Technician A says circuit breakers are used on some circuits to protect them from temporary current overload. Technician B says that circuit breakers work because of a bimetal strip that moves and opens contacts when too much current passes through. Who is right?

• A. A only
• B. B only
• C. Neither
• D. Both A and B

Answer: (D)

Protecting a circuit by interrupting current when it exceeds safe levels. That’s exactly what circuit breakers do to prevent damage or fire from overloads or faults. The action described for the first statement is correct: breakers are used on circuits to protect them from temporary or sustained overloads by tripping when current goes beyond the device’s rating. The second statement is also correct because many breakers rely on a thermal mechanism involving a bimetal strip. When excess current flows, the strip heats up, bends due to the different expansion rates of the metals, and causes the contacts to open, stopping the current. In higher fault situations, a magnetic mechanism can contribute to a faster trip as well. So both statements together describe how circuit breakers function.