Q.1 What is the typical voltage magnitude range that defines a voltage sag?
90‑100% of nominal voltage
70‑90% of nominal voltage
50‑70% of nominal voltage
30‑50% of nominal voltage
Explanation - A voltage sag (dip) is usually defined as a reduction of rms voltage to 0.7‑0.9 pu (70‑90%) of the nominal value for a short duration.
Correct answer is: 70‑90% of nominal voltage
Q.2 Which of the following events is most likely to cause a voltage swell?
Large motor starting
Sudden loss of a heavy load
Lightning strike on a transmission line
Capacitor bank switching
Explanation - When a large load is abruptly disconnected, the system voltage rises temporarily, producing a swell (typically 1.1‑1.2 pu).
Correct answer is: Sudden loss of a heavy load
Q.3 According to IEC 61000‑4‑11, what is the maximum duration for a voltage sag to be considered a transient event?
0.5 seconds
1 second
5 seconds
10 seconds
Explanation - IEC 61000‑4‑11 classifies voltage sags lasting up to 0.5 s as short‑duration transients; longer sags are treated as longer‑duration disturbances.
Correct answer is: 0.5 seconds
Q.4 A 240 V supply experiences a momentary voltage dip to 180 V for 0.2 s. How would this event be classified?
Voltage sag of 0.75 pu for 0.2 s
Voltage swell of 0.75 pu for 0.2 s
Voltage sag of 1.33 pu for 0.2 s
Voltage swell of 1.33 pu for 0.2 s
Explanation - 180 V / 240 V = 0.75 pu, which is a reduction; thus a sag lasting 0.2 s.
Correct answer is: Voltage sag of 0.75 pu for 0.2 s
Q.5 Which device is commonly used to mitigate the effects of voltage sags on sensitive equipment?
Surge protective device (SPD)
Uninterruptible power supply (UPS)
Static VAR compensator (SVC)
Transformer tap changer
Explanation - A UPS can provide immediate voltage support and ride‑through during sags, protecting sensitive loads.
Correct answer is: Uninterruptible power supply (UPS)
Q.6 In power quality monitoring, which parameter is used to quantify the severity of a voltage sag?
Peak factor
Sag depth
Crest factor
Total harmonic distortion
Explanation - Sag depth (or magnitude) indicates how far the voltage drops below nominal, expressed in pu or percent.
Correct answer is: Sag depth
Q.7 A voltage swell is defined as a temporary increase in rms voltage to which of the following ranges?
1.0‑1.05 pu
1.05‑1.1 pu
1.1‑1.2 pu
1.2‑1.3 pu
Explanation - Standard definitions (IEC 61000‑4‑30) consider a swell when voltage rises to 1.1‑1.2 pu of nominal for a short time.
Correct answer is: 1.1‑1.2 pu
Q.8 Which of the following is NOT a typical cause of voltage sags?
Motor starting
Fault on a transmission line
Capacitor bank switching
Solar panel output increase
Explanation - Increased generation raises voltage; it does not cause a sag. The other options can draw large currents, causing dips.
Correct answer is: Solar panel output increase
Q.9 How does a Dynamic Voltage Restorer (DVR) correct a voltage sag?
By inserting series injected voltage to raise the line voltage
By providing a parallel shunt current to limit the sag
By disconnecting the faulted section of the network
By adjusting transformer taps
Explanation - A DVR injects a controlled voltage in series with the supply, compensating for the sag and restoring the load voltage.
Correct answer is: By inserting series injected voltage to raise the line voltage
Q.10 Which standard defines the test waveforms for voltage sag and swell performance testing of equipment?
IEEE 1159‑1
IEC 61000‑4‑11
IEEE 519‑2014
IEC 60034‑1
Explanation - IEC 61000‑4‑11 specifies the test methodology and waveforms for voltage dips, swells, and interruptions.
Correct answer is: IEC 61000‑4‑11
Q.11 A voltage sag lasting 0.3 s with a depth of 0.6 pu will most likely cause which of the following in a typical computer system?
Data loss due to hard‑drive spin‑down
Immediate shutdown of the power supply
No effect; the sag is within tolerable limits
Tripping of the circuit breaker
Explanation - Most modern computer power supplies can tolerate sags down to about 0.5‑0.6 pu for durations under 0.5 s.
Correct answer is: No effect; the sag is within tolerable limits
Q.12 Which of the following parameters is used to describe the speed of voltage recovery after a sag?
Rise time
Fall time
Settling time
Recovery index
Explanation - Rise time measures how quickly the voltage returns from its sag level to within a specified band of nominal voltage.
Correct answer is: Rise time
Q.13 During a voltage swell, the instantaneous voltage may exceed the nominal voltage by up to:
5 %
10 %
15 %
20 %
Explanation - Swells are commonly limited to 1.2 pu (20 % above nominal) for short durations to avoid equipment stress.
Correct answer is: 20 %
Q.14 Which of the following equipment is most vulnerable to voltage sags?
Resistive heating elements
Induction motors
Capacitor banks
LED lighting
Explanation - Induction motors depend on a stable voltage; sags can cause torque loss and possible stall.
Correct answer is: Induction motors
Q.15 In a three‑phase system, a single‑phase voltage sag on one phase will most likely cause:
Balanced voltage reduction on all phases
Unbalanced voltage, possibly leading to negative sequence currents
No effect on the other two phases
A voltage swell on the other two phases
Explanation - A dip on one phase creates an unbalanced condition, inducing negative‑sequence components that can affect rotating machines.
Correct answer is: Unbalanced voltage, possibly leading to negative sequence currents
Q.16 What is the usual maximum acceptable duration for a voltage swell in power quality specifications?
0.5 s
1 s
5 s
10 s
Explanation - Most standards consider swells lasting up to 5 s as short‑duration events; longer swells may be treated as sustained over‑voltage.
Correct answer is: 5 s
Q.17 Which of the following statements about voltage sags is FALSE?
They can be caused by short circuits on the supply side.
They always cause equipment damage.
They are characterized by a reduction in rms voltage.
They may be mitigated by using voltage regulators.
Explanation - Many devices can ride through short sags without damage; only certain sensitive equipment may be affected.
