A thermal overload uses a _______ strip to activate a contact that opens a circuit and removes power to the motor.

A thermal overload uses a _______ strip to activate a contact that opens a circuit and removes power to the motor.

• A. ceramic
• B. polymer
• C. magnetic
• D. bi-metallic

Answer: (D)

A bi-metallic strip is used because it responds directly to heat. When the motor overheats, the two metals with different coefficients of thermal expansion are bonded together and expand by different amounts. This difference causes the strip to bend, which activates the contact mechanism to open the circuit and remove power to the motor. This is exactly how thermal overload protections detect overheating and trip to prevent damage. The other options don’t provide this temperature-driven bending action: a magnetic mechanism trips from high current via magnetic force, while ceramic or polymer strips don’t inherently produce the controlled bending needed to trip the contact in a thermal overload.

A bi-metallic strip is used because it responds directly to heat. When the motor overheats, the two metals with different coefficients of thermal expansion are bonded together and expand by different amounts. This difference causes the strip to bend, which activates the contact mechanism to open the circuit and remove power to the motor. This is exactly how thermal overload protections detect overheating and trip to prevent damage. The other options don’t provide this temperature-driven bending action: a magnetic mechanism trips from high current via magnetic force, while ceramic or polymer strips don’t inherently produce the controlled bending needed to trip the contact in a thermal overload.