D.C. circuit interruption and arc characteristics at low air pressures

This report summarizes work carried out by the National Physical Laboratory at the request of the Ministry of Supply. Measurements have been made of the voltage and current in d.c. arcs over a range of air pressures using silver, copper and tungsten electrodes. The investigation has shown that, although the arc characteristic at low pressures is less favourable for breaking large currents than it is at atmospheric pressure, comparable switch performance may usually be obtained either by an increase of gap length, or by an increase in the number of series breaks. Simple empirical relationships for the arc characteristics based on the experimental data obtained are given for silver and tungsten electrodes. Tests on the interrupting capacity of switches gave results which were in reasonable agreement with predictions made from these empirical formulae.