Which device is immune to temperature changes?

• A. Magnetic overload relay
• B. Thermal overload relay
• C. Electronic overload protector
• D. Mechanical overload switch

Answer: (A)

The key idea here is how overload protection devices react to ambient temperature. A magnetic overload relay trips based on the magnetic force generated by the current flowing through a coil. Its operation depends on the amount of current, not on heat buildup or temperature-sensitive components, so ambient temperature changes have little to no effect on when it trips. That makes it effectively immune to temperature variations within its rated range.

Thermal overload relays, on the other hand, rely on heat build-up in a bi-metal element to bend and trip. Since heat transfer and reaction of the bi-metal are influenced by starting temperature and ambient temperature, these devices can trip earlier or later depending on the surrounding temperature. Electronics-based overload protectors depend on semiconductor circuits, whose thresholds drift with temperature, and mechanical overload switches can have performance changes due to thermal expansion or spring tension shifts.

So the magnetic overload relay is the best choice for being unaffected by temperature changes.