# 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:

## 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.

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.

## 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