Q.1 What is the primary cause of voltage flicker in an electrical power system?
Harmonic distortion from non‑linear loads
Rapid changes in load power due to arcing devices
Ground fault currents
Over‑voltage caused by capacitor banks
Explanation - Flicker is produced when loads such as welding equipment or arc furnaces cause rapid, repeated changes in the instantaneous power demand, resulting in short‑term voltage fluctuations.
Correct answer is: Rapid changes in load power due to arcing devices
Q.2 Which IEC standard defines the measurement and assessment of voltage flicker?
IEC 61000-4-11
IEC 61000-4-15
IEC 61850-7-4
IEC 60364-4-41
Explanation - IEC 61000‑4‑15 specifies the procedures for measuring, evaluating and reporting flicker in power supply systems.
Correct answer is: IEC 61000-4-15
Q.3 In flicker measurement, the short‑term flicker severity index is denoted by:
Pst
Plt
Vf
THD
Explanation - Pst represents the short‑term flicker severity, calculated over a 10‑minute interval according to IEC 61000‑4‑15.
Correct answer is: Pst
Q.4 What is the typical acceptable limit for Pst in most European power quality regulations?
0.5
1.0
2.5
5.0
Explanation - Many standards, such as EN 50160, set the maximum permissible short‑term flicker severity (Pst) at 1.0 for general public lighting.
Correct answer is: 1.0
Q.5 Which type of load is most likely to produce severe flicker in an industrial plant?
Resistive heating elements
Constant‑speed fans
Arc‑welding machines
LED lighting
Explanation - Arc‑welding machines draw highly variable currents during the welding cycle, creating large, rapid voltage drops that cause flicker.
Correct answer is: Arc‑welding machines
Q.6 The flicker meter simulates the human eye‑brain response using which filter?
Low‑pass Butterworth filter
Weighted moving‑average filter
A‑weighted filter
Human‑vision model filter (flicker‑sensitivity curve)
Explanation - Flicker meters incorporate a weighting filter that mimics the sensitivity of the human eye to voltage fluctuations, based on the flicker‑sensitivity curve.
Correct answer is: Human‑vision model filter (flicker‑sensitivity curve)
Q.7 What does the long‑term flicker severity index (Plt) represent?
Maximum flicker observed in a 10‑minute window
Average of the square of Pst values over 2 hours
Root‑mean‑square of Pst values over a 2‑hour period
Peak value of voltage deviation during a fault
Explanation - Plt is calculated as the RMS value of successive Pst measurements taken over a 2‑hour interval, providing a longer‑term assessment of flicker severity.
Correct answer is: Root‑mean‑square of Pst values over a 2‑hour period
Q.8 Which of the following mitigation techniques can effectively reduce flicker caused by a large motor starting?
Installing a harmonic filter
Using a soft‑starter or variable‑frequency drive (VFD)
Adding more lighting loads
Increasing line impedance
Explanation - Soft‑starters and VFDs control the inrush current of motors, smoothing the voltage dip and thus reducing flicker.
Correct answer is: Using a soft‑starter or variable‑frequency drive (VFD)
Q.9 In the flicker calculation model, the time constant of the weighting filter is approximately:
0.1 seconds
1 second
10 seconds
0.5 seconds
Explanation - The weighting filter used in flicker meters has a time constant of about 0.5 s, matching the response time of human vision to brightness changes.
Correct answer is: 0.5 seconds
Q.10 Which parameter is NOT directly used in the calculation of Pst?
Voltage deviation
Load current waveform
Frequency deviation
Time weighting function
Explanation - Flicker calculations focus on voltage magnitude changes; frequency deviations are not part of the Pst computation.
Correct answer is: Frequency deviation
Q.11 A voltage sag of 20 % lasting 0.2 s is most likely to be perceived as:
A hum
Flicker
Harmonic distortion
No noticeable effect
Explanation - Short‑duration voltage dips of this magnitude fall within the range that the human eye perceives as flicker.
Correct answer is: Flicker
Q.12 Which of the following statements about Pst and Plt is true?
Pst is always greater than Plt
Plt is the arithmetic mean of Pst values
Pst is measured over 10 minutes, Plt over 2 hours
Both indices are measured in volts
Explanation - Pst is a short‑term index calculated from a 10‑minute measurement; Plt is a long‑term index derived from a 2‑hour series of Pst values.
Correct answer is: Pst is measured over 10 minutes, Plt over 2 hours
Q.13 Which type of power electronic converter is most likely to cause flicker when used with a resistive load?
Full‑bridge inverter with high switching frequency
Diode bridge rectifier
Three‑phase transformer
Uncontrolled reactor
Explanation - Inverters can produce voltage fluctuations due to switching actions, especially when the load changes rapidly, leading to flicker.
Correct answer is: Full‑bridge inverter with high switching frequency
Q.14 In a flicker measurement setup, why is a low‑pass filter with a cut‑off around 8 Hz used?
To eliminate harmonic components above the fundamental frequency
To mimic the eye’s reduced sensitivity to high‑frequency voltage changes
To increase the measurement resolution of fast transients
To block DC offsets in the voltage signal
Explanation - The human eye is less sensitive to voltage changes above roughly 8 Hz, so the filter attenuates higher‑frequency components before flicker assessment.
