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

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

Explanation: Buchholz relays are installed in the pipe between the main tank and the conservator of an oil-filled transformer. It detects faults by sensing the presence of gas that is produced by the decomposition of transformer oil under electrical faults like short circuits, inter-turn faults, core faults, or overheating, triggering an alarm or tripping the transformer.

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

Explanation: The plug setting of electromagnetic relay can be changed by changing the number of ampere turns.

• An electromagnetic relay operates based on the magnetic field generated by the flow of current through its coil. The strength of the magnetic field depends on the number of ampere-turns, which is the product of the current (in amperes) flowing through the coil and the number of turns of wire in the coil.
• The plug setting in an electromagnetic relay essentially allows for the adjustment of the number of turns (or sometimes indirectly the current), altering the sensitivity and operating characteristics of the relay. By adjusting this setting, you change the ampere-turns, which modifies the level of current required to activate the relay. This is typically done by changing the position of a movable core or adjusting a variable resistor in the relay circuit.

A fuse is never inserted in

A fuse is never inserted in

1. Negative of DC circuit
2. Neutral wire
3. Phase wire
4. Positive of DC circuit

Correct answer: 2. Neutral wire

Explanation: In an electrical system, the live wire (or phase wire) carries the voltage relative to the ground or neutral. The neutral wire completes the circuit by providing a return path to the ground. If a fuse were placed in the neutral wire and the fuse were to blow due to an excessive current, the circuit’s connection to the neutral would be severed. However, all parts of the circuit before the fuse in the neutral wire would still be connected to the live wire. This means that those components, including any appliances connected to the circuit, would still be at a live voltage relative to the ground.

If someone were to touch such an appliance after the fuse in the neutral wire has blown, they could potentially become a path for the current to reach the ground, leading to an electric shock. This happens because, even though the current flow is stopped by the blown fuse in the neutral wire, the voltage potential from the live wire is still present.

Hence, placing the fuse in the live wire is crucial:

• It ensures that when a fuse blows due to an overload or fault, all parts of the circuit downstream of the fuse are disconnected from the high voltage supply, making them safe from electrical shock risks.
• This setup interrupts the power effectively and minimizes the risk of accidents, making it a safer and more effective approach to circuit protection.

Setting of instantaneous relays used for earth fault detection in motors

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 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.

Result of Arc Production in circuit breaker

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

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

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

Explanation: An incipient fault refers to a fault in its initial stages that is usually minor and small in magnitude. These faults begin subtly and may not immediately disrupt the system or cause significant damage. Incipient faults are crucial to identify early because they can evolve into more severe faults if not addressed. While the other options (Fatal, Infinitely damage to the equipment, more dangerous than P-P fault) may describe possible outcomes if an incipient fault is ignored, they do not accurately characterize the fault’s nature at its inception. An incipient fault starts small, potentially growing worse without timely intervention.

Most dangerous type of fault on any power system is

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

Explanation: The most dangerous type of fault on any power system is generally considered to be the “Three phase fault.” This type of fault involves all three phases simultaneously coming into contact with each other or with the earth, leading to a significant short-circuit current. It results in the maximum possible fault current, creating severe stress on the electrical system, potentially causing extensive damage to equipment and significant power outages. This type of fault affects system stability and is the most severe in terms of the immediate impact on the electrical system’s integrity and safety.

Cartridge type fuse can handle current upto

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

Explanation: Cartridge type fuses are designed to handle significant currents and can protect circuits in high-current situations. The maximum current that a typical cartridge type fuse can handle is usually around 100,000 amperes (A). This rating provides a balance between practical current handling capabilities and safety considerations, protecting electrical systems effectively without prematurely melting under normal operating conditions.

The major advantage associated with the fuse is

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

Explanation:

1. It is cheaper to use: Fuses are generally less expensive compared to circuit breakers.
2. It requires no special maintenance: Fuses do not require ongoing maintenance; they simply need to be replaced after they blow.
3. Its time of operation is shorter than CB’s: Fuses typically operate and interrupt a circuit faster than circuit breakers, which can be crucial in preventing damage due to electrical faults.

Thus, all these points combined represent major advantages of using fuses.

The modern electrical power systems are more likely to use

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

Explanation: Modern electrical power systems increasingly use static digital relays due to their higher precision, reliability, and versatility. Unlike electromechanical relays, which rely on mechanical movements, and static analog relays, which use continuous electrical signals, static digital relays use digital processors to evaluate inputs and make decisions. This allows for more complex decision-making processes, integration with modern communication systems, better fault analysis, and adaptability to different situations, making them more suitable for current demands in power system management and protection.

Air circuit breakers are generally used for current values up to

Air circuit breakers are generally used for current values up to:

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

Correct answer: 2. 6000 A

Explanation: Air circuit breakers are typically designed to handle medium voltage applications and are commonly used for current ratings up to 6000 A. This makes them suitable for many industrial and commercial applications where protection of electrical circuits is necessary, but not for extremely high current scenarios which would require different types of breakers.

