What is a bimetallic strip overload relay made of?

A bimetallic strip overload relay is constructed from two jointed pieces of dissimilar metal that have different thermal expansion rates. When electrical current passes through the relay and causes it to heat up, the metal strip bends due to the differing rates of expansion. This bending effect is what triggers the relay to open the circuit, providing overload protection for motors by interrupting the current flow when excessive heat—which indicates excessive current—is detected.

The effectiveness of a bimetallic strip overload relay relies on the principle that the two metals react to heat differently. This characteristic is essential for the relay's operation, allowing it to respond accurately and quickly to variations in current that could potentially damage the motor. The use of dissimilar metals, rather than identical metals or other materials, ensures a significant bending action, making the relay sensitive and reliable in protecting electrical equipment.