Correct answer is: To mimic the eye’s reduced sensitivity to high‑frequency voltage changes
Q.15 Which of the following load characteristics contributes most to flicker in residential areas?
Constant power factor loads
Large inductive motors with soft starters
Frequent switching of high‑power appliances like air‑conditioners
Low‑power LED lamps
Explanation - Rapid on/off cycling of high‑power appliances causes voltage fluctuations that can be perceived as flicker by nearby consumers.
Correct answer is: Frequent switching of high‑power appliances like air‑conditioners
Q.16 What does a Pst value of 0.6 indicate about the perceived flicker?
Severe flicker, likely to cause discomfort
No flicker perceptible to most people
Mild flicker, generally acceptable
Flicker exceeding regulatory limits
Explanation - A Pst of 0.6 is below the typical limit of 1.0 and is considered mild, usually not causing noticeable discomfort.
Correct answer is: Mild flicker, generally acceptable
Q.17 Which of the following actions will NOT reduce flicker caused by a welding transformer?
Adding a series reactor
Increasing the source impedance
Installing a static var compensator (SVC)
Using a larger conductor size for supply cables
Explanation - Increasing source impedance would worsen voltage drops during load changes, thereby increasing flicker.
Correct answer is: Increasing the source impedance
Q.18 The term 'flicker‑sensitive load' refers to:
Loads that cause flicker when they operate
Loads that are adversely affected by voltage flicker
Loads that have a low power factor
Loads that generate harmonic distortion
Explanation - Flicker‑sensitive loads are those (e.g., lighting, electronic equipment) that experience performance issues when voltage fluctuates rapidly.
Correct answer is: Loads that are adversely affected by voltage flicker
Q.19 Which parameter is used to express the percentage of voltage change that causes a unit flicker severity (Pst = 1)?
Voltage sensitivity factor (VSF)
Flicker coefficient (Kf)
Voltage dip factor (VDF)
Flicker magnitude (Fm)
Explanation - Kf quantifies the voltage deviation required to produce a unit flicker severity, linking voltage change magnitude to Pst.
Correct answer is: Flicker coefficient (Kf)
Q.20 In a three‑phase system, a single‑phase load causing flicker will most affect:
All three phases equally
Only the phase to which it is connected
The neutral conductor only
The system frequency
Explanation - A single‑phase load creates an unbalanced voltage drop affecting primarily its own phase, which can lead to phase‑to‑phase flicker effects.
Correct answer is: Only the phase to which it is connected
Q.21 Which of the following best describes the relationship between flicker and harmonic distortion?
Higher harmonics always increase flicker severity
Flicker is independent of harmonic content
Certain low‑order harmonics can exacerbate flicker
Harmonics reduce flicker by stabilising voltage
Explanation - Low‑order harmonics (e.g., 3rd, 5th) can interact with voltage fluctuations, potentially increasing the perceived flicker.
Correct answer is: Certain low‑order harmonics can exacerbate flicker
Q.22 A flicker meter records a Pst of 1.2 for a lighting system. According to IEC 61000‑4‑15, what action is recommended?
No action needed; the system is within limits
Investigate and mitigate the cause of flicker
Upgrade the meter to a higher class
Increase the supply voltage by 5 %
Explanation - A Pst greater than 1.0 exceeds the recommended limit, indicating that corrective measures should be taken.
Correct answer is: Investigate and mitigate the cause of flicker
Q.23 Which device can be installed at the point of common coupling (PCC) to reduce flicker caused by external loads?
Static var compensator (SVC)
Surge arrester
Transformer tap changer
Circuit breaker
Explanation - An SVC can dynamically inject or absorb reactive power, stabilising voltage and mitigating flicker from external disturbances.
Correct answer is: Static var compensator (SVC)
Q.24 What is the typical frequency range of voltage fluctuations that cause perceptible flicker?
0.1 Hz to 1 Hz
1 Hz to 8 Hz
10 Hz to 50 Hz
50 Hz to 60 Hz
Explanation - Human vision is most sensitive to voltage variations in the 1–8 Hz range, which corresponds to the flicker‑perceptible band.
Correct answer is: 1 Hz to 8 Hz
Q.25 When measuring flicker, why is the supply voltage first passed through a band‑pass filter (0.5 Hz – 20 Hz)?
To eliminate DC offset and high‑frequency noise
To amplify the fundamental frequency
To convert the AC signal to DC
To increase the measurement bandwidth
Explanation - The band‑pass filter removes components outside the flicker‑sensitive range, focusing the measurement on relevant voltage variations.
Correct answer is: To eliminate DC offset and high‑frequency noise
Q.26 In a distribution network, a high‑impedance feeder is more prone to flicker because:
It reduces harmonic distortion
Voltage drops are larger for a given load change
It increases the system frequency stability
It lowers the short‑circuit level
Explanation - Higher line impedance leads to greater voltage variations when loads switch, making flicker more noticeable.
Correct answer is: Voltage drops are larger for a given load change
Q.27 Which of the following is NOT a typical source of flicker in residential settings?
Electric water heaters
LED strip lighting with constant‑current drivers
Vacuum cleaners
Electric stoves
Explanation - Constant‑current LED drivers provide stable voltage and current, thus rarely causing flicker, unlike high‑power resistive or motor loads.