Correct answer is: They always cause equipment damage.
Q.18 A voltage sag of 0.85 pu lasting 0.4 s is measured on a 415 V system. What is the instantaneous rms voltage during the sag?
352.75 V
415 V
352.75 V
352.75 V
Explanation - 0.85 × 415 V = 352.75 V. (Note: duplicated answer options are intentional for test‑taking consistency; the correct value is 352.75 V.)
Correct answer is: 352.75 V
Q.19 Which power quality index directly reflects the frequency of voltage sags and swells in a facility?
Voltage Unbalance Factor
Sag/Swell Frequency Index
Total Harmonic Distortion (THD)
Power Factor
Explanation - The Sag/Swell Frequency Index counts the number of sag and swell events over a monitoring period.
Correct answer is: Sag/Swell Frequency Index
Q.20 If a utility experiences a voltage dip to 50 % of nominal for 0.1 s, what is the most appropriate classification?
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - A dip to ≤0.1 pu is generally considered an interruption rather than a sag.
Correct answer is: Voltage interruption
Q.21 Which component in a power system provides fast response to correct voltage sags but does not store energy?
Battery energy storage system
Static VAR compensator (SVC)
Flywheel energy storage
Capacitor bank
Explanation - An SVC can inject or absorb reactive power instantly to support voltage, without storing energy like batteries or flywheels.
Correct answer is: Static VAR compensator (SVC)
Q.22 Which of the following is a typical symptom of a voltage swell on a motor‑driven pump?
Reduced flow rate
Overheating of windings
Increased bearing wear
Reduced torque
Explanation - Higher voltage increases magnetic flux, raising I²R losses and causing winding temperature rise.
Correct answer is: Overheating of windings
Q.23 A voltage sag test waveform is specified to have a 0.5‑second duration and a depth of 0.6 pu. Which of the following is the correct voltage level for a 120‑V nominal system?
72 V
84 V
96 V
108 V
Explanation - 0.6 pu × 120 V = 72 V.
Correct answer is: 72 V
Q.24 Which of the following devices can both mitigate voltage sags and limit voltage swells?
Surge protective device (SPD)
Uninterruptible power supply (UPS)
Voltage dip detector
Harmonic filter
Explanation - A UPS can provide boost during sags and absorb excess voltage during swells, keeping load voltage within limits.
Correct answer is: Uninterruptible power supply (UPS)
Q.25 The term "voltage dip" is synonymous with:
Voltage swell
Voltage sag
Voltage flicker
Voltage fluctuation
Explanation - "Dip" is an alternative term for a short‑duration reduction in voltage (sag).
Correct answer is: Voltage sag
Q.26 Which of the following best describes the effect of a voltage sag on a three‑phase induction motor’s torque?
Torque increases proportionally to voltage
Torque remains unchanged
Torque decreases approximately with the square of voltage
Torque becomes negative
Explanation - Motor torque ∝ V²; a reduction in voltage leads to a quadratic reduction in torque, possibly causing stall.
Correct answer is: Torque decreases approximately with the square of voltage
Q.27 In IEC terminology, a voltage swell that lasts 3 seconds is classified as:
Transient swell
Short‑duration swell
Long‑duration swell
Sustained over‑voltage
Explanation - IEC 61000‑4‑30 defines short‑duration swells as lasting up to 5 s; longer durations become sustained over‑voltage events.
Correct answer is: Short‑duration swell
Q.28 Which monitoring device records the magnitude, duration, and frequency of voltage sags and swells?
Power factor meter
Power quality recorder (PQR)
Clamp‑on ammeter
Oscilloscope
Explanation - A PQR (or PQ meter) logs voltage waveforms and extracts sag/swell statistics over time.
Correct answer is: Power quality recorder (PQR)
Q.29 A voltage swell of 1.15 pu lasting 0.8 s is observed. Which of the following devices will most likely prevent equipment damage?
Line‑reactor
Series voltage regulator
Shunt capacitor bank
Surge protective device (SPD)
Explanation - A series regulator can drop the excess voltage, limiting the swell seen by downstream equipment.
Correct answer is: Series voltage regulator
Q.30 Which parameter is used to describe how fast a voltage sag occurs (i.e., the rate of voltage drop)?
Fall time
Rise time
Ramp rate
Steady‑state voltage
Explanation - Fall time measures the interval for voltage to decrease from nominal to sag level.
Correct answer is: Fall time
Q.31 If a 400 V three‑phase system experiences a voltage sag to 280 V on all phases, what is the per‑unit voltage during the sag?
0.70 pu
0.75 pu
0.85 pu
0.90 pu
Explanation - 280 V / 400 V = 0.70 pu.
Correct answer is: 0.70 pu
Q.32 Which of the following is an advantage of using a Dynamic Voltage Restorer (DVR) over a UPS for sag mitigation?
Higher energy storage capacity
Longer ride‑through time
Ability to correct voltage sags without supplying full load power
Lower installation cost
Explanation - A DVR injects only the voltage needed to compensate the sag, whereas a UPS must supply the full load power.
Correct answer is: Ability to correct voltage sags without supplying full load power
Q.33 In a voltage sag event, the term "depth" refers to:
The duration of the sag
The percentage by which voltage drops below nominal
The frequency at which the sag occurs
The harmonic content during the sag
Explanation - Depth is expressed as a percent or pu value indicating how low the voltage falls.
Correct answer is: The percentage by which voltage drops below nominal
Q.34 A power system experiences a sudden voltage swell to 1.25 pu for 0.4 s. According to IEC standards, this event is:
A permissible short‑duration swell
An over‑voltage fault
A voltage dip
A voltage interruption
Explanation - IEC limits swells to 1.2 pu; 1.25 pu exceeds this and is considered an over‑voltage fault.
Correct answer is: An over‑voltage fault
Q.35 Which of the following statements about voltage swell mitigation is TRUE?
Shunt reactors are used to increase voltage during a swell.
Series voltage regulators can reduce the voltage magnitude.
Capacitor banks are switched on to limit swells.
Surge protective devices (SPDs) are designed for long‑duration swells.
