Arcing of electrical contacts in telephone switching circuits

This is a presentation of a study of the mechanisms of the short arc between closely spaced contacts and its erosion effects. The study is based on optical measurements of the erosion obtained on contacts after repeated arcing on closure or opening. Most experiments reported here are essentially of the probing type designed to test specific postulates and assumptions. For short arcs initiated at 250 volts, clean palladium, iron and nickel contacts have shown a reversal, with arc duration, in the direction of net transfer. Net anode losses were obtained with short duration arcs and net cathode losses with longer duration arcs. This reversal, however, did not occur with silver, gold or copper. For longer arcs initiated as air breakdowns from 500 volts, all the above metals indicated a net loss from the cathode. For arcs initiated at 250 volts between fully activated contacts, shallow cathode losses were generally observed with little or no buildups on the anode. The first section of this paper is a summary of the experimental work done and the results obtained. In the second section, the data are analyzed and a tentative working model is proposed for the short arc and its erosion effects.