All Free Electrical Engineering MCQs with Answers

• A. current wave only
• B. voltage wave only
• C. Both (A) and (B)
• D. power factor wave only

Explanation: Energizing a transmission line creates a traveling electromagnetic wave, consisting of related voltage and current waves that propagate…

• A. required to operate the circuit breaker
• B. absorbed by the circuit of relay
• C. developed by the relay circuit
• D. none of the above

Explanation: Relay burden is the power absorbed by the relay circuit, usually expressed in volt-amperes at the rated current or voltage.

• A. lengthening of the gap
• B. cooling and blast effect
• C. both (A) and (B)
• D. deionizing the oil with forced air
• E. none of the above

Explanation: After the arc current reaches its natural zero, circuit breakers prevent restriking by increasing the gap and removing heat and ionization…

• A. the spring tension
• B. the rate of flux built up
• C. armature core air gap
• D. all of the above

Explanation: Relay speed is influenced by spring tension, the rate at which operating flux builds up, and the armature core air gap.

• A. resistance, reactance or impedance
• B. voltage and current
• C. light intensity
• D. temperature
• E. all above

Explanation: Protective relays can be made responsive to electrical quantities such as current, voltage, resistance, reactance, or impedance, and to…

• A. good selectivity and reliability
• B. economy and simplicity
• C. high speed and selectivity
• D. all of the above

Explanation: Good protective relaying must combine selectivity, reliability, speed, simplicity, and economy.

• A. two currents
• B. two voltages
• C. two or more similar electrical quantities
• D. none of the above

Explanation: A differential relay compares corresponding currents or other similar electrical quantities entering and leaving a protected zone, using…

• A. current during abnormal condition
• B. voltage during abnormal condition
• C. constantly the electrical quantities which differ during normal and abnormal conditions
• D. none of the above

Explanation: A protective relay continuously monitors electrical quantities such as current and voltage and responds when their values or relationships…

• A. recovery
• B. surge
• C. operating
• D. arc
• E. none of the above

Explanation: The recovery voltage is the voltage that appears across the circuit-breaker contacts after the arc has been extinguished.

• A. merz price percentage differential protection
• B. mho and ohm relays
• C. horn gaps and temperature relays
• D. earth fault and positive sequence relays

Explanation: Merz-Price percentage differential protection compares currents entering and leaving the protected zone, so an internal fault produces a…

• A. power
• B. current
• C. voltage wave
• D. all of the above

Explanation: A directional relay responds according to the direction of power flow, determined from the phase relationship between voltage and current.

• A. over currents
• B. short duty
• C. intermittant duty
• D. repeated duty

Explanation: Air-blast circuit breakers are suited to frequent switching because the compressed-air blast rapidly extinguishes the arc and permits…

• A. transient state of short-circuit current
• B. sub-transient state of short-circuit current
• C. steady state of short-circuit current
• D. after D.C. component has ceased

Explanation: A circuit breaker must interrupt a fault before the short-circuit current reaches steady state, while it still has transient behavior and…

• A. few turns of thin wire
• B. few turns of thick wire
• C. many turns of thin wire
• D. many turns of thick wire

Explanation: A DC shunt relay is connected across the supply and must draw relatively small current, so its coil uses many turns of thin wire.

• A. have low contact resistance
• B. be clean and smooth
• C. be of sufficient size and proper shape
• D. have all above properties

Explanation: Relay contacts should have low resistance, clean and smooth surfaces, and adequate size and shape to carry current without excessive…

• A. short-circuits
• B. heavy loads
• C. grounds
• D. all of the above

Explanation: Thermal overload relays respond to heating caused by sustained overcurrent, typically when a motor is carrying a heavy load.

• A. induction
• B. solid state
• C. thermal
• D. electromagnetic
• E. all above

Explanation: Overload relays are available in induction, thermal, electromagnetic and solid-state forms, depending on the sensing and tripping…

• A. short-circuit
• B. temperature
• C. overload
• D. over voltage

Explanation: A thermal protection switch responds to heating caused by excessive current, so it protects against overloads.

• A. is yellow in colour
• B. is lighter than air
• C. is nontoxic
• D. has pungent small
• E. none of the above

Explanation: Pure SF6 is a colourless, odourless and essentially nontoxic gas, although its decomposition products can be hazardous and it can cause…

• A. Water
• B. Oil
• C. Air
• D. SF6

Explanation: An air circuit breaker uses atmospheric air as the arc-extinguishing medium.