Arcing of Electrical Contacts in Telephone 

 Switching Circuits 



Part III — Discharge Phenomena on Break of 

 Inductive Circuits 



By M. M. ATALLA 



(Manuscript received November 16, 1953) 



This is a presentation of a study of the discharge phenomena occurring 

 befiveen contacts on break of an inductive load. The main objectives are: 



(1) to forward some detailed explanations of the main components of a 

 break transient in terms of basic conduction and emission processes, and 



(2) to establish the conditions that determine the nature of the transients. 

 The study covered the following: (1) occurrence of interrupted and steady 

 arcs, {2) initiation of reversed arcs in one breakdown, (3) arc initiation 

 under dynamic conditions, (4) initiation and maintenance of glow dis- 

 charge, and (5) glow-arc transitions. 



INTRODUCTION 



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 may have inductances of the order of tens of henries and may store 

 as much energy as 10 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 combina- 

 tions. In most cases, the energies involved are too high to provide 

 satisfactory contact hfe from the standpoint of electrical erosion. 



The discharge transients obtained are usually complex in nature.' 

 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. 



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