Q.1 What is the primary function of a circuit breaker in a power system?
To increase the voltage
To isolate faults
To regulate frequency
To store energy
Explanation - A circuit breaker quickly interrupts the flow of current during a fault to protect equipment and maintain system stability.
Correct answer is: To isolate faults
Q.2 Which protective device is most suitable for short‑circuit protection in a residential distribution system?
Fuses
Residual Current Devices (RCDs)
Surge Protection Devices (SPDs)
Motor protection relays
Explanation - Fuses are cost‑effective and suitable for the relatively low fault currents in residential systems.
Correct answer is: Fuses
Q.3 A 3‑phase fault current of 30 kA will be cleared by which type of circuit breaker?
Standard air‑circuit breaker
Vacuum circuit breaker
SF6 circuit breaker
Thermal circuit breaker
Explanation - SF6 breakers can withstand very high fault currents, making them suitable for industrial plants with high fault levels.
Correct answer is: SF6 circuit breaker
Q.4 What does the term 'selective tripping' refer to in protective relaying?
Tripping all breakers during a fault
Tripping only the nearest breaker
Sequential tripping of breakers
Delaying the tripping of breakers
Explanation - Selective tripping ensures that only the breaker closest to the fault opens, minimizing system disruption.
Correct answer is: Tripping only the nearest breaker
Q.5 Which of the following is a typical function of a Ground Fault Circuit Interrupter (GFCI)?
Detects phase‑to‑phase faults
Monitors voltage level
Detects imbalance between live conductors
Regulates power factor
Explanation - A GFCI trips when it senses a difference between current entering and leaving a circuit, indicating leakage to ground.
Correct answer is: Detects imbalance between live conductors
Q.6 In a protective relay setting, what is the significance of the 'inverse time characteristic'?
The relay trips faster with higher fault currents
The relay trips slower with higher fault currents
The relay does not trip at all
The relay trips only during normal operation
Explanation - An inverse time characteristic means the relay’s trip time decreases as the fault current increases.
Correct answer is: The relay trips faster with higher fault currents
Q.7 Which type of relay is commonly used for motor protection against overload and short circuits?
Distance relay
Overcurrent relay
Differential relay
Voltage relay
Explanation - Overcurrent relays detect currents exceeding a preset threshold, protecting motors from overload and short‑circuit conditions.
Correct answer is: Overcurrent relay
Q.8 What is the main advantage of using an electromagnetic circuit breaker over a thermal‑magnetic breaker?
Faster opening time
Lower cost
Better fault tolerance
No maintenance required
Explanation - Electromagnetic breakers use a magnetic actuator to open the contacts quickly, which is crucial for high‑power faults.
Correct answer is: Faster opening time
Q.9 In a power system, a 'dead zone' in the relay setting refers to:
A period when the relay is not active
A region where the relay does not trip despite fault current
The time between fault initiation and detection
A region where the relay trips automatically
Explanation - A dead zone is a deliberate non‑sensitive zone to avoid nuisance tripping during small transients.
Correct answer is: A region where the relay does not trip despite fault current
Q.10 Which protective device is primarily used to prevent damage caused by voltage surges?
Thermal fuse
Surge protector
Ground fault relay
Recloser
Explanation - Surge protectors clamp high‑voltage spikes to safe levels, safeguarding downstream equipment.
Correct answer is: Surge protector
Q.11 In a differential relay protecting a transformer, if the sum of currents entering the transformer equals the sum leaving, the relay:
Trips immediately
Opens only during fault
Remains closed
Triggers an alarm
Explanation - Differential protection relies on the difference between input and output currents; if they balance, no fault is detected.
Correct answer is: Remains closed
Q.12 The time‑overlap between two relays in a protection scheme is used to:
Ensure both relays trip together
Prevent any relay from tripping
Create a delay in tripping
Detect faults in two separate areas
Explanation - Overlap is a technique to allow a higher‑level relay to operate while a lower‑level relay is still in its pickup range, preventing unnecessary tripping.
Correct answer is: Prevent any relay from tripping
Q.13 What is a primary feature of a recloser in a distribution system?
It never opens once closed
It can automatically re‑close after a fault clears
It is used for voltage regulation
It is a type of fuse
Explanation - Reclosers attempt to restore power after transient faults, improving reliability.
Correct answer is: It can automatically re‑close after a fault clears
Q.14 Which of the following is a common setting for a distance relay?
Trip at 10% of rated current
Trip at 200% of rated voltage
Trip at a preset impedance value
Trip when frequency drops below 47 Hz
Explanation - Distance relays measure the impedance to the fault; they trip when this value falls below a set threshold.
Correct answer is: Trip at a preset impedance value
Q.15 The term 'inverse‑time' in protection relays means the relay:
Opens faster when fault current is lower
Opens faster when fault current is higher
Never opens during fault
Opens only after a fixed delay
Explanation - Inverse‑time relays have a faster trip time for higher fault currents, allowing quick clearance.
Correct answer is: Opens faster when fault current is higher
Q.16 Which device is used to protect transmission lines from overload due to continuous high currents?
Thermal‑magnetic fuse
Thermal‑magnetic circuit breaker
Surge arrester
Voltage regulator
Explanation - Thermal‑magnetic breakers combine thermal action for overload and magnetic action for fault currents.
Correct answer is: Thermal‑magnetic circuit breaker
Q.17 In the context of protective relays, the term 'pick‑up' refers to:
The minimum current required to trigger the relay
The time delay before the relay trips
The point at which the relay resets
The maximum current the relay can handle
Explanation - Pick‑up is the threshold current or voltage above which the relay starts to react.
Correct answer is: The minimum current required to trigger the relay
Q.18 Which of the following is a typical setting for a ground‑fuse relay in a medium voltage substation?
Trip at 5% of rated current
Trip at 50% of rated current
Trip at 100% of rated current
Trip at 200% of rated current
Explanation - Ground‑fuse relays are often set to trip at the rated current to protect against fault conditions.
Correct answer is: Trip at 100% of rated current
Q.19 What is the main reason for using a differential relay over a current‑comparative relay in a transformer?
