Arcing of Electrical Contacts in Telephone Switching Circuits: Part I - Theory of the Initiation of the Short Arc

Iii the course of study of arcing phenomena between electrical contacts, it has been long established that a condition for sustaining the short arc is to maintain a current through the arc greater than a minimum value called the minimum arcing current. This current is generally a characteristic of the contact material and is appreciably affected by surface contaminations. For clean metals the minimum arcing current is usually equal to a few tenths of an ampere. Before establishing the arc, therefore, there must exist a certain mechanism which accounts for a rapid current build-up from zero to a value as high as the minimum arcing current. For an inductive circuit, a higher inductance should result in a longer period of current build-up. With a sufficiently high circuit inductance this initiation period may be made long enough to be directly observed and to allow an examination of the mechanism involved. Such experiments have been made and observations have indicated that the initiation period consisted of a succession of rapid dis1231