Fog Cut-Out Relays
These relays are designed to switch off the foglamp(s) of a towing vehicle when the foglamp of a towed trailer is on. There are stand-alone fog cut-out relays and some Smart bypass relays have this function built in.
The reason for turning off the vehicle foglamp is to avoid a distracting glare being reflected by the towed trailer, especially if the trailer is a caravan.
A fog cut-out relay is, in fact, a combined monitor and relay: the relay that switches off the towing vehicle's foglamp is switched by a monitoring circuit that operates when it detects current flowing through it to the trailer foglamp.
The switched side is fitted in line in the vehicle foglamp circuit and the monitoring side is fitted in the cable going to pin 2 (Fog) of the trailer socket.
It is necessary, during fitting, to cut the wire that supplies the vehicle foglamp so as to fit the relay's switched circuit in line.