Which protection system compares the current entering and leaving the zone

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

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

Correct answer: 1. Differential protection system

Explanation: Differential protection works by measuring the current entering and leaving a specific zone. If the currents differ beyond a set threshold, it indicates a fault within the zone, triggering a protective response. This method ensures precise fault detection, making it effective for protecting transformers, generators, and other critical equipment.

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

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

Explanation: Frame leakage protection method provides a single ground tank connection over a current transformer (CT) that feeds an overcurrent relay. It connects all the framework, circuit breakers, tanks, and other components, while insulating the bus-supporting structure and associated switchgear from the ground. Every circuit connected to the bus has its breakers tripped by a multi-contact auxiliary relay controlled by the overcurrent relay.

In this protection setup, an overcurrent relay is connected to the secondary of a CT, grounding the only metal supporting structure or fault bus.

The relay remains inactive during normal operation. However, if a fault occurs involving an electrical connection between a conductor and the ground supporting structure, current will flow through the fault bus to the ground, activating the relay.

When the relay operates, it trips all the breakers connecting the equipment to the bus.

The correct equation for an effectively grounded system is

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

Explanation: In a power system, X0 represents the zero-sequence reactance, while X1 represents the positive-sequence reactance. In an effectively grounded system, the zero-sequence reactance (X0) should be much smaller than the positive-sequence reactance (X1). This condition ensures that in the event of a ground fault, a significant portion of the fault current will flow through the ground rather than back into the system, thus effectively limiting the fault current and protecting the system.

Therefore, the correct equation for an effectively grounded system is X0/X1 < 3.0, indicating that the zero-sequence reactance should be less than three times the positive-sequence reactance.

Buchholz relay is used to protect

Buchholz relay is used to protect:

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

Correct answer: 4. Transformers

Explanation: A Buchholz relay is an important protective device primarily used in oil-immersed transformers. It is installed in the piping between the transformer’s main tank and its conservator tank. This relay operates based on the detection of gas or oil flow within the transformer’s oil-filled tank. It serves as an effective protection against internal faults such as insulation failure, overheating, or partial discharges within the transformer.

While Buchholz relays are not typically used for alternators, induction motors, or synchronous motors, they are essential safety components for transformers due to their ability to detect incipient faults and prevent catastrophic damage. Therefore, the correct answer is Transformers.

Carrier current protection scheme is recommended for

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

Explanation: Carrier current protection is a technique used to provide selective and high-speed protection for high-voltage (HV) transmission lines. It involves superimposing high-frequency carrier signals onto the power frequency currents flowing through the transmission lines. These carrier signals carry protection commands and signals between the relays installed at each end of the transmission line.

This scheme is particularly effective for protecting long-distance HV transmission lines, as it offers rapid fault detection and selective tripping of circuit breakers to isolate faulty sections while maintaining the continuity of power supply for the rest of the system.

While high-voltage (HV) cables may also benefit from carrier current protection in certain cases, it is most commonly applied to HV transmission lines due to their length and the need for efficient and selective fault detection over long distances. Therefore, the correct answer is HV transmission lines.

The under-voltage relay is recommended for

The under-voltage relay is recommended for:

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

Correct answer: 2. Motors

Explanation: When the voltage supplied to a motor drops below a certain level, it can lead to various issues such as reduced torque, overheating, and potential damage to the motor windings. The under-voltage relay helps prevent these problems by disconnecting the motor from the power supply when the voltage falls below a safe operating range.

While under-voltage protection can be important for other electrical equipment such as transformers, feeders, and busbars, it is particularly critical for motors because voltage fluctuations can directly affect their performance and longevity. Therefore, the correct answer is Motors.

Which of following is instantaneous relay

____________________ 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

Explanation: PMMC (Permanent Magnet Moving Coil) relays are used as instantaneous relays in electrical systems. They operate based on the principle of the magnetic field produced by a permanent magnet interacting with the current-carrying coil. PMMC relays provide instantaneous response to abnormal conditions such as overcurrent or undercurrent in the system.

SF6 circuit breakers have very short arcing time

SF6 circuit breakers have very short arcing time:

1. True
2. False

Correct answer: 1. True

Explanation: SF6 (sulfur hexafluoride) circuit breakers have very short arcing times because SF6 gas has excellent insulating and arc-quenching properties. When an arc occurs, SF6 gas quickly absorbs the energy, cooling and extinguishing the arc efficiently. This rapid arc extinction contributes to the reduced arcing time in SF6 circuit breakers.

Plug setting multiplier is defined as the ratio of

Plug setting multiplier is defined as the ratio of:

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

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

Explanation: The Plug Setting Multiplier (PSM) is defined as the ratio of the fault current in the relay coil to the relay’s pickup current. The pickup current is the minimum current at which the relay operates. PSM helps determine the operating condition of the relay under fault conditions.

