Up to this time, only a little studies have been undertaken on the effect of sample size on arc resistance of electrical insulating materials. They are usually done with an assembly of electrodes in contact with the testing insulator but are scarcely done with an assembly of electrodes detached from the testing insulator.
In this study, arc resistance tests were carried out with the latter assembly under higher applied voltage and in a longer electrode gap length, that is, in a larger sample size. Furthermore, the results were compared with those of the smaller sample size in order to discuss the effect of sample size.
The experimental method used was similar to the ASTM method, but the plate electrodes were placed horizontally about 1∼3mm apart from the surface of the sample. The electrode gap length was 5∼10mm and the applied voltage was max. 50kV (commercial frequency 60Hz). To prevent the variation of arc current due to the change in electrode shape, electrode gap length, applied voltage, etc., the same arc current as that in the ASTM method was maintained by adjusting the current controlling resistance of the primary circuit of testing circuit. The arc intermittence time was also the same as in the ASTM method.
Generally, during the arc deterioration in these electrode assemblies, no arc failure point was observable. Consequently, the total time required for the deterioration path to progress and stop at the neighbourhood of the under part of tip of electrodes was regarded as the arc resistance time.
Furthermore, the experiments on a larger sample to examine the effect of sample size were done not only for the case that one electrode was placed on the testing insulator and the other was detached from the testing insulator, but also for the case that the air gap between the electrode and the testing insulator was changed.
The change in arc deterioration time of the sample with the variation of electrode gap length shows a V type curve for the both cases of the equal air gap and the different air gap, but the style of the curve differs a little depending upon the kind of samples. This result seems to indicate that the effect of sample size exists on arc resistance of electrical insulating materials.