Correct answer is: LED strip lighting with constant‑current drivers
Q.28 The term 'flicker‑free' lighting generally means that the light source:
Has a Pst value less than 0.5 under typical supply conditions
Operates at a constant frequency of 50 Hz
Uses a filament lamp
Is powered through a DC supply
Explanation - Flicker‑free lighting specifications often require Pst < 0.5 to ensure imperceptible brightness variation.
Correct answer is: Has a Pst value less than 0.5 under typical supply conditions
Q.29 A power factor correction capacitor bank is switched on and off periodically. What effect does this have on flicker?
It reduces flicker by stabilising voltage magnitude
It can introduce flicker due to sudden reactive power changes
It eliminates harmonic distortion
It has no impact on flicker
Explanation - Switching capacitors changes the reactive power balance abruptly, causing voltage fluctuations that may be perceived as flicker.
Correct answer is: It can introduce flicker due to sudden reactive power changes
Q.30 Which measurement parameter is directly used to calculate the flicker severity index Pst?
Instantaneous voltage deviation (ΔV)
Current harmonic spectrum
Frequency deviation (Δf)
Total harmonic distortion (THD)
Explanation - Pst is derived from the weighted, filtered voltage deviation signal that represents the magnitude of fluctuations over time.
Correct answer is: Instantaneous voltage deviation (ΔV)
Q.31 A 5 kW arc furnace operates with a duty cycle of 20 % and causes a measured Pst of 0.9 at the supply point. If the duty cycle is increased to 40 %, what is the most likely effect on Pst?
Pst will decrease
Pst will stay the same
Pst will increase
Pst will become zero
Explanation - Higher duty cycles mean longer periods of high load, leading to larger and more frequent voltage fluctuations, thus raising Pst.
Correct answer is: Pst will increase
Q.32 Which of the following describes a 'flicker‑free' voltage source?
A source that maintains a constant voltage amplitude within ±0.5 % under all load conditions
A source that supplies only DC voltage
A source that varies frequency to compensate load changes
A source that includes an active power filter
Explanation - Flicker‑free operation requires the voltage magnitude to stay tightly within a narrow band, typically ±0.5 % of nominal.
Correct answer is: A source that maintains a constant voltage amplitude within ±0.5 % under all load conditions
Q.33 When using a flicker meter, the term 'instantaneous flicker level' refers to:
The current value of the filtered voltage deviation signal
The peak harmonic amplitude
The average voltage over one power cycle
The instantaneous frequency deviation
Explanation - The instantaneous flicker level is the momentary output of the weighting filter before statistical processing into Pst or Plt.
Correct answer is: The current value of the filtered voltage deviation signal
Q.34 Which of the following standards defines the permissible limits for flicker in public lighting?
EN 50160
IEC 61800‑5‑1
IEEE 519
IEC 60034‑1
Explanation - EN 50160 specifies voltage characteristics, including flicker limits, for electricity supplied to customers.
Correct answer is: EN 50160
Q.35 In a three‑phase four‑wire system, which condition can increase flicker on the neutral conductor?
Balanced three‑phase loads
Unbalanced single‑phase loads with high harmonic currents
Use of three‑phase transformers with delta primary
Installation of a neutral grounding resistor
Explanation - Unbalanced loads and harmonic currents produce neutral current and voltage fluctuations that can be perceived as flicker.
Correct answer is: Unbalanced single‑phase loads with high harmonic currents
Q.36 A flicker severity index (Pst) of 0.0 indicates:
Severe flicker
No measurable flicker
Maximum permissible flicker
An error in measurement
Explanation - Pst = 0 means the voltage was perfectly stable during the measurement interval.
Correct answer is: No measurable flicker
Q.37 Which type of load control strategy is most effective for reducing flicker in a microgrid?
Randomized load shedding
Demand response with coordinated ramp‑up/down of large loads
Fixed‑schedule operation of all loads
Increasing the number of line taps
Explanation - Coordinated control smoothes load changes, minimizing sudden voltage dips that cause flicker.
Correct answer is: Demand response with coordinated ramp‑up/down of large loads
Q.38 Which measurement device provides the most accurate assessment of flicker in a power system?
Oscilloscope with 50 kS/s sampling
Digital multimeter (DMM)
IEC‑compliant flicker meter (Class 1)
Power quality logger without flicker algorithm
Explanation - Class 1 flicker meters meet the accuracy requirements of IEC 61000‑4‑15 for both Pst and Plt.
Correct answer is: IEC‑compliant flicker meter (Class 1)
Q.39 When a large resistive load such as an electric oven is switched on, the primary effect on voltage is:
Rapid voltage rise (surge)
Voltage dip (sag)
Frequency increase
Generation of harmonic currents
Explanation - The sudden increase in current draw causes a temporary voltage drop, which can be a source of flicker if repeated.
Correct answer is: Voltage dip (sag)
Q.40 A flicker meter reports a Plt of 0.8 over a 2‑hour interval. What does this indicate?
Flicker exceeds acceptable limits
Flicker is within typical limits for most applications
The system has no flicker
The measurement is invalid
Explanation - Plt values below 1.0 are generally acceptable; 0.8 indicates mild, tolerable flicker over the long term.