So, the correct option is: Fault current in relay coil / Pick up current.

High-speed circuit breakers result in

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

Explanation: High-speed circuit breakers can lead to decreased stability of the power system. While they quickly isolate faults, the sudden disconnection can cause abrupt changes in system dynamics, potentially leading to transient instability and oscillations that can affect the overall 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

Explanation: The rate of rise of restriking voltage measures how quickly the voltage increases across the contacts of a circuit breaker after an interruption. It’s a crucial factor in the design of circuit breakers and is typically expressed in kilovolts per microsecond (kV/µs) to reflect the high voltage changes over very short time intervals common in high-voltage systems. This unit ensures that the measure is both precise and relevant to the time scales involved in electrical discharges and fault conditions.

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. Heavy Fuse
2. Heavy Circuit breaker
3. Slow Relay
4. Fast acting Switch

Correct answer 4. Fast acting Switch

Explanation: A fault diverter quickly redirects current away from a faulted section of a circuit to protect equipment and maintain system stability. It acts rapidly to minimize damage and interruption, fitting the description of a fast-acting switch. Heavy fuses and circuit breakers also protect against faults but operate differently, while a slow relay does not align with the requirement for rapid action.

The major disadvantage associated with the static digital relay is

The major disadvantage 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

Explanation: Static digital relays offer advantages such as high accuracy, fast performance, and sensitivity, but they tend to be more expensive compared to traditional electromechanical relays. This higher cost is primarily due to the advanced technology and components used in their design and manufacture.

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

Explanation: The fusing factor of a fuse is defined as the ratio of the minimum fusing current (the smallest current at which the fuse will blow) to the current rating of the fuse (the maximum current the fuse can carry without blowing). Mathematically, it is given by:

Fusing Factor = Minimum fusing current/Current rating of the fuse

This ratio helps in determining the safety margin and operational reliability 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

Explanation: The correct statement about Sulphur Hexafluoride gas is: “It absorbs free electrons.” Sulphur Hexafluoride (SF6) is used as an insulating gas in high-voltage electrical equipment due to its excellent dielectric properties. It effectively absorbs free electrons, preventing electrical arcing and enhancing insulation performance.

Equation for 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

Explanation: The breaking capacity of a circuit breaker refers to the maximum current that the breaker can safely interrupt without causing damage or failure. It indicates the highest fault current that the breaker can interrupt under specified conditions, usually expressed in terms of voltage and current, often in units like kA (kiloamperes) or MVA (Megavolt-amperes). It’s a critical parameter in ensuring the safety and reliability of electrical systems, as it determines the breaker’s ability to protect the circuit from overloads and short circuits by quickly disconnecting power when necessary. The correct equation for the breaking capacity of a single-phase circuit breaker is: V × I × 10^-3 MVA. This formula calculates the product of voltage (V) and current (I) multiplied by 10^-3, representing the Mega Volt-Amperes (MVA) unit. It quantifies the maximum power the breaker can interrupt safely. This formula reflects the standard practice in electrical engineering for expressing breaking capacity in terms of voltage, current, and power.

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

Explanation: The correct answer is “Greater, smaller.” When the current is greater, it generates more heat in the fuse, causing it to blow faster. Conversely, with a smaller current, less heat is generated, thus requiring more time for the fuse to blow. This relationship is consistent with the basic principle that higher currents overload the fuse more quickly, resulting in a shorter blow-up time.

Current setting of earth relays

Usually the earth relays have ___________ current settings:

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

Correct answer: 1. Lower

Explanation: Earth relays typically have lower current settings. These settings are designed to detect low levels of fault current, ensuring swift isolation of faulty circuits for safety reasons. Higher current settings might overlook minor faults, risking damage to equipment or personnel safety. So, “Lower” is the correct answer.

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

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

Explanation: A relay’s main purpose is to detect abnormal conditions or faults in an electrical circuit. Once a fault is detected, the relay sends a signal to the circuit breaker to trip the circuit and isolate the faulty section.

Induction Relays Working

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

Affirmation: Induction relays 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: 1. Both statement and affirmation are correct

Explanation: Induction relays operate on the principle of the induction motor, which relies on the interaction of AC magnetic fields to produce motion or actuation. Consequently, they cannot be used with DC quantities because direct current does not create the varying magnetic fields necessary for their operation. Thus, both the statement and the affirmation are correct.

Buchholz relay is type and protection principle

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

Explanation: The Buchholz relay is a gas-actuated protection device specifically used in oil-immersed transformers. It detects gas produced by internal faults such as arcing, which releases gases into the oil. These gases cause the relay to trip and signal an alarm, protecting the transformer from further damage. It is not used for generators, motors, or alternators.

