vol%. This indicates that HFC-227ea has lower acute inhalational toxicity compared to FK-5-1-12.

Cardiac sensitization is the condition in which the heart becomes temporarily more susceptible than usual to the arrhythmogenic effects of endogenous catecholamines when exposed to high concentrations of numerous halogenated and unsubstituted hydrocarbons. It is measured in dogs after they have been exposed to halocarbon agents for 5 min. At a 5 min interval, an external dose of adrenaline is given, and any resulting cardiac sensitization in the dog is documented. Five minutes of exposure is typically selected in advance of the adrenaline challenge dose to simulate the exposure duration involved in “sniffing” situations. The maximum blood level is typically attained after about five minutes of exposure to a given chemical concentration [122]. The lowest dose at which a cardiac response is observed is referred to as the LOAEL (lowest observed adverse effect level). The next lowest dose tested, at which no positive response is observed, is referred to as the NOAEL (no observed adverse effect level). These endpoints are determined in animals that have already been primed for a cardiac response due to the high level of epinephrine challenge. Without an external epinephrine challenge, the concentration required to evoke a response would need to be much higher. The EPA applies the LOAEL and NOAEL endpoints from these dog tests directly to humans, recognizing the high sensitivity of the test procedure. Therefore, the LOAEL and NOAEL levels established by this test procedure are very conservative estimates of the levels that would trigger a cardiac response without an external source of epinephrine [123]. In summary, the NOAEL levels for FK-5-1-12 and HFC-227ea, as shown in Table 9, indicate that cardiac sensitization occurs at 10 vol% and 9 vol%, respectively. This results in a safety margin of 6 to 29 percent for HFC-227ea and 79 to 122 percent for FK-5-1-12 when compared to the minimum design concentration for Class C fires as specified in NFPA 2001 and ISO 14520. The LOAEL for FK-5-1-12 mirrors the safety margin of the NOAEL, as its level is above 10 vol%. For HFC-227ea, the LOAEL occurs at 10.5 vol%, yielding a higher safety margin of 24 to 50 percent compared to the NOAEL. In conclusion, FK-5-1-12 is safer for humans, providing a higher safety margin at both LOAEL and NOAEL levels compared to HFC-227ea. It also ensures minimal hazardous effects on human health in normally occupied spaces with total flooding applications to protect energized electrical equipment.

One scientific approach used to better understand the health effects of chemicals is known as physiologically based pharmacokinetic (PBPK) modeling. A PBPK model relates the amount of chemical exposure to the concentration of the chemical in the blood and organs over time. For instance, PBPK models can help determine whether a toxic level of a chemical would be present in the blood or an organ of a person after exposure to a certain amount of the chemical through drinking water. These models are developed using mathematical values (referred to as “parameters”) and equations that describe various characteristics and processes of the body, such as body weight, blood flow rate, and metabolism rate. The purpose of PBPK models is to interpret animal toxicity data in the context of human health, a process known as extrapolation. By adjusting the physiological parameters in a PBPK model that describes a chemical in a laboratory animal species, the model can be adapted for humans. This allows for more accurate and human-relevant predictions [124]. The time for safe human exposure to HFC-227ea at specified concentrations is detailed in NFPA 2001’s Table 4.3.2.3 (c). According to this table, exposure to HFC-227ea is considered safe for at least five minutes at a concentration of 10.5%, which corresponds to the LOAEL level. It is important to evaluate the duration of safe exposure in relation to the concentration of the agent required to extinguish a fire. By comparing the concentration for safe five-minute exposure to the recommended design concentration of the agent, we can gain insights into safety margins. For HFC-227ea, the safe exposure concentration is 10.5%, while the recommended design concentration for Class C fires is 7.0%. This indicates that even though the safe exposure concentration is at the LOAEL, there is a 3.5% difference between the safe exposure concentration and the recommended design concentration for HFC-227ea [125]. In contrast, since no PBPK model has been conducted for FK-5-1-12, it is not possible to make a comparison between the concentration for safe five-minute exposure and the recommended design concentration of the agent based on the PBPK model.

The extent of thermal decomposition products generated when a halocarbon agent interacts with a flame front depends on three critical factors that affect the time required to extinguish the fire. These factors are the ratio of the fire size to the enclosure volume, the volumetric concentration of the agent, and the discharge time. Testing of thermal decomposition products has demonstrated that significant concentrations of HF can be produced when fluorinated halon replacement agents are used for fire suppression. Typically, HF concentrations for these agents are between 2 and 10 times higher than those for Halon 1301. It is important to note that these peak HF concentrations generally decrease quickly. Tests have indicated that HF concentrations can drop to half of their peak levels within 5 min after the agent is discharged [126]. Hydrogen fluoride (HF) is a colorless, fuming liquid or gas with a strong, irritating odor. The following acute health effects may occur immediately or shortly after exposure to HF. It can severely irritate and burn the eyes, potentially causing permanent damage. Contact with HF can cause irritation and severe burns to the skin and deep tissues. Burns may develop hours after contact, even if no pain is initially felt. HF can irritate the nose and throat. Inhaling HF can irritate the lungs, leading to coughing and/or shortness of breath. Higher exposures may result in a build-up of fluids in the lungs (pulmonary edema), a medical emergency characterized by severe shortness of breath. Exposure to HF can cause headaches, dizziness, nausea, and vomiting. Very high exposure can lead to fluoride poisoning, which includes symptoms such as stomach pain, weakness, convulsions, collapse, and potentially death. The following chronic health effects can occur sometime after exposure to HF and may last for months or years. Repeated inhalation of HF may cause bronchitis, characterized by coughing, phlegm, and/or shortness of breath. HF may damage the liver and kidneys. Long-term exposure to very high concentrations can cause fluoride deposits in the bones and teeth, a condition known as “Fluorosis” (changes in bone structure). This can result in bone pain, fractures, disability, and mottling of the teeth [127]. The Occupational Safety and Health Administration (OSHA) defines the immediate hazardous to life or health (IDLH) for HF concentration as 30 ppm, while the legal airborne permissible exposure limit (PEL) is 3 ppm averaged over an 8 h work shift [128].

It is important to note that HF gases will react with moisture to form hydrofluoric acid on the surfaces of electronic equipment. The corrosion damage caused by HF gas and acid, along with smoke and fire effluents, can lead to metal loss, electrical current leakage, and increased contact resistance. These damages may result in serious electrical short circuits or cause the electronic equipment to receive false data, disrupting electronic systems and potentially leading to system failure. The damage caused by HF, smoke, and fire effluents affects not only the equipment located near the fire but also remote equipment. Even if there is no immediate reaction on the electronic equipment from HF exposure, long-term damage due to HF corrosion is possible. The corrosion process on electronic equipment does not cease once the fire is extinguished. The extent of corrosion damage depends on factors such as HF concentration, temperature, and relative humidity in the enclosure, the duration of exposure, the nature of the combustible material involved, and the properties of the exposed equipment [129]. Kim et al. [130] conducted a study on the use of HFC-227ea as a total flooding–fire suppression agent for in-cabinet cable fires and a small liquid fuel pool fire in a simulated electronic equipment room, focusing on the measurement of acidic gas products. The study found that the extinguishment time of a cable bundle fire is affected by the cabinet’s ventilation conditions, which in turn influence the levels of acidic gases like HF. In a closed fire cabinet, HF concentrations peaked at 800 ppm, whereas in an open or ventilated cabinet, they stayed below 100 ppm due to quicker extinguishment. There was minimal migration of HF to adjacent non-fire cabinets, and HF levels in the test room generally remained below 100 ppm, which is considered safe for short-term exposure. However, in the case of large fires, HF concentrations in the test room reached hazardous levels, averaging 2000 ppm over 10 min, with substantial HF transport to non-fire cabinets. This situation poses a risk of corrosion to electronic equipment and health hazards to individuals during large fires when using HFC-227ea. Therefore, early fire detection is essential to control small fires before they grow, ensuring the safety of both people and property.

