Q.1 Which type of fuse is designed to protect against short‑circuit currents by melting a metal wire element quickly?
Thermal fuse
Cartridge fuse
Time‑delay (slow‑blow) fuse
Thermistor fuse
Explanation - Cartridge fuses have a thin metal wire element that melts rapidly when a short‑circuit current flows, providing fast clearing of the fault.
Correct answer is: Cartridge fuse
Q.2 What does the I²t rating of a protective device represent?
The device’s instantaneous breaking capacity
The energy let‑through during a short‑circuit fault
The maximum continuous current it can carry
The thermal time constant of the device
Explanation - I²t is the integral of the square of the fault current over time, representing the energy that passes through the device before it clears the fault.
Correct answer is: The energy let‑through during a short‑circuit fault
Q.3 Which protective device uses a residual current transformer to detect earth‑leakage currents?
Miniature circuit breaker (MCB)
Residual‑current device (RCD)
Magnetic circuit breaker
Fusible switch
Explanation - RCDs sense the difference between live and neutral currents; any imbalance (earth leakage) trips the device.
Correct answer is: Residual‑current device (RCD)
Q.4 A ‘slow‑blow’ fuse is most suitable for which of the following applications?
Protecting a motor with high inrush current
Protecting a sensitive electronic circuit
Protecting a lighting circuit
Protecting a battery bank
Explanation - Slow‑blow fuses tolerate brief overloads such as motor start‑up currents without nuisance tripping.
Correct answer is: Protecting a motor with high inrush current
Q.5 What is the primary purpose of a time‑current characteristic (TCC) curve for a protective device?
To show the device’s voltage rating
To illustrate the device’s response time at various fault currents
To list the mechanical dimensions of the device
To indicate the device’s cost
Explanation - A TCC curve plots clearing time versus fault current, allowing coordination between protective devices.
Correct answer is: To illustrate the device’s response time at various fault currents
Q.6 Which of the following is NOT a typical characteristic of a magnetic circuit breaker?
Fast tripping for high short‑circuit currents
Adjustable thermal trip setting
Trip based on electromagnetic force
Fixed instantaneous trip threshold
Explanation - Magnetic breakers rely on electromagnetic forces for instantaneous tripping; they do not have adjustable thermal settings.
Correct answer is: Adjustable thermal trip setting
Q.7 In a coordinated protection scheme, the downstream protective device must:
Trip faster than the upstream device for the same fault level
Trip slower than the upstream device for the same fault level
Never trip, leaving all faults to the upstream device
Trip only on overload conditions
Explanation - Selectivity requires the nearest device to the fault to clear it first, preventing unnecessary upstream trips.
Correct answer is: Trip faster than the upstream device for the same fault level
Q.8 What is the standard voltage rating for a typical low‑voltage residential circuit breaker in North America?
120 V
240 V
480 V
600 V
Explanation - Most residential breakers are rated for 240 V, which covers both 120 V single‑phase circuits and 240 V split‑phase loads.
Correct answer is: 240 V
Q.9 A fuse has a marking '250 V 5 A C'. What does the 'C' denote?
The fuse is a ceramic type
The fuse is rated for continuous duty
The fuse has a slow‑blow (time‑delay) characteristic
The fuse is for DC applications only
Explanation - In fuse marking, 'C' indicates a time‑delay or slow‑blow characteristic, suitable for circuits with temporary overloads.
Correct answer is: The fuse has a slow‑blow (time‑delay) characteristic
Q.10 Which protective device is specifically designed to protect against earth‑fault currents in wet locations such as bathrooms?
Miniature circuit breaker (MCB)
Ground‑fault circuit interrupter (GFCI)
Thermal overload relay
Magnetic fuse
Explanation - GFCIs monitor the imbalance between live and neutral and trip quickly when leakage exceeds ~5 mA, ideal for wet areas.
Correct answer is: Ground‑fault circuit interrupter (GFCI)
Q.11 The breaking capacity (Icu) of a circuit breaker is:
The maximum continuous current it can carry
The maximum short‑circuit current it can safely interrupt
The minimum current required to trip the device
The voltage rating of the device
Explanation - Icu (ultimate breaking capacity) defines the highest fault current the breaker can clear without damage.
Correct answer is: The maximum short‑circuit current it can safely interrupt
Q.12 Which of the following best describes a ‘selective’ or ‘cascade’ protection arrangement?
All devices trip simultaneously under any fault
Only the upstream device trips, ignoring downstream devices
The device closest to the fault trips first, leaving upstream devices untouched
Devices operate based on voltage, not current
Explanation - Selectivity ensures that only the protective device nearest the fault clears it, improving system reliability.
Correct answer is: The device closest to the fault trips first, leaving upstream devices untouched
Q.13 A fuse with a ‘fast‑acting’ characteristic is most appropriate for protecting:
A motor with high inrush current
A transformer winding
A sensitive electronic circuit
A lighting circuit with LED lamps
Explanation - Fast‑acting fuses clear faults quickly, preventing damage to delicate electronics that cannot tolerate over‑current transients.
Correct answer is: A sensitive electronic circuit
Q.14 What is the typical rated make‑and‑break time for a standard thermal magnetic circuit breaker?
1 ms
5 ms
30 ms
100 ms
Explanation - Thermal magnetic breakers usually have a make‑and‑break time of about 30 ms for instantaneous tripping.
Correct answer is: 30 ms
Q.15 Which protective device uses a bimetallic strip to provide overload protection?