Which Transformer is used in Electrical power protection schemes

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: Voltage Transformers (VTs) are used to step down high voltages to lower levels for metering and protection. Capacitive Voltage Transformers (CVTs) also serve this purpose but are particularly useful for high-voltage applications, as they combine capacitive voltage dividers and inductive transformers to provide accurate voltage measurements while being more economical for very high voltages. Thus, both types of transformers are applicable in power protection schemes.

CVTs are often used to provide

CVTs are often used to provide:

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

Correct answer: 4. All of these

Explanation: CVTs (Capacitive Voltage Transformers) are versatile devices used in electrical power systems. They provide:

1. Protection: They step down high voltages to lower levels for protective relays.
2. Instrumentation: They supply precise voltage measurements for meters and instruments.
3. Communication: They assist in power line carrier communication (PLCC) systems, enabling data transmission over power lines.

Thus, CVTs serve multiple roles, making “All of these” the most accurate answer.

Equipment having lowest chances of fault occurrence

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

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

Explanation: Faults in electrical systems are categorized based on the balance of the system after the fault. Symmetrical faults involve all three phases equally, maintaining balance. Unsymmetrical faults affect one or two phases, creating an imbalance. A single L-G fault impacts only one phase, causing an imbalance in the system, hence it is an unsymmetrical fault.

Making vs Breaking Capacity of Circuit Breakers

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

Explanation: The correct answer is “Making capacity, Symmetrical breaking capacity.” The making capacity of a circuit breaker refers to its ability to handle the initial surge of current when closing, while the symmetrical breaking capacity denotes its capability to interrupt a fault current under specified conditions. This order ensures the breaker can safely close without damage and effectively interrupt fault currents, prioritizing operational safety and efficiency.

Definite time relay and Amount of Current

A definite time relay is ___________ the amount of current:

1. Dependent on
2. Independent of

Correct answer: 2. Independent of

Explanation: A definite time relay is independent of the amount of current. This type of relay operates based on a predetermined time delay rather than being influenced by the current passing through it. It triggers its response after a specific time period, regardless of the current level, making it suitable for applications where timing is critical regardless of variations in current.

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

Explanation: The correct answer is “All of these.” When experiencing an electric shock, the severity of damage and injuries depends on various factors: the amount of current passing through the body, the path the current takes, and the duration for which the current flows. Each of these factors contributes significantly to the extent of harm caused to the individual.

Over voltage Provided by Alternator

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: While sudden load variations can indeed cause overvoltage in alternators, not all power stations have overvoltage protection systems. Therefore, while the problem statement is accurate, the proposed solution is not universally implemented, making it the correct choice.

Backup protection is used 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

Explanation: Backup protection is installed to increase the protection reliability of the system. It acts as a secondary layer of defense in case the primary protection system fails. This redundancy ensures continuous protection against faults or abnormalities in the system. Therefore, the correct answer is: “Increase the protection reliability of the system.”

Arc resistance and Cross sectional area

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

Explanation: Arc resistance and cross-sectional area are inversely related. As the cross-sectional area decreases, the arc resistance increases because the available space for current flow decreases, causing congestion and higher resistance. This relationship is consistent with Ohm’s Law, where resistance is inversely proportional to the cross-sectional area of the conductor.

Breaking capacity of circuit breaker

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 indeed measured in Root Mean Square (RMS) values, as it indicates the effective value of the current or voltage. This ensures that the circuit breaker can safely interrupt the circuit under normal and fault conditions. The unit of measurement for the breaking capacity is typically in kilo Volt Amperes (kVA), representing the apparent power rating of the circuit breaker.

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

Explanation:

1. Extinguish the arc quickly: When an electrical fault occurs, an arc forms between the contacts of the circuit breaker. Rapidly extinguishing this arc is crucial to prevent damage to the equipment and ensure safety by swiftly cutting off the flow of current.
2. Delays the current interruption process: The presence of an arc can prolong the time it takes for the circuit breaker to fully interrupt the current flow. This delay can result in extended exposure to fault conditions, potentially causing damage to the system.
3. Generates enormous heat: The arc in a circuit breaker generates significant heat due to the high temperatures reached by the electrically ionized gas. This heat can damage the contacts and surrounding components if not controlled effectively, emphasizing the importance of quick extinguishment.

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

Explanation: Arc extinction in an air circuit breaker is achieved using air. When the circuit breaker interrupts the current flow, the arc is drawn into the arc chute where it is rapidly cooled and extinguished by the surrounding air. Oil and SF6 circuit breakers use different mediums for arc extinction, making “None of these” incorrect in this context.

Current Handling Capacity of Semi-open cartridge type fuse

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 typically designed to handle currents up to 4000 A. These fuses provide protection against overcurrent by melting the conductor inside when the current exceeds the rated capacity, interrupting the circuit to prevent damage to equipment or fire hazards. Therefore, the correct answer is: 4000 A

Selectivity of the Power system

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

Explanation: Dividing the system into various protection zones. This increases selectivity by isolating faults to specific zones, allowing for targeted responses without affecting the entire system. Dividing into smaller zones enhances fault detection and minimizes widespread disruptions.

