OPEN-CONTACT PERFORMANCE OF TWIN CONTACTS 



1375 



operation a certain number of the twin contacts will become half -open.* 

 These contacts will behave as if they were single-contacts until either: 

 (1) the open half clears itself by operation, or (2) the other half becomes 

 open leading to a twin-contact open. It is assumed that a failing twin- 

 contact is cleared by an operator and then put back into service. 



In developing the theory it was necessary to represent by analytic 

 expressions the rate of occurrence of opens on single contacts and the 

 rate of their clearing by operation. These are approximations of a fairly 

 large amount of experimental data consistently obtained from a number 

 of tests on a variety of actual telephone relay contacts. 



(1) Rate of opens of single contacts "Ps": For a large number of 

 single contacts at one set of operating conditions, the rate of opens is 

 usually constant. This constant depends primarily on the quality and 

 concentration of the offending foreign matter involved, the design of the 

 contacts and their mechanism of actuation. Fig. 1 shows the results of 

 three tests on single contacts of different design at different test condi- 

 tions. The}'' all substantiate the assumption that the rate of opens of 



1.2 



6 8 10 12 14 



THOUSANDS OF OPERATIONS 



Fig. 1 — Rate of opens of single relay contacts. 



* A half -open is defined as one where only one member, of a pair in a twin-con- 

 tact, is open. In practice, a half -open is not normally detected. Only a simultane- 

 ous open on both members of a twin contact will cause a circuit failure. 



