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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Which relay used bimetallic strip?
Thermal relay
Gas operated relay
Induction cup relay
None of above
Thermal relay
Quick Summary: A thermal relay utilizes the principle of differential thermal expansion of a bimetallic strip to actuate a tripping mechanism when overcurrent causes heating. The bimetallic strip consists of two metals with different coefficients of thermal expansion bonded together, which bend upon heating.
A thermal relay utilizes the principle of differential thermal expansion of a bimetallic strip to actuate a tripping mechanism when overcurrent causes heating. The bimetallic strip consists of two metals with different coefficients of thermal expansion bonded together, which bend upon heating.
ΔL=L0αΔT — Linear expansion formula
ΔT∝I2Rt — Thermal heating principle
As current flows through the heater element associated with the bimetallic strip, I2R losses generate heat proportional to the square of the current. This heat causes the two dissimilar metals to expand at different rates, forcing the strip to deflect. Once the deflection reaches a critical threshold, it triggers the mechanical contact mechanism to break the circuit.
Bimetallic strips act as both a sensor and an actuator.
These relays are primarily used for overload protection, not for short-circuit protection.
The response time of thermal relays is inversely proportional to the magnitude of current overload.
Simple and rugged construction.
Inherent inverse time-current characteristics.
Economical for low-voltage applications.
Slow operation compared to electromagnetic relays.
Affected by ambient temperature variations.
Not suitable for high-speed fault clearing.
Motor overload protection in MCC panels.
Domestic appliances like irons and heaters.
Small-scale circuit breakers.
Gas-operated relays (like Buchholz relay) use gas accumulation principles for transformer protection.
Induction cup relays operate on electromagnetic induction principles for high-speed distance protection.
Thermal relays are specifically designed for 'time-delay' protection to allow for motor starting inrush currents.
A is correct — The thermal relay uses the differential expansion of a bimetallic strip to sense overload conditions through heat.
Always distinguish between thermal relays (slow/overload) and electromagnetic/induction relays (fast/faults) when solving protection system MCQs.