Q.1 What is the primary purpose of a protective relay in a power system?
Increase system voltage
Control power factor
Detect and isolate faults
Regulate transformer tap position
Explanation - A protective relay senses abnormal conditions such as overcurrent, overvoltage, or earth fault and signals circuit breakers to isolate the faulted section.
Correct answer is: Detect and isolate faults
Q.2 In an overcurrent protection scheme, a time delay is introduced to prevent:
Short circuits from occurring
The relay from tripping on the fault
The system from overloading
Voltage spikes during normal operation
Explanation - A time delay allows other protective devices to act first and avoids nuisance tripping for transient overcurrents.
Correct answer is: The relay from tripping on the fault
Q.3 Which type of fault involves a single line coming into contact with the earth?
Three‑phase fault
Line‑line fault
Line‑earth fault
Double line‑earth fault
Explanation - A line‑earth fault occurs when one phase conductor touches the earth or equipment grounded.
Correct answer is: Line‑earth fault
Q.4 The impedance seen by a fault at a point in the network is called:
Fault impedance
Thevenin impedance
Source impedance
Load impedance
Explanation - The network can be reduced to a Thevenin equivalent seen from the fault point, used to compute fault currents.
Correct answer is: Thevenin impedance
Q.5 In a bolted fault, the fault current is primarily limited by:
The source impedance
The transformer impedance
The fault impedance
The protective relay setting
Explanation - Bolted faults have negligible fault impedance, so the fault current is governed by the supply’s internal impedance.
Correct answer is: The source impedance
Q.6 A time‑overcurrent relay uses a time‑current characteristic curve of which type?
Inverse
Inverse‑plus
Inverse‑minus
Direct
Explanation - Inverse‑plus curves provide a time delay that decreases with higher overcurrent, offering selective coordination.
Correct answer is: Inverse‑plus
Q.7 The sum of the squares of the fault current magnitudes on all phases equals:
Three times the total fault current
The total fault power
The square of the total fault current
Zero for balanced faults
Explanation - For balanced faults, the magnitude of the total fault current equals the square root of the sum of squares of the phase currents.
Correct answer is: The square of the total fault current
Q.8 Which of the following is NOT a common type of ground fault protection?
Buchholz relay
Earth fault relay
Impedance protection relay
Ground‑fault resistor
Explanation - Buchholz relays detect gas build‑up in transformers, not ground faults.
Correct answer is: Buchholz relay
Q.9 A bolted fault current at a bus with a source impedance of 0.01 p.u. and a line impedance of 0.05 p.u. is approximately:
20 p.u.
10 p.u.
5 p.u.
0.2 p.u.
Explanation - Fault current ≈ 1/(Zsource+Zline) = 1/(0.01+0.05) ≈ 16.7 p.u.; rounding to nearest option gives 10 p.u.
Correct answer is: 10 p.u.
Q.10 In an under‑voltage relay, a low voltage condition is often caused by:
An overcurrent event
A line‑line fault
A transformer tap change
An isolated fault
Explanation - Line‑line faults draw high fault currents, leading to voltage dips that under‑voltage relays detect.
Correct answer is: A line‑line fault
Q.11 The characteristic curve of a distance relay is based on:
The fault current magnitude
The impedance of the faulted section
The time delay of the relay
The voltage level
Explanation - Distance relays measure apparent impedance to locate faults within their zone.
Correct answer is: The impedance of the faulted section
Q.12 Which parameter is most critical when selecting a fuse for an electric motor circuit?
Time‑current characteristic
Melting point of the fuse element
Fuse temperature rating
Maximum interrupting current
Explanation - The fuse must be able to safely interrupt the worst‑case fault current without damage.
Correct answer is: Maximum interrupting current
Q.13 An instantaneous overcurrent relay will trip if the measured current exceeds:
0.5 times rated current
1.0 times rated current
1.5 times rated current
2.0 times rated current
Explanation - Instantaneous relays are set to trip at or just above the rated current with no time delay.
Correct answer is: 1.0 times rated current
Q.14 The Z‑test is used to check:
System stability margin
Correctness of transformer tap settings
Validity of fault impedance measurements
Co‑ordination of protection devices
Explanation - The Z‑test compares the time‑current curves of two relays to verify selective tripping.
Correct answer is: Co‑ordination of protection devices
Q.15 Which of the following best describes a 'slow‑break' relay?
A relay that trips instantly
A relay with a very short delay
A relay designed to operate in a high‑fault‑current situation
A relay that lags the fault current rise by a small time period
Explanation - Slow‑break relays allow the current to rise slightly before tripping, useful in certain protection schemes.
Correct answer is: A relay that lags the fault current rise by a small time period
Q.16 A three‑phase fault in a 345 kV network results in a fault current of 4000 A. What is the per‑phase fault current?
4000 A
1388 A
1733 A
2500 A
Explanation - For a balanced three‑phase fault, Iph = If / √3 = 4000 / 1.732 ≈ 1388 A.
Correct answer is: 1388 A
Q.17 An impedance relay’s zone 1 setting is typically:
0–0.5 p.u.
0.5–1.0 p.u.
1.0–1.5 p.u.
1.5–2.0 p.u.
Explanation - Zone 1 covers the protected section up to roughly 50 % of the nominal impedance.
Correct answer is: 0–0.5 p.u.
Q.18 The 'inverse time' characteristic of a relay implies that as the fault current increases, the operating time:
Increases
Decreases
Remains constant
Oscillates
Explanation - Inverse time curves provide quicker tripping for larger fault currents.
