**3. Experimental Investigations**

The experimental circuit (Figure 2) adopted for the present study consists of a HV transformer (110 kV, 5 kVA) and voltage and leakage current measuring systems (voltage divider and shunt resistor). The arc discharge's light emission intensity was recorded with a R3896 Hamamatsu Photonics photomultiplier (PMT) directed at the HV electrode. The covered wavelength ranges from ultraviolet light to the near-infrared region (185 to 900 nm), allowing the detection of early discharge inception since corona discharges in air emit light mainly in the 230–405 nm range. The most intense emissions are between 300 and 360 nm, whereas those of the arc phase are between 600 and 800 nm [7]. The PMT tube had a rise time of 22 ns. The applied voltage was increased at a rate of 1 kV/s using a High Volt WSM3 control device. Measurements signals (current, voltage, and PMT) were recorded via a National Instrument data acquisition system (NI PCI 6251).

The leakage current was recorded through a 100 Ω power shunt resistor (RSH).

The preparation of the saline solution, used for the pollution of insulators, was based on the salt fog method according to standard IEC 60507. The level of the pollution layer was chosen on the basis of earlier work by Teguar et al. [22–24,28]. These authors considered that the highly conductive layers have a conductivity greater than 400 μS/cm, while the weakly conductive layers have a conductivity lower than 40 μS/cm. The tap water was referred as median conductivity level.

The physical laboratory model was carefully cleaned with distilled water so that all traces of drifts were removed and dried with papers. The conductivity of the pollution layer was adjusted to a desired value using a conductivity meter of Yokogawa type by adding sodium chloride (NaCl) to deionised water. The pollution layer was deposited by spraying the surface of the insulating plate with the salty solution while respecting the number of sprays and distance to obtain a uniform and reproducible pollution layer.

**Figure 2.** Measuring circuit for AC inception and breakdown test.