Which protection system compares the electrical quantity entering and leaving the zone

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

Explanation: The correct answer is “Differential protection system.” This system compares the electrical quantities entering and leaving a zone. It detects any difference, triggering operation to protect the zone. This method is crucial for safeguarding against faults like short circuits. It’s distinct from balanced voltage and current methods, which focus on equilibrium rather than detecting discrepancies. Differential protection ensures swift action to prevent damage in electrical systems.

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

Explanation: Solkor unit protection is used with transformers. It employs pilot wires to detect faults in the transformer windings and initiate protective actions. Transformers are crucial components in power systems, requiring robust protection to prevent damage and ensure reliability. The Solkor unit, with its pilot wire scheme, effectively safeguards transformers from faults, enhancing system performance and longevity.

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

Explanation: Transformer oil needs sufficient dielectric strength to prevent electrical breakdown. A minimum of 50 kV ensures safe operations, preventing short circuits or damage to the transformer. Lower values may lead to insulation failure and potential hazards.

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. Delayed working action
3. Both of these
4. None of these

Correct answer: 1. Instantaneous working action

Explanation: Magnetic circuit breakers operate based on the magnetic force generated by the current flowing through them. When the current exceeds a certain threshold, the magnetic force triggers the breaker to trip almost instantaneously. This rapid response is crucial for protecting electrical circuits from severe damage due to sudden faults like short circuits or large current surges. Therefore, the primary characteristic of a magnetic circuit breaker is its ability to work instantaneously, without a delay, ensuring quick disconnection of the faulty circuit.

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

Explanation: Air blast circuit breakers extinguish the arc using a high-velocity blast of air, which cools and de-ionizes the arc plasma efficiently, leading to minimal arc energy. This method is more effective compared to the other types.

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

Explanation: Sensitivity of a relay is often inversely related to its VA (Volt-Ampere) rating. A lower VA rating means the relay requires less power to operate, making it more sensitive to changes in the system. In this case, the 5 VA relay requires less power to trigger compared to the 10 VA relay, indicating higher sensitivity. Thus, the 5 VA relay is more sensitive than the 10 VA relay.

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

Solution:

To solve for the pickup value of the overcurrent relay, follow these steps:

1. Understand the relay setting: The relay is set to operate at 200% of its rated current.
2. Determine the transformer ratio: The current transformer (CT) has a ratio of 200/5, meaning primary current of 200 A corresponds to secondary current of 5 A.
3. Calculate the rated current: With the CT ratio, the secondary rated current is 5 A (corresponding to 200 A primary current).
4. Apply the relay setting: The relay pickup value is 200% of the CT secondary rated current, which is 200%×5 A=2×5 A=10 A200%×5 A=2×5 A=10 A.

Thus, the pickup value is 10 A.

The isolator is installed in power system primarily to

The isolator is installed in power system primarily 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

Explanation: The primary purpose of an isolator in a power system is to isolate a part of the system. Isolators are mechanical switches used to ensure that a section of the power system is completely de-energized for maintenance or servicing. Unlike circuit breakers, isolators do not have an arc-quenching mechanism and are not designed to interrupt current. Instead, they are used to safely disconnect portions of the system only when the circuit is already opened by a circuit breaker or when no current flows. This ensures the safety of personnel working on the electrical equipment.

The term inverted running represents an alternator

The term inverted running represents an alternator

1. Which is protected via inversion circuit
2. Which runs at a very high speed
3. Which behaves as a motor
4. That is turbo-alternator and that uses inverter circuits

Correct answer: 3. Which behaves as a motor

Explanation: The term “inverted running” in the context of an alternator refers to “which behaves as a motor.” An alternator typically converts mechanical energy to electrical energy. However, when an alternator runs in an inverted manner, it operates as a motor, converting electrical energy back into mechanical energy. This reversal in function is characteristic of “inverted running” and distinguishes it from normal operation where the alternator generates electricity. It is not about high speed, protection via inversion circuits, or being a turbo-alternator using inverter circuits, but specifically about the alternator acting as a motor.

Making capacity of the circuit breaker is measured in terms of

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

Explanation: The making capacity of a circuit breaker is typically measured in terms of the peak value of the current. This is because the making capacity refers to the highest current that the breaker can safely handle when closing onto a fault. When a circuit breaker closes, the initial current surge (due to the transient condition) can reach a peak value significantly higher than the steady-state current. Therefore, to ensure safety and reliability, the making capacity is rated based on this peak current.

Mathematically the making capacity is defined by the equation

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

Explanation: Making capacity is defined as the peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker.

The making current is equal to the maximum value of asymmetrical current. To find this value, one needs to multiply symmetrical breaking current by √2 to convert this from RMS to peak, and then by 1·8 to include the “doubling effect” of maximum asymmetry. The total multiplication factor becomes √2 × 1·8 = 2·55.

