In a bimetallic overload relay, when current exceeds a threshold, which factor causes the bimetallic strip to spring away and open the motor control circuit?

• A. Light
• B. Pressure
• C. Heat
• D. Sound

Answer: (C)

This question centers on how a bimetallic overload relay uses heat to actuate. When current through the motor exceeds the threshold, resistive heating raises the temperature of the bimetallic strip. The strip is made of two metals bonded together that have different coefficients of thermal expansion. As temperature rises, one metal expands more than the other, causing the strip to bend (spring away). This bending trips the mechanism and opens the motor control circuit to protect the motor.

Light, pressure, and sound aren’t what drives this action in a bimetallic overload relay—the actuation is thermal, produced by the heat from the current.