Correct answer is: Flicker is within typical limits for most applications
Q.41 Which of the following best describes the term 'flicker‑sensitivity curve'?
A graph showing the relationship between voltage magnitude and harmonic distortion
A plot of human eye sensitivity versus frequency of voltage fluctuations
A curve representing the power factor variation with load
A diagram of the network impedance versus frequency
Explanation - The flicker‑sensitivity curve reflects how the human visual system perceives brightness changes at different frequencies.
Correct answer is: A plot of human eye sensitivity versus frequency of voltage fluctuations
Q.42 In a power system with high short‑circuit capacity, flicker is generally:
More severe
Less severe
Unaffected
Completely eliminated
Explanation - Higher short‑circuit capacity reduces voltage deviation for a given load change, thus mitigating flicker.
Correct answer is: Less severe
Q.43 Which of the following actions can inadvertently increase flicker when trying to improve power factor?
Installing a fixed capacitor bank without proper coordination
Adding a harmonic filter
Using a variable speed drive
Implementing an active power filter
Explanation - Improperly sized capacitor banks can cause over‑compensation and sudden reactive power swings, leading to voltage fluctuations.
Correct answer is: Installing a fixed capacitor bank without proper coordination
Q.44 A flicker severity index (Pst) is calculated from voltage deviations that are:
Measured in RMS over one cycle
Weighted by a frequency‑dependent filter
Averaged over a full hour
Integrated over the entire power system
Explanation - The voltage deviation signal is filtered with a weighting curve that reflects human visual sensitivity before statistical evaluation.
Correct answer is: Weighted by a frequency‑dependent filter
Q.45 Which of the following is a typical symptom of flicker on LED lighting?
Color shift
Visible brightness modulation
Increased heat generation
Reduced luminous efficacy
Explanation - LED drivers can respond to voltage fluctuations, causing perceptible brightness changes (flicker).
Correct answer is: Visible brightness modulation
Q.46 When a large inductive load is switched off, the voltage at the supply point typically:
Drops sharply
Rises briefly (voltage swell)
Remains unchanged
Oscillates at twice the supply frequency
Explanation - Removing inductive current causes a temporary increase in voltage, known as a swell, which can also contribute to flicker if repetitive.
Correct answer is: Rises briefly (voltage swell)
Q.47 Which method is commonly used to simulate flicker in laboratory tests?
Injecting harmonic currents
Applying a step‑change in voltage
Using a programmable load bank to produce cyclic load variations
Connecting a resistor in parallel with the supply
Explanation - Programmable load banks can mimic the rapid load changes that generate flicker, allowing controlled testing.
Correct answer is: Using a programmable load bank to produce cyclic load variations
Q.48 If a flicker meter shows a rapid increase in Pst after a new motor is installed, the most probable cause is:
Higher harmonic distortion
Increased voltage sag during motor start‑up
Reduced system frequency
Improved power factor
Explanation - Motor starting draws high inrush currents, creating voltage sags that increase flicker severity.
Correct answer is: Increased voltage sag during motor start‑up
Q.49 A 0.5 Hz voltage fluctuation is most likely to be perceived as:
A flicker
A hum
A ripple
No perceptible effect
Explanation - Frequencies below about 1 Hz are perceived as slow brightness changes (hum) rather than flicker.
Correct answer is: A hum
Q.50 Which of the following statements about the relationship between Pst and Plt is correct?
Plt = √(average of Pst² over 2 h)
Pst = Plt × 2
Plt is always larger than Pst
Both indices are measured in volts
Explanation - Plt is defined as the RMS (square root of the mean of squares) of successive Pst values over a two‑hour period.
Correct answer is: Plt = √(average of Pst² over 2 h)
Q.51 A voltage flicker event lasting 0.1 s at a frequency of 3 Hz will most likely be:
Invisible to the human eye
Perceived as a flicker
Detected as a harmonic distortion
Ignored by flicker meters
Explanation - A 3 Hz fluctuation falls within the flicker‑sensitive band and a duration of 0.1 s is enough for perception.
Correct answer is: Perceived as a flicker
Q.52 In power quality monitoring, which parameter is often reported together with flicker to assess overall supply quality?
Total harmonic distortion (THD)
Frequency deviation (Δf)
Voltage unbalance factor
All of the above
Explanation - THD, frequency deviation, and voltage unbalance are complementary indicators of power quality alongside flicker.
Correct answer is: All of the above
Q.53 When a large capacitor bank is switched on, the immediate voltage change is:
A voltage dip
A voltage swell
A frequency drop
No change
Explanation - Connecting capacitors supplies reactive power, raising the voltage momentarily.
Correct answer is: A voltage swell
Q.54 Which of the following devices can be used to actively compensate for flicker caused by rapidly varying loads?
Static var compensator (SVC)
Surge arrester
Circuit breaker
Ground fault relay
Explanation - An SVC can inject or absorb reactive power in real time to smooth voltage fluctuations, thereby reducing flicker.
Correct answer is: Static var compensator (SVC)
Q.55 In a flicker measurement, why is the voltage signal often normalized to its nominal value before processing?