Enlow [131] illustrated the complete combustion of the FK-5-1-12 agent, which results in the production of hydrogen fluoride and carbon dioxide:

C₆F₁₂O + 2.5O₂ + 6 H₂O→6 CO₂ + 12 HF

(15)

The combustion of the FK-5-1-12 agent will, therefore, generate 12 volumes of hydrogen fluoride per molecule basis. Under dry conditions, the combustion of the FK-5-1-12 agent results in incomplete combustion and the production of carbonyl fluoride. Carbonyl fluoride is a highly reactive gas that reacts with available water to produce carbon dioxide and hydrogen fluoride via the following reaction:

CF₂O + H₂O→CO₂ + 2 HF

(16)

The magnitude of the HF concentration for a “normal” sized room is illustrated in Figure 10. It is a useful tool for estimating the thermal decomposition products for a specific room size. For example, to maintain HF concentrations below an acceptable range in a 1000 ft³ (28 m³) room, fire detection should be activated before the room reaches 3.7 kW in size. The severity of a fire is relative to the size of the enclosure. Increasing both the fire size and the enclosure size may not result in higher concentrations of HF during extinguishing. However, increasing the fire size while maintaining the enclosure size has been shown to increase HF production. Consequently, it is expected that larger fire sizes (one order of magnitude larger within the same enclosure) than those used in this study would have resulted in higher HF production [132]. Ditch et al. [133] demonstrated that FK-5-1-12 exhibits comparable performance to currently available halon alternatives in thermal decomposition product (TDP) testing. The experiments confirmed its effectiveness in extinguishing fires fueled by various fuels under a wide range of conditions. The results revealed a correlation between HF production and factors such as fire size, discharge time, and agent concentration. Notably, shorter exposure times to fire consistently resulted in lower TDP generation. Figure 11 depicts the dangerous toxic loads (DTLs) for humans based on Meldrum’s analyses and compares the level of HF measured during extinguishment of fire less than 25 kW [134]. The DTL corresponds to exposure levels that would result in significant distress for all exposed personnel. The testing results reveal that levels of HF produced in the referenced fires, where the FK-5-1-12 agent is used as an extinguishant, fall significantly below the DTL curve. As a result, the FK-5-1-12 agent can be designed to operate optimally for personnel protection and sensitive assets, reducing the generation of HF that would exacerbate the potential hazard posed by a fire [135]. Thus, the design of FK-5-1-12 systems should focus on quick fire detection and rapid discharge to minimize the risk of excessive HF exposure and maximize the safety of both personnel and sensitive assets.

The thermal decomposition of HFCs in fire suppression systems can produce high concentrations of HF gas. Depending on the specific HFC chemistry and the size of the fire, the IDLH levels of HF gas can be reached in under 45 s. This hazard is particularly dangerous when an HFC suppression system is used indoors. The creation of toxic byproducts during fire scenarios presents additional risks to first responders and any nearby personnel beyond the immediate danger of the fire itself, and they may be unaware of these risks [136]. Hynes et al. [137] conducted a shock tube study on the pyrolysis of CF₃CHFCF₃ within a temperature range of 1200–1500 K and at pressures of 16–18 atm. The study utilized gas chromatography (GC-MS) and FTIR spectroscopy for product analysis, aiming to develop a comprehensive chemical mechanism to describe the decomposition of CF₃CHFCF₃. Experimental results and modeling indicated that the primary initiation reaction involved bond fission (CF₃CHFCF₃→CF₃ + CF₃CHF). Additionally, the abstraction of the secondary hydrogen atom by fluorine atoms was identified as a significant reaction pathway. In contrast, the 1, 2 HF elimination process (CF₃CHFCF₃→CF₃CF=CF₂ + HF) was found to occur at a slower rate. Copeland et al. [138] investigated the thermal decomposition of HFC-227ea at low pressure, highly diluted in argon, across a temperature range of 600–2000 °C using photoelectron spectroscopy. The primary decomposition reaction at lower temperatures (600–900 °C) was identified as the thermodynamically favored reaction producing CF₃CF=CF₂ and HF. At elevated temperatures (900–1200 °C), the decomposition pathway yielding C₂F₄ and CF₃H became more significant. No evidence was found for the reaction CF₃CHFCF₃→CF₃CHF + CF₃, which was anticipated based on a shock tube study conducted at much higher pressures, nor for CF₃CHFCF₃→CF₃CF + CF₃H. For the latter reaction, it is likely that CF₃CF is converted into C₂F₄ under the experimental conditions before photoionization occurs in the ionization region of the photoelectron spectrometer. At even higher temperatures, C₃F₆ was observed to decompose into C₂F₄ and CF₂, while C₂F₄ further decomposed into CF₂. The report by Andersson et al. examines the production of HF and COF₂ during experiments with halocarbon agents HFC-227ea and FK-5-1-12. It was found that at lower application rates, most fluorine from HFC-227ea ends up as HF. As the concentration approaches extinguishing levels, the proportion of COF₂ increases, but total fluorine recovery, including HF, remains below 40% at a 90% application rate. FK-5-1-12 shows lower fluorine recovery as HF and COF₂ compared to HFC-227ea. The yield of HF decreases for both agents as the application rate increases, with HFC-227ea producing a high yield of HF at low rates, while FK-5-1-12 yields significantly less HF at similar rates. No data are available for high application rates for both agents. The yield of COF₂ increases with higher application rates for all agents. At low rates, the extinguishing agent combusts more, transferring fluorine to HF. At higher rates, less combustion occurs, favoring COF₂ formation over HF. Effective fire extinguishing requires sufficient agent quantity and distribution to minimize harmful combustion products [139].

Since inert gases are not suitable for assessing cardiac sensitization like halocarbon gases, the values for the IG-541 agent in Table 6 are derived from the physiological effects in humans in a hypoxic environment and equivalent to the NOAEL and LOAEL values provided for the FK-5-1-12 agent. The IG-541 agent’s 43% and 52% design concentrations correspond to NOAEL and LOAEL values, which equate to 12% minimum oxygen concentration for the no-effect level and 10% minimum oxygen concentration for the low-effect level, respectively. The primary health risk associated with the use of inert agents is hypoxia, which occurs when there is an insufficient level of oxygen reaching the body’s tissues and organs. Hypoxia can result from inhaling air with a low oxygen concentration. IG-541 extinguishes fires by reducing oxygen levels to a point where combustion cannot continue. However, this creates a hypoxic environment that can be harmful to humans. The extent of potential harm depends on oxygen concentration and the duration of exposure to the hypoxic atmosphere. At very low oxygen levels (below 8 percent), exposure can be potentially fatal [140]. Acknowledging the presence of CO₂ in IG-541 and its beneficial effects on blood oxygenation and cerebral blood flow, the New Extinguishants Advisory Group (NEAG) has established the following guidelines for the use of IG-541:

(1)

The oxygen concentration must not drop below 10% at any time while people are present.

