Capability of sealed contact relays

A CIRCUIT designer contemplating the use of a relay must consider its operating characteristics and the reliability of its contacting function. The electric erosion causing contact wear gives rise to contact surface conditions permitting contact failure to open a circuit caused by locking; particularly in talking circuits, reliability can be further impaired by the operating environment which may contain corrosive elements, organic vapors, or mobile dirt, all of which increase the probability of failure to give contact closure. Unfortunately, the incidence of these factors affecting reliability is not predictable with respect to time or place. The considerations involved are not unlike those associated with electron tubes and transistors. Because of the growth in the complexity of electric switching systems increasing emphasis is placed upon the elimination of factors affecting component reliability. In the Bell System one means to achieve improved reliability has been the development of sealed contacts. Two types have reached application, both providing encapsulation to free contacts from the effects of environment; one type, in addition, circumvents the effects of electric erosion by employing liquid contact surfaces. The development of sealed contacts and relays using such contacts has been described elsewhere;^1-4 the purpose of this paper is to consider some of their characteristics from the standpoint of application.