100+ Most Important Power System Protection MCQs

Power system protection refers to the measures and devices implemented to detect and mitigate faults and abnormalities in an electrical power system, ensuring its safe and reliable operation. It involves the application of relays, circuit breakers, fuses, and other protective equipment to identify and isolate faulty components or sections, preventing damage to equipment and minimizing the impact of disruptions. The primary goals of power system protection include safeguarding equipment, minimizing power interruptions, preserving system stability, and protecting personnel and the public from potential hazards. Efficient and effective power system protection is crucial for maintaining the integrity and continuity of electrical supply. Electrical Engineering XYZ presents most 100+ important power system protection MCQs on important topics including fuses, relays, circuit breakers, machines, motors, transformers, alternator, grounding and protection schemes. Also see complete list of Electrical Engineering MCQs here. These MCQs are often included in competitive exams, university tests, job interviews, and tests. Let’s start learning:

Basic function of a relay is to

The basic function of a relay is to

1. Detect the fault
2. Interrupt the circuit
3. Both of these
4. None of these

Correct answer: 1. Detect the fault

Download Relays Handbook here

Explanation: A relay is an electrical device that opens or closes a circuit under controlled conditions. Its primary function is to detect abnormal conditions in the electrical circuit, such as overcurrents, voltage spikes, or other faults. When a relay detects such a fault, it operates to send a signal. This operation can trigger various responses depending on the system design, such as activating an alarm or engaging a circuit breaker to interrupt the current flow and isolate the faulted section of the circuit. Learn about Basics of Relays here

The key point in this Electrical Engineering XYZ MCQs is that while the relay itself detects the fault, it does not directly interrupt the circuit; instead, it triggers mechanisms like circuit breakers to perform the interruption. Thus, the answer “Detect the fault” is the most accurate and specific to the primary function of a relay. The option “Interrupt the circuit” is not correct as the relay does not perform this action directly.

The most dangerous fault on power systems is

The most dangerous fault on power systems is:

1. L-G
2. L-L
3. L-L-L
4. L-L-G

Correct answer: 3. L-L-L

Explanation: L-L-L Faults are considered the most severe form of faults in a power system. This is because they involve all three phases directly, leading to the highest level of unbalance and the most significant disturbances in the system. These faults cause the highest fault currents compared to other types of faults. Learn more about Line to Line to Line faults here

An overcurrent relay having a current setting of 150% is connected through a 400/5 CT. The pickup value of current is

An overcurrent relay having a current setting of 150% is connected through a 400/5 CT. The pickup value of current is:

1. 2.5 A
2. 5 A
3. 7.5 A
4. 10 A

Correct answer: 3. 7.5 A

Solution: 5 * 1.5 = 7.5 A

The instantaneous relays used for earth fault detection in motors is usually set to _____________

The instantaneous relays used for earth fault detection in motors is usually set to _____________:

1. 50% of the RLA
2. 50% of the FLA
3. 20% of the RLA
4. 20% of the FLA

Correct answer: 4. 20% of the FLA

Explanation: The correct answer is “20% of the FLA” (Full Load Amps).

Instantaneous relays used for earth fault detection in motors are typically set to 20% of the Full Load Amps (FLA). FLA refers to the maximum current that a motor is designed to carry under normal operating conditions. By setting the relay to 20% of the FLA, it allows for quick detection of earth faults in the motor circuit.

Earth faults occur when an unintended connection to the earth or ground is established, which can lead to dangerous conditions such as electric shock or damage to equipment. The instantaneous relay operates immediately upon detecting a fault current exceeding its set value (20% of FLA) to isolate the motor from the power supply and prevent further damage or hazards.

Setting the relay to 20% of FLA provides a good balance between sensitivity to detect faults and avoiding false tripping due to normal motor start-up or transient conditions. It ensures that even small earth faults are swiftly detected, protecting the motor and preventing potential harm.

The term effective earthing refers to the earthing which is done through

The term effective earthing refers to the earthing which is done through:

1. Capacitance
2. Reactance
3. Transformer
4. None of these

Correct answer: 4. None of these

Explanation: The correct answer is “None of these.”

Effective earthing refers to the process of providing a low-resistance path for electrical currents to flow into the ground, thereby preventing the buildup of potentially dangerous voltages. It is an essential safety measure to protect people, equipment, and structures from electric shocks and other electrical hazards.