Correct answer is: Decreases
Q.19 In a power system, the term 'selectivity' refers to:
The ability to isolate only the faulted portion of the system
The ability to withstand high fault currents
The ability to adjust tap positions accurately
The ability to synchronize generators
Explanation - Selective tripping ensures minimal impact on the rest of the network.
Correct answer is: The ability to isolate only the faulted portion of the system
Q.20 Which device is typically used to interrupt a fault in a 400 kV transmission line?
Fuse
Circuit breaker
Resistor
Screw‑type relay
Explanation - High‑voltage circuit breakers are designed for breaking large fault currents safely.
Correct answer is: Circuit breaker
Q.21 The primary fault impedance of a transformer is usually:
Very low
Very high
Same as the source impedance
Negligible
Explanation - Transformers have low impedance, resulting in high fault currents when bolted.
Correct answer is: Very low
Q.22 Which protective device is most sensitive to earth fault currents?
Distance relay
Overcurrent relay
Earth fault relay
Under‑voltage relay
Explanation - Earth fault relays detect imbalances between phase and neutral currents.
Correct answer is: Earth fault relay
Q.23 A relay set to 300 A instantaneous will trip if:
The current reaches 200 A
The current reaches 250 A
The current reaches 300 A
The current reaches 350 A
Explanation - Instantaneous relays trip exactly at the set current value.
Correct answer is: The current reaches 300 A
Q.24 For a bolted fault on a 69 kV line with source impedance of 0.02 p.u. and line impedance of 0.08 p.u., the fault current is roughly:
5 kA
10 kA
20 kA
2 kA
Explanation - I_fault ≈ 1/(Zsource+Zline) = 1/(0.10) = 10 kA.
Correct answer is: 10 kA
Q.25 Which parameter does not affect the operating time of a time‑overcurrent relay?
Set current
Source impedance
Ambient temperature
Relay’s internal calibration
Explanation - Relay time characteristics are defined by current and internal calibration; temperature may affect hardware but not the set curve.
Correct answer is: Ambient temperature
Q.26 In a fault analysis, the term 'symmetrical component' refers to:
Components of the load current
Components of the fault current
Decomposition of phase quantities into zero‑, positive‑ and negative‑sequence components
Components of voltage regulation
Explanation - Symmetrical components simplify analysis of unbalanced faults.
Correct answer is: Decomposition of phase quantities into zero‑, positive‑ and negative‑sequence components
Q.27 Which of the following best describes a 'bolted fault'?
A fault with high impedance
A fault caused by a broken breaker
A fault with negligible resistance between phase and ground
A fault that occurs during system startup
Explanation - Bolted faults imply a direct short with very low impedance, yielding high fault currents.
Correct answer is: A fault with negligible resistance between phase and ground
Q.28 Which of these is a typical setting for a 33 kV feeder overcurrent relay?
10 % overcurrent
30 % overcurrent
70 % overcurrent
90 % overcurrent
Explanation - Relays are often set at 70–80 % of rated current to provide a margin.
Correct answer is: 70 % overcurrent
Q.29 A distance relay's zone 2 is designed to:
Protect only the nearest substation
Cover the entire feeder
Provide backup protection for zone 1
Detect earth faults only
Explanation - Zone 2 extends beyond zone 1 to ensure no fault escapes protection.
Correct answer is: Provide backup protection for zone 1
Q.30 The fault clearing time in a 132 kV system must typically be less than:
1 s
5 s
10 s
15 s
Explanation - High‑voltage systems aim to clear faults quickly to protect equipment and maintain stability.
Correct answer is: 5 s
Q.31 What is the primary difference between an instantaneous relay and an inverse‑time relay?
Instantaneous relays are slower
Inverse‑time relays have a time delay that increases with current
Instantaneous relays do not have a time delay
Inverse‑time relays cannot operate in the presence of earth faults
Explanation - Instantaneous relays trip immediately when the set current is exceeded.
Correct answer is: Instantaneous relays do not have a time delay
Q.32 During a three‑phase fault, the fault current is often 1.732 times greater than the phase current. What is the factor by which the total fault current exceeds the rated current of a transformer with a 15 kA rated current?
1.732
1.0
2.0
0.5
Explanation - For balanced faults, fault current = 1.732 × phase current; compared to rated current, this factor determines protection settings.
Correct answer is: 1.732
Q.33 Which protective device is used primarily to protect against over‑voltage conditions?
Surge arrester
Fuse
Distance relay
Ground fault relay
Explanation - Surge arresters clamp transient over‑voltages to safe levels.
Correct answer is: Surge arrester
Q.34 The term 'zero‑sequence current' is significant in:
Over‑voltage protection
Three‑phase fault analysis
Ground‑fault detection
Transformer tap settings
Explanation - Zero‑sequence currents flow between phase and ground in earth faults.
Correct answer is: Ground‑fault detection
Q.35 Which of the following best describes the purpose of a 'trip set' in a relay?
To adjust the relay’s voltage sensitivity
To define the current threshold for tripping
To control the breaker’s opening speed
To monitor system frequency
Explanation - Trip set specifies the current level at which the relay will command a breaker to trip.
Correct answer is: To define the current threshold for tripping
Q.36 What is the typical maximum interrupting capacity of a 500 kV circuit breaker?
10 kA
30 kA
50 kA
70 kA
Explanation - High‑voltage circuit breakers are designed to interrupt fault currents up to around 50 kA.
Correct answer is: 50 kA
Q.37 A relay that trips if the measured current is 80 % of the rated current and remains tripped if it is 90 % of the rated current is called:
Inverse‑time relay
Instantaneous relay
Current‑threshold relay
Differential relay
Explanation - Threshold relays use a fixed percentage of rated current as the trip condition.
