Switchgear & Protection MCQ – 19 previous year questions with solutions

Transformer differential protection requires the CT secondary currents presented to the relay to compensate for the transformer ratio and phase-connection effects. Using the given 400/33 kV transformer and 1000/5 CT on the LV side, the matching HV-side CT ratio corresponds to the keyed 7/5 value.

C — Internal faults. A differential relay compares quantities entering and leaving the protected zone; they balance for external faults and differ only for faults inside the zone.

C — It responds to the direction of current flow. Using a voltage polarising quantity, a directional relay identifies whether fault current/power flows forward or reverse and trips only for one direction.

B — Distance relay. It measures the apparent impedance V/I at the relay point; since line impedance is proportional to length, it operates when a fault lies within its set reach.

Plug setting multiplier is the fault current divided by relay pickup current. PSM=I_fault/I_pickup=30/7.5=4, so the statement claiming 5 is numerically incorrect. Key relation: PSM=I_fault/I_pickup.

Exam focus: Calculate PSM numerically before selecting an option. Common trap: Induction-type relays are not used directly on DC quantities.

C — Protect equipment, isolate the faulted section and maintain supply continuity. Protection detects faults, disconnects only the faulted part and keeps the healthy system running.

First refer the relay setting through the CT ratio: 100/5 = 20, so a 1.5 A relay pickup corresponds to 30 A primary residual current. Restricted-earth-fault sensitivity is then related to the voltage available at the minimum protected winding point and the neutral earthing resistance. The official final key gives 2.12 Ω for the stated 90% protection condition.

D — The voltage difference exceeds a set value. The relay continuously compares the voltages at two monitored points and operates when their difference crosses the setting.

C — To operate only when primary protection fails. Back-up protection is time-delayed so the primary relay clears the fault first with minimum isolation; back-up acts only if it fails.

Air circuit breakers are LV switchgear devices used at high-current incomers/feeders, not small domestic branch circuits. Modern trip units provide overcurrent/short-circuit and often earth-fault protection. Key relation: ACB applications commonly include main LT switchboards and large feeders.

Exam focus: Domestic low-current circuits normally use MCB/RCBO class devices. Common trap: ACB does not mean compressed-air blast breaker in this LT-panel context.

D — Ionisation of the medium between the separating contacts. As the gap opens, current is forced through a tiny gap with a very high field and temperature, ionising the medium so it conducts as an arc.

C — The maximum current it can safely interrupt. Breaking capacity is the highest prospective fault current the breaker can interrupt at rated voltage under specified conditions (expressed in kA or MVA).

MCCBs provide switching, isolation and overcurrent protection for higher-current circuits than typical MCBs. Many have adjustable long-time/instantaneous or electronic trip settings. Key relation: Trip settings must coordinate with cable ampacity and downstream protection.

Exam focus: MCCB trip adjustability is a major distinction from many small MCBs. Common trap: Device frame rating and actual trip setting are not the same thing.

D — Higher dielectric strength. SF₆ is highly electronegative: it captures free electrons, has about 2–3 times the dielectric strength of air and recovers strength quickly after current zero.

C — System stability and safety of the alternator. Excess current from a fault or severe overload overheats the winding and creates damaging electromagnetic forces; overcurrent protection disconnects the machine in time.

A Buchholz relay is a gas- and oil-surge-operated relay on oil-immersed transformers with a conservator. It alarms on slow internal faults and trips on severe internal faults.

A — Reverse Power relay. It detects active power flowing from the energized system into the alternator when the prime mover can no longer supply sufficient mechanical power.

C — Balanced earth fault. This is the protection scheme associated with the 1–5 MVA generator range in the supplied paper.

Rupturing (breaking) capacity is the largest prospective short-circuit current a device can interrupt safely. It is a fault-interruption rating — it does not change regulation, inductance or insulation resistance.