(2)

For oxygen concentrations between 10% and 12%, exposure should be limited to no more than 2 min.

(3)

For oxygen concentrations between 12% and 15%, exposure should be limited to no more than 20 min.

(4)

The final CO₂ levels should be maintained between 2.5% and 5%.

It is important to note that in a fire situation, elevated CO₂ levels can help maintain normal cognitive function due to their positive effects on blood oxygen saturation and cerebral blood flow, which can aid in evacuating the room. However, the increased ventilation and potential reduction in breath-hold time induced by higher CO₂ levels may also enhance the body’s uptake of toxic materials from the fire [141]. In comparison to the NFPA 2001 guidelines for human exposure limitations, as shown in

Table 10. Human exposure limitations for IG-541 and halocarbon agents used in total flooding application as specified in NFPA 2001 [22].

Agent Design Concentration (vol%)	Occupancy	Exposure Time Limit	Remarks
HFC-227ea
≤9.0	Normally occupied	Up to 5 min	No PBPK data needed
>9.0 and ≤10.5	Normally occupied	Up to 5 min	PBPK data
>10.5 and ≤11.0	Not occupied	Up to 1.13 min	PBPK data
>11.0 and ≤11.5	Not occupied	Up to 0.60 min	PBPK data
>11.5 and ≤12.0	Not occupied	Up to 0.49 min	PBPK data
FK-5-1-12
≤10.0	Normally occupied	Up to 5 min	No PBPK data needed
>10.0	Not occupied	Up to 30 s	No PBPK data available
IG-541
≤43.0	Normally occupied	Up to 5 min	≥12% oxygen concentrations
>43.0 and ≤52.0	Normally occupied	Up to 3 min	<12.0% and ≥10.0% oxygen concentrations
>52.0 and ≤62.0	Not occupied	Up to 30 s	<10.0% and ≥8.0% oxygen concentrations
>62.0	Not occupied	No exposure allowed	<8.0 oxygen% concentrations

, there are notable differences in the exposure limits for oxygen concentrations between 10% and 12%. While NFPA 2001 allows an exposure limit of up to 3 min, the NEAG guideline restricts this to no more than 2 min. Additionally, NFPA 2001 permits up to 5 min of human exposure for oxygen concentrations above 12%, whereas the NEAG guideline extends the exposure limit to 20 min for oxygen concentrations between 12% and 15%. When suppressing Class C fires in accordance with NFPA 2001 and ISO 14520 standards, the safety margin relative to the NOAEL value of the IG-541 agent is reduced to only 3% to 12%, despite the improved breathing effects due to the addition of CO₂ in the IG-541 agent. Although the safety margin can be increased from 25% to 35% when compared to the LOAEL value of the IG-541 system, the personnel exposure time limit must be reduced to 3 min in normally occupied spaces, as indicated in Table 10.

Table 10 presents the human exposure limits for IG-541 and halocarbon agents, as specified in NFPA 2001. These limits specify the agent design concentration in relation to the type of occupancy and the allowable exposure time, specifically for use in total flooding applications. To prevent unnecessary exposure to the agents, both inert and halocarbon agents have a maximum exposure time of 5 min. Additionally, unprotected personnel are prohibited from entering a protected space during or after agent discharge. The FK-1-12 agent, with a design concentration above 10%, is permitted on the condition that personnel may evacuate the area within 30 s and that it is utilized exclusively in unoccupied spaces.

Table 11. The overall effect of reduced O₂ concentrations [142].

O2 (vol%)	Effects and Symptoms
18–21	No discernible symptoms can be detected by the individual. A risk assessment must be undertaken to understand the causes and determine whether it is safe to continue working.
11–18	Reduction of physical and intellectual performance without the sufferer being aware.
8–11	Possibility of fainting within a few minutes without prior warning. Risk of death below 11%.
6–8	Fainting occurs after a short time. Resuscitation possible if carried out immediately.
0–6	Fainting almost immediate. Brain damage, even if rescued.

summarizes the overall effects of decreased oxygen concentration. Inert gases such as nitrogen, argon, and helium are odorless, colorless, and tasteless, making them particularly dangerous because they provide no warning of their presence or the resulting decrease in oxygen levels. For individuals unaware of their surroundings, the onset of asphyxiation can occur without warning symptoms and may happen very quickly, sometimes in just seconds, if oxygen levels are extremely low. As such, you may not realize you are in danger until it is too late [142]. According to the NFPA standard, as shown in Table 10, human exposure to IG-541 agents is limited to concentrations below 43%, which corresponds to an oxygen concentration of 12%, for no more than 5 min in normally occupied spaces. When gases other than oxygen are introduced or mixed with the air we breathe, the oxygen concentration decreases, leading to oxygen deficiency. If the IG-541 agent causes this deficiency, a person’s physical and mental performance can decline without realizing it. Fainting can occur unexpectedly when oxygen levels drop to around 10%. If oxygen concentration falls below 10%, death from asphyxia can occur within minutes unless resuscitation is administered promptly [143]. Therefore, Table 10 shows the installation of IG-541 in a normally occupied space is prohibited if the agent design concentration exceeds 52% to ensure that it is safe for human exposure. At a design concentration exceeding 62%, the installation of the IG-541 agent is restricted to an unmanned room and human exposure is prohibited due to its equivalence to an oxygen concentration of less than 8%. Profound fainting upon exposure to oxygen concentrations below 8% can lead to severe consequences, such as brain damage in individuals, even if they are promptly rescued. In conclusion, the HF gas generated in fires as a breakdown product of the halocarbon agents can cause toxic effects on humans and corrosion damage to electronic equipment. However, the IG-541 does not decompose measurably during the extinguishment of a fire. When comparing the NOAEL and LOAEL values, the design concentration of the FK-5-1-12 agent has a much larger safety margin than the IG-541 and HFC-227ea.

