The Sensata Dominican REP (or Mexico depending on where made) RP17A is a thermal protector* for lighting fixtures. It has a dedicated voltage of 220Vac and has high sensitivity. It has three leads, black, white, and violet. 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 RP17A 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.