Circuit breakers trip due to which mechanism described in the material?

• A. A bimetal strip that moves and opens a set of contacts when too much current passes through it
• B. A fuse wire that melts
• C. A magnetic coil that releases a latch
• D. An electronic sensor with a transistor

Answer: (A)

When current exceeds the breaker’s rating, heat builds up in a bimetal strip due to I^2R losses. The strip is made of two metals that expand at different rates, so the heat causes it to bend. That bending moves a linkage and releases the mechanism, opening the contacts and interrupting the circuit. This thermal trip is a fundamental way many breakers detect overloads and shut off power, and they can reset after the fault is cleared. Some breakers also have a magnetic trip for very fast response to short circuits, but the described mechanism—bimetal strip heating and bending driving the trip—is the hallmark of how the device senses and interrupts overcurrent. A fuse melts, which is a different protective device; a magnetic coil or an electronic sensor describe alternative concepts not the basic thermal trip mechanism.