Overcurrent protective devices can operate on heating; which mechanism causes this operation?

• A. Increase in magnetic field
• B. Increase in current causing heating and bending of a bi-metal strip
• C. Decrease in current causing cooling
• D. Voltage sensing

Answer: (B)

Overcurrent protection that relies on heating uses the power dissipated by the current to trigger the trip. When current increases, I^2R losses generate heat in the protective element. In devices with a bi-metal strip, two metals with different thermal expansion rates are bonded together. As the strip heats unevenly, one metal expands more than the other, causing the strip to bend. This bending releases a latch or opens the contacts, stopping the current flow. This heating-based mechanism provides a time-delayed response that protects wiring from overheating.

The magnetic trip mechanism, which acts instantly when a strong fault current creates a strong magnetic field, does not rely on heating. The other options do not describe a heating-based operation, and thus aren’t applicable here.