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Examoogle • User • info@examoogle.com • EE-2024-8821
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Which is measuring relay?
Current relay
Voltage relay
Power relay
All of above
All of above
Quick Summary: A measuring relay is a protective device that operates when the electrical quantity it monitors (current, voltage, power, or impedance) exceeds or falls below a pre-set threshold value. Since current, voltage, and power are all fundamental electrical quantities that can be measured and compared against reference values, all the listed options qualify as measuring relays.
A measuring relay is a protective device that operates when the electrical quantity it monitors (current, voltage, power, or impedance) exceeds or falls below a pre-set threshold value. Since current, voltage, and power are all fundamental electrical quantities that can be measured and compared against reference values, all the listed options qualify as measuring relays.
Irelay=CTRIprimary — Current relay sensing principle
P=VIcosϕ — Power relay sensing principle
Measuring relays consist of a sensing element (like a CT or PT) that converts high-power system quantities into low-power signals. An electromagnetic, electronic, or numerical comparator then evaluates this signal against a fixed reference value. If the monitored quantity exceeds the relay setting, an output contact closes to trigger the circuit breaker.
Measuring relays are defined under IEC 60255 standards.
They provide quantitative protection rather than binary logic.
Sensitivity is determined by the threshold settings (pick-up values).
High precision in detecting fault conditions.
Allows for graded protection schemes.
More complex circuitry compared to simple auxiliary relays.
Susceptible to errors from CT/PT saturation.
Overcurrent protection in distribution lines.
Under-voltage protection for sensitive loads.
Reverse power protection for alternators.
Current relays (e.g., IDMT relays) are used for overcurrent protection.
Voltage relays are used for over/under voltage protection of busbars.
Power relays are specifically used to detect the direction of power flow.
D is correct — Current, voltage, and power are all measurable electrical quantities monitored by specific relay types to initiate protective actions.
Remember that while all listed are measuring relays, relays like 'Auxiliary' or 'Signaling' relays are NOT measuring relays because they do not monitor an electrical quantity against a threshold.