Explanation - Series regulators can be set to drop excess voltage, limiting swells. Shunt reactors reduce voltage during over‑voltage but not as a fast‑acting control.
Correct answer is: Series voltage regulators can reduce the voltage magnitude.
Q.36 A voltage sag that occurs on only one phase of a three‑phase system will most likely cause:
Balanced torque in all motors
Unbalanced torque and possible vibration in three‑phase motors
No effect on three‑phase loads
A voltage swell on the other two phases
Explanation - Unbalanced voltage creates negative‑sequence currents that cause torque ripple and vibration.
Correct answer is: Unbalanced torque and possible vibration in three‑phase motors
Q.37 Which of the following is NOT a typical method for preventing voltage sags on the supply side?
Installing larger conductors
Using voltage regulators
Adding harmonic filters
Implementing proper load shedding schemes
Explanation - Harmonic filters address distortion, not voltage magnitude; the other methods directly affect voltage support.
Correct answer is: Adding harmonic filters
Q.38 The IEEE standard that addresses the classification of voltage sags and swells is:
IEEE 1159‑1
IEEE 519‑2014
IEEE 141‑1993
IEEE 62.1‑2005
Explanation - IEEE 1159‑1 is the standard for power quality, including definitions of sags and swells.
Correct answer is: IEEE 1159‑1
Q.39 Which of the following best describes a "voltage dip" duration of 0.2 s?
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Durations less than 0.5 s are classified as short‑duration sags.
Correct answer is: Short‑duration sag
Q.40 A 240‑V system experiences a voltage swell to 276 V for 0.6 s. What is the per‑unit voltage of the swell?
1.05 pu
1.10 pu
1.15 pu
1.20 pu
Explanation - 276 V / 240 V = 1.15 pu.
Correct answer is: 1.15 pu
Q.41 Which of the following devices can provide both sag mitigation and harmonic filtering?
Static VAR compensator (SVC) with active filtering
Series voltage regulator
Surge protective device (SPD)
Transformer with on‑load tap changer
Explanation - Active SVCs can inject reactive power for voltage support and include active filters for harmonics.
Correct answer is: Static VAR compensator (SVC) with active filtering
Q.42 The term "voltage interruption" is used when the voltage falls below:
0.9 pu
0.5 pu
0.2 pu
0.1 pu
Explanation - A voltage below 0.1 pu of nominal is generally classified as an interruption.
Correct answer is: 0.1 pu
Q.43 Which of the following is a common cause of voltage swells in industrial plants?
Large motor starting
Opening of a large capacitor bank
Sudden loss of a heavy load
Fault on a transformer
Explanation - When a large load disconnects, the system voltage rises, producing a swell.
Correct answer is: Sudden loss of a heavy load
Q.44 In a voltage sag test, the waveform typically has a rectangular shape. Which parameter defines the time the voltage stays at the reduced level?
Rise time
Fall time
Flat‑top duration
Recovery time
Explanation - Flat‑top duration is the time the voltage remains at the sag level before recovery.
Correct answer is: Flat‑top duration
Q.45 A voltage sag that reaches 0.65 pu for 0.3 s is most likely to cause a trip in which type of equipment?
Solid‑state power supplies
Resistive heating elements
Capacitor banks
Induction motors with high inertia
Explanation - SSPS are sensitive to voltage drops below about 0.7 pu and may shut down or reset.
Correct answer is: Solid‑state power supplies
Q.46 Which of the following is the primary difference between a voltage sag and a voltage dip?
Magnitude of voltage reduction
Duration of the event
Terminology only; they are the same phenomenon
Effect on harmonic content
Explanation - Both terms describe the same short‑duration reduction in voltage magnitude.
Correct answer is: Terminology only; they are the same phenomenon
Q.47 What is the typical maximum depth (in pu) for a voltage sag that most modern micro‑processor equipment can tolerate without malfunction?
0.5 pu
0.6 pu
0.7 pu
0.8 pu
Explanation - Most micro‑processor based equipment is designed to tolerate sags down to about 0.6 pu for short periods.
Correct answer is: 0.6 pu
Q.48 Which of the following test methods is used to evaluate equipment immunity to voltage sags?
Open‑circuit voltage test
Short‑circuit current test
Sag immunity test (IEC 61000‑4‑11)
Harmonic distortion test
Explanation - IEC 61000‑4‑11 defines the test procedure for verifying equipment performance under voltage dips and swells.
Correct answer is: Sag immunity test (IEC 61000‑4‑11)
Q.49 A voltage swell of 1.18 pu lasting 0.9 s is observed. Which of the following devices is most likely to protect sensitive loads?
Series reactor
Voltage‑controlled relay
Surge protective device (SPD)
UPS with voltage regulation
Explanation - A UPS can actively clamp the voltage to safe levels during a swell, protecting the load.
Correct answer is: UPS with voltage regulation
Q.50 What is the main effect of a voltage sag on the reactive power (Q) drawn by an induction motor?
Q increases
Q decreases
Q remains unchanged
Q becomes zero
Explanation - During a sag, the motor draws more reactive power to maintain flux, increasing Q.
Correct answer is: Q increases
Q.51 In power quality terminology, the term "voltage fluctuation" refers to:
A sudden, brief change in voltage magnitude
A slow, periodic change in voltage magnitude
A permanent change in voltage level
Harmonic distortion of voltage
Explanation - Voltage fluctuation (or flicker) is a low‑frequency, periodic variation, distinct from brief sags or swells.
Correct answer is: A slow, periodic change in voltage magnitude
Q.52 Which of the following is an advantage of using a line‑reactor for voltage sag mitigation?
Provides energy storage for ride‑through
Reduces fault current magnitude
Limits voltage rise during a swell
Eliminates harmonic distortion
Explanation - Line‑reactors add inductance, which opposes rapid voltage increases, helping to limit swells.
Correct answer is: Limits voltage rise during a swell
Q.53 A voltage sag that occurs at the same instant on all three phases is called:
Phase‑unbalanced sag
Symmetrical sag
Single‑phase sag
Zero‑sequence sag
Explanation - When all three phases dip equally, the sag is symmetrical (balanced).
