High Voltage-Low Current Arc Resistance Test Using the Electrodes Detached From Testing Insulator

Abstract

The ASTM D 495-61 method has been regarded as a representative test for the high voltage-low current arc resistance of insulating materials. In this method, however, the electrodes are placed directly on the sample, so that the heat delivered to the fire spot at the tip of advancing track is of the orders of magnitude greater than that on the surface. Accordingly, the failure by the heat delivered to the fire spot appears to be emphasized in comparison with the failure by the heat delivered to the surface from the body side of arc.From these view points, it is considered that, when the electrodes are detached from the surface, the formation of fire spot can be avoided and consequently the failure caused by only the body of arc can be taken out and examined under various conditions excluding the influence of electrode position and its state in order to evaluate the arc resistance. In fact, under such conditions, the formation of tracking is not observed and the burning part spreads out widely on the surface.The experimental method used in this study was similar to the ASTM D 495-61 method, but the electrodes were placed horizontally about 1∼2mm apart from the surface of the sample as shown in Fig. 1. The elapsed time of failure were selected to be 10, 20, 30sec and so on. After allowing 30 minutes of cooling, the surface resistance was measured in order to detect the failure. Various types of thermosetting resins were used as the test samples.The following results were obtained from these experiments:(1) At more than 10∼40 seconds of failure time, the relation between the failure time and the ratio of the insulating resistance after failure to that before failure begins to deteriorate. From this relationship the characteristics of failure can be derived.(2) For the evaluation of arc resistance, the measurement of insulating resistance after the failure is superior than that of breakdown voltage.(3) The time dependency of insulating resistance during failure, which can be followed by the measuring circuit arranged between the electrodes (Fig. 1), shows that the process of failure is extremely complicated.(4) The present method is found to be useful as a practical testing method from the comparison between the present results and the results obtained by various arc resistance tests.