What are the two classifications of overload relays?

• A. Mechanical and electrical
• B. Thermal and magnetic
• C. Standard and advanced
• D. Adjustable and fixed

Answer: (B)

The correct classification of overload relays is thermal and magnetic. This classification is based on the operating principle of the relays.

Thermal overload relays function by utilizing a bimetallic strip that bends and trips the relay when the temperature rises beyond a certain threshold due to excessive current. This mechanism allows them to respond gradually to overcurrent conditions, providing protection in a way that mimics the thermal characteristics of electric motors.

Magnetic overload relays, on the other hand, operate using an electromagnetic coil that generates a magnetic field. When the current exceeds the rated level, the magnetic force becomes sufficient to operate a trip mechanism that disconnects the load. This type of relay reacts more quickly compared to thermal relays, making it suitable for applications where instantaneous response is necessary.

Understanding these two classifications allows technicians to choose the appropriate overload relay based on their specific application requirements, taking into account factors such as response time and the characteristics of the equipment being protected.