The options provided in the multiple-choice question are not appropriate for describing effective earthing:

Capacitance: Capacitance refers to the ability of a capacitor to store electrical energy in an electric field. While capacitance can be present in certain electrical systems, it is not directly related to effective earthing. Effective earthing is primarily concerned with creating a low-resistance connection to the ground.

Reactance: Reactance is a property of electrical circuits that describes the opposition to the flow of alternating current (AC). It can include both inductive reactance (due to inductors) and capacitive reactance (due to capacitors). While reactance is relevant to electrical circuits, it does not specifically address effective earthing.

Transformer: A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Transformers are primarily used for voltage transformation and isolation purposes. While transformers may be involved in certain aspects of electrical systems, they do not directly relate to effective earthing.

In summary, effective earthing is not accomplished through capacitance, reactance, or transformers. It involves the establishment of a low-impedance path to the ground, typically achieved through the use of grounding electrodes, conductors, and grounding systems.

The component which provides a signal to circuit breaker under the fault condition

The component which provides a signal to circuit breaker under the fault condition:

1. Isolator
2. Fuse
3. Relay
4. CT

Correct answer: 3. Relay

Explanation: In electrical power systems, a relay is a device that detects abnormal conditions or faults in the circuit and provides a signal to activate the circuit breaker. When a fault occurs, such as a short circuit or overload, the relay senses the abnormal current or voltage and triggers a response to protect the system. Also see Electrical Engineering XYZ article on Power System Automation here.

Relays are designed to detect specific fault conditions and respond accordingly. They can be electromechanical or solid-state devices, depending on the application. When a fault is detected, the relay typically energizes a coil, which in turn activates the circuit breaker to open and interrupt the flow of current, thereby preventing further damage or hazards.

Isolators: Also termed as disconnect switches or switches, are used to physically disconnect or isolate a circuit from the power supply. They do not provide a signal to the circuit breaker under fault conditions.

Also see: What is Breaking Capacity of Circuit Breakers and How it is expressed

Fuses are protective devices that contain a metal element that melts when the current exceeds a certain limit. While fuses play a role in protecting the circuit, they do not provide a signal to the circuit breaker. Instead, they themselves act as a sacrificial component, breaking the circuit when the current exceeds the rated value.

CT (Current Transformer) is an instrument used to assist the metering instruments & relays for measuring electric current. It is not directly involved in providing a signal to the circuit breaker under fault conditions.

Therefore, the component that specifically provides a signal to the circuit breaker under fault conditions is the relay.

Also see: 20 Different types of relays used in Power System

The correct statement about Buchholz relay

The correct statement about Buchholz relay

1. It can only be used with largely sized alternators
2. It is installed with largely sized transformers which have no conservator tank
3. It can detect incipient faults
4. It should never be used with oil immersed transformers

Correct answer: 3. It can detect incipient faults

Explanation: The Buchholz relay is a protection relay primarily employed with oil-filled transformers. It is designed to detect and provide early warning of incipient faults such as internal short circuits, partial discharges, and oil leaks within the transformer. The relay is installed in the pipe connecting the main tank and conservator tank of the transformer.

The relay operates based on the principle of gas accumulation. It consists of a float and a mercury switch, which are immersed in the oil. When a fault occurs inside the transformer, it generates gases, usually due to the decomposition of oil or the formation of arcs. These gases accumulate in the relay’s upper chamber, causing the float to rise. When the float reaches a certain level, it activates the mercury switch, which triggers an alarm or initiates a trip signal to disconnect the transformer from the power source.

The size of the alternator or the presence of a conservator tank in the transformer is not directly related to the use of Buchholz relay. The relay can be used with transformers of various sizes, as long as they are oil-filled and have the appropriate piping arrangement for installing the relay. Therefore, the statements suggesting that the relay can only be used with largely sized alternators or should never be used with oil-immersed transformers are incorrect.

Incipient faults refer to early or developing faults within an electrical system that have not yet fully manifested or caused significant damage. These faults are often characterized by subtle changes, such as small changes in current flow, temperature, or insulation resistance. Incipient faults can arise due to various factors, including insulation deterioration, loose connections, partial discharges, or the presence of contaminants. Detecting incipient faults is crucial as it allows for timely intervention and preventive measures to avoid more severe failures or disruptions. Monitoring systems, such as relays or sensors, are employed to identify these initial signs of fault development and enable proactive maintenance or corrective actions.

The correct statement is that the Buchholz relay can detect incipient faults, making it a valuable protective device for monitoring the condition of oil-filled transformers.