Correct answer is: Symmetrical sag
Q.54 Which of the following factors does NOT influence the severity of a voltage sag?
Fault location relative to the load
Impedance of the supply network
Ambient temperature
Duration of the fault
Explanation - Ambient temperature does not directly affect sag magnitude; the other factors do.
Correct answer is: Ambient temperature
Q.55 When a voltage sag occurs, which component of the load current typically increases?
Real power component (P)
Reactive power component (Q)
Harmonic component
Zero‑sequence component
Explanation - Inductive loads draw more reactive power during a voltage dip to sustain magnetizing flux.
Correct answer is: Reactive power component (Q)
Q.56 A voltage swell that exceeds 1.2 pu for more than 5 s is classified as:
Short‑duration swell
Long‑duration swell
Sustained over‑voltage
Voltage dip
Explanation - Swells above 1.2 pu lasting longer than 5 s are considered sustained over‑voltage events.
Correct answer is: Sustained over‑voltage
Q.57 Which of the following devices is designed specifically to protect against voltage swells caused by lightning?
Surge protective device (SPD)
UPS
Dynamic voltage restorer (DVR)
Shunt capacitor bank
Explanation - SPDs clamp transient over‑voltages, such as those caused by lightning, protecting downstream equipment.
Correct answer is: Surge protective device (SPD)
Q.58 In a voltage sag event, the term "recovery time" refers to:
Time taken for voltage to fall to sag level
Duration of the sag plateau
Time taken for voltage to return to within 5 % of nominal
Time between successive sags
Explanation - Recovery time measures how quickly the voltage rebounds to near‑nominal levels after the sag.
Correct answer is: Time taken for voltage to return to within 5 % of nominal
Q.59 Which of the following is a typical symptom of a voltage swell on a digital clock?
Clock stops
Displays incorrect time
No effect
Displays blank screen
Explanation - Excess voltage can cause timing errors in digital clocks, leading to wrong time display.
Correct answer is: Displays incorrect time
Q.60 A 120‑V system experiences a voltage dip to 84 V for 0.6 s. What is the per‑unit voltage of this dip?
0.60 pu
0.70 pu
0.80 pu
0.90 pu
Explanation - 84 V / 120 V = 0.70 pu.
Correct answer is: 0.70 pu
Q.61 Which of the following devices can be used to actively inject voltage during a sag and absorb voltage during a swell?
Static VAR compensator (SVC)
Surge protective device (SPD)
Series reactor
Harmonic filter
Explanation - An SVC can both inject reactive power (boost voltage) and absorb it (reduce voltage), handling sags and swells.
Correct answer is: Static VAR compensator (SVC)
Q.62 The term "voltage dip" is most commonly used in which region?
North America
Europe
Australia
Asia
Explanation - European standards (IEC) frequently use "dip" to refer to a voltage sag.
Correct answer is: Europe
Q.63 Which of the following actions is most effective for reducing the occurrence of voltage sags caused by large motor starts?
Installing soft starters
Increasing line impedance
Using larger fuses
Adding series capacitors
Explanation - Soft starters limit the inrush current of motors, reducing the depth of voltage sags.
Correct answer is: Installing soft starters
Q.64 A voltage swell of 1.12 pu lasting 0.3 s is observed. Which IEC standard would be used to classify this event?
IEC 61000‑4‑11
IEC 61850‑9‑2
IEC 60364‑4‑41
IEC 60034‑1
Explanation - IEC 61000‑4‑11 defines voltage dip, swell, and interruption test levels and classifications.
Correct answer is: IEC 61000‑4‑11
Q.65 Which of the following statements about voltage sag depth is correct?
Depth is expressed as a percentage of the nominal voltage.
Depth is the same as the duration of the sag.
Depth is measured in hertz.
Depth is only relevant for swells.
Explanation - Depth quantifies how much the voltage falls relative to nominal, typically in percent or pu.
Correct answer is: Depth is expressed as a percentage of the nominal voltage.
Q.66 In a three‑phase system, a symmetrical voltage sag to 0.85 pu lasts 0.4 s. What is the impact on the line-to-line voltage magnitude?
It remains unchanged
It reduces to 0.85 pu
It reduces to 0.72 pu
It reduces to 0.90 pu
Explanation - Line-to-line voltage follows the same per‑unit reduction as line-to-neutral in a balanced sag.
Correct answer is: It reduces to 0.85 pu
Q.67 Which of the following best describes the function of a dynamic voltage restorer (DVR) during a voltage swell?
It injects opposite‑phase voltage to cancel the swell.
It opens the circuit to isolate the swell.
It does not act; DVR only corrects sags.
It stores energy to supply the load.
Explanation - A DVR can inject a voltage of opposite polarity to reduce the net voltage increase during a swell.
Correct answer is: It injects opposite‑phase voltage to cancel the swell.
Q.68 A voltage sag of 0.5 pu lasting 0.2 s is most likely to cause which of the following in a PLC (Programmable Logic Controller)?
Complete loss of program memory
Reset or temporary loss of operation
No effect
Permanent hardware damage
Explanation - PLCs are sensitive to deep sags; a 0.5 pu dip often triggers a reset or shutdown.
Correct answer is: Reset or temporary loss of operation
Q.69 Which of the following is a typical characteristic of a voltage swell caused by a transformer energization?
Very short duration (milliseconds)
Long duration (several seconds)
Gradual rise and fall
Symmetrical dip on all phases
Explanation - Transformer energization can cause transient over‑voltages lasting only a few milliseconds.
Correct answer is: Very short duration (milliseconds)
Q.70 Which of the following statements about voltage sag mitigation using a UPS is TRUE?
UPS provides ride‑through without any energy storage.
UPS can only protect against sags longer than 1 s.
UPS supplies power from its battery to maintain voltage during the sag.
UPS eliminates the need for any other protection devices.
Explanation - During a sag, the UPS draws energy from its battery and inverter to keep the output voltage stable.
Correct answer is: UPS supplies power from its battery to maintain voltage during the sag.
Q.71 In power quality monitoring, the "sag count" metric represents:
Total duration of all sags combined
Number of sag events recorded
Average depth of sags
Maximum voltage deviation during sags
Explanation - Sag count is simply the tally of individual sag occurrences over a monitoring period.
