Which type of overload relay uses a bimetallic strip as its operational sensing device?

• A. Magnetic overload relay
• B. Electronic overload relay
• C. Bimetallic overload relay
• D. Automated overload relay

Answer: (C)

The correct answer is the bimetallic overload relay. This type of relay operates based on the principle of thermal expansion of metals. It utilizes a bimetallic strip, which is made from two different metals that expand at different rates when heated. As the current flowing through the relay exceeds its designed limit, the bimetallic strip heats up and bends due to the differential expansion. This bending action ultimately opens or trips the circuit, thus protecting the connected equipment from excessive current.

The bimetallic overload relay is commonly used in motor protection applications because of its simplicity and reliability. It provides a time-delay feature that allows for brief surges of current without tripping, which is important for handling inrush currents commonly seen in motors during startup.

In contrast, magnetic overload relays operate based on electromagnetic principles, using coils and mechanical actions to trip the circuit, while electronic overload relays rely on electronic components to detect overload conditions and typically offer more precise and programmable features. Automated overload relays typically incorporate additional automation features but do not specifically use a bimetallic strip as their primary sensing mechanism.