Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
Which type of protect fault protection does the thermal overload relay provide?
Overload
Electric heating
Over voltage
None
Electric heating
Quick Summary: A thermal overload relay functions on the principle of thermal expansion caused by electric heating. When current flows through the relay's bimetallic strips, it generates heat proportional to $I^2R$, which eventually trips the relay when the heat exceeds a predefined safety limit.
A thermal overload relay functions on the principle of thermal expansion caused by electric heating. When current flows through the relay's bimetallic strips, it generates heat proportional to I2R, which eventually trips the relay when the heat exceeds a predefined safety limit.
H=I2Rt — Heat generated in the bimetallic strip where I is current, R is resistance, and t is time.
ΔL=L0αΔT — Linear expansion principle where ΔL is change in length and α is the thermal expansion coefficient.
The relay contains bimetallic strips made of two different metals with varying thermal expansion coefficients. As current passes through the heating element associated with the strips, the heat produced causes differential expansion, leading the strip to bend. This mechanical deflection triggers a trip mechanism that opens the circuit, protecting the motor from continuous overcurrent damage.
Thermal relays provide protection specifically against sustained overloads.
The bimetallic strip acts as both the sensing element and the switching actuator.
These relays have an inverse time-current characteristic, meaning they trip faster as the overload current increases.
They are not suitable for instantaneous short-circuit protection.
Cost-effective solution for small to medium motors.
Mimics the heating characteristic of the protected motor windings.
Simple mechanical construction with high reliability.
Subject to ambient temperature variations affecting calibration.
Requires manual reset after a trip event.
Slower response time compared to electronic or magnetic relays.
Motor Control Centers (MCCs).
HVAC system motor protection.
Industrial conveyor and pump motor starters.
Standard: IEC 60947-4-1 governs the performance of low-voltage electromechanical contactors and motor starters.
Option A is a functional outcome, but 'Electric heating' describes the physical phenomenon (principle) causing the protection.
Option C refers to voltage surges, which require surge arresters or voltage relays, not thermal overload relays.
B is correct — The thermal overload relay operates based on the principle of electric heating causing bimetallic element deflection.
Always distinguish between thermal relays (slow, overload-based) and magnetic relays (instantaneous, short-circuit-based) in your power system study.