The Sensata Dominican REP (or Mexico depending on where it is made) RP9A is a thermal protector* for lighting fixtures. It has a heater voltage of 12Vac and has low sensitivity. It has three leads, black, white, and yellow. When the temperature of the fixture is exceeded it will cause the light to blink. It is auto resetting for when the fault is corrected. The Sensata RP9A is designed to fit into a 1/2" KO.
*A thermal protector is a safety device designed to prevent lighting fixtures from overheating, which can lead to damage, fire hazards, and reduced lifespan of the fixture.
The black lead wire is always connected to the input live wire The white wire is the common which is permanently connected at the factory with an extra six inch piece of wire of the same color The third wire of the RP is permanently connected to the live side of the load