Higher cost
Better sensitivity to internal faults
Simplicity of installation
Ability to detect voltage sag
Explanation - Differential relays compare currents entering and leaving; they are highly sensitive to internal faults within the transformer.
Correct answer is: Better sensitivity to internal faults
Q.20 The 'inverse‑time' characteristic of a relay can be mathematically represented by which function?
t = k / I
t = k * I
t = k * I^2
t = I / k
Explanation - Inverse‑time means trip time t is inversely proportional to fault current I (t = k/I).
Correct answer is: t = k / I
Q.21 Which protective device is specifically designed to guard against lightning strikes?
Surge arrester
Ground‑fuse relay
Thermal‑magnetic fuse
Voltage regulator
Explanation - Surge arresters clamp high‑voltage spikes caused by lightning, protecting equipment.
Correct answer is: Surge arrester
Q.22 A 'reclosable' fuse will:
Never open
Open once and never reclose
Open and then automatically reclose after a set time
Only close during maintenance
Explanation - Reclosable fuses allow for automatic recovery after transient faults, improving reliability.
Correct answer is: Open and then automatically reclose after a set time
Q.23 Which setting on a relay is used to prevent nuisance tripping during normal operation?
Dead‑time setting
Pickup setting
Trip delay setting
Zero‑current setting
Explanation - Dead‑time prevents the relay from tripping during brief current spikes that are not faults.
Correct answer is: Dead‑time setting
Q.24 Which of the following best describes a 'time‑overlap' in a protection scheme?
Two relays operate simultaneously
One relay’s trip time overlaps with another’s
Both relays never operate
The time delay of a relay is zero
Explanation - Time‑overlap ensures a higher‑level relay does not trip before a lower‑level relay has cleared the fault.
Correct answer is: One relay’s trip time overlaps with another’s
Q.25 What is the primary purpose of a surge protector in an industrial facility?
Regulate voltage to 120 V
Protect equipment from overcurrent
Protect equipment from overvoltage spikes
Increase power factor
Explanation - Surge protectors clamp high voltage transients that could damage sensitive equipment.
Correct answer is: Protect equipment from overvoltage spikes
Q.26 Which type of relay is most commonly used for line‑to‑line fault detection on transmission lines?
Voltage relay
Distance relay
Thermal relay
Ground‑fuse relay
Explanation - Distance relays measure impedance and detect line faults based on fault distance.
Correct answer is: Distance relay
Q.27 In a power system, an inverse‑time overcurrent relay has a pickup set at 120% of the rated current. If the fault current is 240% of rated, the relay will trip in:
Shorter time than with 120% fault current
Longer time than with 120% fault current
Same time regardless of fault current
Never trip
Explanation - Higher fault current leads to a shorter trip time in inverse‑time relays.
Correct answer is: Shorter time than with 120% fault current
Q.28 Which of the following devices can operate at a speed of milliseconds for fault clearance?
Thermal fuse
Thermal‑magnetic circuit breaker
SF6 circuit breaker
Standard fuse
Explanation - SF6 breakers have rapid magnetic opening, suitable for clearing high‑current faults in milliseconds.
Correct answer is: SF6 circuit breaker
Q.29 A differential relay is most effective when the impedance between the two terminals is:
High
Zero
Variable
Moderate
Explanation - Differential relays detect differences in current; if internal impedance is low, the relay is highly sensitive to faults.
Correct answer is: Zero
Q.30 Which protective device is designed to sense a difference of 30 mA between live conductors?
Residual Current Device (RCD)
Ground fault relay
Surge protector
Voltage relay
Explanation - An RCD trips when the imbalance current exceeds its sensitivity threshold, often 30 mA in residential circuits.
Correct answer is: Residual Current Device (RCD)
Q.31 Which of the following is a characteristic of a magnetic circuit breaker?
It uses a thermal element for opening
It operates by an electromagnetic actuator
It is used only for low voltage systems
It has no contact wear
Explanation - Magnetic circuit breakers use a magnetic field to separate the contacts rapidly during a fault.
Correct answer is: It operates by an electromagnetic actuator
Q.32 In a protective relay, the 'dead zone' is implemented to avoid:
Short circuits
Long‑term degradation
Nuisance tripping
Over‑voltage surges
Explanation - Dead zones prevent relays from tripping on small, transient disturbances that are not actual faults.
Correct answer is: Nuisance tripping
Q.33 Which device is used to detect and clear a ground fault in a 11 kV substation?
Ground fault relay
Voltage relay
Current transformer
Distance relay
Explanation - Ground fault relays monitor the ground path and trip breakers when fault currents exceed a set threshold.
Correct answer is: Ground fault relay
Q.34 Which protective device is most suitable for handling 15 kA fault currents in a high‑voltage network?
Standard fuse
Vacuum circuit breaker
SF6 circuit breaker
Thermal‑magnetic fuse
Explanation - SF6 circuit breakers are designed for very high fault currents, typical of high‑voltage networks.
Correct answer is: SF6 circuit breaker
Q.35 What does the term 'selective coordination' imply in protective relaying?
All relays operate simultaneously
Only the nearest relay operates to clear a fault
No relay operates under any condition
All relays operate in sequence
Explanation - Selective coordination ensures that only the relay closest to the fault trips, maintaining system stability.
Correct answer is: Only the nearest relay operates to clear a fault
Q.36 Which of the following is a common parameter to set in a distance relay for a 220 kV line?
Pickup impedance of 0.2 ohms
Trip delay of 5 seconds
Zero‑current setting of 200 A
Pickup voltage of 220 V
Explanation - Distance relays use impedance thresholds; a typical pickup for a 220 kV line is around 0.2 ohms.
Correct answer is: Pickup impedance of 0.2 ohms
Q.37 Which protective device is designed to interrupt short circuits by a magnetic mechanism?
Thermal‑magnetic fuse
Electromagnetic circuit breaker
Voltage relay
Ground fault relay
Explanation - Electromagnetic circuit breakers use a magnetic actuator to separate contacts during a fault.
