**7. Conclusions**

This paper extensively reviewed polymer materials, thermoplastics, and thermosets for application in electrical apparatus. Polyethylene has been the widely applied material for manufacturing cables at the medium and high voltage range due to its high electrical strength and low production costs. Using a cross-linking process, good thermal and mechanical characteristics can be achieved with XLPE, HDPE, and EPR. On the other hand, at low voltage for indoor applications, PVC is replaced with superior polymeric insulation due to safety and public health regulations. For HVDC applications, polymeric insulated cables have not been as successful as oil-impregnated paper cables because of some operational conditions that reduce reliability and increase the functioning costs.

On the other hand, smart polymeric materials were more useful in biological and medicinal applications due to their sensitivity to the environment. However, it has been observed that more research is required in the mechanical, electrical, and thermal stresses of polymers to increase the reliability and power density. Besides, residual stresses are a common phenomenon that affects the production of injected molded polymers. These stresses come up due to deformations/bends, twists, or pressure. The high pressure during molding also leads to complex situations such as chain reaction, stretching, and relaxation. These residuals are usually quantified using techniques with lower accuracy. To improve the accuracy, new techniques such as indentation are typically employed.

It is also important to note that for polymers, thermal conductivity refers to the polymers and their fillers. It is interesting to note that thermal conductivities are significant at low filler loadings due to the polymer matrix's thermally conductive fillers' disjointing. However, polymers' properties can be changed by increasing the temperature according to the procedure mentioned in UL 746B. Additionally, several factors affect the dielectric strength of polymers, which include: (1) environmental; (2) electrode effects; (3) temperature; and (4) voltage application and frequency.

However, current research concentrates on developing novel materials (e.g., nanofilleradded polymers) that possess additional capabilities such as improved mechanical strength and electrical erosion reduction. In general, PD has already been established to evaluate the quality of high voltage insulation systems. Future development trends for nanocomposites should concentrate on developing nanostructured materials for ultra-capacitors, electrooptic, discharge resistant high voltage equipment insulation, sensors and actuators.

**Author Contributions:** S.K.M.H. formulated the study and contributed with the abstract, introduction, polymer characteristics, and conclusion; J.A.A.-R. and A.A.M. involved the abstract, introduction, electrical properties, and conclusion; Y.U. and H.R. contributed the abstract, types, synthesis, thermal properties of the polymer, and conclusion; B.H.J. helped with introducing mechanical properties and N.A.B. with electrical properties and abstracts; F.M.-S. contributed the thermal properties, applications, conclusions as well as references. All the authors extensively reviewed the paper, provided areas of improvement, and added substantive information. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to express their appreciation to Agencia Nacional de Investigación y Desarrollo (ANID) for the support received through the projects Fondecyt regular 1200055 and Fondef ID19I10165 and the UTFSM for the project PI\_m\_19\_01. Nurul Aini Bani would like to thank the Ministry of Higher Education (MOHE), Malaysia via Universiti Teknologi Malaysia (UTM) (Research cost center no. Q.K130000.3556.06G43) for the support received.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
