The Sensata Dominican REP (or Mexico depending on where made) RP18A is a thermal protector* for lighting fixtures. It has a dedicated voltage of 277Vac and has high sensitivity. It has four leads, black, white, blue and orange. 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 RP18A is designed to fit into a 1/2" KO.
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 all common wires The orange wire of the RP-18A is connected to the 277 Volt tap of the ballast The blue wire is then connected to the tap of the ballast which will match the input voltage Although the primary usage is 120/277V, the thermal protector inside of these units is rated up to 480 VAc
*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.