THE BELL SYSTEM 



TECHNICAL JOURNAL 



V o L u M E X X X 1 1 1 J A N U A H Y 1 9 5 4 n u M b e R 1 



Copyright, 1954, American Telephone and Telegraph Company 



Design of Relays 



Introduction 



(Manuscript received September 28, 1953) 



The electromechanical relay is the basic building block of modern 

 dial switching systems and also of various automatic control systems 

 and computers. 



All of these systems depend on the action of the relay which is simple 

 in its functions, but has to meet complex requirements placed on it by 

 the systems in which it is used. Perhaps the best illustration of this 

 apparent conflict is to note that a relay has simply to close or open 

 electrical contacts when its coil is energized or deenergized. However, 

 these simple functions must be performed equally well by millions of 

 relays, and each of these must continue to perform reliably for millions 

 and in some cases for more than a billion operations during its lifetime. 

 Furthermore, in many cases the relay must function in a few thousandths 

 of a second, or use little electrical power. The reliability re(}uired in 

 telephone switching systems would be considered unreasonable in many 

 other types of equipment and can be judged from the fact that a single 

 failure in 5,000,000 operations is considered to be poor performance. 



Stated another way, satisfactory operation for the average relay in 

 modern telephone switching systems is less than one failure in forty 

 years of operation. The measurement of such low troulile rates is, in 

 itself, a difficult and challenging problem. 



The need for such a high degree of reliability and for the associated 

 requirement of high speed is evident from a few figures relating to 



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