∴ Making capacity =2·55 × Symmetrical breaking capacity

The desired quantity of fuse

The desired quantity of fuse

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

Correct answer: 1. High conductivity

Explanation: A fuse should possess high conductivity to ensure minimal resistance to the flow of electricity under normal operating conditions. This allows the circuit to function efficiently without unnecessary voltage drops. However, when an overcurrent occurs, the high conductivity of the fuse ensures that it heats up rapidly, causing it to melt and break the circuit, thereby protecting the electrical system from damage.

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

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

Explanation:

The relationship between the fusing current and the diameter of a wire is derived from Preece’s Law. According to this law, the fusing current I is proportional to the diameter d of the wire raised to the power of 1.5

Mathematically, it is expressed as:

𝐼 ∝ 𝑑^1.5

𝐼 = k𝑑^1.5

where k is a constant that depends on the material properties of the wire, such as resistivity and melting point.

Symmetrical Breaking Current

A three phase 1000 MVA having 11 kVA circuit breaker 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

Explanation: A bushing in power equipment is an insulating device that allows a conductor to pass safely through a grounded conducting barrier, such as the casing of a transformer or circuit breaker, without making electrical contact with it. This prevents short circuits and electrical faults by ensuring the conductor remains isolated from the metal parts of the equipment. The primary function is to insulate and provide a safe passage for the conductor, preventing direct contact with metal components that could cause damage or safety hazards.

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

Explanation: Instrument transformers, including current transformers (CTs) and voltage transformers (VTs), serve multiple critical functions in electrical systems. They protect electrical equipment by providing accurate measurements for protective relays, ensuring timely fault detection and isolation. They also isolate measurement instruments (such as ammeters and multimeters) from high voltage circuits, enhancing safety for personnel and equipment. Additionally, they isolate relays to ensure that high voltage does not directly affect the relay operation, improving system reliability and safety. Thus, instrument transformers perform all the listed functions.

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

Explanation: Arc resistance is directly proportional to the length of the arc. As the length of the arc increases, the resistance of the arc also increases. This is because a longer arc provides a greater distance for the current to travel, encountering more resistance along its path due to the ionized particles and heat. Consequently, the longer the arc, the higher the resistance, which impacts the voltage and current characteristics of the arc. Therefore, the correct answer is that arc resistance increases with the length of the arc.

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

Explanation: SF6 (sulfur hexafluoride) circuit breakers are preferred for EHV applications due to their superior insulation and arc-quenching properties. SF6 gas has excellent dielectric strength, which allows for the construction of compact breakers that can handle high voltages and currents. Additionally, SF6 circuit breakers have a long service life, minimal maintenance requirements, and are effective in interrupting fault currents quickly and efficiently. In contrast, oil and air blast circuit breakers are less efficient at handling the high demands of EHV installations.

Air used in 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

Explanation:

The preferred air for air blast circuit breakers is moisture-free air. This is because moisture in the air can reduce the dielectric strength, leading to potential electrical breakdowns and arc formation. Moisture-free air ensures higher insulation properties and prevents corrosion of internal components. The presence of moisture can also result in condensation, which further compromises the breaker’s performance and reliability. Therefore, maintaining dry air is crucial for the effective and safe operation of air blast circuit breakers. Other options, like air with nitrogen or hydrogen, are not standard choices and do not provide the same benefits as 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

Explanation: In the context of circuit breakers, the current zero method involves extinguishing the arc when the current naturally falls to zero during an AC cycle. This method takes advantage of the natural zero-crossing points of the AC waveform, at which the current is momentarily zero, making it easier to extinguish the arc. By reducing the resistance, the method aims to minimize energy loss and efficiently interrupt the current. This technique is contrasted with methods that require high or low current conditions to extinguish the arc.

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

Explanation: Temperature-sensitive relays are used in large-sized alternators for providing protection against overloads. These relays monitor the temperature of the alternator and activate protective measures if the temperature rises above a safe threshold, which is often due to prolonged overload conditions. By doing so, they help prevent damage caused by overheating, which can result from sustained high current loads. This makes temperature-sensitive relays particularly effective for maintaining the safe operation and longevity of large alternators.

The arc in circuit breaker converts oil into

The arc in circuit breaker converts oil into:

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

Correct answer: 2. Hydrogen gas

Explanation: When the circuit breaker interrupts the current flow, it creates an arc. The heat from this arc vaporizes the surrounding insulating oil, converting it into hydrogen gas and other byproducts. The generated hydrogen gas helps to rapidly quench the arc by displacing the surrounding air and thus interrupting the electrical current effectively. This process ensures the safe and efficient operation of the circuit breaker in high-voltage systems where oil is used as the insulating medium.

