990 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



operation is "open" contacts due to small insulating particles present 

 in the atmosphere becoming trapped between the contacts. This causes 

 high resistance or open circuit and consetiuent circuit failure. Many at- 

 tempts have been and are being made to reduce "open" contact troubles. 

 Examples are filtering the air supply to the central office, enclosing the 

 relay ec[uipments in closed cabinets, pressurizing the enclosing cabinets, 

 co\'ering smaller groups of contacts by independent covers, employing 

 twin contacts rather than single contacts, and enclosing the relay or its 

 contacts in a hermetically sealed chamber. E\'en going to the extreme 

 of completely isolating the relay from its surroundings is not a complete 

 answer. There is always the possibility of failure by wear particles gen- 

 erated within the enclosure by the relay actuation. 



The most widely employed method to reduce dust failures in the tele- 

 phone plant is the use of twin contacts in combination with some of the 

 above mentioned types of enclosures. If the incidence of dust failures fol- 

 lowed the laws of probability, then elementary considerations would lead 

 us to predict that if single contacts failed at the rate of once in 1,000 oper- 

 ations, the simultaneous failure of the two such contacts comprising the 

 twin would be once in 1,000,000 operations. This is the so-called "sciuare" 

 law. However there are a number of reasons why this is not realized 

 in practice and why the figure of merit for the twin contact is very much 

 less than that indicated by the "square" law. In the first place, when 

 foreign matter becomes lodged on a contact, it seldom falls out on the 

 first subsequent operation, but will reciuire a number of operations before 

 it cleans itself; in fact, it may remain inoperative indefinitely. When this 

 happens to one of the twin contacts, during this period of time, the twin 

 contact is no better than the single contact. In practice, twin contacts 

 are generallj^ used with the same total force as the single contact, being 

 nominally divided equally between the two contacts. This reduction in 

 force per contact on the twin contacts is of considerable importance in 

 reducing its effectiveness. Relay designs -employing twin contacts that 

 have been used in the past do not have complete mechanical inde- 

 pendence of the two members to which the contacts are attached. 

 Foreign material or protrusions under one contact can adversely influence 

 the performance of its mate. In a new design of relay which is about to go 

 into production, the design criterion that twin contacts to be most 

 effective should be completely and mutually independent has been met. 

 Laboratory tests and field experience obtained to date show a marked 

 improvement over the former designs in regard to the incidence of open 

 contact failures. 



Tests have been made repeatedly in the laboratory for comparing the 