Correct answer is: Current‑threshold relay
Q.38 In a single‑line‑to‑ground fault, the fault current flows through:
All three phases equally
Only the faulted phase
The faulted phase and the neutral
The faulted phase and the earth
Explanation - Current returns to ground, completing the circuit.
Correct answer is: The faulted phase and the earth
Q.39 The time required for a relay to trip is influenced by:
Relay power supply voltage
Load factor of the system
Fault current magnitude
Frequency of the system
Explanation - Higher fault currents lead to faster relay operation in inverse‑time relays.
Correct answer is: Fault current magnitude
Q.40 Which of the following is an advantage of a time‑overcurrent relay over a direct overcurrent relay?
It is cheaper
It provides faster tripping
It eliminates the need for a fuse
It allows selective tripping in the presence of multiple faults
Explanation - Time delay permits coordination between relays for selective protection.
Correct answer is: It allows selective tripping in the presence of multiple faults
Q.41 The 'inverse‑plus' time‑current characteristic is most suitable for:
Very small systems
Large power grids
Low‑voltage distribution
Ground fault detection only
Explanation - Inverse‑plus curves offer fine coordination for high‑voltage systems.
Correct answer is: Large power grids
Q.42 When performing a fault analysis, the first step is usually to:
Identify the fault type
Set the relay settings
Calculate system frequency
Check transformer tap positions
Explanation - Knowing the fault type dictates the sequence of calculations.
Correct answer is: Identify the fault type
Q.43 If a relay is set to 70 % of rated current, a fault current of 100 % of rated current will cause the relay to:
Trip immediately
Trip after a delay
Not trip
Trip only if the fault persists for more than 10 s
Explanation - The fault current exceeds the set threshold; relay will activate.
Correct answer is: Trip immediately
Q.44 A distance relay’s zone 3 typically provides protection for:
The feeder only
The substation only
The entire system beyond zone 2
Ground faults only
Explanation - Zone 3 acts as a backup protection covering any fault outside zone 2.
Correct answer is: The entire system beyond zone 2
Q.45 Which protective device is most commonly used in low‑voltage distribution to isolate a faulted circuit?
Circuit breaker
Fuse
Surge arrester
Ground fault relay
Explanation - Fuses provide a simple and inexpensive means of fault isolation in distribution.
Correct answer is: Fuse
Q.46 The operating time of an inverse‑time relay for a 150 % overcurrent setting is:
Longer than the 100 % setting
Shorter than the 100 % setting
The same as the 100 % setting
Undefined
Explanation - Higher overcurrent reduces the operating time on inverse‑time curves.
Correct answer is: Shorter than the 100 % setting
Q.47 When a relay trips, it sends a signal to:
The system operator
The transformer
The circuit breaker
The power plant
Explanation - Relays communicate with breakers to initiate fault isolation.
Correct answer is: The circuit breaker
Q.48 Which of the following is a characteristic of a 'slow‑break' relay?
It trips instantaneously
It delays tripping by several seconds
It has no time delay
It only operates in low‑fault‑current situations
Explanation - Slow‑break relays provide a controlled delay before tripping.
Correct answer is: It delays tripping by several seconds
Q.49 The fault current magnitude for a three‑phase fault at a bus with 0.04 p.u. source impedance and 0.12 p.u. line impedance is approximately:
6.67 kA
8 kA
5 kA
4 kA
Explanation - I_fault = 1/(0.04+0.12) = 1/0.16 = 6.25 p.u.; assuming 1.25 MVA base gives ~8 kA.
Correct answer is: 8 kA
Q.50 Which of the following best defines the 'impedance relay'?
A relay that measures current magnitude
A relay that uses voltage measurement
A relay that calculates impedance between the fault and the source
A relay that detects frequency variations
Explanation - Impedance relays use the ratio of voltage to current to detect fault impedance.
Correct answer is: A relay that calculates impedance between the fault and the source
Q.51 The function of a 'co‑ordination study' in protection schemes is to:
Determine transformer tap positions
Set the relay’s time‑current curves
Align protection devices to clear faults in the correct order
Measure system frequency
Explanation - Coordination ensures that upstream relays act after downstream ones.
Correct answer is: Align protection devices to clear faults in the correct order
Q.52 In a line‑to‑line fault, the fault current is approximately equal to the:
Phase‑phase impedance
Phase‑neutral impedance
Neutral‑neutral impedance
Zero‑sequence impedance
Explanation - Line‑to‑line faults involve two phases, so phase‑phase impedance dominates.
Correct answer is: Phase‑phase impedance
Q.53 A relay with a 'delay' setting of 3 seconds will:
Trip after 3 s regardless of current
Trip after 3 s if current is over set value
Trip immediately when current is over set value
Trip only if the fault lasts for 3 s
Explanation - The delay allows a timed interval before tripping.
Correct answer is: Trip after 3 s if current is over set value
Q.54 The primary protective device for a transformer is:
Circuit breaker
Ground fault relay
Distance relay
Surge arrester
Explanation - Transformers are typically protected by breakers to isolate them during faults.
Correct answer is: Circuit breaker
Q.55 Which type of relay is designed to detect differential currents between two terminals?
Overcurrent relay
Differential relay
Earth fault relay
Under‑voltage relay
Explanation - Differential relays compare currents entering and leaving an equipment section.
Correct answer is: Differential relay
Q.56 The 'fault current' in a 400 kV system is typically expressed in terms of:
Voltage
Power
Per unit
Frequency
Explanation - Per‑unit (p.u.) values simplify calculations across systems of different bases.
Correct answer is: Per unit
Q.57 A 'double line‑to‑earth fault' involves:
Two phases shorted together
Two phases shorted to earth
One phase shorted to earth
All three phases shorted to earth
Explanation - In double line‑to‑earth, two phases simultaneously short to ground.
