The client has a house with a newly renovated bathroom and three existing light switches which are to be used to control a mains powered extractor fan in the new bathroom. If any of the three light switches is on, the fan is to run.
If the output from these three switches were to be connected directly to the fan, then the fan would turn on if any of the switches is on, but it would also result in the lights controlled by the three switches (in different rooms) all turning on simultaneously. This would be a problem.
Therefore, the output from the three switches is connected to three independent relays with 240V AC coils on the board pictured. These three relays each switch a single 12V signal from a nearby mains transformer to energise a 12V DC coil relay which in turn switches mains live to supply power to the fan when one or more of the light switches is on.
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