Definition–A thermal over load relay is an electro mechanical relay which is used to protect motors from overloading (or overheating). The relay is operated by heat developed in the relay when current exceeds its preset value of the relay.

Construction – It consists of 3nos bi-metallic strips which are placed in a housing/case. These 3 bi-metallic strips are wounded with wires (3-phases,one for each phase, R,Y,B). These bimetallic strips are connected with a common lever which is further connected with NO/NC contacts of relay.

Working principle – Under normal load condition, bimetallic strips don’t as these strips are designed to carry rated current of the motor. When motor draws more current due to overloading, the exceeded current when passes through the bimetallic strips, these strips bend & moves lever which changes the NO/NC contacts of relay. When NO/NC contacts change, starter trips motor as these NO/NC contacts are connected with trip circuit of the starter.

(Note – Bend of metallic strips depend of the amount of current. Strips bend faster on higher current & vice versa.)