ABB thermal overload relays are 3-phase relays with bi-metallic tripping elements (1 per pole). The motor current flows through the tripping elements and heats them directly and indirectly. In case of an overload (over-current), the bimetal elements become bent as a result of the heating. This leads to a release of the relay and a change of the contacts switching position (95-96 / 97-98). The contact 95-96 is used to control the load contactor.
The overload relays have a setting scale in amperes, which allows the direct adjusting of the relay without any additional calculation. In compliance with international standards, the setting current is the rated current of the motor and not the tripping current (no tripping at 1.05 x I, tripping at 1.2 x I (I = setting current). The relays are constructed in a way that they protect themselves in the event of an overload. The overload relay has to be protected against short-circuit.