All Free Electrical Engineering MCQs with Answers

• A. impedance
• B. directional
• C. non-directional
• D. none of the above

Explanation: An admittance relay, commonly called a mho relay, has a directional characteristic and responds to both impedance magnitude and phase…

• A. 100 microsecond
• B. 50 millisecond
• C. 0.5 sec
• D. 0.1 sec

Explanation: High-voltage air-blast circuit breakers are designed for rapid interruption, typically within about 50 milliseconds at this voltage level.

• A. relay
• B. rewirable fuse
• C. H.R.C. only
• D. all of the above

Explanation: During a fault, the relay detects the abnormal condition and sends a trip signal to the circuit-breaker.

• A. zero sequence currents only
• B. positive sequence currents only
• C. negative and zero sequence currents
• D. zero, negative and positive sequence currents

Explanation: A balanced three-phase set has equal magnitudes separated by 120°, which forms a positive-sequence set only.

• A. no current flow
• B. heavy current flow
• C. voltage drop
• D. voltage rise

Explanation: A short circuit creates a very low-impedance path, so the current rises sharply and may become several times the normal load current.

• A. earth wires
• B. isolators
• C. H.R.C. fuses
• D. reactors

Explanation: Reactors add inductive reactance to the circuit, limiting the rate and magnitude of short-circuit current.

• A. fault occurs on the bus itself
• B. fault occurs on neutral line
• C. two or more faults occur simultaneously
• D. fault occurs with respect to earthing

Explanation: In a single-bus arrangement, every circuit is connected to the same bus, so a bus fault can disconnect or shut down all connected…

• A. distribution transformer
• B. generator transformer of the power plant
• C. auto-transformer of the power plant
• D. station transformer of the power plant

Explanation: Overfluxing protection is especially important for the generator transformer because excessive volts-per-hertz can saturate its magnetic…

• A. constant for all distances
• B. lower than that of if fault occurs away from the relay
• C. higher than that of if fault occurs away from the relay
• D. none of the above

Explanation: Interpreting “VII ratio” as the V/I impedance ratio, a fault nearer the impedance relay gives a shorter line impedance and therefore a…

• A. Open circuit
• B. Short-circuit
• C. Long
• D. None of the above

Explanation: For an open-circuit termination, the reflected voltage is in phase with the incident voltage, giving a voltage reflection coefficient of…

• A. Lightning arresters
• B. Relays
• C. Step-down transformer
• D. Switchgear

Explanation: A lightning arrester is connected at the line entry point to divert a travelling surge to earth before it reaches sensitive equipment.

• A. Impedance
• B. Reactance
• C. Either of the above
• D. None of the above

Explanation: Impedance relays are commonly used for phase-fault protection, particularly on short and medium transmission lines.

• A. above 30 kVA.
• B. equal to and above 5 MVA
• C. equal to and above 25 MVA
• D. none of the above

Explanation: Transformer differential protection is normally applied to larger transformers because its cost and complexity are justified at higher…

• A. hydro-electric generators
• B. steam turbine generators
• C. gas turbine generators
• D. all of the above
• E. none of the above

Explanation: Overvoltage protection is applied to generators of all these prime-mover types because excessive voltage can damage insulation and…

• A. Zero
• B. Unity
• C. Infinity
• D. None of the above

Explanation: The reflection coefficient is Γ = (ZL − Z0)/(ZL + Z0). When the load impedance equals the line surge impedance, the numerator is zero, so…

• A. zero current
• B. maximum current
• C. minimum voltage
• D. maximum voltage

Explanation: An AC circuit-breaker arc naturally reaches its weakest point when the current passes through zero, allowing the dielectric strength…

• A. single phase motors only
• B. two phase motors only
• C. two single phase motors running in parallel
• D. three phase motors

Explanation: Single phasing means one phase of a three-phase supply is lost, which can overheat and damage a three-phase motor.

• A. low resistance
• B. high resistance
• C. low impedance
• D. high impedance

Explanation: A series reactor limits fault current mainly through its inductive reactance, so it should have high impedance but very low resistance to…

• A. Inductance
• B. Capacitance
• C. Resistance
• D. Reactance

Explanation: For a short transmission line, shunt capacitance is small because the line length is short, so it is neglected in the model.

• A. generators
• B. busbars
• C. transformers
• D. motors
• E. all of the above

Explanation: Under-voltage relays can initiate protection or control actions in generators, busbars, transformers, and motors, such as tripping or load…