Free Power Systems MCQs with Answers
334 Power Systems MCQs from Electrical Engineering, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
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334 questions · page 4 of 17
- 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…
Correct answer: merz price percentage differential protection- 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.
Correct answer: power- 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…
Correct answer: repeated duty- 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…
Correct answer: transient state of short-circuit current- 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.
Correct answer: 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…
Correct answer: have all above properties- 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.
Correct answer: heavy loads- 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…
Correct answer: all above- 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.
Correct answer: overload- 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…
Correct answer: is nontoxic- A. Water
- B. Oil
- C. Air
- D. SF6
Explanation: An air circuit breaker uses atmospheric air as the arc-extinguishing medium.
Correct answer: Air- A. Oil circuit breakers
- B. Air blast circuit breakers
- C. Minimum oil circuit breakers
- D. All of the above
Explanation: Routine tests such as insulation, mechanical operation and timing checks are performed on oil, air-blast and minimum-oil circuit breakers.
Correct answer: All of the above- A. copper tungsten alloy
- B. porcelain
- C. electrolytic copper
- D. aluminium alloy
Explanation: Arcing contacts must withstand high temperature, erosion and repeated arcing, so copper-tungsten alloys are commonly used.
Correct answer: copper tungsten alloy- A. a capackive reactance
- B. an inductive reactance
- C. a resistance increasing with voltage rise across the arc
- D. a resistance decreasing with voltage rise across the arc
Explanation: An arc has a negative resistance characteristic: as the voltage across the arc rises, its effective resistance generally falls because…
Correct answer: a resistance decreasing with voltage rise across the arc- A. fuses
- B. relays
- C. fast switches
- D. circuit breakers
Explanation: Fault diverters are high-speed switches that divert or interrupt fault current before serious equipment damage occurs.
Correct answer: fast switches- A. delayed
- B. instantaneous
- C. both of the above
- D. none of the above
Explanation: A magnetic circuit breaker uses the strong magnetic field produced by a fault current to trip immediately.
Correct answer: instantaneous- A. delayed trip action
- B. instantaneous trip action
- C. both of the above
- D. none of the above
Explanation: A thermal circuit breaker uses a bimetallic strip that heats and bends under sustained excess current, producing a time delay before…
Correct answer: delayed trip action- A. Battery charging work
- B. Arc welding
- C. Electrolytic and electro-chemical processes
- D. Arc lamps for search lights
- E. All of the aboveCompare Local Schools
Explanation: DC supplies are used for battery charging, arc welding, electrochemical processes, and certain arc-lamp applications such as searchlights.
Correct answer: All of the aboveCompare Local Schools- A. earth
- B. conduction
- C. sheath
- D. insulator
Explanation: In a cable capacitor, the electric field varies inversely with radius, so it is greatest at the surface of the inner conductor.
Correct answer: conduction- A. insulation
- B. conductor
- C. outer sheath
- D. uniformly all over
Explanation: In a cable, the electric stress is greatest at the conductor surface and decreases outward, reaching its minimum at the inner surface of…
Correct answer: insulation