Abstract
The experimental and theoretical investigations were made on the arc instability phenomena for the silver-tungsten-carbide (Ag-WC) contacts used in low-surge vacuum circuit breakers. After the arc instabilities started, the large and high-frequency (HF) oscillations were observed, which were imposed on both the arc voltage and current waveforms. In the experiments using the test circuit with some parallel capacitances, we found that the value of the chopping current increases with an increase of the HF current related both to the parallel capacitances and to the arc instability startings. In the measurements of the chopping current values for some kinds of Ag-WC contacts, we found that the value of the chopping current decreases with a decrease of its Ag content. This tendency agreed with the results of the analysis on the initiation of the arc instability, taking account of some thermal effects of the WC component. These results show that the low chopping current characteristic of Ag-WC depends on the thermal effect of the WC component which was subjected to high temperatures by the cathode spots.