Correct answer is: Electromagnetic circuit breaker
Q.38 When a relay is set to a 'trip time' of 0.5 seconds at 150% of rated current, what will be the trip time at 200% of rated current assuming inverse‑time operation?
0.5 seconds
0.375 seconds
0.625 seconds
0.75 seconds
Explanation - Inverse‑time means trip time decreases as current increases; 200% is higher than 150%, thus time reduces.
Correct answer is: 0.375 seconds
Q.39 Which of the following best describes the purpose of a 'thermal‑magnetic fuse'?
To detect over‑voltage
To provide short‑circuit protection only
To provide both overload and short‑circuit protection
To regulate frequency
Explanation - Thermal‑magnetic fuses combine a thermal element for overload and a magnetic element for fault currents.
Correct answer is: To provide both overload and short‑circuit protection
Q.40 Which protective relay setting is critical for preventing simultaneous tripping of two relays during a fault?
Dead‑time
Pickup setting
Trip delay setting
Zero‑current setting
Explanation - Dead‑time prevents the relay from tripping immediately, allowing a downstream relay to operate first.
Correct answer is: Dead‑time
Q.41 A 33 kV substation uses a distance relay with a pickup impedance of 0.3 ohms. The fault occurs at a distance of 50 km. What is the fault impedance?
15 ohms
30 ohms
45 ohms
60 ohms
Explanation - Impedance = Voltage / Current; using the line impedance per km (e.g., 0.6 ohms/km), 50 km gives 30 ohms. The distance relay will detect it if it is below the pickup of 0.3 ohms times rated current.
Correct answer is: 15 ohms
Q.42 Which device is specifically designed for detecting earth faults in a 11 kV distribution system?
Ground fault relay
Voltage relay
Distance relay
Thermal‑magnetic fuse
Explanation - Ground fault relays monitor earth fault currents and trip breakers when they exceed a preset value.
Correct answer is: Ground fault relay
Q.43 The 'pickup time' of a relay is:
The time taken for the relay to detect a fault
The time delay before the relay trips after detection
The time for the relay to reset
The maximum operating time of the relay
Explanation - Pickup time is the delay between detection and actuation, allowing for coordination.
Correct answer is: The time delay before the relay trips after detection
Q.44 Which of the following is NOT a typical function of a protective relay?
Detect overcurrent
Detect undervoltage
Adjust transformer tap settings
Detect ground faults
Explanation - Relay protection functions include fault detection; tap settings are adjusted by a tap changer, not a relay.
Correct answer is: Adjust transformer tap settings
Q.45 Which relay type uses both current transformers (CTs) and voltage transformers (VTs) to determine the presence of a fault?
Current‑comparative relay
Voltage relay
Distance relay
Differential relay
Explanation - Current‑comparative relays compare currents from different CTs to detect faults.
Correct answer is: Current‑comparative relay
Q.46 The term 'inverse‑time overcurrent relay' is best described as:
A relay that trips only when current is below a threshold
A relay that trips faster with higher current
A relay that trips slower with higher current
A relay that does not trip at all
Explanation - Inverse‑time relays have a trip time inversely proportional to fault current.
Correct answer is: A relay that trips faster with higher current
Q.47 Which protective device is specifically designed to open contacts when the temperature rises beyond a set limit?
Thermal relay
Magnetic relay
Voltage relay
Ground fault relay
Explanation - Thermal relays open when the thermal element, such as a bimetal strip, deforms due to heat.
Correct answer is: Thermal relay
Q.48 A relay that monitors the difference between input and output currents is called a:
Voltage relay
Differential relay
Overcurrent relay
Ground fault relay
Explanation - Differential relays compare the currents flowing into and out of a protected zone.
Correct answer is: Differential relay
Q.49 In a power system, which protective device is most suitable for handling over‑current conditions in a low‑voltage distribution network?
SF6 circuit breaker
Thermal‑magnetic fuse
Vacuum circuit breaker
Surge arrester
Explanation - Thermal‑magnetic fuses are commonly used for low‑voltage distribution to protect against over‑current faults.
Correct answer is: Thermal‑magnetic fuse
Q.50 A 'time‑overlap' technique is used to ensure that:
All relays trip at the same time
No relay trips during a fault
The downstream relay clears the fault before the upstream relay trips
The upstream relay clears the fault before the downstream relay trips
Explanation - Time‑overlap allows a more selective, downstream relay to act before the upstream relay opens.
Correct answer is: The downstream relay clears the fault before the upstream relay trips
Q.51 Which of the following is a typical feature of a vacuum circuit breaker?
Uses a magnetic actuator
Uses a gas (SF6) to extinguish arc
Has a large contact wear rate
Is unsuitable for high‑fault currents
Explanation - Vacuum breakers use a magnetic actuator and a vacuum interrupter to extinguish arcs quickly.
Correct answer is: Uses a magnetic actuator
Q.52 What is the main advantage of using a ground‑fuse relay in a medium voltage network?
It provides high‑speed tripping
It does not require a CT
It has low cost
It is maintenance‑free
Explanation - Ground‑fuse relays quickly detect and clear earth fault currents to protect equipment.
Correct answer is: It provides high‑speed tripping
Q.53 Which type of relay is typically used for detecting overload in a motor?
Current relay
Thermal relay
Distance relay
Voltage relay
Explanation - Thermal relays use heat generated by excess current to detect motor overload.
Correct answer is: Thermal relay
Q.54 The function of a 'trip curve' in a protective relay is to:
Show the relationship between current and trip time
Indicate the voltage rating of the relay
Display the relay’s temperature
Determine the relay’s life expectancy
Explanation - Trip curves illustrate how quickly a relay will trip at various current levels.
Correct answer is: Show the relationship between current and trip time
Q.55 In a typical protection scheme, which relay is responsible for detecting a high‑voltage fault on a 400 kV line?
Voltage relay
Distance relay
Current relay
Thermal relay
Explanation - Distance relays detect faults based on impedance; they are used for high‑voltage transmission lines.
Correct answer is: Distance relay
Q.56 What is the main purpose of a 'recloser' in an electrical distribution system?
To continuously open and close the circuit
To automatically reclose after a transient fault clears
To measure voltage levels
To regulate frequency
Explanation - Reclosers improve reliability by attempting to restore power after brief fault conditions.
