Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. Starter combinations are setup together with block contactors and mini contactors.
The thermal overload relays are three pole relays with bimetal tripping elements. The motor current flows through the bimetal tripping elements and heats them directly and indirectly. In case of an overload (over current), the bimetal elements bent as a result of the heating. This leads to a release of the relay and a change of the contacts switching position.
The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.
L&T provides the MN series of overload relays for its MNX series of contactors and ML series of relays for the ML series of contactors. Siemens provides the 3UA series of overload relays for its 3TF series of contactors. Schneider LC1D series of contactors use the LRD series relays and LC1E series use the LRE series of overload relays.For ABB, the A series contactors are used with the TA..DU series of thermal overload relays.