Arcing of Electrical Contacts in Telephone 

 Switching Circuits 



Part I — Theory of the Initiation of the Short Arc 



By M. M. ATALLA 



(Manuscript received April 1, 1953) 



This is a presentation of a theory for the mechanism of the initiation of 

 the short arc commonly observed on the closure and opening of electrical 

 contacts. The theory is based on the experimental evidence that an established 

 arc is generally preceded by a period of local high frequency discharges at the 

 contacts. During this period the circuit current builds up. If and when this 

 current reaches the arc initiation current of the contact a steady arc is es- 

 tablished. It is shown that this initiation period of the arc is directly deter- 

 minable from the circuit conditions and the contact condition. This mecha- 

 nism furnishes rather simple explanations to some complex phenomena 

 commonly observed before the establishment of the arc. The mechanism of 

 the initiation of an individual discharge, however, still remains uncertain. 



INTRODUCTION 



In the course of study of arcing phenomena between electrical con- 

 tacts, it has been long established that a condition for sustaining the 

 short arc is to maintain a current through the arc greater than a mini- 

 mum 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 mini- 

 mum 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 in- 

 volved. Such experiments have been made and observations have in- 

 dicated that the initiation period consisted of a succession of rapid dis- 



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