Correct answer is: To automatically reclose after a transient fault clears
Q.57 A protective relay that uses both CT and VT inputs to detect a fault is typically called a:
Current‑comparative relay
Voltage relay
Distance relay
Differential relay
Explanation - Current‑comparative relays compare CT and VT signals to determine fault conditions.
Correct answer is: Current‑comparative relay
Q.58 Which of the following is a typical setting for an overload relay protecting a transformer?
Set at 110% of rated current
Set at 50% of rated current
Set at 200% of rated current
Set at 10% of rated current
Explanation - Overload relays are usually set slightly above the normal operating current to avoid nuisance tripping.
Correct answer is: Set at 110% of rated current
Q.59 Which device is commonly used to provide arc‑suppression for high‑voltage circuit breakers?
Arc‑suppression electrode
Thermal fuse
Ground fault relay
Voltage regulator
Explanation - Arc‑suppression electrodes help extinguish the arc during the opening of high‑voltage breakers.
Correct answer is: Arc‑suppression electrode
Q.60 In an inverse‑time relay, increasing the fault current from 150% to 200% of rated current will:
Increase the trip time
Decrease the trip time
Keep the trip time unchanged
Prevent tripping entirely
Explanation - Inverse‑time relays trip faster with higher fault currents.
Correct answer is: Decrease the trip time
Q.61 A protective scheme for a 132 kV transmission line typically includes:
Only a distance relay
A voltage relay and a thermal relay
A distance relay, a ground fault relay, and a voltage relay
Only a current relay
Explanation - A comprehensive protection scheme uses multiple relay types to cover various fault conditions.
Correct answer is: A distance relay, a ground fault relay, and a voltage relay
Q.62 Which of the following is a characteristic of a 'magnetic circuit breaker'?
Uses a thermal element to open
Opens in milliseconds for high fault currents
Has no magnetic field
Is used for low voltage only
Explanation - Magnetic circuit breakers can interrupt large currents quickly due to magnetic action.
Correct answer is: Opens in milliseconds for high fault currents
Q.63 In a protection scheme, the purpose of a 'dead‑time' setting is to:
Ensure rapid tripping
Prevent nuisance tripping during short‑term transients
Increase the sensitivity of the relay
Delay the relay reset
Explanation - Dead‑time provides a window to avoid false tripping due to brief disturbances.
Correct answer is: Prevent nuisance tripping during short‑term transients
Q.64 A 220 kV substation uses a distance relay with a pickup impedance of 0.25 ohms. If the fault occurs 80 km away, what is the fault impedance?
16 ohms
20 ohms
24 ohms
28 ohms
Explanation - Using the line impedance per km (approx. 0.25 ohms/km), the fault impedance is 80 * 0.25 = 20 ohms.
Correct answer is: 20 ohms
Q.65 Which type of relay is used to protect a transformer against an internal short between its phases?
Current‑comparative relay
Differential relay
Voltage relay
Thermal relay
Explanation - Differential relays detect imbalances caused by internal transformer faults.
Correct answer is: Differential relay
Q.66 A 'ground‑fuse' relay is designed to handle:
High fault currents in high‑voltage lines
Small earth faults in low voltage circuits
Overvoltage transients
High‑frequency noise
Explanation - Ground‑fuse relays protect against earth faults in low‑voltage distribution networks.
Correct answer is: Small earth faults in low voltage circuits
Q.67 Which of the following best describes the term 'overcurrent relay'?
A relay that trips on overvoltage
A relay that trips on currents above a preset value
A relay that trips on short circuits only
A relay that regulates frequency
Explanation - Overcurrent relays detect when current exceeds a preset threshold and trip the circuit breaker.
Correct answer is: A relay that trips on currents above a preset value
Q.68 In a typical power system, a 'surge arrester' is primarily used to protect against:
Long‑duration overcurrent
Short‑duration overvoltage transients
Continuous overload
Under‑frequency conditions
Explanation - Surge arresters clamp high‑voltage spikes, such as from lightning or switching transients.
Correct answer is: Short‑duration overvoltage transients
Q.69 A 'distance relay' in a transmission line system is based on:
Voltage magnitude
Current magnitude
Impedance between the relay and fault
Temperature of the conductors
Explanation - Distance relays measure the impedance to determine fault distance.
Correct answer is: Impedance between the relay and fault
Q.70 Which protective device is most suitable for protecting a high‑power motor against phase imbalance?
Overcurrent relay
Differential relay
Voltage relay
Thermal relay
Explanation - Differential relays can detect imbalance by comparing currents entering and leaving the motor.
Correct answer is: Differential relay
Q.71 The 'pickup' setting of a relay is defined as:
The maximum current the relay can withstand
The minimum current required for the relay to start operating
The time delay before the relay trips
The maximum voltage the relay can handle
Explanation - Pickup is the threshold current above which the relay becomes active.
Correct answer is: The minimum current required for the relay to start operating
Q.72 Which of the following is a feature of an SF6 circuit breaker?
Uses a magnetic actuator
Uses a gas medium to extinguish arcs
Has no maintenance requirements
Is unsuitable for high fault currents
Explanation - SF6 gas is used in the breaker to extinguish arcs quickly and safely.
Correct answer is: Uses a gas medium to extinguish arcs
Q.73 Which protective device is used to detect a phase‑to‑ground fault on a 33 kV circuit?
Ground fault relay
Voltage relay
Distance relay
Thermal‑magnetic fuse
Explanation - Ground fault relays monitor for fault currents to ground and trip the breaker.
Correct answer is: Ground fault relay
Q.74 In a protective relay setting, what is the purpose of a 'trip delay'?
To speed up tripping
To delay tripping until after a fault clears
To reset the relay after operation
To increase the pickup current
Explanation - Trip delay allows the fault to be cleared by downstream devices before the relay trips.
Correct answer is: To delay tripping until after a fault clears
Q.75 Which of the following best describes a 'dead zone' in a relay?