8. Economic Factor

It is imperative for business owners who seek to prioritize safety within budget constraints to understand the several factors that affect the cost of installing a clean agent fire suppression system. The main considerations that impact the economic aspect of implementing the system are building size and layout, system type and complexity, compliance requirement, installation labor, and material cost, as well as maintenance and testing. Gordon and Preston [144] evaluated the comparative costs of the IG-541 system against alternative gaseous fire suppression systems for hydroelectric generator applications. The cost of IG-541 as a gaseous agent is significantly lower than that of HFC-227ea and FK-5-1-12, as it uses naturally occurring atmospheric gases. However, IG-541 requires a larger quantity of gas compared to halocarbon agents. When comparing FK-5-1-12 to HFC-227ea, FK-5-1-12 is typically about 10% more expensive. The equipment cost of the IG-541 system is higher due to the substantial number of bottles and extensive manifolding required, whereas halocarbon systems involve far fewer valves. Annual maintenance costs for IG-541 and halocarbon systems are similar, driven by the testing requirements outlined in relevant standards and codes. However, over a ten-year period, maintenance costs can vary significantly due to gas bottle retesting expenses, which are considerably lower—by a factor of 5 to 10 times—for systems with fewer bottles. Capital budget differences between systems are narrowing due to increased competition and the availability of similar generic products. Additional factors such as manifold configurations and redundancy requirements also impact system costs. Robin [145] evaluated the quantities of inert gas and halocarbon agents required to extinguish Class A and Class B fires in a 100 m³ enclosure. The findings showed that HFC-227ea requires the least mass for extinguishment compared to IG-541 and FK-5-1-12. While FK-5-1-12 requires a larger mass than HFCs, it also has a higher cost per unit mass compared to both HFCs and inert gases, making FK-5-1-12 the least cost-effective option among clean agent systems. Inert gas agents, due to their physical properties, cannot be compressed into a liquid state and must be stored as high-pressure gases. This necessitates high-pressure storage cylinders and piping, significantly increasing the cost of inert gas systems. Additionally, their low volumetric efficiency and inability to store liquids require a larger number of cylinders, leading to increased storage space, a larger system footprint, and higher overall costs. In contrast, halocarbon agents, except HFC-23, can be stored as liquids in standard low-pressure cylinders, allowing a greater mass of agents to be stored in a given cylinder volume. This reduces the number of cylinders and the associated storage requirements, making these systems more space-efficient and cost-effective. As a result, system costs for inert gas agents escalate more rapidly with increasing system size compared to halocarbon systems.

The IPCC [146] documented Wickham’s study, which compared the average values of space, weight, and cost for IG-541 and halocarbon agent systems against a Halon 1301 system. The comparison focused on protecting room volumes ranging from 500 m³ to 5000 m³ in compliance with IMO regulations for shipboard machinery spaces, as detailed in

Table 12. Comparison of average values in the 500 to 5000 m³ range, using the Halon 1301 system as the basis [138].

Agent	Concentration (vol%)	Weight (kg/m3)	Footprint (104 m2/m3)	Cube (104 m3/m3)	System Cost (USD/m3)	Agent Cost (USD/m3)
HFC-227ea	8.7	1.1	6.8	13.1	28.05	19.08
FK-5-1-12	5.5	1.2	7.3	13.8	35.98	26.59
IG-541	40.0	4.3	28.2	56.6	34.07	9.62

. The weight includes the storage containers and their contents, excluding piping and hangers. Footprint refers to the area occupied by the agent containers, defined by the dimensions of the cylinder bank. Cube represents the volume occupied by the containers, calculated as the footprint area multiplied by the cylinder height, measured to the top of the valves. System cost includes the agent, storage containers, actuators, brackets, hoses, valves, controls, alarms, and other components sold by manufacturers to distributors or installers. It excludes distribution piping, supports, fittings, electrical components, installation labor, packing, and shipping costs. The analysis revealed that HFC-227ea systems had the lowest average values for weight, footprint, cube, and overall system cost. However, IG-541 systems had the lowest agent cost compared to FK-5-1-12 and HFC-227ea. While FK-5-1-12 systems required less weight, footprint, and cube than IG-541 systems, IG-541 maintained a cost advantage in terms of agent pricing over FK-5-1-12. Further details on the comparison of IG-541 and halocarbon agent systems for 500 m³, 1000 m³, 3000 m³, and 5000 m³ volumes, focusing on weight, footprint, cube, and cost, are available in Wickham’s report [147]. In terms of system weight, the HFC-227ea system is the most weight-efficient, with the IG-541 system being three times heavier. FK-5-1-12 and HFC-227ea systems are nearly identical in weight. Regarding system footprint, the IG-541 system requires significantly more floor space compared to halocarbon agent systems, with cubic space requirements closely aligning with the footprint. When comparing costs, HFC-227ea emerges as the most cost-effective option. However, cost evaluations can vary significantly when considering factors such as selector valve systems, bulk storage configurations, and combined-use systems. The IG-541 system is frequently configured with selector valve arrangements, allowing a single agent supply to protect multiple hazards. This configuration is highly cost-effective, as the expense of the agent supply can be distributed across several applications. In both land-based and marine systems, there is a point at which transitioning from high-pressure cylinder storage to low-pressure bulk storage becomes more economical due to the increasing agent weight. Additionally, some regulatory bodies and local authorities mandate combined system use to ensure comprehensive protection for equipment and room spaces. For instance, SOLAS regulations mandate a local application water system for hazardous machinery spaces alongside a total flooding system for machinery spaces in marine environments.

The IPCC [148] also documented a comparison of primary clean agent fire extinguishing systems currently in use, under development, or demonstrated for fixed systems suitable for occupied spaces. The analysis highlights relative cost differences among the options. FK-5-1-12 has the highest initial investment cost, as well as higher annual service and end-of-life recovery costs, compared to IG-541 and HFC-227ea. Annual service costs account for replacing 2% of the agent lost per year. For halocarbon agents, the end-of-life value is positive, as recovered agents can be recycled and resold. This cost recovery is equivalent to 50% of the agent’s initial cost, enabling reuse in new systems or replenishment of existing ones. Wolf’s [149] cost study provides a more comprehensive analysis than Wickham’s, as it includes installation labor and recharge costs. The cost comparison for IG-541 and halocarbon agents, considering new system installation and one or two recharge events, is shown in

Table 13. System cost comparisons of IG-541 and halocarbon agents for the 4000 and 60,000 cubic-foot spaces considering new installations with and without recharges [149].

Agent	New System Installation	New System Installation and One Recharge	New System Installation and Two Recharges
Cost Over Life of Small System—4000 Cubic Foot (USD)
IG-541	42,000	45,088	48,176
FK-5-1-12	36,000	51,696	57,840
HFC-227ea	35,000	40,276	45,552
Cost Over Life of Large System—60,000 Cubic Foot (USD)
IG-541	220,000	246,700	273,400
FK-5-1-12	130,500	198,640	266,796
HFC-227ea	109,500	162,580	215,660

for 4000 and 6000-cubic-foot systems. The analysis indicates that for smaller spaces, the cost of a new system installation is similar for IG-541 and halocarbon agents, assuming no discharges over the system’s lifespan. For larger spaces, the initial cost of installing an IG-541 system is significantly higher if no discharges occur. If there is one recharge during the system’s life, the total cost for small spaces remains comparable across all agents. For large spaces with one recharge, the costs for IG-541 and FK-5-1-12 are similar, while HFC-227ea is less expensive. For smaller systems, if there are two discharges, FK-5-1-12 becomes more expensive than both IG-541 and HFC-227ea. In larger systems with two recharges, IG-541 and FK-5-1-12 have the highest costs. Additionally, Bank et al. [150] conducted a life-cycle assessment comparing the environmental impacts of HFC-227ea and IG-541 in a typical UK-based fire suppression system. The study revealed that approximately six times the amount of IG-541 is required compared to HFC-227ea to achieve equivalent performance. Consequently, six cylinders of IG-541 would be needed in place of a single HFC-227ea cylinder.