Correct answer is: Two phases shorted to earth
Q.58 Which device is commonly used to interrupt fault currents in a 110 kV network?
Fuse
Circuit breaker
Surge arrester
Ground fault relay
Explanation - Circuit breakers can interrupt high fault currents typical of 110 kV systems.
Correct answer is: Circuit breaker
Q.59 The time‑current characteristic of a relay is often plotted on a graph with:
Current on the horizontal axis and time on the vertical axis
Voltage on the horizontal axis and current on the vertical axis
Frequency on the horizontal axis and time on the vertical axis
Current on the horizontal axis and voltage on the vertical axis
Explanation - This plot shows how time to trip varies with current level.
Correct answer is: Current on the horizontal axis and time on the vertical axis
Q.60 In protection coordination, the upstream relay must have a time delay of at least how many times the downstream relay’s time delay?
1/2
1
2
3
Explanation - A common rule of thumb is that the upstream device should be at least twice as slow.
Correct answer is: 2
Q.61 The primary function of an earth fault relay is to:
Detect voltage dips
Detect overcurrent between phases
Detect imbalance between phase and neutral currents
Detect frequency variations
Explanation - Earth faults create a mismatch that earth‑fault relays sense.
Correct answer is: Detect imbalance between phase and neutral currents
Q.62 The impedance of a 345 kV transmission line is typically expressed in:
Ohms per kilometer
Per unit per kilometer
Per unit per system base
Kilohms
Explanation - Impedance is often normalized to the system base for simplicity.
Correct answer is: Per unit per system base
Q.63 Which of the following is a common fault type that involves only two phases?
Single line‑to‑ground fault
Line‑to‑line fault
Three‑phase fault
Double line‑to‑ground fault
Explanation - Line‑to‑line faults short two phases together.
Correct answer is: Line‑to‑line fault
Q.64 A relay that uses a fixed time delay before tripping is referred to as a:
Instantaneous relay
Time‑overcurrent relay
Delay relay
Differential relay
Explanation - Delay relays have a predetermined delay time before acting.
Correct answer is: Delay relay
Q.65 A protective relay that trips when the voltage falls below a certain level is an:
Overcurrent relay
Under‑voltage relay
Earth fault relay
Distance relay
Explanation - Under‑voltage relays detect voltage sags.
Correct answer is: Under‑voltage relay
Q.66 The impedance of a transformer secondary is typically:
High
Low
Zero
Same as the primary
Explanation - Transformers have low secondary impedance to limit fault currents.
Correct answer is: Low
Q.67 A fault analysis that uses symmetrical components is used to solve:
Balanced load calculations
Unbalanced fault conditions
Frequency response
Voltage regulation
Explanation - Symmetrical components break unbalanced phasors into balanced sets.
Correct answer is: Unbalanced fault conditions
Q.68 The setting of a relay to 120 % of rated current is commonly used for:
Ground fault detection
Instantaneous tripping
Selective overcurrent protection
Voltage regulation
Explanation - A higher setting allows downstream relays to act first.
Correct answer is: Selective overcurrent protection
Q.69 Which of the following is NOT a typical fault in power systems?
Three‑phase fault
Line‑line fault
Neutral‑neutral fault
Single line‑to‑ground fault
Explanation - Neutral‑neutral faults are impossible as neutrals are the same conductor.
Correct answer is: Neutral‑neutral fault
Q.70 The fault current in a 69 kV line with a source impedance of 0.03 p.u. and line impedance of 0.07 p.u. is approximately:
5 kA
10 kA
15 kA
20 kA
Explanation - I_fault ≈ 1/(0.10) = 10 kA.
Correct answer is: 10 kA
Q.71 The function of a 'resettable fuse' is to:
Break a fault permanently
Break a fault temporarily and then reset automatically
Detect overcurrent only
Regulate voltage
Explanation - Resettable fuses, or polyswitches, reset once current returns to normal.
Correct answer is: Break a fault temporarily and then reset automatically
Q.72 A fault that involves only one phase and the ground is known as:
Three‑phase fault
Single line‑to‑ground fault
Line‑line fault
Double line‑to‑ground fault
Explanation - Single line‑to‑ground faults are the most common earth fault type.
Correct answer is: Single line‑to‑ground fault
Q.73 A relay that provides protection against both overcurrent and earth fault is called a:
Dual‑function relay
Inverse‑time relay
Differential relay
Distance relay
Explanation - Dual‑function relays combine multiple protection functions into one device.
Correct answer is: Dual‑function relay
Q.74 The term 'inverse‑time' indicates that the relay will:
Trip faster as current increases
Trip slower as current increases
Trip at a constant time regardless of current
Trip only after a fixed delay
Explanation - Inverse‑time relays have a decreasing time‑current curve.
Correct answer is: Trip faster as current increases
Q.75 The time‑delay setting in a protection relay is mainly used to achieve:
Selectivity
Fast tripping
Lower cost
Higher sensitivity
Explanation - Delays ensure that the nearest protective device acts first.
Correct answer is: Selectivity
Q.76 A relay that compares the currents at two points of a transformer is a:
Current‑comparison relay
Differential relay
Ground fault relay
Overvoltage relay
Explanation - Differential relays detect imbalance between currents entering and leaving a device.
Correct answer is: Differential relay
Q.77 The most common fault type in distribution systems is:
Three‑phase fault
Single line‑to‑ground fault
Line‑line fault
Double line‑to‑ground fault
Explanation - Ground faults are the majority in distribution networks.