A zone where the relay trips immediately
A zone where the relay does not trip despite fault current
A zone where the relay resets automatically
A zone where the relay is offline
Explanation - Dead zone protects against nuisance tripping during short, non‑fault transients.
Correct answer is: A zone where the relay does not trip despite fault current
Q.76 The term 'selective coordination' in protection schemes ensures that:
All relays operate simultaneously
Only the nearest relay to the fault operates
No relays operate during a fault
All relays operate sequentially
Explanation - Selective coordination avoids unnecessary disconnections by allowing only the affected portion to trip.
Correct answer is: Only the nearest relay to the fault operates
Q.77 Which type of relay would be most appropriate for detecting a 400 kV transmission line fault?
Voltage relay
Current relay
Distance relay
Thermal relay
Explanation - Distance relays measure impedance, making them ideal for high‑voltage fault detection.
Correct answer is: Distance relay
Q.78 A 'recloser' in a distribution system typically:
Never opens once closed
Closes and opens automatically after a fault is cleared
Requires manual operation to reclose
Only operates during maintenance
Explanation - Reclosers attempt to restore power after transient faults, improving reliability.
Correct answer is: Closes and opens automatically after a fault is cleared
Q.79 Which device is used to protect a transformer against internal short between its windings?
Differential relay
Ground fault relay
Voltage relay
Thermal relay
Explanation - Differential relays compare currents to detect internal faults inside the transformer.
Correct answer is: Differential relay
Q.80 In a power system, the 'pickup current' of an overcurrent relay is set at 120% of the rated current to:
Trip during normal operation
Avoid nuisance tripping during overload
Increase the sensitivity to short circuits
Ensure fast tripping during faults
Explanation - Setting pickup above normal operating current prevents tripping under normal conditions.
Correct answer is: Avoid nuisance tripping during overload
Q.81 Which of the following is a typical feature of a 'ground‑fuse relay' used in low voltage circuits?
High trip speed for fault currents
Long trip delay for overload detection
No requirement for CTs
Used only for voltage regulation
Explanation - Ground‑fuse relays rapidly detect and clear earth fault currents to protect equipment.
Correct answer is: High trip speed for fault currents
Q.82 In a protection scheme, a 'time‑overlap' is used to ensure that:
All relays trip simultaneously
The downstream relay clears the fault before the upstream relay trips
No relay trips during a fault
The upstream relay trips before the downstream relay
Explanation - Time‑overlap allows selective protection by letting the more selective relay act first.
Correct answer is: The downstream relay clears the fault before the upstream relay trips
Q.83 Which relay is typically used to detect a phase‑to‑phase fault on a 132 kV line?
Distance relay
Voltage relay
Thermal relay
Ground fault relay
Explanation - Distance relays detect faults by measuring the impedance between the relay and the fault.
Correct answer is: Distance relay
Q.84 A 'thermal relay' protects a motor by:
Monitoring voltage levels
Detecting temperature rise due to overcurrent
Measuring frequency changes
Detecting phase imbalance
Explanation - Thermal relays use a bimetal element that deforms under heat, opening the contacts to protect against overload.
Correct answer is: Detecting temperature rise due to overcurrent
Q.85 What is the main advantage of a 'distance relay' over a 'current relay' for transmission line protection?
Higher sensitivity to overvoltage
Capability to detect fault distance
Lower cost
Faster response time
Explanation - Distance relays measure the impedance to the fault, allowing them to detect fault distance and type.
Correct answer is: Capability to detect fault distance
Q.86 In a protective relay, the term 'trip curve' illustrates:
The relay's temperature over time
The relationship between fault current and trip time
The relay's voltage tolerance
The relay's reset time
Explanation - Trip curves show how quickly a relay will trip at various fault current levels.
Correct answer is: The relationship between fault current and trip time
Q.87 Which device is typically used for providing ground fault protection in a 33 kV substation?
Ground fault relay
Voltage relay
Distance relay
Thermal‑magnetic fuse
Explanation - Ground fault relays monitor fault currents to earth and trip the breaker when they exceed the set threshold.
Correct answer is: Ground fault relay
Q.88 Which type of protective relay uses both a CT and a VT to determine the presence of a fault?
Current‑comparative relay
Voltage relay
Distance relay
Differential relay
Explanation - Current‑comparative relays compare currents from CTs and VTs to detect faults.
Correct answer is: Current‑comparative relay
Q.89 A 'reclosable fuse' is designed to:
Never open once closed
Open once and never reclose
Open and then automatically reclose after a set time
Only close during maintenance
Explanation - Reclosable fuses allow for automatic recovery after transient faults, improving reliability.
Correct answer is: Open and then automatically reclose after a set time
Q.90 Which of the following best describes a 'ground fault relay'?
Detects voltage sag
Detects current imbalance to ground
Detects phase‑to‑phase faults
Detects overvoltage spikes
Explanation - Ground fault relays monitor the current flowing to ground and trip when it exceeds a preset threshold.
Correct answer is: Detects current imbalance to ground
Q.91 In an SF6 circuit breaker, the gas used for arc extinguishment is:
SF6
SF5
SF4
SF3
Explanation - SF6 (sulfur hexafluoride) is the gas commonly used in SF6 circuit breakers for efficient arc suppression.
Correct answer is: SF6
Q.92 Which of the following is the primary function of a 'surge arrester'?
Regulate voltage
Detect overcurrent
Clamp high‑voltage transients
Measure frequency
Explanation - Surge arresters protect equipment by clamping high‑voltage spikes to safe levels.
Correct answer is: Clamp high‑voltage transients
Q.93 A 'distance relay' typically uses a characteristic impedance of:
0.5 ohms
1 ohm
5 ohms
10 ohms
Explanation - Distance relays are set to a characteristic impedance that matches the line impedance per km.
Correct answer is: 0.5 ohms
Q.94 Which protective relay is most suitable for detecting overload in a motor?
Thermal relay
Current relay
Voltage relay
Ground fault relay
Explanation - Thermal relays use heat from excess current to detect overload conditions.
Correct answer is: Thermal relay
Q.95 In a high‑voltage substation, the 'pickup' of a distance relay is usually set at:
1.5 times the line impedance
0.5 times the line impedance
2 times the line impedance
0.1 times the line impedance
Explanation - Pickup is often set to half the characteristic impedance to ensure detection of faults.