The breaking capacity of the circuit breaker is in _______ and is measured in ________

The breaking capacity of the circuit breaker is in _______ and is measured in ________:

1. R.M.S, kVA
2. R.M.S, MVA
3. Peak, kVA
4. Peak, MVA

Correct answer: 2. R.M.S, MVA

Explanation: The breaking capacity of a circuit breaker is typically measured in R.M.S (Root Mean Square) and is expressed in MVA (Mega Volt-Amperes) or kVA (kilo Volt-Amperes), depending on the application. Therefore, the correct option would be:

R.M.S, MVA

The breaking capacity of a circuit breaker refers to its ability to interrupt or break the flow of electrical current under abnormal conditions, such as a fault or short circuit. When a fault occurs in an electrical system, the circuit breaker needs to open quickly to disconnect the faulty circuit and prevent further damage to the equipment and potential hazards.

The breaking capacity of a circuit breaker is a measure of its ability to interrupt or “break” the current flow without causing excessive arcing or damage. It indicates the maximum fault current that the circuit breaker can safely interrupt under specific conditions.

The breaking capacity is typically specified as a maximum current rating, expressed in kVA or MVA, depending on the magnitude of the electrical system.

Semi-open cartridge type fuse can handle current up to ___________

Semi-open cartridge type fuse can handle current up to ___________:

1. 1000 A
2. 2500 A
3. 4000 A
4. 10000 A

Correct answer: 3. 4000 A

Explanation: Semi-open cartridge type fuses are designed to handle high current levels in electrical systems. They consist of a cylindrical housing with a fuse element inside, which is typically made of a conductive material with a low melting point. When the current flowing through the fuse exceeds its rated capacity, the fuse element melts, interrupting the circuit and protecting the electrical equipment from damage.

Interturn faults on an alternator are _________ and are ___________ detectable

Interturn faults on an alternator are _________ and are ___________ detectable:

1. Common, easily
2. Uncommon, easily
3. Uncommon, not easily
4. Common, not easily

Correct answer: 3. Uncommon, not easily

Explanation: Interturn faults refer to faults that occur between the turns or windings of the coils in an alternator. These faults can occur due to insulation breakdown, mechanical stress, or other reasons. When an interturn fault occurs, it disrupts the proper functioning of the alternator and can lead to issues such as reduced output voltage, increased temperature, or even complete failure.

The term “uncommon” in this Electrical Engineering xyz mcq on Power System Protection indicates that interturn faults are not frequently encountered in alternators. They are relatively rare compared to other types of faults. However, when they do occur, they can have significant consequences for the operation and performance of the alternator.

The term “not easily” in the answer choice suggests that detecting interturn faults is challenging. Interturn faults can be elusive because they often result in subtle changes in the electrical characteristics of the alternator. These faults may not be easily visible during routine inspections or standard monitoring methods. Specialized diagnostic techniques, such as partial discharge testing or insulation resistance measurement, may be required to detect interturn faults accurately.

Overall, interturn faults on an alternator are infrequent occurrences and pose difficulties in their detection, making them uncommon and not easily detectable.

MCQ

i. Declaration: Ground wires can easily protect the equipment from direct lightning thunders.

X: Ground wires are used to provide protection against traveling waves

1. X is an application of declaration
2. Both declaration and X are incorrect
3. The declaration is correct, X is incorrect
4. Declaration and X explain the same fact in different words

Correct answer: 3. The declaration is correct, X is incorrect

Buchholz Relay is used for providing protection to

Buchholz Relay is used for providing protection to

1. Bus bars
2. Transmission lines
3. Transformers
4. All of these

Correct answer: 3. Transformers

The plug setting of electromagnetic relay can be changed by

The plug setting of electromagnetic relay can be changed by:

1. Changing the number of ampere turns
2. By changing the gauge number of wire
3. Adjusting the PWM
4. None of these

Correct answer: 1. Changing the number of ampere turns

The structure of TRIAC matches two SCR’s which are joined back to back in a parallel fashion. Thus we can relate a TRIAC with a bi-directional SCR.

tion of arc in circuit breaker

a. The production of arc in circuit breaker:

1. Delays the current interruption process
2. Generates enormous heat
3. Both of these
4. None of these

Correct answer: 3. Both of these

The basic requirement of any power system protection scheme is

b. The basic requirement of any power system protection scheme is:

1. Selectivity
2. Stability
3. Speed
4. All of these

Correct answer: 4. All of these

The incipient fault in beginning is

c. The incipient fault in beginning is:

1. Small
2. Fatal
3. Infinitely damage to the equipment
4. more dangerous than P – P fault

Correct answer: 1. Small

Most dangerous type of fault on any power system is

d. Most dangerous type of fault on any power system is:

1. Phase to earth
2. Phase to Phase
3. Phase to Phase to Earth
4. Three phase fault

Correct answer: 3. Three phase fault

Reactance in any system exists due to

e. Reactance in any system exists due to:

1. Resistors
2. Capacitors
3. Inductors
4. 2 & 3

Correct answer: 4. 2 & 3

Cartridge type fuse can handle current upto

f. Cartridge type fuse can handle current upto:

1. 50,000 A
2. 1,00,000 A
3. 2,00,000 A
4. 5,00,000 A

Correct answer: 2. 1,00,000 A

The major advantage associated with the fuse is

g. The major advantage associated with the fuse is:

1. It is cheaper to use
2. It requires no special maintenance
3. Its time or operation is shorter than CB’s
4. All of these

Correct answer: 4. All of these

The modern electrical power systems are more likely to use

h. The modern electrical power systems are more likely to use:

1. Electromechanical relays
2. Static analog relays
3. Static digital relays
4. None of these

Correct answer: 3. Static digital relays

Air circuit breakers can be used for current values up to

i. Air circuit breakers can be used for current values up to:

1. 3000 A
2. 6000 A
3. 18000 A
4. 32000 A

Correct answer: 2. 6000 A

The protection system which compares the current entering and leaving the protection zone

j. The protection system which compares the current entering and leaving the protection zone:

1. Different protection system
2. Integral protection system
3. Back protection system
4. Subtraction protection system

Correct answer: 1. Different protection system

In case of fault it is often desired that frame leakage protection should trip

a. In case of fault it is often desired that frame leakage protection should trip:

1. Half the installed breakers
2. Only one breaker
3. All connected breakers
4. Two breakers

Correct answer: 3. All connected breakers

The correct equation for an effectively grounded system is

b. The correct equation for an effectively grounded system is:

1. X₀/X₁ < 3.0
2. X₀/X₁ > 3.0
3. X₀/X₁ > 2.0
4. 3.0 < X₀/X₁< 9.0

Correct answer: 1. X0/X1 < 3.0

Buchholz relay is used to protect

c. Buchholz relay is used to protect:

1. Alternator
2. Induction Motors
3. Synchronous motors
4. Transformers

Correct answer: 4. Transformers

Generally, the carrier current protection scheme is recommended for

d. Generally, the carrier current protection scheme is recommended for:

1. HV cables
2. HV transmission lines
3. Both of these
4. None of these

Correct answer: 2. HV transmission lines

SF6 gas is ____________

e. SF6 gas is ____________:

1. Highly inflammable
2. Non-inflammable

Correct answer: 3. Non-inflammable

The under-voltage relay is recommended for

f. The under-voltage relay is recommended for:

1. Transformers
2. Motors
3. Feeder
4. Busbars

Correct answer: 2. Motors

____________________ is/are an instantaneous relay/s

g. ____________________ is/are an instantaneous relay/s:

1. PMMC type
2. Shaded pole type
3. Both of these
4. None of these

Correct answer: 1. PMMC type

Statement: SF6 circuit breakers have very short arcing time

h. Statement: SF6 circuit breakers have very short arcing time.

Utterance: SF6 circuit breakers are costly.

1. Both statement and utterance are correct
2. Both are wrong
3. The statement is correct, the utterance is wrong
4. The statement is wrong, the utterance is correct

Correct answer: 1. Both statement and utterance are correct

Plug setting multiplier is defined as the ratio of

i. Plug setting multiplier is defined as the ratio of:

1. Fault current in relay coil x Pick up current
2. Fault current in relay coil / Pick up current
3. Fault current in relay coil x Transformer secondary turns
4. Fault current in relay coil / Transformer secondary turns

Correct answer: 2. Fault current in relay coil / Pick up current

High-speed circuit breakers result in

j. High-speed circuit breakers result in:

1. Increased stability of power systems
2. Decreased stability of the power system

Correct answer: 2. Decreased stability of the power system

The rate of rise of restriking voltage is usually expressed in terms of

The rate of rise of restriking voltage is usually expressed in terms of:

1. V / s
2. kV / ms
3. kV / µs
4. MV/ns

Correct answer: 3. kV / µs

It is desired to chop 20 A current in a circuit having L = 4 mH, C = 0.2 µC. Find the induced voltage

It is desired to chop 20 A current in a circuit having L = 4 mH, C = 0.2 µC. Find the induced voltage.

