In this diagram we are going to use a temperature sending unit to determine when to switch the fan on and off. We normally wire an additional manual switch used to override the sending unit in the event we have a sending unit failure, or we simply decide to turn the fan on.

The only problem with this setup is that if for some reason we have a fuse blow, we will loose the fan and may not know it until it is too late.

In this diagram we still use a temperature sending unit to determine when to switch the fan on and off.

The difference with this diagram is that we power the fan with a circuit breaker and pass a ground thru the relay to turn the fan on.

If the circuit breaker should trip, the fan will shut off until the breaker resets.