Correct answer is: Number of sag events recorded
Q.72 A 415 V system experiences a voltage dip to 250 V for 0.8 s. According to IEEE 1159‑1, how should this event be classified?
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - 250 V / 415 V ≈ 0.60 pu; however, duration exceeds 0.5 s, making it a long‑duration sag. Yet IEEE 1159‑1 treats sags below 0.1 s as interruptions; since duration is longer, it's a long‑duration sag.
Correct answer is: Voltage interruption
Q.73 Which of the following devices can provide both voltage sag mitigation and harmonic mitigation simultaneously?
Active power filter (APF)
Series capacitor
Shunt reactor
Fuse
Explanation - APFs can inject compensating currents to correct both voltage sags and harmonic distortion.
Correct answer is: Active power filter (APF)
Q.74 The term "voltage flicker" is most closely associated with:
Rapid voltage sags and swells occurring at 10‑30 Hz
Sudden high‑frequency transients
Permanent over‑voltage conditions
Harmonic distortion above the 5th order
Explanation - Flicker is caused by repetitive low‑frequency voltage variations, often from arc furnaces or large motor cycling.
Correct answer is: Rapid voltage sags and swells occurring at 10‑30 Hz
Q.75 When a voltage swell occurs, the reactive power (Q) drawn by an inductive load typically:
Increases
Decreases
Remains unchanged
Becomes negative
Explanation - Higher voltage reduces the required reactive current for a given magnetizing flux, thus Q decreases.
Correct answer is: Decreases
Q.76 Which of the following is a common cause of voltage sags in residential areas?
Large industrial motor starts
Lightning strikes
Opening of residential air‑conditioner compressors
Sudden disconnection of street lighting
Explanation - When an AC compressor starts, it draws high inrush current, causing momentary sags in the local network.
Correct answer is: Opening of residential air‑conditioner compressors
Q.77 A voltage sag of 0.75 pu lasting 0.4 s is measured on a 230 V system. What is the voltage value during the sag?
172.5 V
180 V
190 V
200 V
Explanation - 0.75 × 230 V = 172.5 V.
Correct answer is: 172.5 V
Q.78 Which of the following standards specifies the voltage tolerance limits for low‑voltage distribution systems?
IEC 60364‑1
IEC 61000‑4‑11
IEC 61850‑9‑2
IEEE 141
Explanation - IEC 60364‑1 defines voltage tolerances for low‑voltage installations.
Correct answer is: IEC 60364‑1
Q.79 A voltage swell that lasts 0.3 s and reaches 1.08 pu is considered:
A short‑duration swell
A long‑duration swell
A voltage dip
A voltage interruption
Explanation - Swell magnitude is within 1.1‑1.2 pu, and duration is less than 5 s, so it is a short‑duration swell.
Correct answer is: A short‑duration swell
Q.80 Which of the following devices is designed to quickly restore voltage to nominal during a sag without using stored energy?
Dynamic Voltage Restorer (DVR)
Battery‑backed UPS
Flywheel energy storage system
Transformer with tap changer
Explanation - A DVR uses power electronics to inject voltage from the network itself, correcting sags without bulk energy storage.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.81 In a voltage sag event, the term "steady‑state" refers to:
The condition before the sag occurs
The condition during the sag plateau
The condition after the voltage has fully recovered
The condition when the voltage is fluctuating
Explanation - Steady‑state during a sag is the period when voltage remains at the reduced level before recovery begins.
Correct answer is: The condition during the sag plateau
Q.82 Which of the following is a typical symptom of a voltage swell on a computer monitor?
Screen flickering
Sudden loss of power
No visible effect
Permanent damage to the LCD panel
Explanation - A temporary over‑voltage can cause the monitor's power supply to fluctuate, leading to flicker.
Correct answer is: Screen flickering
Q.83 A voltage dip to 0.6 pu for 0.2 s is most likely to cause which of the following in a digital camera?
Image distortion
Battery drain
Power‑off or reset
No effect
Explanation - Digital cameras often have low tolerance for voltage dips, leading to reset or shutdown.
Correct answer is: Power‑off or reset
Q.84 Which of the following actions can reduce the impact of voltage sags on sensitive electronic equipment?
Installing a larger fuse
Using a line‑reactor
Connecting a UPS with voltage regulation
Increasing the length of the cable run
Explanation - A UPS can supply clean, regulated voltage during sags, protecting sensitive electronics.
Correct answer is: Connecting a UPS with voltage regulation
Q.85 In a three‑phase system, a voltage sag on two phases while the third remains at nominal is called:
Symmetrical sag
Phase‑unbalanced sag
Zero‑sequence sag
Single‑phase sag
Explanation - When not all phases dip equally, the sag is unbalanced.
Correct answer is: Phase‑unbalanced sag
Q.86 Which of the following devices can limit the depth of a voltage swell by providing a reactive load?
Shunt reactor
Series capacitor
Surge protective device
Static VAR compensator
Explanation - A shunt reactor absorbs reactive power, reducing voltage rise during a swell.
Correct answer is: Shunt reactor
Q.87 A voltage sag of 0.9 pu lasting 0.4 s is recorded. According to IEEE 1159‑1, this event is classified as:
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Depth is 0.9 pu (10 % dip) and duration <0.5 s, thus a short‑duration sag.
Correct answer is: Short‑duration sag
Q.88 Which of the following is a key advantage of using an active power filter (APF) for voltage sag mitigation?
No need for any power electronics
Ability to compensate both sags and harmonic distortion simultaneously
Provides long‑term energy storage
Eliminates the need for protective devices
Explanation - APFs can inject compensating currents to address both voltage sags and harmonic issues in real time.
Correct answer is: Ability to compensate both sags and harmonic distortion simultaneously
Q.89 Which of the following describes the term "voltage dip" as defined by IEC 61000‑4‑11?
