Arcing of Electrical Contacts in Telephone Switching Circuits: Part III - Discharge Phenomena on Break of Inductive Circuits

An important phase in the study of discharge phenomena between contacts is that involving the break of an inductive circuit. A typical switching circuit in its simplest form consists of a battery in series with a coil (electro-magnet), a cable or lead and a pair of contacts. Coils now in use maj r have inductances of the order of tens of henries and may store as much energy as 106 ergs. On break of the circuit an appreciable portion of this energy may be dissipated between the contacts through a steady arc, a series of interrupted arcs, a glow discharge or any of their combinations. In most cases, the energies involved are too high to provide satisfactory contact life from the standpoint of electrical erosion. The discharge transients obtained are usually complex in nature. 1 A close examination of these transients reveals a great deal of rather curious effects that have not been previously considered in detail. This is a presentation of a recent study of the break transient with the primary objective of furnishing some explanation of the more pertinent phenomena involved in terms of the basic concepts of surface emission and gas conduction. 535