33 real Protection & Switchgear questions from the Electrical Design bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is Differential protection?
Junior
A.voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
B.compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults
C.arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
D.overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)
2. Which term means: "compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults"?
A.Differential protection — high-impedance differential scheme on a transformer winding that detects earth faults near the neutral which biased differential cannot see
B.Differential protection — interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
C.Differential protection — compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults
D.Differential protection — gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
A.interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
B.arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
C.voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
D.protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current
5. Which term means: "arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas"?
A.SF6 circuit breaker — high-impedance differential scheme on a transformer winding that detects earth faults near the neutral which biased differential cannot see
B.SF6 circuit breaker — voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
C.SF6 circuit breaker — arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
D.SF6 circuit breaker — protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current
A.contacts part inside a sealed vacuum interrupter and the arc dies at the first current zero; the dominant choice from 3.3 kV to 33 kV
B.arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
C.voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
D.gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
8. Which term means: "contacts part inside a sealed vacuum interrupter and the arc dies at the first current zero; the dominant choice from 3.3 kV to 33 kV"?
A.Vacuum circuit breaker — contacts part inside a sealed vacuum interrupter and the arc dies at the first current zero; the dominant choice from 3.3 kV to 33 kV
B.Vacuum circuit breaker — overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)
C.Vacuum circuit breaker — interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
D.Vacuum circuit breaker — gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
A.arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
B.gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
C.interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
D.contacts part inside a sealed vacuum interrupter and the arc dies at the first current zero; the dominant choice from 3.3 kV to 33 kV
11. Which term means: "gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge"?
A.Buchholz relay — high-impedance differential scheme on a transformer winding that detects earth faults near the neutral which biased differential cannot see
B.Buchholz relay — peer-to-peer Ethernet multicast message that carries trips and interlocks between IEDs within about 4 ms, replacing hard-wired copper links
C.Buchholz relay — compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults
D.Buchholz relay — gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
14. Which term means: "overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)"?
A.IDMT relay — contacts part inside a sealed vacuum interrupter and the arc dies at the first current zero; the dominant choice from 3.3 kV to 33 kV
B.IDMT relay — interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
C.IDMT relay — arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
D.IDMT relay — overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)
A.interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
B.Zone 1 covers about 80% of the line instantaneously, Zone 2 reaches 120% with 0.3–0.5 s delay, Zone 3 backs up the adjacent line
C.high-impedance differential scheme on a transformer winding that detects earth faults near the neutral which biased differential cannot see
D.arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
17. Which term means: "Zone 1 covers about 80% of the line instantaneously, Zone 2 reaches 120% with 0.3–0.5 s delay, Zone 3 backs up the adjacent line"?
A.Distance relay zones — Zone 1 covers about 80% of the line instantaneously, Zone 2 reaches 120% with 0.3–0.5 s delay, Zone 3 backs up the adjacent line
B.Distance relay zones — voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
C.Distance relay zones — compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults
D.Distance relay zones — protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current
20. Which term means: "protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current"?
A.CT class 5P20 — gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
B.CT class 5P20 — protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current
C.CT class 5P20 — arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
D.CT class 5P20 — Zone 1 covers about 80% of the line instantaneously, Zone 2 reaches 120% with 0.3–0.5 s delay, Zone 3 backs up the adjacent line
23. Which term means: "peer-to-peer Ethernet multicast message that carries trips and interlocks between IEDs within about 4 ms, replacing hard-wired copper links"?
A.IEC 61850 GOOSE — peer-to-peer Ethernet multicast message that carries trips and interlocks between IEDs within about 4 ms, replacing hard-wired copper links
B.IEC 61850 GOOSE — arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
C.IEC 61850 GOOSE — overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)
D.IEC 61850 GOOSE — Zone 1 covers about 80% of the line instantaneously, Zone 2 reaches 120% with 0.3–0.5 s delay, Zone 3 backs up the adjacent line
26. Which term means: "voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike"?
A.Transient recovery voltage — gas-actuated relay in the pipe between tank and conservator that alarms on slow gas accumulation and trips on a sudden oil surge
B.Transient recovery voltage — voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
C.Transient recovery voltage — overcurrent relay whose operating time falls as fault current rises, the IEC standard-inverse curve being t = 0.14·TMS/(I^0.02 − 1)
D.Transient recovery voltage — compares current entering and leaving a zone and trips on the difference, with a bias slope to restrain against CT mismatch on through-faults
29. Which term means: "interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors"?
A.Grading margin — voltage that appears across breaker contacts immediately after current interruption, which must stay below the breaker withstand to avoid re-strike
B.Grading margin — interval of 0.2–0.4 s kept between successive IDMT relays to cover breaker operating time, relay overshoot and CT/relay errors
C.Grading margin — protection CT with 5% composite error at 20 times rated current, unlike metering classes 0.2 or 0.5 that are accurate only near rated current
D.Grading margin — arc is quenched in sulphur hexafluoride, whose dielectric strength is about 2.5 times air; standard at 66 kV and above despite being a potent greenhouse gas
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