Break arcs in inductive circuits and the minimum arc current

A simulation method for voltage and current waveforms of break arcs in inductive circuits is developed, based on Holm´s diagrammatical method. In this method, the minimum arc current plays an important role, limiting the lowest value of the arc current. The calculation was carried out for a tungsten contact and compared with the experimental results. The difference between the calculated and measured values seems to be caused by the use of the 1 A in the calculation, which is generally accepted as the minimum arc current of tungsten in the air. To investigate the reason for the difference, break arc ignition rate and its duration were measured for tungsten contact at lower currents than 1 A. The condition was 24-V DC source, 10-mH inductance, and 1000-m/s² contact separating speed. The ignition rate increased gradually from 0% at 0.2 A to 100% at 1.0 A, through 50% at 0.45 A. The arc duration also increased from 10 μs at 0.3 A to 50 μs at 0.8 A and rapidly up to 100~200 μs at 1.0 A. No appreciable difference was observed under the other experimental conditions such as 12-V or 36-V DC sources or 3000-m/s² contact separating speed. 0.4 A was used as the minimum arc current in this simulation method and brought closer agreement with the experimental results, although 1.0 A is a characteristic current for steady and long duration arc ignition