In conclusion, each fire suppression system offers distinct advantages, making the installation costs of IG-541 and halocarbon systems generally comparable. Among the options, the HFC-227ea system is the least expensive for initial installation compared to IG-541 and FK-5-1-12. However, the high agent cost of FK-5-1-12 leads to higher overall costs than IG-541 and HFC-227ea when future recharge expenses are factored in. The IG-541 system requires a significantly larger footprint and greater weight than halocarbon systems due to its high volumetric requirements, necessitating a larger number of cylinders. Inert gas systems are commonly designed to protect multiple areas, using a cylinder bank with enough capacity to safeguard the largest enclosure. Directional valves are employed to direct the gas to the appropriate hazard area, with gas supplied either through individual piping from the cylinder bank or via a high-pressure manifold extending throughout the protected building [151]. While the FK-5-1-12 system also supports selector valve configurations, its 10 s maximum discharge period limits its application to nearby hazards unless the protected rooms are closely situated. In data centers and substations, raised floors are often built to provide a flexible and efficient environment for managing cables and cooling systems. According to NFPA 75, A.9.1.1.3 notes that halocarbon agents should not be used to protect the space below a raised floor unless the space above the floor is also protected. If a fire exists in the equipment above the raised floor, agents below the extinguishing concentration may be exposed, and halocarbon agents may undergo significant decomposition. An inert gas system must be used to protect the space under a raised floor if only the space is to be protected by a total flooding system [152].

9. System Design Consideration

Table 14 provides a comparison of IG-541 and halocarbon agents based on system characteristics and design requirements. IG-541 is stored as a compressed gas in steel cylinders at storage pressures of either 200 bar or 300 bar. In contrast, halocarbon agents are stored as compressed liquids in steel containers, super-pressurized with nitrogen to 25 bar, 42 bar, or 50 bar to improve discharge flow characteristics. Consequently, the minimum design working pressure for system piping upstream of the pressure reducer must align with the storage pressure. This necessitates the use of higher-grade piping materials for IG-541 systems compared to halocarbon agent systems. For halocarbon agent systems, the maximum permitted discharge time is 10 s, which is defined as the period during which all liquid agents pass through the nozzle. The total discharge time is extended due to the expulsion of nitrogen and agent vapor from the system. The 10 s discharge time limit for halocarbon agents aims to achieve four key objectives [153]:

(1)

Ensure high flow rates through nozzles for adequate mixing of the agent with air inside the enclosure.

(2)

Provide sufficient velocity through pipes to maintain a homogeneous flow of liquid and vapor.

(3)

Limit the formation of agent thermal decomposition products.

(4)

Minimize direct and indirect fire damage, especially in rapidly developing fire scenarios.

Among these objectives, minimizing the formation of agent thermal decomposition products is the most critical concerning discharge. For Class C fires, the IG-541 system has a discharge time requirement of 120 s. The primary reasons for this limitation, despite IG-541 producing no decomposition products, are the following:

(1)

To limit direct and indirect fire damage.

(2)

To minimize the duration that the fire burns in a depleted oxygen atmosphere.

Table 14. System characteristic and design requirements comparison between IG-541 and halocarbon agents.

Agent	Stored Pressure (bar)	Discharge Time (s) for Class C Fire	Pipe Run Distance	Visibility After Discharge	Pressure Effect	Combustion Product Ventilation	Concentration Hold Time	Agent Recovery, Recycling, and Reclamation
HFC-227ea	25/42/50	10	Short	Low	Positive + Negative	Yes	Weak	Yes
FK-5-1-12	25/42/50	10	Short	Low	Positive + Negative	Yes	Weak	Yes
IG-541	200/300	120	Long	High	Positive	No	Good	No

Table 14. System characteristic and design requirements comparison between IG-541 and halocarbon agents.

Agent	Stored Pressure (bar)	Discharge Time (s) for Class C Fire	Pipe Run Distance	Visibility After Discharge	Pressure Effect	Combustion Product Ventilation	Concentration Hold Time	Agent Recovery, Recycling, and Reclamation
HFC-227ea	25/42/50	10	Short	Low	Positive + Negative	Yes	Weak	Yes
FK-5-1-12	25/42/50	10	Short	Low	Positive + Negative	Yes	Weak	Yes
IG-541	200/300	120	Long	High	Positive	No	Good	No

The IG-541 agent is stored and discharged as a gas, which minimizes energy loss due to phase change and results in lower friction losses. This enables the IG-541 agent to travel longer distances through smaller-diameter pipes. In contrast, the halocarbon agent stores the agent as compressed liquid within the cylinder, pressurized with dry nitrogen to propel the agent through the piping system. The liquid is transformed into a gas as it travels under pressure through the discharge piping due to generated friction. Compressed liquids do not flow as well as inert gases since their pressure in the pipe is lower. Consequently, the halocarbon system’s discharge pipe length is more constrained than the IG-541 system. Discharging halocarbon agents creates the appearance of fog, which may reduce visibility. However, this typically clears quickly and should not obstruct personnel’s ability to safely escape the protected area. There is no significant condensation of air moisture during the IG-541 agent discharge; therefore, the gases do not reduce escape route visibility [154].

The effectiveness of a gaseous total flooding fire suppression system relies on retaining the air–extinguishant mixture within the protected area for a certain period. This requires minimizing gas exchange (leakage) between the enclosure and the outside environment, necessitating a high degree of enclosure integrity. If the enclosure is too tightly sealed during extinguishant discharge, the resulting pressure change could exceed the structural limits of its surfaces, such as windows and doors. Conversely, if there is too much vent area, rapid gas exchange will shorten the retention time of the extinguishant. Therefore, gaseous fire suppression systems must manage pressure within the protected area during discharge and ensure the extinguishant–air mixture is retained for a specified period afterward. Pressure venting of enclosures is crucial for designing a clean agent fire prevention system because it reduces the forces caused by changes in enclosure pressure when gaseous agents are released into an enclosure, hence preventing the likelihood of structural integrity failure. Furthermore, the operation of pressure vent or extract systems requires the removal of mixtures of air/gaseous media, combustion products, and extinguishant breakdown products from a protected enclosure into the environment [155]. Figure 12 depicts typical pressure excursions on the structure of the protected enclosure during inert gas and halocarbon agent discharge. The peak positive and negative pressures generated during the discharge of halocarbon fire suppression systems can potentially damage the structural components of the protected area, such as walls, studs, and windows, if not properly installed. If these structural components are damaged during system discharge, it can no longer be assured that the enclosure will maintain the necessary concentration of the agent long enough to effectively suppress the fire. While the choice of agent is a significant factor, it is only one of many that influence the magnitude of pressure peaks within the protected space. Other critical factors include agent concentration, wall construction, the size and location of enclosure leakage areas, fire size, humidity, and retention time, all of which play a role in the overall pressure dynamics. The discharge characteristics of the halocarbon agent system typically include an increase in negative pressure at the beginning of the discharge, which is partly caused by the rapid temperature decrease, followed by a rise in positive pressure. Therefore, a dual-flow damper with both a positive (outward) and a negative (inward) opening damper is needed [156]. The initial negative pressure spike results from the cooling effect due to the agent’s high heat capacity, which absorbs sensible heat during its rapid vaporization upon discharge. The subsequent positive pressure spike occurs because of the relatively rapid introduction of the gaseous agent and propellant mass into the fixed volume of the enclosure [157]. The discharge of inert gas agents is characterized by a rapid increase in pressure to a peak level, followed by a relatively slow decrease over time. For this type of discharge, an outward-opening damper is needed, as positive pressure is consistently generated during an IG-541 system discharge. The pressure dynamics of inert gas agent discharges are notably different from those of halocarbon-based agent discharges. The thermodynamic properties of a halocarbon-based agent determine the extent of heat transfer during discharge, resulting in varying pressure dynamics for different halocarbon-based agents [158]. The IG-541 system’s constant flow design prevents peak pressure increases during discharge and introduces lower pressure into the piping system. The valve is specially designed to evenly distribute the gas flow into the protected area, reducing the need for venting and minimizing the risk of over-pressurization. This design offers an excellent alternative to standard high-pressure inert gas systems, which typically have a distinctive peak flow and pressure spike, necessitating larger, higher-specification pipework and a greater venting area [159].