Differential Protection Schemes for Longer Lines

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

Explanation: “Logic defends the statement” is the correct answer. It indicates that although the statement itself is incorrect, the reasoning provided to support it is valid. In this case, the logic that “differential protection scheme should not be employed for longer transmission lines because it is very expensive” is considered a valid defense, even though the statement itself is incorrect.

Instantaneous relays Settings

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 setting for instantaneous relays used in motor earth fault detection is typically 20% of the Full Load Amps (FLA). This setting ensures rapid response to earth faults while minimizing the risk of nuisance tripping. By setting it at 20% of the FLA, the relay can detect faults promptly without being overly sensitive to normal operational fluctuations. This balance between sensitivity and reliability enhances the overall protection of the motor and associated equipment against earth faults.

Ground Wires Purpose

Declaration: Ground wires 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

Explanation: Ground wires are indeed installed to safeguard equipment from direct lightning strikes by providing a low-resistance path to the ground, mitigating potential damage. However, X’s assertion about protecting against traveling waves is incorrect. Traveling waves refer to disturbances in a medium, often related to signal transmission, and are not directly addressed by ground wires. Therefore, while the declaration is accurate, X’s statement is not applicable in this context.

The term switchgear refers to

The term switchgear refers to equipment used for:

1. Switching circuits
2. Controlling circuits
3. Protecting circuits
4. All of these

Correct answer: 4. All of these

Explanation: The term switchgear refers to the apparatus used for switching, controlling and protecting the electrical circuits, components, and equipment. The switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions.

Winding which connects in series with CT

Which winding of the current transformer connects in series with the circuit to be protected:

1. Primary winding
2. Secondary winding
3. Any of these
4. None of these

Correct answer: 1. Primary winding

Explanation:

• The primary winding of a current transformer (CT) is connected in series with the circuit to be protected. The primary winding often comprises of the main conductor itself.
• The secondary winding of current transformer is connected to the relay operating coil

Use of Reactors in Power System Protection

The use of reactors allows installation of circuit breakers of:

1. Lower ratings
2. Higher ratings

Correct answer: 1. Lower ratings

Explanation: With the fast expanding and modern power systems, the fault level is also rising. The circuit breakers connected in the electrical power system should be capable of dealing with maximum possible short-circuit currents that can occur at their points of connection.

Usually, the reactance of the system under fault conditions is low and fault currents may rise to a dangerously high value. If no steps are taken to limit the value of these short-circuit currents, not only will the duty required of circuit breakers be excessively heavy, but also damage to lines and other equipment will almost certainly occur.

Reactors are used in power systems to limit the short-circuit currents to a value which the circuit breakers can handle.

Which of following is unsymmetrical fault

Which of following is unsymmetrical fault

1. Single line-to-ground fault (L—G)
2. Line-to-line fault (L—L)
3. Double line-to-ground fault (L—L—G)
4. All of these
5. None of these

Correct answer: 4. All of these

Explanation: The faults on the power system that give rise to unequal fault currents in the lines with unequal phase displacement are known as unsymmetrical faults. All the three faults mentioned in this Electrical Engineering MCQ are unsymmetrical faults.

Magnitude of zero sequence components in 3 Phase 4 Wire Unbalanced System

The magnitude of zero sequence components in a 3-phase, 4 wire unbalanced system, is

1. One-third of the current in the neutral wire
2. Equal to current in neutral wire
3. Twice the current in neutral wire
4. 1.73 times the current in neutral wire

Correct answer: 1. One-third of the current in the neutral wire

Explanation: In case of a 3-phase, 4 wire unbalanced system, the magnitude of zero sequence components is one-third of the current in the neutral wire.

Zero sequence current = 1/3 × Current in neutral wire

Arc extinguishing in circuit breakers can be carried out using

Arc extinguishing in circuit breakers can be carried out using:

1. Low resistance method
2. Current zero method
3. High resistance method
4. All of these

Correct answer: 4. All of these

Explanation: All three methods mentioned above can be used for extinguishing the arc. In high resistance method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc.

Low resistance method or current zero method is used by modern high power AC circuit breakers for arc extinction. In this method arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking inspite of the rising voltage across the contacts.

The fuse rating is expressed in terms of

The fuse rating is expressed in terms of

1. Current
2. kVA
3. VAR
4. Voltage

Correct answer: 1. Current

Explanation: A fuse is a protective device used in electrical circuits to protect the wiring and equipment from overcurrent or short-circuit conditions. The rating of a fuse specifies the maximum current that can safely pass through the fuse without causing it to blow (melt). This is important because if the current exceeds the fuse’s rating, the fuse will blow to protect the rest of the circuit from damage.