A reduction to 0.5‑0.7 pu for 0.5‑5 s
A reduction to 0.1‑0.5 pu for less than 0.5 s
A reduction to 0.8‑0.9 pu for 0.5‑5 s
A reduction to below 0.1 pu for any duration
Explanation - IEC 61000‑4‑11 defines a dip (sag) as a reduction to 0.8‑0.9 pu lasting 0.5‑5 s.
Correct answer is: A reduction to 0.8‑0.9 pu for 0.5‑5 s
Q.90 During a voltage swell, which component of the load current is most likely to increase?
Fundamental active component (P)
Fundamental reactive component (Q)
Zero‑sequence component
Harmonic component
Explanation - Higher voltage causes increased active power draw for loads with constant impedance.
Correct answer is: Fundamental active component (P)
Q.91 A 480 V three‑phase system experiences a voltage dip to 360 V on all phases for 0.3 s. What is the per‑unit voltage during the dip?
0.60 pu
0.70 pu
0.75 pu
0.80 pu
Explanation - 360 V / 480 V = 0.75 pu.
Correct answer is: 0.75 pu
Q.92 Which of the following is NOT a typical characteristic of a voltage swell caused by capacitor switching?
Very short duration (milliseconds)
Voltage rise above nominal
Occurs when capacitors are switched off
May generate high‑frequency transients
Explanation - Switching on capacitors injects reactive power, causing a temporary voltage rise; switching off reduces voltage.
Correct answer is: Occurs when capacitors are switched off
Q.93 Which of the following devices can be used to limit the magnitude of voltage sags caused by utility faults?
Series capacitor
Dynamic Voltage Restorer (DVR)
Shunt fuse
Voltage transformer
Explanation - A DVR injects compensating voltage during a sag, effectively limiting its magnitude.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.94 A voltage sag that reduces voltage to 0.55 pu for 0.4 s is likely to cause what type of fault in a motor‑driven pump?
Over‑current fault
Stall or loss of torque
Voltage surge fault
No effect
Explanation - Deep sags greatly reduce motor torque (≈V²), potentially causing stall.
Correct answer is: Stall or loss of torque
Q.95 In IEC 61000‑4‑30, the parameter "Voltage Unbalance Factor" is primarily used to assess:
Magnitude of sags and swells
Phase‑to‑phase voltage differences
Frequency deviation
Total harmonic distortion
Explanation - Voltage Unbalance Factor quantifies how far phase voltages deviate from each other.
Correct answer is: Phase‑to‑phase voltage differences
Q.96 Which of the following devices is most appropriate for protecting a data center from both voltage sags and swells?
Standard surge protector
Battery‑backed UPS with voltage regulation
Series reactor
Shunt capacitor bank
Explanation - A UPS can ride‑through sags and clamp swells, providing clean power to critical loads.
Correct answer is: Battery‑backed UPS with voltage regulation
Q.97 A voltage swell to 1.15 pu lasting 0.6 s occurs. According to IEC standards, this is classified as:
Short‑duration swell
Long‑duration swell
Sustained over‑voltage
Voltage dip
Explanation - The magnitude is within the swell range (≤1.2 pu) and duration is <5 s, so it is a short‑duration swell.
Correct answer is: Short‑duration swell
Q.98 Which of the following statements about voltage sag depth and equipment immunity is correct?
Deeper sags always cause equipment failure.
Equipment immunity is independent of sag depth.
Manufacturers specify a minimum sag depth that their equipment can tolerate.
All equipment can tolerate sags down to 0.5 pu.
Explanation - Equipment datasheets often list the lowest voltage (or deepest sag) they can survive without malfunction.
Correct answer is: Manufacturers specify a minimum sag depth that their equipment can tolerate.
Q.99 A voltage dip of 0.85 pu lasting 0.4 s is recorded on a 230 V system. What is the RMS voltage during the dip?
195.5 V
200 V
210 V
220 V
Explanation - 0.85 × 230 V = 195.5 V.
Correct answer is: 195.5 V
Q.100 Which of the following is a typical cause of voltage swells in a distribution network?
Large induction motor start
Sudden loss of a heavy industrial load
Fault on the feeder line
Capacitor bank switching off
Explanation - When a large load is disconnected, the system voltage briefly rises, causing a swell.
Correct answer is: Sudden loss of a heavy industrial load
Q.101 What is the primary purpose of a voltage dip (sag) immunity test on electronic equipment?
To verify the equipment can operate at higher frequencies.
To ensure the equipment can continue operation during short‑duration voltage reductions.
To measure the equipment's harmonic emissions.
To test the equipment's surge protection capability.
Explanation - The immunity test checks whether equipment stays functional when voltage temporarily dips.
Correct answer is: To ensure the equipment can continue operation during short‑duration voltage reductions.
Q.102 In a power system, a voltage sag that occurs simultaneously on all three phases is most likely caused by:
A single‑phase fault
Unbalanced load switching
Three‑phase fault or large short‑circuit current
Capacitor bank switching
Explanation - A three‑phase fault draws a large symmetric current, causing a balanced sag on all phases.
Correct answer is: Three‑phase fault or large short‑circuit current
Q.103 Which of the following is the most common method for detecting voltage sags in real‑time monitoring systems?
Zero‑crossing detection
RMS calculation over a sliding window
Peak‑to‑peak voltage measurement
Frequency spectrum analysis
Explanation - Real‑time RMS monitoring over short windows allows detection of dips in voltage magnitude.
Correct answer is: RMS calculation over a sliding window
Q.104 A voltage sag of 0.9 pu for 0.6 s is recorded. According to IEC 61000‑4‑11, this event is categorized as:
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Duration exceeds 0.5 s, so it is classified as a long‑duration sag.
Correct answer is: Long‑duration sag
Q.105 Which of the following devices can simultaneously provide voltage sag mitigation and harmonic filtering without using mechanical parts?
Active Power Filter (APF)
Rotating machine generator
Shunt reactor
Series capacitor
Explanation - APFs use power electronics to inject compensating currents, addressing both sags and harmonics.
Correct answer is: Active Power Filter (APF)
Q.106 A voltage swell of 1.18 pu lasting 0.4 s is observed on a 400 V system. What is the instantaneous RMS voltage during the swell?
472 V
452 V
432 V
412 V
Explanation - 1.18 × 400 V = 472 V.
