Theory of Open- Contact Performance 

 of Twin Contacts 



By M. M. ATALLA and MISS R. E. COX 



(Manuscript Received June 17, 1954) 



The first 'part is a presentation of an analytical study of the open-contact 

 performance of twin contacts. It provides means for predicting their per- 

 formance from single contact data. It is shown that the prohability of 

 failure of twin contacts is generally apprcciahly greater than the square of 

 the prohahility of failure of single contacts. This is supplemented with the 

 results of an experimental study which determines the effects of a few design 

 parameters on the performance of single contacts. These are the parameters 

 that determine the magnitude of improvement in performance obtained hy 

 replacing single contacts hy twin contacts. 



INTRODUCTION 



The present s^\itching apparatus normally operates in atmospheres 

 that may be contaminated with dust particles and foreign matter. Some 

 apparatus components, particularly the contacts, are relatively sensitive 

 to such contaminations which may interrupt the proper functioning of a 

 pair of contacts. Normally, a single switching operation in a central office 

 requires the operation of as many as a thousand relays or 10,000 contacts. 

 To secure the high level of performance desired, it is evident that superla- 

 tive performance and high degree of reliability of the contacts are es- 

 sential. 



Many attempts have been and are being made to reduce the so-called 

 "open" contact troubles due to foreign matter. Examples of environ- 

 mental precautions are filtering the air supply to the central office, en- 

 closing apparatus in cabinets, limiting personnel activities in the office, 

 etc. An additional precaution incorporated in the apparatus design is the 

 use of twin contacts. Such a scheme, when properly used, should result 

 in substantial improvement in performance since an open can only take 

 place when both members become open simultaneously. It may occur to 

 one that the probability of a twin-contact open is the square of the prob- 



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