Two types of thermal overload relays in common use today are

• A. The bimetal relay and the melting alloy relay.
• B. The melting alloy relay and the Mercury bulb.
• C. The bimetal relay and the magnetic relay.
• D. The bimetal relay and the trimetal relay.

Answer: (A)

Thermal overload protection relies on sensing heat generated by motor current and using that heat to trigger a trip before the motor overheats. The two most common forms you’ll encounter are a bimetal element and a melting alloy element.

A bimetal relay uses a strip made from two metals with different coefficients of thermal expansion bonded together. As the strip heats, the metals expand at different rates, causing the strip to bend. That bending moves the trip mechanism and opens the circuit after a delay, providing a gradual, time-delayed response that can tolerate short overloads but will eventually trip if heat continues.

A melting alloy relay relies on a small fusible alloy that melts at a specific temperature. When the alloy reaches its melting point, it releases a spring or latch, causing the relay to trip quickly and decisively. This gives a fixed-temperature trip, but can be more sensitive to sustained overloads and less forgiving of brief overloads than the bimetal type.

Mercury bulb trip devices are older and less common today due to safety and environmental concerns. Magnetic relays operate on magnetic principles rather than heat, so they aren’t considered thermal overload types. Trimetal elements exist in some designs but are far less common than the two main forms described above.

Thus, the two commonly used thermal overload relay types are the bimetal and the melting alloy relay.