Mechanical extraction systems may be necessary for both halocarbon and inert gas agents, albeit for different reasons. Halocarbon agents can decompose when exposed to flames and hot surfaces, producing acidic byproducts that may cause corrosion if not promptly removed. Therefore, mechanical extraction is essential when using halocarbon agents. In contrast, inert gases do not generate acidic decomposition products, but mechanical extraction is still preferred to eliminate smoke and soot deposits dispersed throughout the room by the force of the inert gas discharge. To prevent the fire from re-igniting, the extinguishing gas should be retained for a period of time after discharge, which typically requires a well-sealed enclosure. Upon fire detection, the compressed agent, whether a halocarbon or an inert gas, is released into the enclosure, causing a peak pressure of approximately 250 to 1250 Pascals for a brief moment. The exact pressure depends on the total leakage area of the enclosure. Once the enclosure is fully flooded with the clean agents, they will begin to leak out at a rate primarily determined by the leakage area in the lower part of the enclosure. The distribution of the remaining agent will either stay consistent throughout the enclosure due to continuous mixing or form an interface with the air above and the agent below, which will gradually descend over time [160]. The IG-541 has a density similar to that of air [63], resulting in slower leakage and allowing the gaseous agent to be stored for longer periods. In contrast, the FK-5-1-12 has a higher density than air [161], leading to a weak concentration hold time and faster leakage. Although the density of HFC-227ea [162] is lower than FK-5-1-12, it is still higher than IG-541, which also results in a weak concentration hold time and faster leakage. Consequently, a higher volume of halocarbon agent is required to compensate for the faster leakage while maintaining the minimum hold time.

To minimize the environmental impact of halocarbon agents, recovery and recycling should be performed safely and responsibly, adhering to established quality standards. The FIA [163] provides guidance outlining the criteria for recycled halocarbon agents and their use in discharged or service-exchange containers. When containers are returned from service, the agent should be reclaimed during servicing or decommissioning using a process that reverses the original filling procedure. Closed-loop recovery systems, which prevent contact with ambient air, are preferred to minimize losses and avoid contaminant introduction. If the halocarbon agent requires reconditioning to meet the required standards, it should undergo filtering, drying, distillation, or other appropriate processes before reuse. After processing, the agent must be tested to ensure compliance with relevant standards. Agents that fail to meet the required purity levels should not be reused and must be destroyed in an environmentally friendly manner or returned to the manufacturer in accordance with applicable regulations. Halocarbon fire extinguishing agents should only be reused if they have been properly recovered, tested, and processed to meet the necessary quality requirements. The FSTOC [164] has established standard methods for the treatment of HFC-227ea. Reuse involves transferring an agent cylinder from one application to another without additional processing. Recovery entails removing the agent, in any condition, from an extinguishing system cylinder and storing it in an external container without testing or further processing. Recycling focuses on cleaning recovered agents for reuse without meeting all the stringent requirements for reclamation. This process typically includes removing nitrogen, pressurizing the agent, as well as reducing moisture, particulate matter, and non-volatile residue content to meet relevant standards. Reclamation, the preferred method, involves reprocessing the agent to achieve the highest level of purity specified by applicable standards. This process requires specialized separation or distillation equipment to eliminate impurities, including other halocarbon contaminants. The HTOC [165] recommends strategies to reduce halocarbon agent releases that exceed those required for fire protection and explosion prevention. Key recommendations include adopting recycling equipment to recover surplus or reusable fire extinguishing agents, discontinuing the use of fire extinguishants as test gases in discharge testing for fire protection systems, and amending regulations that mandate such practices. Additional measures involve utilizing well-managed central storage facilities for halocarbon extinguishant reserves and avoiding the release of system cylinder contents into the atmosphere during equipment servicing. Since the composition of IG-541 consists of naturally occurring atmospheric gases, it can be safely released into the atmosphere without environmental concerns. Furthermore, atmospheric discharge is a cost-effective option for disposing of IG-541 during the decommissioning of fire extinguishing systems.

10. Practical Implementation

10.1. Past Electrical Fire Incidents

The research report conducted from 2017 to 2021 by U.S. Fire Departments identified equipment or heat source failure as a leading cause of structure fires in industrial and manufacturing properties. Electrical distribution, lighting, and power transfer equipment were frequently involved in ignitions, with wiring and related equipment accounting for 12% of these fires. A significant 24% of structure fires in industrial properties were attributed to equipment or heat source failures, including electrical and mechanical malfunctions or heat sources too close to combustibles. Notably, arcing, unclassified heat from powered equipment, radiated or conducted heat from operating equipment, and sparks or embers were common heat sources [166]. Additionally, a separate report on non-home electrical fires from 2012 to 2016 revealed that electrical distribution and lighting equipment were often implicated in these fires. About 46% of these fires resulted from electrical failures or malfunctions, with arcing being a significant contributor, highlighting the dangers posed by unintended electrical discharges between conductors [167]. Uptime Institute’s outages database shows that fires in data centers are relatively rare and have a minimal impact on operations, accounting for only 1% of significant outages [168]. While the frequency of fires is not increasing relative to the number of data centers or IT loads, they still pose a serious risk. In the case of the OVHcloud data storage center fire on 9–10 March 2021, the fire started in the UPS2 area and spread rapidly across the SBG2 building. The lack of a VESDA (very early smoke detection apparatus) system and an automatic fire suppression system, such as water or gas, was a significant factor that allowed the fire to escalate. In the aftermath, the Bureau of Investigation and Analysis of Industrial Risks recommended the implementation of inert gas systems as a potential fire suppression solution for such facilities [169]. The incident underlines the importance of advanced fire suppression systems, particularly in high-risk areas like data centers and substations, where electrical equipment failure can have catastrophic consequences. Inert gas systems, such as IG-541, could have potentially mitigated the fire’s impact by providing rapid, efficient fire suppression without causing harm to sensitive electronic equipment.