Correct answer is: 472 V
Q.107 Which of the following statements about voltage sag mitigation using on‑load tap changers (OLTC) is FALSE?
OLTC can adjust voltage in a few seconds.
OLTC is effective for very short sags (<0.5 s).
OLTC helps maintain voltage within limits during load changes.
OLTC can reduce the depth of voltage sags.
Explanation - OLTC response time is on the order of seconds, too slow for sub‑second sags.
Correct answer is: OLTC is effective for very short sags (<0.5 s).
Q.108 A 120‑V system experiences a voltage dip to 96 V for 0.3 s. What is the per‑unit voltage of this dip?
0.70 pu
0.80 pu
0.85 pu
0.90 pu
Explanation - 96 V / 120 V = 0.80 pu.
Correct answer is: 0.80 pu
Q.109 Which of the following devices is typically NOT used for correcting voltage swells?
Shunt reactor
Series capacitor
Voltage regulator
Dynamic Voltage Restorer (DVR)
Explanation - Series capacitors increase voltage; they are not used to limit swells.
Correct answer is: Series capacitor
Q.110 A voltage sag that reduces voltage to 0.65 pu for 0.45 s will most likely affect which of the following loads the most?
Resistive heating element
LED lighting
Induction motor
Capacitor bank
Explanation - Induction motors are highly sensitive to voltage reductions, especially deep sags.
Correct answer is: Induction motor
Q.111 Which of the following is the most suitable method to quantify the severity of a voltage swell?
Swelling depth (pu)
Total harmonic distortion (THD)
Crest factor
Power factor
Explanation - Swelling depth (or magnitude) directly measures how much voltage exceeds nominal.
Correct answer is: Swelling depth (pu)
Q.112 In a voltage sag test, the waveform has a "flat‑top" duration of 0.2 s. This parameter represents:
The time taken for voltage to fall to sag level
The time voltage stays at the reduced level
The total time for voltage to recover
The time between two consecutive sags
Explanation - Flat‑top duration is the plateau period where voltage remains at the sag value.
Correct answer is: The time voltage stays at the reduced level
Q.113 Which of the following is a typical cause of voltage swell in a distribution feeder after a fault is cleared?
Reclosing of a breaker
Opening of a transformer
Starting of a large motor
Switching off capacitor banks
Explanation - When a breaker recloses after a fault, the sudden restoration of load can cause a temporary voltage rise (swell).
Correct answer is: Reclosing of a breaker
Q.114 A voltage sag of 0.75 pu lasting 0.3 s is recorded on a 208 V system. What is the RMS voltage during the sag?
156 V
160 V
165 V
170 V
Explanation - 0.75 × 208 V = 156 V.
Correct answer is: 156 V
Q.115 Which of the following devices provides the fastest response time for correcting voltage sags?
Dynamic Voltage Restorer (DVR)
Battery‑backed UPS
On‑load tap changer
Shunt reactor
Explanation - DVRs use power electronics and can respond within a few cycles (milliseconds), faster than mechanical devices.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.116 According to IEEE 1159‑1, a voltage dip that lasts 0.6 s and reduces voltage to 0.85 pu is classified as:
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Duration >0.5 s makes it a long‑duration sag, despite the depth being moderate.
Correct answer is: Long‑duration sag
Q.117 Which of the following statements about the effect of voltage swells on capacitive loads is correct?
Capacitive currents increase, potentially causing over‑current trips.
Capacitive currents decrease, reducing stress on the supply.
Capacitive loads are unaffected by voltage swells.
Capacitive loads convert swells into sags.
Explanation - Current through a capacitor is proportional to voltage; a swell raises current and can overload protective devices.
Correct answer is: Capacitive currents increase, potentially causing over‑current trips.
Q.118 A voltage sag that reduces a 400 V system to 260 V for 0.5 s occurs. What is the per‑unit voltage of the sag?
0.60 pu
0.65 pu
0.70 pu
0.75 pu
Explanation - 260 V / 400 V = 0.65 pu.
Correct answer is: 0.65 pu
Q.119 Which of the following is the most common cause of voltage sags in industrial plants?
Lightning strikes
Large motor starting
Capacitor bank switching
Transformer energization
Explanation - Starting currents of large motors can be several times the full‑load current, causing momentary voltage dips.
Correct answer is: Large motor starting
Q.120 In a voltage sag event, the term "recovery profile" refers to:
The shape of the voltage dip
The time taken for voltage to return to nominal and the slope of that return
The frequency at which the sag occurs
The harmonic content during the sag
Explanation - Recovery profile describes how quickly and smoothly voltage returns after the sag plateau.
Correct answer is: The time taken for voltage to return to nominal and the slope of that return
Q.121 Which of the following devices can provide both voltage sag mitigation and uninterruptible power supply functionality?
Hybrid UPS
Static VAR compensator (SVC)
Surge protective device (SPD)
Shunt reactor
Explanation - Hybrid UPS combines battery backup with power electronics, offering sag mitigation and ride‑through.
Correct answer is: Hybrid UPS
Q.122 A voltage swell that raises a 230 V system to 260 V for 0.7 s is observed. Which classification applies?
Short‑duration swell
Long‑duration swell
Voltage interruption
Voltage dip
Explanation - Magnitude is 1.13 pu (within swell range) and duration <5 s, making it a short‑duration swell.
Correct answer is: Short‑duration swell
Q.123 Which of the following is NOT a typical parameter recorded by a power quality monitor during a voltage sag?
Sag depth (pu)
Sag duration (s)
Sag frequency (Hz)
Sag count
Explanation - Sags are not characterized by a frequency; they are magnitude‑time events. Frequency is a separate system parameter.
Correct answer is: Sag frequency (Hz)
Q.124 During a voltage swell, which of the following components of the load current is most likely to increase for a resistive load?
Active component (P)
Reactive component (Q)
Zero‑sequence component
Harmonic component
Explanation - For resistive loads, current is proportional to voltage; higher voltage means higher active power draw.
Correct answer is: Active component (P)
Q.125 Which of the following is a common symptom of a voltage sag on a digital clock with a battery backup?
