IEC 60587-2007 pdf Electrical insulating materials used under severe ambient conditions – Test methods for evaluating resistance to tracking and erosion

5.3 Method 2: stepwise tracking voltage Select a starting voltage, being a multiple of 250 V, such that failure according to criterion A of 5.4 (current exceeding 60 mA) does not occur sooner than the third voltage step (a preliminary trial test may be necessary). With the contaminant flowing uniformly at the specified rate, switch on and raise the voltage to the selected value. Maintain this voltage for 1 h and increase the voltage by a step of 250 V for each subsequent hour until failure by criterion A is recorded. As the voltage is increased the contaminant flow rate and the resistance value of the series resistor are increased according to Table 1 . The stepwise tracking voltage is the highest voltage withstood by all five specimens for 1 h without failure. Classification of the material is as follows: Class 2A x or 2B x, where x is the highest voltage, in kilovolts, withstood by the material under test. NOTE 1 Effective scintillation is essential and if not obtained, the electrical circuit, the contaminant flow characteristics and contaminant resistivity should be carefully checked. Scintillation means the existence of small yellow to white (with some materials occasionally blue) arcs just above the teeth of the lower electrode, within a few minutes of application of the voltage. These discharges should occur in an essentially continuous manner, although they may jump from one tooth to another. These discharges will burn away the specimen surface and may ultimately lead to tracking failure. Discharges which move rapidly over the surface between the two electrodes are not likely to produce tracks. The condition of effective scintillation may also be observed with a cathode-ray oscilloscope. The signal may be picked up across a resistor (e.g. 330 Ω, 2 W) placed in series with the overcurrent device. Proper scintillation is observed as a continual, but non-uniform, break-up of the power frequency current wave during each half cycle.