Overcurrent protective devices operate on two principles; which pair is correct?

• A. Thermal and Electrical
• B. Thermal and Magnetic
• C. Mechanical and Hydraulic
• D. Optical and Thermal

Answer: (B)

Overcurrent protection relies on how current affects the device: heat from the current acts on a thermal element, and the magnetic field from a fault current acts on a magnetic mechanism. A thermal element heats up with sustained overload until a bimetallic strip or similar part bends and trips, while a magnetic coil responds quickly to high fault currents by pulling a mechanism to trip. Many protective devices combine both principles, using the thermal part for gradual overload protection and the magnetic part for fast response to short circuits. That combination—thermal and magnetic—is the correct pairing. The other options describe mechanisms not used for sensing overcurrent in standard protective devices.