A relay is defined as: A device which opens or closes an auxiliary circuit under predetermined electrical conditions in the main circuit.
The object of a relay is generally to act as a sort of electrical multiplier, that is to say, it enables a comparatively weak current to bring into operation a much stronger current.
Fig. 2,276.—Diagram of connections of General Electric shunt trip coil with and without circuit opening auxiliary switch.
Ques. For what service are relays largely used?
Ans. They are employed in connection with high voltage switches where the small amount of energy derived from an ordinary instrument transformer is insufficient for tripping.
The connections between relays and circuit opening devices are usually electrical. Combinations of this nature are extremely flexible since they permit the use of a number of devices, each having a different function, with a single circuit breaker or oil switch as well as with two or more switches, to secure the desired operation and protection.
Selection.—In all electrical installations protection of apparatus is important, but in some large central stations this is secondary to continuity of service.
To combine maximum protection without interruptions of service is not always possible, but these requirements can be approximated very closely by the use of reliable and simple controlling or protecting devices if proper care be taken to select the relays suited to the special conditions of the installation. To do this intelligently, a knowledge of the various types of relay is necessary.