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Electrical device used in heavy duty automotive circuits. There are two circuits in a relay: one is a light duty switching circuit which controls a different heavy duty circuit. For example, in a headlamp circuit, the light duty switching circuit is controlled by the headlamp switch. This in turn controls the heavy duty supply that serves the headlamps. The main advantage of this arrangement is that the heavier wires that serve the headlamps do not have to be run into the cab and up to the driver's dashboard. Only the light duty switching wires have to go here. The heavy duty wires can go directly from the live source, to the relay (terminal 30), out of the relay (terminal 87) and off to feed the load-in this case, the headlamps.

Relay contacts and Mick Ryan holding them.

Electromagnetic relay.

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