An isolator is installed

An isolator is installed

1. Always independent of the position of circuit breaker
2. As a substitute for circuit breaker
3. To operate the relay of circuit breaker
4. Usually on both sides of a circuit breaker

Correct answer: 4. Usually on both sides of a circuit breaker

Explanation: Isolators are mechanical switches used to ensure that a circuit is completely de-energized for maintenance or service. Unlike circuit breakers, they do not have any arc-quenching capabilities and cannot interrupt current. Their primary function is to provide a safe isolation point in the electrical circuit. By placing isolators on both sides of a circuit breaker, maintenance personnel can safely isolate the breaker from both the supply and load sides, ensuring no accidental flow of current when the breaker is being serviced. This enhances safety during maintenance operations.

For cost and safety, the outdoor substations are installed for voltages beyond

For cost and safety, the outdoor substations are installed for voltages beyond

1. 11 kV
2. 110 kV
3. 33 kV
4. 60 kV

Correct answer: 3. 33 kV

Explanation: Outdoor substations are generally installed for voltages beyond 33 kV. This is due to both cost and safety considerations. For voltages above 33 kV, the equipment becomes larger and requires more space, making indoor installations impractical and expensive. Additionally, the higher voltage equipment necessitates better ventilation and cooling, which is more easily managed outdoors. The construction and maintenance costs are also lower for outdoor setups, and the risk of arc flash or other hazards is reduced due to better dissipation of energy in the open environment.

If fault occurs near the impedance relay, the V/I ratio will be

If fault occurs near the impedance relay, the V/I ratio will be

1. Constant for all distances
2. Higher than that of if fault occurs away from the relay
3. Lower than that of if fault occurs away from the relay
4. None of these

Correct answer: 3. Lower than that of if fault occurs away from the relay

Explanation: If a fault occurs near the impedance relay, the V/I ratio will be lower than that if the fault occurs away from the relay. This is because the impedance relay operates based on the impedance it measures, which is the voltage (V) to current (I) ratio. When a fault is closer to the relay, the impedance is lower due to the shorter distance, resulting in a lower V/I ratio. Conversely, if the fault is further away, the impedance is higher due to the greater distance, leading to a higher V/I ratio.

As compared to a rewireable fuse, the HRC fuse has

As compared to a rewireable fuse, the HRC fuse has

1. High rupturing capacity
2. High speed of operation
3. No ageing effect
4. All of these

Correct answer: 4. All of these

Explanation: The correct answer is All of these.

High Rupturing Capacity (HRC) fuses have several advantages over rewireable fuses:

1. High Rupturing Capacity: HRC fuses can safely interrupt high fault currents without damage.
2. High Speed of Operation: They operate more quickly to interrupt the circuit during fault conditions, protecting equipment and reducing damage.
3. No Ageing Effect: Unlike rewireable fuses, HRC fuses do not deteriorate with time and usage, ensuring consistent performance and reliability.

These features make HRC fuses more efficient and reliable for protecting electrical circuits.

A thermal protection switch can protect against

A thermal protection switch can protect against

1. Over voltage
2. Overload
3. Short-circuit
4. Temperature

Correct answer: 2. Overload

Explanation: A thermal protection switch is designed to protect electrical devices by interrupting the circuit when excessive current (overload) generates heat that could potentially damage the equipment. This mechanism is sensitive to the temperature increase caused by the overload current but is not meant to address over voltage, short-circuits, or directly monitor temperature.

Oil as an arc quenching medium has disadvantage that it has

Oil as an arc quenching medium has disadvantage that it has:

1. Risk of a fire
2. Explosion formation
3. Quality deterioration over time
4. All of these

Correct answer: 4. All of these

Explanation: Oil as an arc quenching medium in electrical equipment like transformers carries several disadvantages. Firstly, it poses a risk of fire due to its flammability. Secondly, in the event of a fault, explosive gases can form within the equipment. Lastly, over time, the quality of the oil deteriorates due to factors like oxidation and contamination, affecting its effectiveness in extinguishing arcs. Hence, all options listed present potential drawbacks of using oil as an arc quenching medium.

Oil in Low oil circuit breakers serves to

Oil in Low oil circuit breakers serves to:

1. Extinguish arc
2. Insulate conducting parts
3. Both of these
4. None of these

Correct answer: 1. Extinguish arc

Explanation: Low oil circuit breakers which use minimum amount of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts are insulated by air or porcelain or organic insulating material.

Role of Instrument transformers is to

Role of Instrument transformers is to

1. Transform voltages
2. Transform currents
3. Both of these
4. None of these

Correct answer: 3. Both of these

Explanation: The correct answer is “Both of these.” Instrument transformers transform high voltages and currents into manageable levels for measurement and protection devices. They ensure safety and accuracy in electrical systems by providing scaled-down representations of these parameters. Voltage transformers (VTs) step down high voltages to levels suitable for instruments, while current transformers (CTs) step down high currents. Both types are essential in various applications, such as power transmission, distribution, and industrial processes.

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

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.

An overcurrent relay having a current setting of 150% Numerical

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

Reactance in any system exists due to

Reactance in any system exists due to:

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

Correct answer: 4. 2 & 3

Oil in a circuit breaker is used to

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