Understanding electrics: switches and relays

Pat Manley and Oliver Ballam demystify boat electrics, starting with switches and relays

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Switches allow a circuit to be 'made' or 'broken' so that a light, motor or whatever can be activated or isolated.

A switch has at least one pair of contacts that can be made or broken but it can be much more complicated. It can make or break several circuits at a time, or can be made to switch between one or more circuits, either individually or in unison.

Obviously the DPDT (double pole, double throw) switch may also be found with a centre OFF position.

Some switches might be latching (stay where they are put-like a domestic light switch) or momentary (return to off when pressure is released-like a horn switch). You don't have to use all the terminals, so you can tailor the switch to suit your circuit.

Switches may have solder terminals or 'spade' terminals, the latter being easier to install in boat circuits as you can use crimp terminals to fit the wires.

Troubleshooting switches

You can check the operation of a switch using a multimeter-with the switch 'open' the resistance should be infinite and with the switch 'closed' the resistance should be zero.