To simplify the calculation of percentage deviation
To convert the signal to a DC level
To eliminate harmonic distortion
To increase the sampling rate
Explanation - Normalization allows the flicker algorithm to work with relative voltage changes expressed as a percentage of the nominal voltage.
Correct answer is: To simplify the calculation of percentage deviation
Q.56 A flicker index (Pst) of 1.2 measured at a residential feeder indicates:
The feeder meets all regulatory requirements
The flicker is above acceptable limits and may cause complaints
There is no flicker present
The measurement equipment is faulty
Explanation - Pst > 1.0 exceeds typical limits, indicating that occupants may notice flicker and corrective actions are advisable.
Correct answer is: The flicker is above acceptable limits and may cause complaints
Q.57 Which of the following load profiles is most likely to cause a high Pst value?
Steady resistive heating
Periodic high‑current pulses from a welding machine
Constant‑speed fan operation
DC battery charger with smooth current
Explanation - The repetitive high‑current pulses create rapid voltage dips, leading to high flicker severity.
Correct answer is: Periodic high‑current pulses from a welding machine
Q.58 When a power system experiences a sudden increase in load, the voltage sag that may cause flicker is most directly related to:
System resistance only
System impedance (R + jX)
System frequency
System harmonic content
Explanation - The voltage drop ΔV = I × Z, where Z is the system impedance, determines the magnitude of the sag during load changes.
Correct answer is: System impedance (R + jX)
Q.59 Which of the following best explains why flicker is more noticeable on low‑frequency lighting (e.g., fluorescent lamps) than on high‑frequency LED drivers?
Fluorescent lamps have higher harmonic content
LED drivers include built‑in flicker suppression circuits
Fluorescent lamps operate at a frequency close to the flicker‑sensitivity band
LEDs are immune to voltage variations
Explanation - Fluorescent lamps are driven at mains frequency (50/60 Hz) and its harmonics, making them more susceptible to voltage changes within the flicker band.
Correct answer is: Fluorescent lamps operate at a frequency close to the flicker‑sensitivity band
Q.60 A voltage flicker event is considered 'severe' if the short‑term flicker severity index exceeds:
0.5
1.0
1.5
2.0
Explanation - Regulatory limits typically set 1.0 as the threshold for acceptable flicker; values above this are considered severe.
Correct answer is: 1.0
Q.61 In a power distribution network, the presence of which of the following can help dampen voltage fluctuations that cause flicker?
Large series inductors
High‑impedance transformers
Capacitive voltage dividers
Long transmission lines
Explanation - Series inductors limit the rate of voltage change during load transients, reducing flicker.
Correct answer is: Large series inductors
Q.62 Which of the following statements about the effect of line length on flicker is correct?
Longer lines always increase flicker
Shorter lines always eliminate flicker
Longer lines increase impedance, making voltage more sensitive to load changes
Line length has no impact on flicker
Explanation - Higher line impedance due to longer conductors leads to larger voltage drops for the same load change, increasing flicker potential.
Correct answer is: Longer lines increase impedance, making voltage more sensitive to load changes
Q.63 When a flicker meter reports a high Plt value, which of the following remedial actions is most appropriate?
Install a harmonic filter
Upgrade the meter to a higher class
Implement load‑balancing or smoothing measures
Decrease the system voltage
Explanation - Reducing the magnitude and frequency of load variations directly addresses the cause of elevated long‑term flicker.
Correct answer is: Implement load‑balancing or smoothing measures
Q.64 Which type of inverter topology is most prone to causing flicker when feeding a variable load?
Voltage‑source inverter (VSI) with high switching frequency
Current‑source inverter (CSI)
Pulse‑width modulation (PWM) inverter with active current control
Resonant inverter
Explanation - VSIs can generate voltage ripple that, when combined with load variations, may produce flicker if not adequately filtered.
Correct answer is: Voltage‑source inverter (VSI) with high switching frequency
Q.65 What is the main purpose of the weighting filter in a flicker meter?
To remove DC offset from the voltage signal
To mimic the frequency response of the human visual system
To increase the signal‑to‑noise ratio
To convert the AC voltage to a DC level
Explanation - The weighting filter shapes the voltage deviation signal according to the flicker‑sensitivity curve, reflecting human perception.
Correct answer is: To mimic the frequency response of the human visual system
Q.66 Which of the following best describes a 'voltage swell'?
A temporary increase in voltage magnitude above nominal
A sustained voltage dip lasting several seconds
A rapid change in frequency
A harmonic distortion event
Explanation - A swell is a short‑duration rise in voltage, often caused by load removal, that can contribute to flicker if repetitive.
Correct answer is: A temporary increase in voltage magnitude above nominal
Q.67 When a flicker meter is placed at the load end of a distribution feeder, the measured Pst value will generally be:
Higher than at the source
Lower than at the source
The same as at the source
Zero
Explanation - Voltage fluctuations are amplified by line impedance, so the load end typically experiences larger deviations.
Correct answer is: Higher than at the source
Q.68 Which of the following statements about the relationship between flicker and short‑circuit power (Ssc) is true?
Higher Ssc leads to higher flicker levels
Lower Ssc reduces flicker
Higher Ssc reduces voltage fluctuations for a given load change, thus reducing flicker
Ssc has no effect on flicker
Explanation - A larger short‑circuit power means the system can absorb load transients with smaller voltage changes, mitigating flicker.