Correct answer is: Single line‑to‑ground fault
Q.78 If a fault current of 200 % of rated current is applied to an inverse‑time relay set at 100 % rated current, the relay will trip in approximately:
Longer
Shorter
Same time
It will not trip
Explanation - Higher fault current shortens the operating time on an inverse‑time curve.
Correct answer is: Shorter
Q.79 The 'fault clearing time' refers to:
The time taken to detect a fault
The time taken for the fault to occur
The time taken for the fault to be isolated
The time taken for the system to restore frequency
Explanation - Fault clearing time is the interval between fault occurrence and isolation.
Correct answer is: The time taken for the fault to be isolated
Q.80 The term 'Z‑test' is used to:
Determine system impedance
Check relay coordination
Measure fault current
Calibrate transformers
Explanation - Z‑test compares relay characteristics for selective operation.
Correct answer is: Check relay coordination
Q.81 Which of the following is NOT typically a component of a protection scheme?
Relay
Breaker
Transformer
Circuit breaker
Explanation - Transformers provide isolation and voltage transformation but are not protection devices.
Correct answer is: Transformer
Q.82 A 'bolted fault' is characterized by:
High resistance between phase and earth
Zero resistance between phase and earth
High resistance between two phases
Zero resistance between two phases
Explanation - Bolted faults have negligible fault impedance.
Correct answer is: Zero resistance between phase and earth
Q.83 Which of the following is a primary function of a 'distance relay'?
Detect fault current magnitude
Measure the apparent impedance to the fault
Measure voltage level
Detect frequency changes
Explanation - Distance relays calculate apparent impedance to locate faults.
Correct answer is: Measure the apparent impedance to the fault
Q.84 In a 400 kV transmission line, the impedance of 5 km is 0.02 p.u. per km. The total line impedance is:
0.10 p.u.
0.05 p.u.
0.20 p.u.
0.02 p.u.
Explanation - 0.02 p.u/km × 5 km = 0.10 p.u.
Correct answer is: 0.10 p.u.
Q.85 A fault that involves all three phases simultaneously is a:
Single line‑to‑ground fault
Line‑to‑line fault
Three‑phase fault
Double line‑to‑ground fault
Explanation - All three phases short together in a three‑phase fault.
Correct answer is: Three‑phase fault
Q.86 A 'differential relay' operates by comparing:
Voltage levels at two points
Current entering and leaving a device
Voltage between phases
Ground fault current
Explanation - Differential relays detect differences indicating fault conditions.
Correct answer is: Current entering and leaving a device
Q.87 Which of the following is an example of a 'selective' protection scheme?
All relays trip at the same time
Only the nearest relay trips to a fault
All relays operate independently
Relays do not coordinate at all
Explanation - Selective tripping isolates only the faulted section.
Correct answer is: Only the nearest relay trips to a fault
Q.88 A 'time‑overcurrent relay' uses a setting of 200 % of rated current and a time delay of 1 s. If a fault occurs with 300 % of rated current, the relay will trip in:
Approximately 0.5 s
Approximately 1 s
Approximately 1.5 s
Approximately 2 s
Explanation - Higher current reduces time; 300 % is 1.5× 200 %, so time ≈ 1 s / 1.5 = 0.67 s (~0.5 s).
Correct answer is: Approximately 0.5 s
Q.89 The main purpose of a 'surge arrester' is to protect against:
Under‑voltage conditions
Over‑current faults
Transient over‑voltages due to lightning
Ground faults
Explanation - Surge arresters clamp high transient voltages to safe levels.
Correct answer is: Transient over‑voltages due to lightning
Q.90 Which of the following best describes a 'zone‑1' setting on a distance relay?
It covers the entire system
It covers the protected section only
It covers up to 150 % of the nominal impedance
It covers only the faulted phase
Explanation - Zone‑1 typically covers up to 90–120 % of the nominal impedance.
Correct answer is: It covers the protected section only
Q.91 A 'fault clearing time' of less than 5 s is desirable for:
Low‑voltage distribution systems
High‑voltage transmission systems
Acoustic monitoring systems
Digital signal processors
Explanation - Rapid clearing protects equipment in high‑voltage grids.
Correct answer is: High‑voltage transmission systems
Q.92 The 'inverse‑plus' curve is most commonly used for:
Low‑voltage protection
High‑voltage protection
Frequency regulation
Voltage regulation
Explanation - Inverse‑plus curves provide precise coordination in high‑voltage systems.
Correct answer is: High‑voltage protection
Q.93 In a fault analysis, the 'short‑circuit impedance' is used to calculate:
System frequency
Fault current
Load power
Voltage drop
Explanation - Short‑circuit impedance directly determines the magnitude of fault currents.
Correct answer is: Fault current
Q.94 The primary advantage of a 'dual‑function relay' is that it:
Reduces the number of devices
Provides faster tripping
Has a longer life span
Is cheaper to produce
Explanation - Combining functions into one relay saves space and cost.
Correct answer is: Reduces the number of devices
Q.95 The 'fault clearing time' for a 33 kV feeder is typically limited to:
30 s
10 s
5 s
2 s
Explanation - Short clearing times protect equipment and reduce downtime.
Correct answer is: 5 s
Q.96 Which relay setting is used to specify the minimum fault current that will cause the relay to trip?
Hold‑over setting
Trip setting
Delay setting
Pickup setting
Explanation - Pickup setting determines the current at which the relay becomes active.
Correct answer is: Pickup setting
Q.97 In a protection coordination study, the 'Z‑test' uses a test current of:
10 % of rated current
50 % of rated current
90 % of rated current
100 % of rated current
Explanation - Z‑test typically uses 90 % rated current to evaluate relay performance.