10.2. Case Studies of IG-541 System

Several prominent organizations in different industries have turned to the IG-541 fire suppression system to protect their critical infrastructure and sensitive equipment, demonstrating its effectiveness and environmental advantages. The world’s largest media conglomerates in Southern California, USA, selected the IG-541 system to safeguard their data processing network from both fire and collateral damage. This solution was crucial in protecting multi-million-dollar network processors within their data processing hub. The system met the client’s stringent requirements by safeguarding sensitive electronic equipment and valuable data in an environmentally friendly manner. The client specifically sought an alternative to the ozone-depleting chemical suppression agents previously in use, and IG-541, made up of atmospheric gases, provided the ideal solution without environmental harm [170]. Similarly, Blix Solutions AS chose IG-541 to align with their commitment to sustainability and environmental responsibility. The IG-541 system, which is composed of naturally occurring gases like nitrogen, argon, and carbon dioxide, offers a non-toxic and environmentally friendly fire suppression solution. This makes it an ideal choice for data centers where protecting sensitive electronic equipment is crucial. This decision was especially beneficial for protecting sensitive electronic equipment in data centers, where water-based systems might cause severe damage. The rapid detection and suppression capabilities of the IG-541 system also minimize the spread of fire, safeguard operations, and ensure business continuity while maintaining the company’s commitment to reducing its carbon footprint [171]. Sindal Biogas, focused on green transition goals, also selected IG-541 to protect their technical rooms, which house critical components like switchboards and servers. These rooms are prone to fire risks that could lead to significant production downtime. As a biogas plant plays a crucial role in sustainability, Sindal Biogas prioritized an environmentally neutral solution that avoids harmful chemicals like PFAS. IG-541’s environmentally friendly nature reassured the company and improved operational stability and risk management, also positively impacting their relationship with their insurance provider [172]. Copenhagen Airport has relied on IG-541 for over 30 years to protect critical IT and technology rooms. The system has been deployed successfully in two minor incidents, including a short circuit and a fan heater fire. In both cases, IG-541 effectively extinguished the fire without causing damage to buildings or IT equipment, ensuring no cleanup was required and maintaining operational continuity [173]. E.ON’s Kemsley Combined Heat and Power (CHP) plant uses IG-541 to safeguard control and electrical switch rooms. This system protects both personnel and equipment, automatically deploying when fire is detected in the protected zones. In a recent incident, the system successfully extinguished a fire in a switchgear room, preventing potential damage and allowing production to continue without disruption. In a notable incident, the IG-541 system automatically triggered when a fire broke out in a switchgear room. The system efficiently extinguished the fire, averting a potentially dangerous situation. Importantly, the IG-541 agent did not interfere with any electrical equipment, and no cleanup was necessary, allowing the plant to resume production immediately. This case exemplifies the system’s ability to maintain safety and ensure operational continuity without causing any disruptions [174]. These case studies highlight the versatility and reliability of IG-541 in protecting critical infrastructure, offering both safety and operational continuity while maintaining environmental responsibility.

10.3. Effectiveness and Limitations of IG-541 System

The IG-541 system is recognized for its effectiveness as a pre-engineered fire suppression solution and offers the flexibility to suppress various classes of fires. As outlined in NFPA 2001’s Section 4.2.1.1 [22], all pre-engineered systems shall be installed to protect hazards within the limitations that have been established by the listing. Those consisting of system components designed to be installed according to pre-tested limitations by a testing laboratory. Automatic extinguishing units incorporate special nozzles, flow rates, methods of application, nozzle placement, actuation techniques, piping materials, discharge times, mounting techniques, and pressurization levels that could differ from those detailed elsewhere in this standard. A key advantage of these systems is their ability to provide early fire detection and rapid response. Depending on the installed detection package, the system can activate immediately or with a programmed delay, ensuring prompt containment of fires at their earliest stage. Additionally, pre-engineered systems are safe and user-friendly, as they operate automatically without requiring human intervention, though a manual override option is available for emergencies. Their pre-tested design ensures reliable performance, meeting NFPA standards and effectively extinguishing specific fire types in specialized environments. Another significant benefit is their ability to eliminate the fuel source of a fire automatically, particularly in cases involving electrical equipment, where the fuel or electrical source might otherwise continue to feed the flames [175]. The IG-541 system, in particular, has demonstrated its effectiveness in various fire scenarios. It can suppress Class C fires effectively while also showing strong performance against Class A and Class B fires. Laboratory evaluations confirm its efficacy through standardized testing protocols, such as the wood crib test for Class A fires and the cup burner method for Class B fires [176]. In real-world applications, the IG-541 system has gained popularity for its reliability in protecting against these types of fires, as demonstrated in various projects, including machinery spaces, museums, libraries, and heritage sites [177,178,179]. This adaptability highlights the IG-541 system’s ability to adjust design concentrations to meet the requirements for suppressing different fire classes, making it a versatile and efficient fire suppression solution.

The IG-541 fire suppression system has notable limitations, particularly in its inability to suppress fires involving specific materials and the potential effects of acoustical noise. Clean agent fire suppression systems shall not be used on fires involving specific materials unless the agents have been tested and approved by the authority having jurisdiction. These materials include chemicals or mixtures capable of rapid oxidation in the absence of air, such as cellulose nitrate and gunpowder. Reactive metals, including lithium, sodium, potassium, magnesium, titanium, zirconium, uranium, and plutonium, are also unsuitable for clean agent suppression due to their unique chemical properties. Additionally, metal hydrides and chemicals capable of autothermal decomposition, such as certain organic peroxides, pyrophoric materials, and hydrazine, present significant risks when interacting with clean agents [22]. Li-ion batteries, widely used in data centers and substations as backup power for uninterruptible power supply (UPS) systems, pose specific challenges despite their advantages, including high energy density, longer lifespan, and lower maintenance. One key risk is thermal runaway, where a lithium-ion cell enters an uncontrollable, self-heating state due to damage, overcharging, or exposure to high temperatures. This process involves exothermic reactions releasing heat that accelerates the reaction if not dissipated effectively. The IG-541 system, while capable of reducing oxygen concentration to suppress fires, cannot halt thermal runaway as the reaction is primarily driven by internal chemical processes rather than oxygen availability. Consequently, the fire may re-ignite after suppression. Addressing this limitation requires fire suppression methods that incorporate active cooling to mitigate the risks of Li-ion battery fires effectively [180]. The potential effects of acoustical noise produced by activation of the IG-541 system shall be considered where such systems are used in an occupancy containing noise-sensitive equipment. Several reports have been recorded on a yearly basis about IG-541 gas discharges conducting to faults in hard disk drives (HDDs) operating in data centers protected by such a solution from fire due to the level of noise produced. The sound power level emitted by nozzles can vary depending on their height and the humidity profile at the measurement site. To minimize noise impact, it is recommended to position nozzles on the facets of the concrete supporting walls, ensuring they do not face HDD racks directly or other nozzles [181]. The sound output of fire suppression systems is dependent on many factors. The factors that influence this include discharge duration, peak agent flow rate, valve technology, and many others. Traditional inert gas suppression systems often use metering orifices, with nozzle sound power levels sometimes exceeding 145 dB. The acoustic nozzle is a substantial advancement in nozzle technology for data center markets, translating into a solution that can reduce sound exposure to sensitive HDDs and requires significantly fewer nozzles and piping. For optimal protection, Johnson Controls recommends maintaining sound pressure levels at 110 dBZ or below across the 500 Hz–10k Hz frequency range at HDD locations [182].