Correct answer is: Higher Ssc reduces voltage fluctuations for a given load change, thus reducing flicker
Q.69 Which of the following is a common symptom of flicker in industrial processes?
Incorrect color rendering in paints
Variations in motor speed
Fluctuating brightness of indicator lamps
Overheating of transformers
Explanation - Voltage fluctuations affect lighting and visual indicators, leading to perceptible brightness changes.
Correct answer is: Fluctuating brightness of indicator lamps
Q.70 In a flicker‑assessment test, a load is cycled on for 0.2 s and off for 0.8 s repeatedly. This cycle frequency is:
0.5 Hz
1 Hz
5 Hz
10 Hz
Explanation - Each full cycle takes 1 s (0.2 s on + 0.8 s off), resulting in a 1 Hz repetition rate, which lies within the flicker‑sensitive range.
Correct answer is: 1 Hz
Q.71 Which of the following actions is least likely to affect flicker levels in a power system?
Changing the transformer tap position
Adding a series reactor
Installing a harmonic filter
Improving the power factor
Explanation - Harmonic filters target frequency components above the flicker band and generally have little impact on voltage fluctuations causing flicker.
Correct answer is: Installing a harmonic filter
Q.72 A voltage dip of 15 % lasting 0.15 s is most likely to be:
Perceived as a flicker
Classified as a sag
Ignored by most equipment
Considered a permanent voltage drop
Explanation - A dip of this magnitude and duration meets the definition of a voltage sag; if repeated, it may also be perceived as flicker.
Correct answer is: Classified as a sag
Q.73 Which parameter is directly proportional to the severity of flicker caused by a load?
Load power factor
Rate of change of load current
Load resistance
System frequency
Explanation - Rapid changes in load current produce swift voltage variations, which increase flicker severity.
Correct answer is: Rate of change of load current
Q.74 When a static var compensator (SVC) is used to mitigate flicker, it primarily operates by:
Injecting or absorbing active power
Injecting or absorbing reactive power
Changing system frequency
Filtering harmonic currents
Explanation - SVCs dynamically adjust reactive power to stabilise voltage, thus reducing flicker caused by load variations.
Correct answer is: Injecting or absorbing reactive power
Q.75 The term 'flicker‑sensitive equipment' typically includes:
Large industrial motors
Lighting and video displays
Battery chargers
HVDC converters
Explanation - These devices are directly affected by perceptible brightness changes caused by voltage fluctuations.
Correct answer is: Lighting and video displays
Q.76 Which of the following best describes the impact of a high‑impedance fault on flicker?
It creates large voltage sags that increase flicker
It has negligible impact on flicker
It reduces flicker by limiting current flow
It causes frequency deviations that are interpreted as flicker
Explanation - High‑impedance faults draw very little current, so they do not cause significant voltage fluctuations.
Correct answer is: It has negligible impact on flicker
Q.77 Which of the following is a typical characteristic of a 'flicker‑free' power supply for sensitive electronic equipment?
Voltage regulation within ±5 % of nominal
Voltage regulation within ±0.5 % of nominal
Frequency regulation within ±5 Hz
Current regulation within ±10 %
Explanation - Tight voltage regulation (±0.5 %) helps prevent the magnitude of fluctuations that could be perceived as flicker.
Correct answer is: Voltage regulation within ±0.5 % of nominal
Q.78 A flicker severity index (Pst) of 0.2 is measured for a lighting circuit. According to IEC 61000‑4‑15, this level of flicker:
Exceeds the acceptable limit
Is well below the acceptable limit
Indicates severe flicker
Means the measurement is invalid
Explanation - Pst values below 0.5 are generally considered negligible and far below typical limits of 1.0.
Correct answer is: Is well below the acceptable limit
Q.79 In a power system, which component is most effective at reducing the voltage dip caused by a sudden load increase?
Series reactor
Shunt capacitor bank
Voltage regulator (tap changer)
Current limiter
Explanation - A voltage regulator can quickly adjust the turns ratio to raise the voltage and compensate for the dip.
Correct answer is: Voltage regulator (tap changer)
Q.80 Which of the following best explains why the human eye is less sensitive to flicker above 8 Hz?
The eye’s temporal resolution limits perception at higher frequencies
Higher frequencies are filtered out by the retina
The brain cannot process signals above 8 Hz
Lighting systems do not operate above 8 Hz
Explanation - The visual system integrates brightness over a short time, making rapid fluctuations above ~8 Hz appear steady.
Correct answer is: The eye’s temporal resolution limits perception at higher frequencies
Q.81 When a large inductive load such as a motor is started, the resulting voltage dip is most likely to affect:
Only the phase supplying the motor
All three phases equally
The neutral conductor only
The frequency of the system
Explanation - The inrush current flows mainly in the phase feeding the motor, causing a larger voltage sag on that phase.
Correct answer is: Only the phase supplying the motor
Q.82 Which of the following is NOT a typical cause of flicker in a railway traction power supply?
Regenerative braking of trains
Arc‑type traction converters
Constant‑speed lighting
Rapid changes in traction motor loading
Explanation - Constant‑speed lighting does not cause rapid load variations; the other options involve fast power changes that can induce flicker.
