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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 mercury?
Thermal relay
Gas operated relay
Induction cup relay
None of above
Gas operated relay
Quick Summary: The Buchholz relay, a type of gas-operated relay, utilizes mercury switches as a primary sensing mechanism for detecting faults in oil-immersed transformers. When gas accumulates or oil surges occur, a float or vane tilts, causing the mercury within a glass tube to bridge two electrical contacts, thereby triggering the trip circuit.
The Buchholz relay, a type of gas-operated relay, utilizes mercury switches as a primary sensing mechanism for detecting faults in oil-immersed transformers. When gas accumulates or oil surges occur, a float or vane tilts, causing the mercury within a glass tube to bridge two electrical contacts, thereby triggering the trip circuit.
Pfault=21ρv2 — Force exerted by oil surge on relay vane
Vgas=k⋅t — Volume of gas accumulation over time t
Inside a Buchholz relay, two mercury switches are mounted on floats. The upper switch detects the slow accumulation of gas (incipient faults) and triggers an alarm. The lower switch, activated by a sudden surge of oil pressure during severe internal faults, closes the tripping circuit to isolate the transformer immediately.
Buchholz relays are exclusively used for oil-immersed transformers.
They provide protection against inter-turn faults, core faults, and oil leaks.
The mercury switch ensures reliable contact closing even under vibration.
Modern relays may use magnetic reed switches instead of mercury due to environmental concerns.
Simple and rugged design.
Provides both alarm (minor faults) and trip (major faults) functionality.
Detects incipient faults before they escalate into major failures.
Limited to oil-immersed transformers only.
Mercury is toxic and environmentally hazardous.
Susceptible to false operations due to mechanical shocks or bubbles in the oil.
Protection of power transformers.
Monitoring of oil-filled reactors.
The mercury switch is housed in a glass bulb filled with an inert gas like Argon to prevent oxidation of the mercury contacts.
Option A: Thermal relays operate based on the expansion of bimetallic strips, not mercury.
Option C: Induction cup relays rely on electromagnetic torque developed in a rotating disc or cup, typical for directional or distance protection.
B is correct — The Buchholz relay is a gas-operated relay that uses mercury-filled switches to detect internal transformer faults.
Always remember that Buchholz relays are 'gas-actuated' and are fundamentally different from electrical relays like overcurrent or distance relays which operate on electromagnetic induction.