1. 10 kV
2. 3.72 kV
3. 2.8 kV
4. 1.7 kV

Correct answer: 3. 2.8 kV

Solution = i √(L/C) = 20 A * √(4 mH/0.2 uC) = 2.8 kV

Fault diverter is actually a

Fault diverter is actually a:

1. Fuse
2. Circuit breaker
3. Relay
4. Switch

Correct answer 4. Switch

3. The fault diverter is actually a fast switch.

The major advantage associated with the static digital relay is

The major advantage associated with the static digital relay is:

1. Less sensitive
2. Performance speed is slow
3. It is expensive
4. It is not accurate

Correct answer: 3. It is expensive

Fusing factor of a fuse is mathematically defined by the equation

Fusing factor of a fuse is mathematically defined by the equation:

1. Current rating of fuse * 4.44
2. 1/Current rating of the fuse
3. Min fusing current / Current rating of the fuse
4. Min fusing current * Current rating of the fuse

Correct answer: 3. Min fusing current / Current rating of the fuse

The correct statement about Sulphur Hexaflouride gas

The correct statement about Sulphur Hexaflouride gas:

1. It provides free electrons to the breaker
2. It absorbs free electrons
3. It increases current flow the through the arc
4. None of these

Correct answer: 2. It absorbs free electrons

The correct equation which explains the breaking capacity of single phase circuit breaker

The correct equation which explains the breaking capacity of single phase circuit breaker:

1. V × I × 10⁻¹ MVA
2. V × I × 10⁻³ MVA
3. V × I × 10⁻⁶ MVA
4. V × I × 10⁻⁹ MVA

Correct answer: 3. V × I × 10⁻⁶ MVA

The __________ the current, the ___________ is the time required by fuse to blow up

The __________ the current, the ___________ is the time required by fuse to blow up:

1. Greater, smaller
2. Greater, Greater
3. Smaller, Smaller
4. All of these are possible

Correct answer: 1. Greater, smaller

Usually the earth relays have ___________ current settings

Usually the earth relays have ___________ current settings:

1. Lower
2. Higher
3. Both
4. None

Correct answer: 1. Lower

The correct equation which explains the breaking capacity of 3 phase circuit breaker

The correct equation which explains the breaking capacity of 3 phase circuit breaker:

1. √3 × V × I × 10⁻¹ MVA
2. √3 × V × I × 10⁻³ MVA
3. √3 × V × I × 10⁻⁶ MVA
4. √3 × V × I × 10⁻⁹ MVA

Correct answer: 3. √3 × V × I × 10⁻⁶ MVA

The basic purpose of a relay is to

a. The basic purpose of a relay is to:

1. Detect the fault
2. Remove the fault
3. Trip the circuit
4. All of these

Correct answer: 1. Detect the fault

MCQ#2

b. Statement: The induction relays operate on the principle of the induction motor.

Affirmation: Induction motors can’t be used with dc quantities.

1. Both statement and affirmations are correct
2. Both are wrong
3. The statement is correct but affirmation is wrong
4. The statement is wrong but affirmation is correct

Correct answer: 3. The statement is correct but affirmation is wrong

Buchholz relay is a _________ relay which is used for protection of _________

c. Buchholz relay is a _________ relay which is used for protection of _________:

1. Gas actuated, transformers
2. Gas actuated, generators
3. Gas actuated, motors
4. Digital, alternator

Correct answer: 1. Gas actuated, transformers

The Insulation test voltage should be done with the parameters

d. The Insulation test voltage should be done with the parameters:

1. 2 kV, 50 Hz, 1 min
2. 5 kV, 50 Hz, 5 min
3. 10 kV, 50 Hz, 10 min
4. 20 kV, 50 Hz, 20 min

Correct answer: 1. 2 kV, 50 Hz, 1 min

In Electrical power protection schemes ______________ transformer is used

e. In Electrical power protection schemes ______________ transformer is used:

1. VT
2. CVT
3. Both of these
4. None of these

Correct answer: 3. Both of these

Explanation: VT stands for voltage transformer and CVT stands for capacitive voltage transformers

CVTs are often used to

f. CVTs are often used to:

1. Protection
2. Instrumentation
3. Communication
4. All of these

Correct answer: 4. All of these

The equipment which has lowest chances of fault occurrence

g. The equipment which has lowest chances of fault occurrence:

1. Alternator
2. Transformer
3. Switchgear
4. Induction motors

Correct answer: 1. Alternator

Single L – G fault is classified as

h. Single L – G fault is classified as:

1. Symmetrical fault
2. Unsymmetrical fault
3. Linear fault
4. None of these

Correct answer: 2. Unsymmetrical fault

Oil in a circuit breaker is used to

i. Oil in a circuit breaker is used to:

1. Quench arc
2. Insulate the breaker contacts
3. Increase the breaking capacity
4. None of these

Correct answer: 1. Quench arc

The _______________ of circuit breaker is always greater than _____________

j. The _______________ of circuit breaker is always greater than _____________:

1. Symmetrical breaking capacity, Making capacity
2. Making capacity, Symmetrical breaking capacity
3. Making capacity, 50000 A
4. None of these

Correct answer: 3. Making capacity, Symmetrical breaking capacity

The component which provides a signal to circuit breaker under the fault condition

The component which provides a signal to circuit breaker under the fault condition:

1. Isolator
2. Fuse
3. Relay
4. CT

Correct answer: 3. Relay

A definite time relay is ___________ the amount of current

A definite time relay is ___________ the amount of current:

1. Dependent on
2. Independent of

Correct answer: 2. Independent of

The extent of damage and injuries caused due to a shock depends on

The extent of damage and injuries caused due to a shock depends on:

1. Amount of current
2. Path of the current
3. Time for which current flows
4. All of these

Correct answer: 4. All of these

Usually, the overvoltage alternator results due to sudden load variation

Problem: Usually, the overvoltage alternator results due to sudden load variation.

Solution: The over-voltage protection system is installed on all power stations.

1. The solution is used to solve the problem
2. Problem is correct but the solution is not always applied
3. Both solution and problem misfit the context
4. The problem never happens on alternators

Correct answer: 2. Problem is correct but the solution is not always applied

Explanation: The over-voltage protection system is only installed on those power stations where there is no attendee.

Backup protection is installed to

Backup protection is installed to:

1. Increase the protection reliability of the system
2. To increase the speed of protection system
3. Both of these
4. None of these

Correct answer: 1. Increase the protection reliability of the system

The correct statement about the cross-section area of the arc and the resistance

The correct statement about the cross-section area of the arc and the resistance:

1. The arc resistance increases with the increase of cross-section area
2. The arc resistance increases with the decrease of cross-section area
3. Both are not related to other
4. None of above

Correct answer: 2. The arc resistance increases with the decrease of cross-section area

The breaking capacity of the circuit breaker is

The breaking capacity of the circuit breaker is in _______ and is measured in ________:

1. R.M.S, kVA
2. R.M.S, MVA
3. Peak, kVA
4. Peak, MVA

Correct answer: 2. R.M.S, MVA

The correct statement about the arc in circuit breaker

The correct statement about the arc in circuit breaker:

1. It is desired to extinguish the arc as quickly as possible
2. Delays the current interruption process
3. Generates enormous heat
4. All of these

Correct answer: 4. All of these

Arc extinction is achieved in the air circuit breaker by using

Arc extinction is achieved in the air circuit breaker by using:

1. Air
2. Oil
3. SF6
4. None of these

Correct answer: 1. Air

Semi-open cartridge type fuse can handle current up to

Semi-open cartridge type fuse can handle current up to ___________:

1. 1000 A
2. 2500 A
3. 4000 A
4. 10000 A

Correct answer: 3. 4000 A

The selectivity of the system is increased by

The selectivity of the system is increased by:

1. Considering the system as a single block
2. Dividing the system into various protection zones
3. Dividing system into two large blocks
4. None of these

Correct answer: 2. Dividing the system into various protection zones

The protection system which compares the electrical quantity which enters and leaves any zone and then operates is

The protection system which compares the electrical quantity which enters and leaves any zone and then operates is:

1. Balanced voltage
2. Balanced current
3. Differential protection system
4. All of these

Correct answer: 4. All of these

Solkor unit protection is used with

Solkor unit protection is used with

1. Stranded nonmetallic buses
2. Solid metallic pilot wires
3. Induction motors
4. Transformers

Correct answer: 2. Solid metallic pilot wires

For safe operations, the transformer oil should have a minimum dielectric strength of

For safe operations, the transformer oil should have a minimum dielectric strength of:

1. 10 KV
2. 20 kV
3. 50 kV
4. 100 KV

Correct answer: 3. 50 kV

The protection relay which has inherent directional characteristics

The protection relay which has inherent directional characteristics:

1. Mho relay
2. Reactance relay
3. Distance relay
4. All of these

Correct answer: 1. Mho relay

The magnetic circuit breaker has

The magnetic circuit breaker has:

1. Instantaneous working action
2. Delays working action
3. Both of these
4. None of these

Correct answer: 1. Instantaneous working action

The correct statement about the unbalanced condition in alternator

The correct statement about the unbalanced condition in alternator:

1. The same current flows through different phases in the unbalanced alternator
2. Unbalanced loading is not dangerous for the alternator
3. Unbalanced loading introduces eddy current in the alternator
4. None of these is correct

Correct answer: 3. Unbalanced loading introduces eddy current in the alternator

The circuit breaker which produces least arc energy

The circuit breaker which produces least arc energy:

1. Air Blast
2. Air break
3. Bulk oil
4. Plain oil

Correct answer: 1. Air Blast

The correct statement about Buchholz relay

The correct statement about Buchholz relay

1. It can only be used with largely sized alternators
2. It is installed with largely sized transformers which have no conservator tank
3. It can detect incipient faults
4. It should never be used with oil immersed transformers

Correct answer: 3. It can detect incipient faults

Generally, earth relays have _____________ current setting

Generally, earth relays have _____________ current setting:

1. Lower
2. Higher

Correct answer: 1. Lower

A power system contains two relays of 5 VA and 10 VA. The correct statement about the sensitivity of relays

A power system contains two relays of 5 VA and 10 VA. The correct statement about the sensitivity of relays:

1. 5 VA relay is more sensitive than 10 VA
2. 10 VA relay is more sensitive than 5 VA
3. Both are equally sensitive
4. One can’t determine sensitivity from VA

Correct answer: 1. 5 VA relay is more sensitive than 10 VA

An overcurrent relay has setting of 200% and is connected to a transformer of 200/5. Find the pickup value

An overcurrent relay has setting of 200% and is connected to a transformer of 200/5. Find the pickup value.

1. 2.5 A
2. 5 A
3. 10 A
4. 20 A

Correct answer: 3. 10 A

The isolator is installed in power system to:

1. Protect against high current faults
2. Isolate a part of the system
3. Protect the buses
4. Separate a CT from PT

Correct answer: 2. Isolate a part of the system

The term inverted running represents an alternator:

1. Which is protected via inversion circuit
2. Which runs at a very high speed
3. A turboalternator which uses inverter circuits
4. Which behaves as a motor

Correct answer: 3. Which behaves as a motor

The making capacity of the circuit breaker is measured in terms of ______:

1. R.M.S
2. Peak
3. Both
4. None

Correct answer: 2. Peak

Mathematically the making capacity is defined by the equation:

1. 1.88 × Symmetrical breaking capacity
2. 2.55 × Symmetrical breaking capacity
3. 3.87 × Symmetrical breaking capacity
4. 4.99 × Symmetrical breaking capacity

Correct answer: 2. 2·55 × Symmetrical breaking capacity

The desired quantity in a fuse is:

1. High conductivity
2. Low conductivity
3. High melting point
4. High deterioration rate

Correct answer: 1. High conductivity

The correct relationship between the fusing current and diameter of the wire is:

1. I = k d
2. I = k d^1.5
3. I = k d²
4. I = k d³

Correct answer: 2. I = k d^1.5

A three phase 1000 MVA circuit breaker, 11 kVA has symmetrical breaking current:

1. 90909 A
2. 11000 A
3. 52486 A
4. 44088 A

Correct answer: 3. 52486 A

Solution: I = 1000 MVA / (11 kVA * √3) = 52486 A

The bushing is used with power equipment to

The bushing is used with power equipment to:

1. Prevent contact between the conductor and metal equipment
2. To facilitate the current flow between conductor and chassis ground
3. Increase the life of the equipment
4. None of these

Correct answer: 1. Prevent contact between the conductor and metal equipment

Instrument transformers are used

Instrument transformers are used:

1. To Protect the electrical equipment
2. To isolate the relays
3. To isolate ammeter and multimeters
4. All of these

Correct answer: 4. All of these

The correct statement about arc resistance

The correct statement about arc resistance:

1. It increases with the length of the arc
2. It decreases with the length of the arc
3. Length and resistance of arc are never related
4. None of above

Correct answer: 1. It increases with the length of the arc

The breaking capacity of the circuit breaker is

The breaking capacity of the circuit breaker is:

1. R.M.S
2. Peak
3. Both
4. None

Correct answer: 1. R.M.S

The circuit breaker which is recommended for EHV Installations

The circuit breaker which is recommended for EHV Installations:

1. Low oil breaker
2. Bulk oil circuit breakers
3. SF6 circuit breaker
4. Air blast circuit breaker

Correct answer: 3. SF6 circuit breaker

The air which is preferred for air blast circuit breakers

The air which is preferred for air blast circuit breakers:

1. Moisture free air
2. Air with 50% nitrogen
3. Air with 25% hydrogen
4. Air with higher moisture content

Correct answer: 1. Moisture free air

The natural frequency of a 60 Hz generator having inductance

The natural frequency of a 60 Hz generator having inductance  10 mH and capacitance 50 µF has a natural frequency of:

1. 2 Hz
2. 60 Hz
3. 225 Hz
4. 22500 Hz

Correct answer: 3. 225 Hz

Solution: f = 1/2π √10 m * 50 u = 225 Hz

Low resistance method of arc extinction is also known as

Low resistance method of arc extinction is also known as:

1. High current method
2. Low current method
3. Current zero method
4. Current infinite method

Correct answer: 3. Current zero method

Relay used for large sized alternators

________________ relays are used in large-sized alternator for providing protection against overloads:

1. Temperature sensitive
2. Overcurrent
3. Impedance
4. Induction relays

Correct answer: 1. Temperature relays

The arc in circuit breaker converts oil into:

1. Fire
2. Hydrogen gas
3. SF6 Gas
4. Nitrogen gas

Correct answer: 2. Hydrogen gas

Power System Protection MCQs Part 10 MCQ#1

Statement: Differential protection scheme should not be employed for longer transmission lines.

Logic: It is very expensive.

1. Both statement and logic are wrong
2. The statement is correct, but logic is incorrect
3. The statement is incorrect, but logic is correct
4. Logic defends the statement

Correct answer: 4. Logic defends the statement

The instantaneous relays used for earth fault detection in motors is usually set to

The instantaneous relays used for earth fault detection in motors is usually set to _____________:

1. 50% of the RLA
2. 50% of the FLA
3. 20% of the RLA
4. 20% of the FLA

Correct answer: 4. 20% of the FLA

MCQ#3

Declaration: Ground wires can easily protect the equipment from direct lightning thunders.

X: Ground wires are used to provide protection against traveling waves

1. X is an application of declaration
2. Both declaration and X are incorrect
3. The declaration is correct, X is incorrect
4. Declaration and X explain the same fact in different words

Correct answer: 3. The declaration is correct, X is incorrect

The method in which earthing is done through a transformer is referred as

The method in which earthing is done through a transformer is referred as:

1. Effective earthing
2. Non-effective earthing
3. Peterson coil earthing
4. Flick method

Correct answer: 2. Non-effective earthing

The term non-effective earthing refers to the earthing which is done through

The term non-effective earthing refers to the earthing which is done through:

1. Resistance
2. Reactance
3. Transformer
4. All of these

Correct answer: 4. All of these

MCQ#6

Argument: Resistance earthing can’t be used for single phase loads.

Solution: Households always employ solid earthing.

1. Argument and solution are not connected in the present context
2. Argument leads to the solution
3. The argument is wrong, the solution is correct
4. Both solution and argument are wrong

Correct answer: 2. Argument leads to a solution

Explanation: The resistance earthing can’t be used for single phase loads due to which the solid earthing method is used by households.

Striker pin fuse is used for

Striker pin fuse is used for:

1. LV circuits
2. LV and MV circuits
3. MV and HV circuits
4. HV circuits

Correct answer: 2. LV and MV circuits

R.R.R.V depends on the

R.R.R.V depends on the:

1. Inductance
2. Capacitance
3. Both of these
4. None of these

Correct answer: 3. Both of these

The arc voltage is always _______________ the current

The arc voltage is always _______________ the current:

1. Leading
2. Lagging
3. In phase with

Correct answer: 3. In phase with

If the time required to reach the peak value is 100 µsec and the peak re-striking voltage is 200 kV, then average RRRV is

If the time required to reach the peak value is 100 µsec and the peak re-striking voltage is 200 kV, then average RRRV is:

1. 20 MV/µsec
2. 2 kV/µsec
3. 200 MV/µsec
4. 20 kV/µsec

Correct answer: 2. 2 kV/µsec