Correct answer is: 0.5 times the line impedance
Q.96 A 'magnetic circuit breaker' uses a magnetic actuator to:
Detect overvoltage
Open contacts quickly during a fault
Regulate current
Maintain temperature
Explanation - Magnetic circuit breakers use a magnetic field to separate contacts rapidly during fault conditions.
Correct answer is: Open contacts quickly during a fault
Q.97 Which device is used to protect a transformer against a short circuit between its windings?
Differential relay
Overcurrent relay
Voltage relay
Thermal relay
Explanation - Differential relays compare currents entering and exiting the transformer to detect internal faults.
Correct answer is: Differential relay
Q.98 A 'dead‑time' setting in a relay is used to:
Prevent nuisance tripping during short transients
Speed up the trip time
Increase the pickup current
Extend the relay reset time
Explanation - Dead‑time allows a brief period after fault detection to prevent false tripping.
Correct answer is: Prevent nuisance tripping during short transients
Q.99 Which protective device is used to prevent over‑current faults in a 11 kV substation?
Overcurrent relay
Ground fault relay
Voltage relay
Surge arrester
Explanation - Overcurrent relays detect excessive current and trip breakers to protect equipment.
Correct answer is: Overcurrent relay
Q.100 Which type of relay uses a bimetal strip to detect overheating due to overload?
Thermal relay
Magnetic relay
Voltage relay
Distance relay
Explanation - Thermal relays use a bimetal element that deforms when heated by excess current.
Correct answer is: Thermal relay
Q.101 What is the main advantage of an SF6 circuit breaker over a vacuum circuit breaker?
Lower cost
Higher fault current capability
No gas required
Smaller size
Explanation - SF6 breakers can handle higher fault currents due to the gas’s superior arc extinguishing properties.
Correct answer is: Higher fault current capability
Q.102 A 'recloser' is used primarily to:
Detect overvoltage
Automatically reclose after a fault clears
Regulate frequency
Measure voltage rise
Explanation - Reclosers restore power after a transient fault, improving reliability.
Correct answer is: Automatically reclose after a fault clears
Q.103 Which relay is most suitable for detecting a phase‑to‑ground fault in a 33 kV distribution system?
Ground fault relay
Distance relay
Voltage relay
Overcurrent relay
Explanation - Ground fault relays monitor fault currents to the earth and trip when thresholds are exceeded.
Correct answer is: Ground fault relay
Q.104 An inverse‑time overcurrent relay will:
Trip slower with higher fault currents
Trip faster with higher fault currents
Trip at a fixed time regardless of current
Never trip
Explanation - Inverse‑time relays reduce trip time as fault current increases.
Correct answer is: Trip faster with higher fault currents
Q.105 Which device is used to detect and clear faults caused by lightning strikes in a distribution network?
Surge arrester
Thermal relay
Ground fault relay
Overcurrent relay
Explanation - Surge arresters clamp high‑voltage spikes from lightning to protect the system.
Correct answer is: Surge arrester
Q.106 The 'trip curve' of a relay is a graph that shows:
Voltage vs. time
Current vs. temperature
Fault current vs. trip time
Frequency vs. voltage
Explanation - Trip curves depict how quickly a relay will trip at different fault current levels.
Correct answer is: Fault current vs. trip time
Q.107 Which of the following is NOT a typical function of a protective relay?
Detecting overcurrent
Detecting undervoltage
Adjusting transformer tap settings
Detecting ground faults
Explanation - Relay protection focuses on fault detection; tap settings are controlled by other devices.
Correct answer is: Adjusting transformer tap settings
Q.108 The 'dead‑zone' in a relay setting is used to:
Increase sensitivity to faults
Prevent nuisance tripping during short transients
Ensure immediate tripping during faults
Reduce the relay’s pickup current
Explanation - Dead‑zone prevents the relay from acting on minor, non‑fault disturbances.
Correct answer is: Prevent nuisance tripping during short transients
Q.109 Which protective device is primarily used for detecting overload in a low‑voltage motor?
Thermal relay
Voltage relay
Ground fault relay
Overcurrent relay
Explanation - Thermal relays monitor heat from overcurrent to protect motors from overload.
Correct answer is: Thermal relay
Q.110 Which relay uses a magnetic field to quickly open the contacts during a fault?
Thermal relay
Magnetic relay
Voltage relay
Distance relay
Explanation - Magnetic relays rely on magnetic force to separate contacts rapidly.
Correct answer is: Magnetic relay
Q.111 A 'ground fault relay' is most effective when set to trip at:
Half the rated current
The rated current
Twice the rated current
Ten times the rated current
Explanation - Setting the relay at the rated current ensures quick response to ground faults.
Correct answer is: The rated current
Q.112 Which of the following best describes an 'inverse‑time overcurrent relay'?
Trips faster with lower fault currents
Trips faster with higher fault currents
Trips at a constant time
Does not trip at all
Explanation - Inverse‑time operation means the relay’s trip time decreases as fault current increases.
Correct answer is: Trips faster with higher fault currents
Q.113 What is the main advantage of a 'distance relay' for high‑voltage transmission lines?
High speed operation
Low cost
Ability to detect fault distance
Simple installation
Explanation - Distance relays measure impedance to determine the fault’s location along the line.
Correct answer is: Ability to detect fault distance
Q.114 Which protective device is used to protect a transformer against short circuits between its windings?
Differential relay
Ground fault relay
Voltage relay
Overcurrent relay
Explanation - Differential relays compare input and output currents to detect internal transformer faults.
Correct answer is: Differential relay
Q.115 A 'surge arrester' protects equipment by:
Clamping high‑voltage transients to safe levels
Regulating voltage
Detecting overcurrent
Maintaining temperature
Explanation - Surge arresters absorb and clamp transient over‑voltages, safeguarding equipment.
Correct answer is: Clamping high‑voltage transients to safe levels
Q.116 Which of the following is the primary function of a 'thermal‑magnetic fuse'?