10.4. Future of IG-541 System

The future trend of IG-541 systems is being shaped by technological advancements, stricter fire safety standards, environmental priorities, and the specialized needs of critical infrastructure that involve energized electrical fires. IG-541 offers key benefits such as being electrically non-conductive, leaving no residue, and having zero ozone depletion potential (ODP) and global warming potential (GWP). These characteristics make it particularly valuable for high-risk and specialized applications. The rising global demand for fire protection systems like IG-541 is driven by the growth of data storage facilities and electrical power supply facilities. IG-541’s non-conductive and environmentally friendly properties make it an ideal choice for these applications. High-tech industries, such as automation and electronics manufacturing, are also expected to adopt IG-541 due to its ability to protect sensitive equipment without leaving residue or conducting electricity. IG-541 systems offer flexibility through integration with other suppression agents and smart technologies. Combining IG-541 with water sprinklers, water mist systems, foam, or chemical agents enhances fire suppression effectiveness, especially for complex hazards like Li-ion batteries or high energy electrical equipment. Integrating Internet of Things (IoT) technology enables real-time monitoring and smart activation, improving system responsiveness and minimizing false alarms. Artificial intelligence (AI) further boosts system readiness by optimizing design and providing predictive insights through AI-based fire behavior modeling. Environmentally, IG-541 stands out for its sustainability. Composed of naturally occurring gases, it reduces carbon footprints while meeting environmental regulations and green building certifications. Its eco-friendly profile supports companies in improving their environmental, social, and governance (ESG) performance, demonstrating commitment to sustainability and responsible operations. By blending environmental responsibility, technological adaptability, and robust fire suppression capabilities, IG-541 systems are positioned as a future-proof solution for modern fire safety challenges.

11. Conclusions and Future Works

The evolution of fire suppression technologies is critical in addressing the growing complexities of protecting high-value assets, such as data centers and electrical substations, while meeting stringent environmental and safety standards. This study comprehensively evaluates IG-541, an inert gas mixture, as a cleaner and safer alternative to halocarbon-based agents like HFC-227ea and FK-5-1-12 in energized electrical fire suppression. IG-541’s composition offers unique advantages, including efficient fire suppression through oxygen reduction, absence of harmful by-products, and a zero impact on ozone depletion and global warming. These attributes make it a strong candidate for adoption in industries prioritizing environmental sustainability and regulatory compliance. In comparison, halocarbon agents, while effective, face increasing scrutiny due to their significant global warming potential, extended atmospheric lifetimes, and toxic decomposition by-products, which present risks to both human health and sensitive equipment. The study also emphasizes the importance of safety in fire suppression, particularly in occupied spaces. IG-541 offers a significant safety advantage due to its inert nature and lack of hazardous decomposition products. Halocarbon agents, on the other hand, can produce hydrogen fluoride (HF) during decomposition, posing risks to human health and electronic equipment. IG-541 also maintains respiratory-friendly conditions, allowing safe evacuation during its deployment. The regulatory landscape is increasingly favoring sustainable fire suppression solutions. International agreements like the Montreal Protocol and local environmental standards are driving the phase-out of halocarbon agents. IG-541 meets stringent safety and environmental requirements outlined by standards such as NFPA 2001 and ISO 14520. This positions it as an ideal solution for future applications, especially in critical industries like data centers, substations, and telecommunications facilities. In summary, IG-541 presents a transformative opportunity for the fire suppression field by balancing high performance, environmental responsibility, and safety. Its adoption can lead the industry toward a future where fire protection supports both operational resilience and global sustainability goals.

Future Works

While IG-541 shows great promise as a sustainable and efficient alternative to halocarbon-based fire suppression agents, further research and development are essential to fully optimize its effectiveness across diverse fire scenarios. A critical area of study involves evaluating IG-541’s extinguishing performance in high-energy electrical fires. As discussed earlier, higher agent concentrations are required to suppress fires involving energized electrical equipment at elevated energy levels. If IG-541 fails to completely remove heat from the equipment, re-ignition may occur due to the continuous presence of intense heat and potential arcing. Additional research should focus on understanding the persistent fire hazards in such scenarios, including ignition and combustion behaviors, as well as the overall fire suppression process. Key factors to consider include compartment fire conditions (e.g., temperature and pressure), flame propagation to adjacent electrical components (such as cables, switchgear, and transient combustibles), and the production of ionized soot, which could significantly contribute to secondary arcing. Further studies should also explore the potential ignition of secondary combustibles, fire growth dynamics, and the intensity of the blaze following an enduring fire event [183]. Assessing the effectiveness and timing of fire suppression interventions will also be crucial in enhancing the reliability of IG-541 in these challenging conditions. Battery systems, critical for backup power in data centers and substations, present additional challenges, as IG-541 is less effective against fires involving reactive metals like Li-ion batteries. Research should focus on improving suppression performance by integrating pressure relief mechanisms and combining IG-541 with cooling-based methods, such as water [184]. Additionally, further safety evaluations are necessary to assess the impact of IG-541 in occupied spaces, particularly as higher agent concentrations are required for high-energy fire scenarios and the protection of complex hazards such as battery systems. Ensuring occupant safety includes understanding the physiological effects of prolonged exposure to reduced oxygen levels. Additionally, safety protocols should be developed to enhance evacuation procedures and implement system fail-safes in the event of an IG-541 discharge within hazardous areas. An optimal IG-541 concentration, delivered through a muffled nozzle system, has been selected for a dual cab fire inerting system based on its effectiveness in ensuring safe evacuation times [185]. To maximize safety and efficiency during fire emergencies, operator training and task synchronization should accompany the system’s implementation. Furthermore, an early fire detection system is essential in IG-541 systems to prevent oxygen levels from dropping dangerously low in confined spaces. If early suppression is not feasible, sufficient evacuation time must be ensured [186].

A comprehensive cost–benefit analysis should be conducted to evaluate the long-term economic viability of IG-541 systems compared to halocarbon and traditional fire suppression agents. This analysis should incorporate lifecycle studies of materials and system components to solidify IG-541’s environmental and operational benefits. These recommendations aim to refine IG-541’s application, reinforcing its position as a sustainable and reliable solution for fire protection in the face of future challenges. In fire safety engineering, a cost–benefit analysis provides a structured approach to determining whether the benefits of a fire safety measure outweigh its costs. Various reference methodologies can be applied to conduct this assessment effectively [187,188]. Further research is needed to reduce the acoustic noise generated during the activation of the IG-541 system to minimize the risk of damage to sensitive equipment and implement effective noise control measures. This includes investigating the combined effect of multiple nozzles—both standard and silent—on noise levels, particularly their impact on HDD performance. Additionally, expanding the range of tested silent nozzles, including designs with sound-absorbing layers, could further mitigate noise-related issues [189]. Additionally, research should focus on optimizing nozzle quantity and spacing, avoiding very short discharge times, and eliminating the use of pneumatic sirens to reduce the impact of sound levels on HDD functionality [190]. Moreover, further research should focus on minimizing or eliminating IG-541 system leakage through ventilation or structural improvements to mitigate over-pressure risks that could damage structures and equipment. Ventilation systems play a crucial role not only in reducing the impact of IG-541 over-pressure but also in optimizing the quantity of required agent needed to achieve the desired oxygen concentration. Studies should investigate IG-541’s extinguishing efficiency across different fire sizes and the thermal pressure effects caused by fires in enclosed spaces. In addition to mechanical ventilation, natural ventilation such as open doors, windows, and leaks also influences airflow and system performance [191,192]. Additionally, research should assess the required IG-541 quantity in scenarios with larger building openings to determine system effectiveness [193]. Furthermore, leakage performance between raised floors and equipment should be analyzed based on opening ratios to verify the reliability of IG-541 when used exclusively for raised floor protection [194].