Telephone Relay

a relay used in telephone central offices as a component of control and other types of apparatus and as a device for switching telephone channels. An automatic central office using step-by-step switching and serving 10,000 subscribers can have approximately 70,000 relays. The relays in widest use are the armature type and the reed type, the latter comprising ferreed switches and relays built with hermetic contacts. Armature relays are distinguished by the large number of contact springs (up to 24) in their actuating component, which form contact groups for various functions; these groups are controlled by the currents in the nonstandard coil. Relays built with hermetic contacts usually have one or two coils, within which there are ten or more hermetic contacts. In relays having two coils, one of the coils serves as a control device, and the other, called the holding coil, magnetizes the reeds (contact strips). Ferreed switches do not have holding coils.

Telephone relays are classified according to the time required for triggering as high-speed (up to 10 milliseconds [msec]), normal (10–50 msec), or low-speed (50 msec to 1 sec). The service life is a function of the number of triggerings; for industrially produced relays subjected to rated loads, this number is ~10⁷ for the armature types and ~10⁹ for the reed types.