Detect overvoltage
Provide short‑circuit protection only
Provide overload and short‑circuit protection
Regulate frequency
Explanation - Thermal‑magnetic fuses combine a thermal element for overload and a magnetic element for fault currents.
Correct answer is: Provide overload and short‑circuit protection
Q.117 A 'distance relay' in a 220 kV line uses a characteristic impedance of:
0.2 ohms
0.4 ohms
0.6 ohms
0.8 ohms
Explanation - Distance relays set their characteristic impedance close to the line impedance per km for accurate fault detection.
Correct answer is: 0.4 ohms
Q.118 Which type of relay is most suitable for detecting a fault between two phases of a 33 kV circuit?
Current relay
Distance relay
Voltage relay
Thermal relay
Explanation - Distance relays can detect faults by measuring the impedance between the relay and the fault point.
Correct answer is: Distance relay
Q.119 The term 'pick‑up time' in a relay refers to:
The delay before the relay resets
The delay before the relay trips after detecting a fault
The time to open the contacts
The maximum operating time of the relay
Explanation - Pick‑up time is the delay from fault detection to relay actuation, allowing coordination.
Correct answer is: The delay before the relay trips after detecting a fault
Q.120 Which protective device is designed to provide high‑speed tripping for ground faults in a medium‑voltage system?
Ground‑fuse relay
Voltage relay
Overcurrent relay
Thermal relay
Explanation - Ground‑fuse relays quickly detect and clear earth fault currents, enhancing protection.
Correct answer is: Ground‑fuse relay
Q.121 A 'recloser' can be described as:
A device that never opens once closed
A device that opens and closes manually only
A device that automatically reopens after a fault clears
A device that closes permanently during faults
Explanation - Reclosers attempt to restore power after transient faults, improving reliability.
Correct answer is: A device that automatically reopens after a fault clears
Q.122 Which relay setting is used to determine the maximum fault current that can be cleared by the relay?
Trip delay
Dead‑time
Pickup
Zero‑current setting
Explanation - Pickup setting defines the minimum current that will trigger the relay, thus limiting the fault current.
Correct answer is: Pickup
Q.123 In a protective scheme for a 33 kV substation, which relay would be most suitable for detecting a fault between phase A and phase B?
Ground fault relay
Distance relay
Voltage relay
Thermal‑magnetic fuse
Explanation - Distance relays measure impedance and are effective for detecting phase‑to‑phase faults.
Correct answer is: Distance relay
Q.124 Which type of circuit breaker is used when high fault currents up to 20 kA are expected?
Standard air‑circuit breaker
Vacuum circuit breaker
SF6 circuit breaker
Thermal‑magnetic fuse
Explanation - SF6 breakers are designed to handle very high fault currents safely.
Correct answer is: SF6 circuit breaker
Q.125 A 'dead zone' in a relay setting is primarily used to:
Ensure rapid tripping
Prevent tripping during short transients
Speed up the trip time
Increase the pickup current
Explanation - Dead zones avoid nuisance tripping from brief non‑fault disturbances.
Correct answer is: Prevent tripping during short transients
Q.126 What does a 'trip curve' typically represent in protective relaying?
The relationship between current and trip time
The relationship between voltage and trip time
The temperature of the relay over time
The frequency of the system
Explanation - Trip curves show how the relay will respond to different fault current levels.
Correct answer is: The relationship between current and trip time
Q.127 Which relay would most likely be used to protect a transformer against a phase‑to‑ground fault?
Differential relay
Ground fault relay
Voltage relay
Thermal relay
Explanation - Ground fault relays monitor currents to earth and trip when they exceed the threshold.
Correct answer is: Ground fault relay
Q.128 A 'distance relay' is typically set with a characteristic impedance that is:
Half of the line impedance per km
Double the line impedance per km
Equal to the line impedance per km
Ten times the line impedance per km
Explanation - Setting the characteristic impedance at half helps to detect faults reliably.
Correct answer is: Half of the line impedance per km
Q.129 Which device is commonly used to provide protection against high‑voltage surges in a distribution network?
Surge arrester
Ground fault relay
Voltage relay
Overcurrent relay
Explanation - Surge arresters clamp transients to safe levels, protecting equipment from over‑voltage.
Correct answer is: Surge arrester
Q.130 In a protective scheme, a 'trip delay' setting is used to:
Speed up the relay operation
Delay the relay operation after fault detection
Reset the relay immediately
Increase the pickup threshold
Explanation - Trip delay allows downstream devices to clear the fault before the relay opens.
Correct answer is: Delay the relay operation after fault detection
Q.131 A 'ground fault relay' is most effective when it is set to trip at:
Half the rated current
The rated current
Twice the rated current
A fixed low current
Explanation - Setting it at rated current ensures a prompt response to ground faults.
Correct answer is: The rated current
Q.132 The primary purpose of a 'recloser' in an electrical distribution system is to:
Never open once closed
Automatically reclose after a fault clears
Regulate voltage
Measure current
Explanation - Reclosers attempt to restore power after transient faults, improving system reliability.
Correct answer is: Automatically reclose after a fault clears
Q.133 Which of the following is NOT typically used for overcurrent protection in a high‑voltage transmission system?
SF6 circuit breaker
Thermal‑magnetic fuse
Distance relay
Thermal‑magnetic circuit breaker
Explanation - SF6 circuit breakers interrupt current but are not used as overcurrent relays; they rely on relays for fault detection.
Correct answer is: SF6 circuit breaker
Q.134 Which device uses a bimetallic strip to open when it overheats?
Thermal relay
Magnetic relay
Voltage relay
Distance relay
Explanation - Thermal relays use a bimetal strip that bends under heat to trigger the opening of contacts.
Correct answer is: Thermal relay
Q.135 The 'dead‑time' setting in a protective relay is used to:
Prevent false tripping during short transients
Increase the trip speed
Decrease the pickup current
Reset the relay automatically
Explanation - Dead‑time allows a brief period after fault detection to avoid unnecessary tripping.
Correct answer is: Prevent false tripping during short transients
Q.136 What is the primary function of a 'ground fault relay' in a low‑voltage distribution network?
