Theory of Open-Contact Performance of Twin Contacts

The present switching 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 superlative performance and high degree of reliability of the contacts are essential. Many attempts have been and are being made to reduce the so-called "open" contact troubles due to foreign matter. Examples of environmental precautions are filtering the air supply to the central office, enclosing 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 prob1373