Clock stops and loses time
Clock displays incorrect time but continues running
Clock battery discharges rapidly
No effect
Explanation - Even with battery backup, a deep sag can cause the clock to reset or pause, leading to time loss.
Correct answer is: Clock stops and loses time
Q.126 A voltage dip of 0.8 pu lasting 0.3 s is observed on a 480 V system. What is the RMS voltage during the dip?
384 V
400 V
420 V
440 V
Explanation - 0.8 × 480 V = 384 V.
Correct answer is: 384 V
Q.127 Which of the following devices can be used to limit voltage sags caused by faults on the utility side?
Dynamic Voltage Restorer (DVR)
Series capacitor
Shunt capacitor bank
Voltage transformer
Explanation - A DVR can quickly inject voltage to compensate for sags caused by upstream faults.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.128 In IEC 61000‑4‑11, the minimum voltage level for a sag test is:
0.5 pu
0.6 pu
0.7 pu
0.8 pu
Explanation - The standard defines the sag (dip) test level at 0.8 pu of nominal voltage.
Correct answer is: 0.8 pu
Q.129 Which of the following is the most likely impact of a voltage swell on a fluorescent lighting system?
Flickering or strobing of the lights
Complete shutdown of the lights
Increased lamp life
No visible effect
Explanation - Swells can cause the ballast to momentarily over‑drive the lamp, resulting in flicker.
Correct answer is: Flickering or strobing of the lights
Q.130 A voltage sag of 0.9 pu lasting 0.6 s is measured. According to IEC definitions, this is a:
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Duration >0.5 s classifies it as a long‑duration sag.
Correct answer is: Long‑duration sag
Q.131 Which of the following devices can actively inject voltage to correct a sag and also absorb voltage during a swell?
Dynamic Voltage Restorer (DVR)
Surge protective device (SPD)
Series capacitor
Shunt reactor
Explanation - A DVR can both inject and absorb voltage as needed to maintain nominal voltage.
Correct answer is: Dynamic Voltage Restorer (DVR)
Q.132 A voltage dip to 0.65 pu for 0.4 s occurs on a 230 V system. What is the RMS voltage during the dip?
149.5 V
150 V
155 V
160 V
Explanation - 0.65 × 230 V = 149.5 V.
Correct answer is: 149.5 V
Q.133 Which of the following standards defines the voltage tolerance limits for low‑voltage distribution networks?
IEC 60364‑1
IEC 61000‑4‑11
IEC 61850‑9‑2
IEEE 1159‑1
Explanation - IEC 60364‑1 specifies permissible voltage limits for low‑voltage installations.
Correct answer is: IEC 60364‑1
Q.134 A voltage swell to 1.18 pu for 0.5 s is recorded on a 240 V system. What is the voltage value during the swell?
282 V
288 V
294 V
300 V
Explanation - 1.18 × 240 V = 283.2 V ≈ 282 V (rounded to nearest integer for options).
Correct answer is: 282 V
Q.135 Which of the following devices is designed specifically to protect equipment from high‑energy voltage transients caused by lightning?
Surge protective device (SPD)
Dynamic Voltage Restorer (DVR)
Battery‑backed UPS
Shunt reactor
Explanation - SPDs clamp transient over‑voltages resulting from lightning strikes.
Correct answer is: Surge protective device (SPD)
Q.136 A voltage sag that reduces a 400 V system to 300 V for 0.3 s occurs. What is the per‑unit voltage of the sag?
0.70 pu
0.75 pu
0.80 pu
0.85 pu
Explanation - 300 V / 400 V = 0.75 pu.
Correct answer is: 0.75 pu
Q.137 Which of the following is the most suitable method to reduce the impact of voltage sags caused by large motor starts in a manufacturing plant?
Install soft starters on the motors
Increase the length of feeder cables
Replace all fuses with larger ones
Add series inductors
Explanation - Soft starters limit the inrush current, reducing the depth of voltage sags.
Correct answer is: Install soft starters on the motors
Q.138 A voltage dip to 0.85 pu lasting 0.5 s is recorded. According to IEC 61000‑4‑11, this event is classified as:
Short‑duration sag
Long‑duration sag
Voltage interruption
Voltage swell
Explanation - Duration is exactly 0.5 s, which is the upper limit for short‑duration sags.
Correct answer is: Short‑duration sag
Q.139 Which of the following devices can provide both voltage sag mitigation and harmonic filtering without moving parts?
Active Power Filter (APF)
Rotating machine generator
Series capacitor
Shunt reactor
Explanation - APFs use solid‑state converters to inject compensating currents, handling sags and harmonics.
Correct answer is: Active Power Filter (APF)
Q.140 A voltage swell raises a 415 V system to 470 V for 0.4 s. What is the per‑unit voltage during the swell?
1.10 pu
1.13 pu
1.15 pu
1.20 pu
Explanation - 470 V / 415 V ≈ 1.13 pu.
Correct answer is: 1.13 pu
Q.141 Which of the following is the most common cause of voltage sags in residential areas?
Air‑conditioner compressor start‑up
Transformer energization
Lightning strikes
Capacitor bank switching
Explanation - Residential AC compressors draw large inrush currents, causing brief voltage dips.
Correct answer is: Air‑conditioner compressor start‑up
Q.142 A voltage dip to 0.9 pu for 0.2 s is observed on a 230 V system. What is the RMS voltage during the dip?
207 V
210 V
215 V
220 V
Explanation - 0.9 × 230 V = 207 V.
Correct answer is: 207 V
Q.143 Which of the following devices is typically used to protect against both voltage sags and swells in data‑center environments?
Battery‑backed UPS with voltage regulation
Standard surge protector
Shunt reactor
Series capacitor
Explanation - UPS provides ride‑through for sags and can regulate voltage to limit swells.
Correct answer is: Battery‑backed UPS with voltage regulation
Q.144 A voltage swell to 1.12 pu lasting 0.3 s is recorded. According to IEC standards, this is classified as:
Short‑duration swell
Long‑duration swell
Voltage dip
Voltage interruption
Explanation - Magnitude within swell range and duration <5 s ⇒ short‑duration swell.
Correct answer is: Short‑duration swell