This is the trinary safety switch, which not only cuts off the compressor clutch at 30 psi or 406 psi, but also sends an electric fan engagement signal at 254 psi. Connect the trinary switch fan trigger to the same relay as your engine temperature switch. This arrangement will engage the fan with either engine temperature or A/C pressure. If you're running a mechanical fan, you only need a binary safety switch to cut off the compressor clutch at the high or low pressures. The trinary switch is mounted on the outside of the bulkhead fitting in the line between the dryer and the evaporator. It can be installed anywhere in the high-pressure liquid line between the condenser and the evaporator.