Arcing of Electrical Contacts in Telephone Switching Circuits - Part II -Characteristics of the Short Arc

The electrical erosion of contacts presents an important problem in the design of telephone switching apparatus. There are several physical phenomena that occur between contacts and contribute to their erosion. The short arc,* which may occur on both make and break of a contact, * The short arc is characterized by its constant voltage, independent of the current, which is of the order of the ionizing potential of the contact material. 1493 1494 T H E BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1 9 5 3 is generally considered to be the major contributor. For illustration, a palladium contact, 1CT4 cm3 in volume, will last for more than 109 operations* only if the arc energy per operation is less than 2.5 ergs. This is based on an erosion rate of 4 X 10~14 cm'1 per erg.1 Furthermore, a short arc with a half ampere current, lasting for only one microsecond, will dissipate as much energy as 70 ergs. Contact erosion may also take place, though at much lower rates for the usual ranges of current and voltage in switching circuits, due to molten bridges 2 on contact break and due to glow discharge. 3 In Part I of this series,4 was discussed the mechanism of the initiation of the short arc as determined by contact and circuit conditions. Three characteristics of the arc were used in the presentation without elaboration as to their nature: (1) the arc initiation voltage, (2) the voltage drop across the arc, and (3) the arc initiation and the arc termination currents. These characteristics have been the subject of a recent study to which this part of the series is mainly devoted.