Correct answer is: 90 % of rated current
Q.98 A 'single line‑to‑ground fault' on a 132 kV system has a fault impedance of 0.02 p.u. The fault current is approximately:
20 kA
10 kA
5 kA
2 kA
Explanation - I_fault = 1/(0.02) = 50 p.u.; for a 1 MVA base this yields about 10 kA.
Correct answer is: 10 kA
Q.99 The 'inverse‑time' curve of a relay ensures that:
The relay does not trip
The relay trips after a fixed time
The relay trips faster for higher currents
The relay trips slower for higher currents
Explanation - Higher currents reduce operating time on inverse‑time curves.
Correct answer is: The relay trips faster for higher currents
Q.100 Which device is used to disconnect a transformer during an earth fault?
Ground fault relay
Overcurrent relay
Under‑voltage relay
Distance relay
Explanation - Ground fault relays detect earth faults and command breakers to disconnect equipment.
Correct answer is: Ground fault relay
Q.101 The 'fault current' in a balanced system is calculated as:
I_fault = V / Z
I_fault = 1 / Z
I_fault = Z / V
I_fault = V × Z
Explanation - For per‑unit systems, fault current equals the reciprocal of fault impedance.
Correct answer is: I_fault = 1 / Z
Q.102 A 'delay relay' is used to:
Trip immediately
Trip after a fixed time delay
Measure voltage
Measure frequency
Explanation - Delay relays incorporate a pre‑defined delay before tripping.
Correct answer is: Trip after a fixed time delay
Q.103 In a protection scheme, the 'Z‑test' ensures that the relay:
Trips at the lowest possible current
Co‑ordinates with upstream and downstream relays
Has the longest possible tripping time
Is set to zero current
Explanation - The Z‑test verifies that each relay clears its zone before the next.
Correct answer is: Co‑ordinates with upstream and downstream relays
Q.104 An 'earth fault relay' is most sensitive to:
Zero‑sequence current
Positive‑sequence current
Negative‑sequence current
Voltage amplitude
Explanation - Earth faults generate zero‑sequence currents flowing to ground.
Correct answer is: Zero‑sequence current
Q.105 The 'fault clearing time' of a circuit breaker is limited to:
5 min
30 s
10 s
1 s
Explanation - Circuit breakers must open within a few seconds to limit fault effects.
Correct answer is: 1 s
Q.106 The 'time‑delay' in a relay is usually expressed in:
Milliseconds
Microseconds
Seconds
Minutes
Explanation - Time delays are typically set in seconds for coordination.
Correct answer is: Seconds
Q.107 A 'distance relay' uses the ratio of:
Voltage to current
Current to impedance
Voltage to impedance
Frequency to phase angle
Explanation - Distance relays compute apparent impedance using V/I.
Correct answer is: Voltage to current
Q.108 A 'fault clearing time' of 5 s is acceptable for:
Low‑voltage distribution systems
High‑voltage transmission systems
Power plant control systems
Industrial motors
Explanation - Lower voltage systems can tolerate slightly longer fault clearing times.
Correct answer is: Low‑voltage distribution systems
Q.109 The main advantage of a 'selective' protection scheme is that it:
Increases system cost
Is harder to implement
Is less reliable
Reduces unnecessary outages
Explanation - Selective tripping isolates only the faulted portion, minimizing impact.
Correct answer is: Reduces unnecessary outages
Q.110 An 'inverse‑time relay' set at 70 % of rated current will operate faster if the fault current increases to:
80 % of rated current
90 % of rated current
100 % of rated current
110 % of rated current
Explanation - Higher overcurrent leads to quicker operation in inverse‑time relays.
Correct answer is: 110 % of rated current
Q.111 The 'fault current' in a 345 kV system with source impedance 0.02 p.u. and line impedance 0.08 p.u. is:
3 kA
5 kA
10 kA
15 kA
Explanation - I_fault = 1/(0.10) = 10 kA on a 1 MVA base.
Correct answer is: 10 kA
Q.112 The main purpose of a 'fault clearing time' in protection planning is to ensure:
All devices trip at the same time
The fault is isolated before equipment is damaged
The fault persists as long as possible
All relays are synchronized
Explanation - Fast fault clearing protects equipment from damage.
Correct answer is: The fault is isolated before equipment is damaged
Q.113 A 'bolted fault' causes:
Low fault current
High fault current
Moderate fault current
Zero fault current
Explanation - Bolted faults have negligible impedance, resulting in high currents.
Correct answer is: High fault current
Q.114 Which of the following is a common fault type in a power system?
Line‑to‑line fault
Ground fault
All of the above
None of the above
Explanation - All listed faults can occur in power systems.
Correct answer is: All of the above
Q.115 In a protection system, the 'trip setting' is the:
Maximum current the relay can measure
Minimum time delay
Current threshold for tripping
Maximum voltage tolerance
Explanation - Trip setting defines the current level that triggers a trip signal.
Correct answer is: Current threshold for tripping
Q.116 Which relay is typically used for 'over‑voltage' protection?
Over‑current relay
Under‑voltage relay
Over‑voltage relay
Earth fault relay
Explanation - Over‑voltage relays detect voltage levels above a set threshold.
Correct answer is: Over‑voltage relay
Q.117 Which of the following best describes a 'line‑to‑ground fault'?
Two phases short together
One phase short to ground
All three phases short together
Neutral short to ground
Explanation - Line‑to‑ground faults involve a single phase contacting earth.
Correct answer is: One phase short to ground
Q.118 In a fault analysis, the 'fault impedance' is typically expressed in:
Ohms
Per unit
Volts
Amps
Explanation - Per‑unit representation simplifies calculations across different voltage levels.
Correct answer is: Per unit
Q.119 The primary function of a 'circuit breaker' is to:
Detect fault currents
Break the circuit during a fault
Regulate voltage
Measure current
Explanation - Circuit breakers disconnect the circuit to isolate faults.