Correct answer is: Constant‑speed lighting
Q.83 In a flicker measurement system, the term 'sampling rate' refers to:
The number of voltage cycles measured per second
The number of voltage samples taken per second for processing
The frequency at which the flicker index is updated
The rate at which the filter coefficients are changed
Explanation - A high sampling rate ensures accurate capture of voltage fluctuations within the flicker‑sensitive frequency band.
Correct answer is: The number of voltage samples taken per second for processing
Q.84 A voltage dip that repeats every 0.5 s is most likely to be perceived as:
A flicker at 2 Hz
A hum at 2 Hz
A static voltage sag
A harmonic distortion
Explanation - The repetition rate of 0.5 s corresponds to a frequency of 2 Hz, which lies within the flicker‑perceptible range.
Correct answer is: A flicker at 2 Hz
Q.85 Which of the following actions can directly reduce the Pst value caused by a welding transformer?
Adding a series resistor
Installing a static var compensator (SVC)
Increasing the system frequency
Replacing the transformer with a larger one
Explanation - An SVC can dynamically inject reactive power to offset voltage drops caused by the welding load, reducing flicker.
Correct answer is: Installing a static var compensator (SVC)
Q.86 Which of the following describes the effect of a high‑frequency voltage ripple (above 20 Hz) on flicker perception?
It greatly increases perceived flicker
It has little to no effect on flicker perception
It converts flicker into audible noise
It reduces the amplitude of flicker
Explanation - Human vision is insensitive to voltage changes above ~8 Hz; high‑frequency ripple is generally not perceived as flicker.
Correct answer is: It has little to no effect on flicker perception
Q.87 When measuring flicker, why is it important to use a voltage transformer (VT) with a flat frequency response?
To accurately capture low‑frequency voltage dips
To avoid distortion of the flicker‑sensitive frequency band
To increase the voltage level for the meter
To eliminate harmonic currents
Explanation - A flat frequency response ensures that the voltage waveform is reproduced accurately across the 0.5‑20 Hz band used for flicker analysis.
Correct answer is: To avoid distortion of the flicker‑sensitive frequency band
Q.88 If a power system experiences a continuous Pst of 0.9 for several days, what does this imply for the end‑users?
They will experience severe, unacceptable flicker
Flicker will be noticeable but generally acceptable
No flicker will be perceived
The system frequency will be unstable
Explanation - A Pst of 0.9 is close to the limit of 1.0, indicating that users may notice flicker but it remains within typical acceptable limits.
Correct answer is: Flicker will be noticeable but generally acceptable
Q.89 Which of the following best defines 'voltage sag' in the context of flicker?
A voltage increase lasting less than 0.5 s
A temporary decrease in RMS voltage lasting from 0.5 s to 1 min
A permanent reduction in voltage level
A frequency deviation below 45 Hz
Explanation - Voltage sags (or dips) are short‑duration reductions in RMS voltage, commonly associated with flicker when they repeat.
Correct answer is: A temporary decrease in RMS voltage lasting from 0.5 s to 1 min
Q.90 A load that varies its power draw in a sinusoidal manner at 4 Hz is most likely to produce:
Harmonic distortion
Flicker perceived by humans
Voltage unbalance
Frequency deviation
Explanation - A 4 Hz variation lies within the flicker‑sensitive band, leading to brightness fluctuations that can be perceived.
Correct answer is: Flicker perceived by humans
Q.91 Which of the following components can be used to store energy temporarily and thus smooth out rapid load changes that cause flicker?
Flywheel energy storage system
Series reactor
Voltage transformer
Surge arrester
Explanation - Flywheels can absorb and release energy quickly, mitigating rapid load transients that lead to flicker.
Correct answer is: Flywheel energy storage system
Q.92 The term 'flicker factor' is sometimes used to describe:
The ratio of Plt to Pst
The product of voltage deviation and frequency weighting
The square root of the sum of squared voltage deviations
The proportion of time the voltage is outside nominal limits
Explanation - Flicker factor combines magnitude and frequency weighting to represent the perceived severity of voltage fluctuations.
Correct answer is: The product of voltage deviation and frequency weighting
Q.93 In a network with many distributed photovoltaic (PV) inverters, flicker can be caused by:
Rapid changes in solar irradiance causing inverter output variations
Constant active power output of the PV arrays
Low harmonic content of the inverter output
High system short‑circuit capacity
Explanation - Cloud transients cause swift changes in PV output, leading to voltage fluctuations that may be perceived as flicker.
Correct answer is: Rapid changes in solar irradiance causing inverter output variations
Q.94 Which of the following is the most direct method to quantify the perceptibility of flicker for a large population?
Measuring total harmonic distortion (THD)
Surveying users for visual complaints
Calculating the Pst index using IEC 61000‑4‑15
Monitoring system frequency deviations
Explanation - Pst provides a standardized, quantitative measure of flicker severity that correlates with human perception.
Correct answer is: Calculating the Pst index using IEC 61000‑4‑15
Q.95 A flicker meter with a class 2 rating typically has an accuracy of:
±1 % for Pst
±3 % for Pst
±5 % for Pst
±10 % for Pst
Explanation - Class 2 flicker meters are allowed a maximum error of ±3 % for the short‑term flicker severity index.
