Which statement describes the difference between thermal and magnetic circuit breakers?

• A. Thermal (heat) type air circuit breakers are used primarily for protection against overcurrent; a typical thermal circuit breaker operates on the principle of metal expanding when heated.
• B. Magnetic circuit breakers rely on heat expansion when heated.
• C. Thermal breakers have no magnetic coil.
• D. Magnetic breakers rely on constant current to prevent trips.

Answer: (A)

The main idea is that thermal and magnetic breakers use different mechanisms to trip. In a thermal (heat) breaker, current flow heats a metal element, typically a bimetal strip, which bends as it heats and eventually causes the contacts to open. This is a delayed response that protects against sustained overcurrents. In a magnetic breaker, the current creates a strong magnetic field in a coil, and a magnetic force trips the mechanism instantly or very quickly when a short-circuit current is detected.

The statement about thermal breakers operating on metal expansion when heated is the best way to capture the essential mechanism of how they trip, which makes it the correct distinction from magnetic breakers. The other statements miss the core difference: magnetic breakers do not rely on heat expansion, and they trip on overcurrent rather than a constant current state.