Correct answer is: Break the circuit during a fault
Q.120 In a protection scheme, the 'delay setting' of a relay is used to:
Reduce the time for tripping
Increase the time for tripping
Set the current threshold
Measure fault current
Explanation - Delay settings allow coordination by adding a time lag.
Correct answer is: Increase the time for tripping
Q.121 The 'fault current' in a 110 kV line with a source impedance of 0.03 p.u. and line impedance of 0.07 p.u. is approximately:
5 kA
10 kA
15 kA
20 kA
Explanation - I_fault = 1/(0.10) = 10 kA.
Correct answer is: 10 kA
Q.122 Which relay setting is used to determine the minimum fault current that will cause the relay to trip?
Trip setting
Pickup setting
Delay setting
Hold‑over setting
Explanation - Pickup setting defines the threshold current for relay activation.
Correct answer is: Pickup setting
Q.123 The 'fault current' in a balanced system can be expressed as:
I_fault = V / Z
I_fault = 1 / Z
I_fault = V × Z
I_fault = Z / V
Explanation - Short‑circuit current equals the reciprocal of fault impedance (p.u.).
Correct answer is: I_fault = 1 / Z
Q.124 The function of an 'earth fault relay' is to:
Detect overcurrent between phases
Detect zero‑sequence current imbalance
Detect voltage sags
Detect frequency dips
Explanation - Earth fault relays monitor the imbalance caused by ground faults.
Correct answer is: Detect zero‑sequence current imbalance
Q.125 Which of the following is NOT a typical protection device?
Relay
Circuit breaker
Transformer
Fuse
Explanation - Transformers are equipment, not protective devices.
Correct answer is: Transformer
Q.126 In a 345 kV system, the impedance of a line section is 0.02 p.u. per km. The total impedance of a 10 km section is:
0.02 p.u.
0.1 p.u.
0.2 p.u.
0.5 p.u.
Explanation - 0.02 p.u/km × 10 km = 0.20 p.u.
Correct answer is: 0.2 p.u.
Q.127 The main purpose of a 'time‑overcurrent relay' is to:
Trip immediately on any overcurrent
Trip after a preset delay if overcurrent persists
Trip only on undercurrent
Trip based on voltage
Explanation - Time‑overcurrent relays coordinate protection by delaying tripping.
Correct answer is: Trip after a preset delay if overcurrent persists
Q.128 The 'fault clearing time' for a 33 kV distribution feeder is typically limited to:
30 s
10 s
5 s
1 s
Explanation - Short fault clearing times protect equipment and minimize outages.
Correct answer is: 5 s
Q.129 A 'distance relay' uses a zone‑2 setting of 1.5 p.u. What does this represent?
The relay will trip for faults up to 1.5 p.u. of impedance
The relay will trip for faults beyond 1.5 p.u. of impedance
The relay will trip after 1.5 s delay
The relay will trip after 1.5 % of rated current
Explanation - Zone‑2 extends protection beyond the primary section.
Correct answer is: The relay will trip for faults up to 1.5 p.u. of impedance
Q.130 Which of the following is a typical fault in a 110 kV transmission line?
Three‑phase fault
Line‑to‑line fault
Ground fault
All of the above
Explanation - All these fault types can occur on a 110 kV line.
Correct answer is: All of the above
Q.131 The 'fault current' in a 132 kV system with source impedance 0.02 p.u. and line impedance 0.08 p.u. is:
10 kA
5 kA
2 kA
1 kA
Explanation - I_fault = 1/(0.10) ≈ 10 kA.
Correct answer is: 10 kA
Q.132 Which of the following best describes the purpose of an 'earth fault relay' in a distribution system?
Detect high voltage
Detect low voltage
Detect faults to earth
Detect frequency variations
Explanation - Earth fault relays monitor current imbalance indicating earth faults.
Correct answer is: Detect faults to earth
Q.133 In protection coordination, the 'time‑delay' of the upstream relay must be:
Shorter than the downstream relay
Equal to the downstream relay
Longer than the downstream relay
Irrelevant to the downstream relay
Explanation - Upstream devices act after downstream ones for selective protection.
Correct answer is: Longer than the downstream relay
Q.134 A 'fault current' that flows to the earth during a ground fault is called:
Zero‑sequence current
Positive‑sequence current
Negative‑sequence current
Balanced current
Explanation - Zero‑sequence current returns to ground in earth faults.
Correct answer is: Zero‑sequence current
Q.135 Which of the following is a common protective device in high‑voltage systems?
Fuse
Surge arrester
Circuit breaker
All of the above
Explanation - All listed devices are used for protection in high‑voltage networks.
Correct answer is: All of the above
Q.136 The 'Z‑test' in protection coordination involves:
Comparing relay time‑current curves
Measuring fault impedance
Testing transformer tap positions
Testing circuit breaker speed
Explanation - Z‑test checks that relay curves are coordinated for selective operation.
Correct answer is: Comparing relay time‑current curves
Q.137 In a 132 kV system, a fault impedance of 0.02 p.u. results in a fault current of:
10 kA
5 kA
20 kA
15 kA
Explanation - I_fault = 1/(0.02) = 50 p.u.; for 1 MVA base equals 10 kA.
Correct answer is: 10 kA
Q.138 An 'instantaneous relay' will trip:
After a set delay
Immediately when the set current is exceeded
Only after a fault persists for 10 s
Only when voltage drops below a threshold
Explanation - Instantaneous relays have zero delay and trip on any overcurrent.
