Contact Phenomena in Telephone Switching Circuits* 



By A. M. CURTIS 



The phenomena occurring at the closing and opening of con- , 

 tacts carrying weak currents have been investigated by means 

 which include a study of the high-frequency transient voltages and 

 currents. These influence the erosion in a complex manner which 

 varies with contact materials, surface conditions and surrounding 

 atmosphere. Three principal classes of effect have been distin- 

 guished. These are: (1) Disruptive sparkovers initiating 9. series 

 of metallic arcs lasting less than a microsecond each; (2) A nitrogen 

 gas glow discharge at about 300 volts, preceded by a brief group 

 of disruptive sparkovers; (3) High field breakdowns due to cold 

 point discharges which cause transient metallic closures of ap- 

 proaching contacts and similar transient reclosures of separating 

 contacts. 



' I '*HE operation of a telephone system depends on the proper per- 

 -■- formance of many millions of electrical contacts, a large propor- 

 tion of which are in relays. The relays must be designed for a life 

 during which they operate from as few as five thousand to as many as 

 four hundred million times. Although the nominal currents and 

 voltages carried by the contacts are rather low, the large number of 

 operations may cause erosion which in a very small percentage of cases 

 leads to failures to close or open the circuit. The difficulties caused by 

 even very rare failures make the control of contact erosion a problem 

 of major importance for the telephone companies. 



Research and development work on contacts has of course been car- 

 ried on continuously since very early in the development of the tele- 

 phone system. The aim is to design contacts to have a life at least 

 equal to that of the apparatus of which they form a part and to require 

 a minimum of maintenance. Although this aim has in general been 

 successfully met there have been some cases in which the contacts have 

 worn out too rapidly. 



Although it had long been realized that contact operation necessarily 

 involved the generation of high-frequency transients, there was at first 

 no apparatus available which would permit these transients to be 

 studied. The Dufour oscillograph was for a long time the only instru- 

 ment which covered the range of frequencies involved. It was em- 



* Prest-ntcd at Winter Convention of A. I. E. E., New York, N. Y., fanuarv 22-26, 

 1940. 



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