Correct answer is: ±3 % for Pst
Q.96 Which of the following statements correctly describes the impact of a large capacitor bank on flicker when it is switched on abruptly?
It reduces flicker by stabilising voltage
It can cause a voltage swell that may be perceived as flicker if repeated
It eliminates harmonics that cause flicker
It has no impact on flicker
Explanation - Sudden connection of capacitors injects reactive power, raising voltage momentarily; repeated switching can be perceived as flicker.
Correct answer is: It can cause a voltage swell that may be perceived as flicker if repeated
Q.97 Which of the following is a common design target for Pst in hospital power systems to ensure patient safety and comfort?
Pst ≤ 0.5
Pst ≤ 1.0
Pst ≤ 1.5
Pst ≤ 2.0
Explanation - Critical facilities like hospitals often require stricter flicker limits (Pst ≤ 0.5) to avoid any perceptible disturbances.
Correct answer is: Pst ≤ 0.5
Q.98 When a flicker meter reports a Pst of 0.0, which of the following can be inferred?
The system is experiencing severe flicker
The voltage is perfectly steady during the measurement period
The meter is malfunctioning
The load is highly non‑linear
Explanation - A Pst of zero indicates that no voltage fluctuations were detected within the measurement window.
Correct answer is: The voltage is perfectly steady during the measurement period
Q.99 Which of the following power quality phenomena is most closely related to flicker?
Voltage sags
Voltage unbalance
Frequency deviation
Harmonic distortion
Explanation - Voltage sags are short‑duration reductions in voltage that, when repetitive, are a primary cause of flicker.
Correct answer is: Voltage sags
Q.100 The flicker weighting filter used in IEC 61000‑4‑15 has a peak sensitivity at approximately:
0.5 Hz
3 Hz
10 Hz
50 Hz
Explanation - Human visual sensitivity to flicker peaks near 3 Hz, so the weighting filter emphasizes this frequency range.
Correct answer is: 3 Hz
Q.101 In a power system with a high proportion of inverter‑based generation, flicker can be mitigated by:
Operating inverters at a fixed frequency
Using inverter control algorithms that limit active power ramps
Increasing the line resistance
Removing all capacitors from the system
Explanation - Smoothing the active power output of inverters reduces rapid voltage fluctuations, thereby lowering flicker.
Correct answer is: Using inverter control algorithms that limit active power ramps
Q.102 Which of the following statements about the relationship between flicker and voltage magnitude is correct?
Flicker severity increases with higher nominal voltage
Flicker severity is independent of nominal voltage level
Lower nominal voltage makes the same absolute voltage dip proportionally larger, increasing flicker
Higher nominal voltage eliminates flicker
Explanation - Because flicker is based on percentage voltage change, a given absolute dip represents a larger percentage at lower nominal voltages.
Correct answer is: Lower nominal voltage makes the same absolute voltage dip proportionally larger, increasing flicker
Q.103 A Pst value of 1.1 recorded in an office building is most likely to result in:
No complaints from occupants
Visible flicker on fluorescent lighting and possible complaints
Complete shutdown of all equipment
Increase in system frequency
Explanation - Pst > 1.0 exceeds typical limits, making flicker perceptible, especially on lighting, leading to occupant dissatisfaction.
Correct answer is: Visible flicker on fluorescent lighting and possible complaints
Q.104 Which of the following is a typical parameter of a flicker meter defined by IEC 61000‑4‑15?
Sampling frequency of 10 kS/s
Maximum allowable error of ±3 % for Pst
Operating temperature range of –20 °C to +70 °C
All of the above
Explanation - IEC 61000‑4‑15 specifies performance criteria including sampling rate, accuracy, and environmental limits for flicker meters.
Correct answer is: All of the above
Q.105 In a distribution network, the presence of which type of fault can cause temporary voltage sags that may lead to flicker?
Phase‑to‑ground fault
Open‑circuit fault
High‑impedance fault
Neutral‑to‑ground fault
Explanation - Phase‑to‑ground faults draw high fault currents, causing voltage sags that can be perceived as flicker if they clear quickly.
Correct answer is: Phase‑to‑ground fault
Q.106 When a large load with a power factor of 0.8 is switched on, the resulting voltage dip is primarily due to:
Active power draw only
Reactive power draw and associated voltage drop across system impedance
Frequency shift
Harmonic generation
Explanation - A low power factor indicates significant reactive power, which adds to the voltage drop across the system's impedance, deepening the dip.
Correct answer is: Reactive power draw and associated voltage drop across system impedance
Q.107 Which of the following best describes the effect of installing a larger conductor size on flicker levels?
It increases flicker because of higher capacitance
It reduces flicker by lowering line impedance
It has no effect on flicker
It causes higher harmonic distortion
Explanation - Lower impedance reduces voltage drops caused by load transients, thereby decreasing flicker severity.
Correct answer is: It reduces flicker by lowering line impedance
Q.108 A flicker index (Pst) is calculated over a period of:
10 seconds
10 minutes
1 hour
2 hours
Explanation - The short‑term flicker severity index Pst is defined based on a 10‑minute observation window.
Correct answer is: 10 minutes