Detect phase‑to‑phase faults
Detect overvoltage
Detect imbalance to ground
Detect overcurrent
Explanation - Ground fault relays monitor the difference between live and neutral currents.
Correct answer is: Detect imbalance to ground
Q.137 Which protective device is used to protect a transformer against an internal fault?
Differential relay
Overcurrent relay
Voltage relay
Thermal relay
Explanation - Differential relays detect internal transformer faults by comparing currents.
Correct answer is: Differential relay
Q.138 In a protection scheme, the 'dead zone' is a:
Zone where the relay trips instantly
Zone where the relay does not trip
Zone where the relay resets
Zone where the relay is offline
Explanation - Dead zone prevents tripping on minor disturbances.
Correct answer is: Zone where the relay does not trip
Q.139 A 'recloser' in an electrical distribution system primarily operates:
Never opening once closed
Reclosing after transient faults clear
Regulating voltage
Detecting overvoltage
Explanation - Reclosers restore power automatically after a fault is cleared.
Correct answer is: Reclosing after transient faults clear
Q.140 Which device is commonly used for overcurrent protection in a 33 kV distribution system?
Overcurrent relay
Voltage relay
Ground fault relay
Surge arrester
Explanation - Overcurrent relays detect excessive current and trip the breaker to protect equipment.
Correct answer is: Overcurrent relay
Q.141 A 'distance relay' measures impedance to detect faults. Which of the following is the most important parameter for setting it?
Pickup voltage
Pickup impedance
Pickup current
Pickup temperature
Explanation - Distance relays rely on impedance thresholds to detect faults.
Correct answer is: Pickup impedance
Q.142 The 'trip delay' in a protective relay is used to:
Speed up tripping
Delay tripping until downstream devices clear the fault
Reset the relay
Reduce the pickup current
Explanation - Trip delay allows more selective protection by giving downstream relays time to act first.
Correct answer is: Delay tripping until downstream devices clear the fault
Q.143 Which device is used to provide protection against overvoltage surges in a distribution network?
Surge arrester
Overcurrent relay
Thermal relay
Voltage regulator
Explanation - Surge arresters clamp high‑voltage spikes to safe levels.
Correct answer is: Surge arrester
Q.144 A 'ground fault relay' operates by detecting:
Imbalance in phase currents
Overcurrent in the line
Imbalance to ground
High voltage transients
Explanation - Ground fault relays monitor the difference between live and neutral currents.
Correct answer is: Imbalance to ground
Q.145 Which of the following is a primary feature of an SF6 circuit breaker?
Uses a gas medium for arc suppression
Uses a vacuum for arc suppression
No gas required
Only suitable for low fault currents
Explanation - SF6 gas provides excellent arc quenching capability for high fault currents.
Correct answer is: Uses a gas medium for arc suppression
Q.146 In a protection scheme, the 'dead zone' prevents the relay from:
Trip during short transients
Trip during normal operation
Trip during large faults
Trip during high voltage
Explanation - Dead zones are designed to avoid nuisance tripping.
Correct answer is: Trip during short transients
Q.147 A 'recloser' in an electrical distribution system is primarily used to:
Never open once closed
Automatically reclose after a fault clears
Regulate voltage
Detect overvoltage
Explanation - Reclosers restore power automatically after transient faults.
Correct answer is: Automatically reclose after a fault clears
Q.148 Which protective device is used to protect a transformer against phase‑to‑phase faults?
Differential relay
Ground fault relay
Voltage relay
Thermal relay
Explanation - Differential relays detect faults by comparing currents entering and leaving the transformer.
Correct answer is: Differential relay
Q.149 A 'distance relay' is typically set with a characteristic impedance of:
Half of the line impedance
Double the line impedance
Equal to the line impedance
Ten times the line impedance
Explanation - The characteristic impedance is often set at half the line impedance to improve fault detection accuracy.
Correct answer is: Half of the line impedance
Q.150 The 'pickup' setting of a relay is:
The maximum current the relay can withstand
The minimum current required to trigger the relay
The time delay before tripping
The maximum voltage the relay can handle
Explanation - Pickup is the threshold at which the relay becomes active.
Correct answer is: The minimum current required to trigger the relay
Q.151 Which relay is most suitable for detecting overcurrent in a low‑voltage motor?
Thermal relay
Voltage relay
Ground fault relay
Overcurrent relay
Explanation - Thermal relays use heat from excess current to detect overload conditions.
Correct answer is: Thermal relay
Q.152 In a high‑voltage substation, the 'dead zone' of a relay helps to:
Speed up tripping
Avoid nuisance tripping during small disturbances
Increase the pickup current
Delay the reset time
Explanation - Dead zones prevent relay activation for minor transients that are not actual faults.
Correct answer is: Avoid nuisance tripping during small disturbances
Q.153 Which device is used to provide high‑speed tripping for ground faults in a medium‑voltage system?
Ground‑fuse relay
Voltage relay
Overcurrent relay
Thermal relay
Explanation - Ground‑fuse relays detect and clear earth fault currents quickly.
Correct answer is: Ground‑fuse relay
Q.154 Which of the following is a typical setting for an overcurrent relay protecting a 33 kV distribution line?
Pickup at 110% of rated current
Pickup at 50% of rated current
Pickup at 200% of rated current
Pickup at 10% of rated current
Explanation - Overcurrent relays are set slightly above normal operating current to avoid nuisance tripping.
Correct answer is: Pickup at 110% of rated current
Q.155 A 'distance relay' in a 132 kV system typically uses a characteristic impedance of:
0.2 ohms
0.4 ohms
0.6 ohms
0.8 ohms
Explanation - The characteristic impedance is chosen to match the line’s impedance per km for accurate fault detection.
Correct answer is: 0.4 ohms
Q.156 What is the primary advantage of a 'thermal‑magnetic fuse' over a standard fuse?
Lower cost
Provides overload protection in addition to short‑circuit protection
Requires no maintenance
Can handle higher fault currents
Explanation - The thermal element protects against overloads while the magnetic element handles fault currents.
Correct answer is: Provides overload protection in addition to short‑circuit protection