Correct answer is: Immediately when the set current is exceeded
Q.139 The primary function of an 'earth fault relay' is to:
Detect voltage sags
Detect overcurrent between phases
Detect imbalance between phase and neutral currents
Detect frequency dips
Explanation - It senses the difference indicating an earth fault.
Correct answer is: Detect imbalance between phase and neutral currents
Q.140 The 'fault clearing time' in a power system is critical because:
It determines the system frequency
It limits the duration of a fault to protect equipment
It controls transformer tap changes
It sets the voltage regulation
Explanation - Short fault clearing times prevent equipment damage.
Correct answer is: It limits the duration of a fault to protect equipment
Q.141 In a fault analysis, the 'source impedance' is typically expressed in:
Ohms
Per unit
Volts
Amps
Explanation - Per‑unit representation simplifies fault calculations.
Correct answer is: Per unit
Q.142 A 'bolted fault' has:
High impedance
Low impedance
Moderate impedance
Variable impedance
Explanation - Bolted faults short directly, so impedance is very low.
Correct answer is: Low impedance
Q.143 Which protective device is most suitable for a 69 kV feeder to isolate a fault?
Fuse
Circuit breaker
Surge arrester
Ground fault relay
Explanation - Circuit breakers can interrupt high fault currents typical of 69 kV feeders.
Correct answer is: Circuit breaker
Q.144 The 'fault current' in a 33 kV system with source impedance 0.04 p.u. and line impedance 0.12 p.u. is approximately:
5 kA
10 kA
15 kA
20 kA
Explanation - I_fault = 1/(0.16) ≈ 6.25 p.u.; for a 1 MVA base this is about 10 kA.
Correct answer is: 10 kA
Q.145 Which of the following is a type of fault that involves three phases simultaneously?
Single line‑to‑ground fault
Line‑to‑line fault
Three‑phase fault
Double line‑to‑ground fault
Explanation - All three phases are shorted together in a three‑phase fault.
Correct answer is: Three‑phase fault
Q.146 In a protection scheme, the 'relay' is typically connected to:
The source only
The load only
Both the source and load
Neither source nor load
Explanation - Relays monitor currents in both directions to detect faults.
Correct answer is: Both the source and load
Q.147 A 'fault clearing time' that exceeds the relay’s operating time will result in:
Successful fault isolation
Equipment damage
No effect
Reduced system frequency
Explanation - Delays in clearing a fault can lead to overheating and failure.
Correct answer is: Equipment damage
Q.148 The 'fault impedance' of a transmission line is usually expressed in:
Per unit
Ohms
Volts
Amps
Explanation - Per‑unit values are used for ease of calculation.
Correct answer is: Per unit
Q.149 An 'over‑current relay' is most sensitive to:
Voltage level
Current level
Frequency
Neutral temperature
Explanation - Over‑current relays trigger when current exceeds the set level.
Correct answer is: Current level
Q.150 A 'distance relay' calculates its setting based on:
Voltage only
Current only
Apparent impedance
Frequency only
Explanation - Distance relays use V/I ratio to determine apparent impedance to the fault.
Correct answer is: Apparent impedance
Q.151 Which of the following is NOT typically a parameter in a time‑overcurrent relay setting?
Pickup current
Trip current
Delay time
Voltage sensitivity
Explanation - Time‑overcurrent relays are set based on current parameters, not voltage.
Correct answer is: Voltage sensitivity
Q.152 A 'fault clearing time' of less than 5 s is considered:
Slow
Moderate
Fast
Unacceptable
Explanation - Fast clearing times are desirable to limit fault impact.
Correct answer is: Fast
Q.153 In a 345 kV network, the fault current at a bus with a source impedance of 0.02 p.u. and line impedance of 0.08 p.u. is:
10 kA
5 kA
15 kA
20 kA
Explanation - I_fault = 1/(0.10) = 10 kA on a 1 MVA base.
Correct answer is: 10 kA
Q.154 In protection coordination, the upstream relay’s time delay should be:
Shorter than the downstream relay
Equal to the downstream relay
Longer than the downstream relay
Irrelevant to the downstream relay
Explanation - Upstream devices should act after downstream devices for selective tripping.
Correct answer is: Longer than the downstream relay
Q.155 The 'fault current' is calculated by dividing:
Voltage by impedance
Impedance by voltage
Voltage by current
Current by voltage
Explanation - Short‑circuit current = V/Z (in per‑unit or actual units).
Correct answer is: Voltage by impedance
Q.156 Which of the following best describes a 'bolted fault' in a transmission line?
A fault with high impedance
A fault with negligible impedance
A fault with moderate impedance
A fault that never occurs
Explanation - Bolted faults have very low impedance, resulting in high fault currents.
Correct answer is: A fault with negligible impedance
Q.157 The primary function of a 'circuit breaker' in a power system is to:
Detect faults
Interrupt fault currents
Regulate voltage
Measure frequency
Explanation - Circuit breakers disconnect the circuit to isolate faults.
Correct answer is: Interrupt fault currents
Q.158 Which of the following is a typical fault type in a 110 kV transmission line?
Line‑to‑line fault
Single line‑to‑ground fault
Three‑phase fault
All of the above
Explanation - All listed faults can occur in a 110 kV line.
Correct answer is: All of the above
Q.159 In a fault analysis, the 'source impedance' is typically measured in:
Ohms
Per unit
Volts
Amps
Explanation - Per‑unit representation is standard for fault analysis.
Correct answer is: Per unit
Q.160 Which of the following best explains the term 'selective tripping'?
All relays trip simultaneously
Only the nearest relay to the fault trips
Relays do not coordinate
Relays trip only on over‑voltage
Explanation - Selective tripping isolates the faulted portion only.
Correct answer is: Only the nearest relay to the fault trips