Magnetic fuse
Thermal overload relay
Residual‑current device (RCD)
Electronic circuit breaker
Explanation - A bimetallic strip bends with temperature rise caused by overload current, opening contacts to protect the circuit.
Correct answer is: Thermal overload relay
Q.16 In an industrial setting, a 10 kA breaking capacity is required for a protective device. Which statement is true?
The device can only interrupt currents up to 10 A
The device can safely interrupt fault currents up to 10 kA
The device must be installed in a 10 kV system
The device will trip instantly at 10 A
Explanation - A 10 kA breaking capacity indicates the maximum short‑circuit current the device can safely interrupt.
Correct answer is: The device can safely interrupt fault currents up to 10 kA
Q.17 Which type of fuse is typically used in automotive applications?
Blade fuse
Cartridge fuse
Thermal fuse
High‑rupture‑capacity (HRC) fuse
Explanation - Blade fuses (ATC/ATF) are standard in automotive circuits due to their compact size and easy replacement.
Correct answer is: Blade fuse
Q.18 The term ‘N‑rated fuse’ refers to:
A fuse that can be replaced without interrupting service
A fuse rated for non‑interruptible loads
A fuse that does not require a protective conduit
A fuse that is non‑polarized
Explanation - N‑rated (or ‘non‑interruptible’) fuses are designed for hot‑swap replacement in live circuits, often used in high‑availability systems.
Correct answer is: A fuse that can be replaced without interrupting service
Q.19 A device that trips when the current exceeds 100 % of its rated current for more than 5 seconds is known as:
Instantaneous trip
Inverse‑time characteristic
Time‑delay (slow‑blow) characteristic
Thermal trip
Explanation - Slow‑blow (time‑delay) devices allow a temporary overload before tripping, often specified by a time‑current relationship.
Correct answer is: Time‑delay (slow‑blow) characteristic
Q.20 Which protective device is most commonly used to protect three‑phase motor circuits from short‑circuit and overload conditions?
Miniature circuit breaker (MCB)
Thermal magnetic motor circuit breaker (MCB)
Residual‑current device (RCD)
Fuse link
Explanation - Motor circuit breakers combine thermal overload protection with magnetic short‑circuit tripping, suited for three‑phase motors.
Correct answer is: Thermal magnetic motor circuit breaker (MCB)
Q.21 The ‘inverse‑time’ characteristic of an overload relay means:
The higher the current, the longer the relay takes to trip
The higher the current, the shorter the relay takes to trip
Trip time is independent of current magnitude
The relay trips only after a fixed delay
Explanation - Inverse‑time relays trip faster as the overload current increases, providing protection proportional to severity.
Correct answer is: The higher the current, the shorter the relay takes to trip
Q.22 Which of the following protective devices provides both over‑current and earth‑leakage protection in a single unit?
Miniature circuit breaker (MCB)
Residual‑current circuit breaker (RCCB)
Molded case circuit breaker (MCCB)
Fusible switch
Explanation - RCCBs combine over‑current (typically via an integrated MCB) and residual‑current detection, offering dual protection.
Correct answer is: Residual‑current circuit breaker (RCCB)
Q.23 A protective device with a ‘tripping curve’ that lies entirely to the left of another device’s curve is said to be:
Slower
Faster
More selective
Less sensitive
Explanation - A curve to the left indicates that the device trips at lower currents or faster times compared to the other.
Correct answer is: Faster
Q.24 Which rating indicates the maximum voltage a fuse can safely interrupt without flashover?
Current rating (A)
Breaking capacity (kA)
Voltage rating (V)
Power rating (W)
Explanation - The voltage rating specifies the highest system voltage at which the fuse can operate safely.
Correct answer is: Voltage rating (V)
Q.25 In a three‑phase system, the protective device must be rated for the:
Phase‑to‑ground voltage only
Line‑to‑line voltage only
Higher of phase‑to‑ground or line‑to‑line voltage
Neutral current only
Explanation - Protective devices must withstand the maximum voltage present in the system, which is the higher of the two.
Correct answer is: Higher of phase‑to‑ground or line‑to‑line voltage
Q.26 A fuse labeled ‘250 V 10 A S’ is a:
Fast‑acting fuse
Slow‑blow (time‑delay) fuse
Special safety fuse
Silicon‑controlled fuse
Explanation - The 'S' suffix denotes a slow‑blow characteristic, allowing temporary overloads.
Correct answer is: Slow‑blow (time‑delay) fuse
Q.27 What is the purpose of a ‘fusible link’ in an electrical installation?
To provide a disconnect switch
To protect wiring from overload by melting before the main fuse
To regulate voltage
To act as a surge protector
Explanation - A fusible link is a short, low‑voltage-rated fuse that opens before the main protective device, safeguarding the wiring.
Correct answer is: To protect wiring from overload by melting before the main fuse
Q.28 Which protective device is required by code for outdoor receptacles in residential dwellings?
Miniature circuit breaker (MCB)
Ground‑fault circuit interrupter (GFCI)
Thermal overload relay
Time‑delay fuse
Explanation - Code mandates GFCI protection for outdoor receptacles to reduce shock hazards.
Correct answer is: Ground‑fault circuit interrupter (GFCI)
Q.29 The term ‘short‑time withstand rating’ of a circuit breaker indicates:
The maximum voltage it can sustain
The duration it can carry a specified overload current without tripping
The time it takes to reset after a fault
The minimum current required to trip
Explanation - Short‑time withstand rating defines how long a breaker can tolerate an overload before opening.
Correct answer is: The duration it can carry a specified overload current without tripping
Q.30 A device that disconnects a circuit when the line‑to‑ground voltage exceeds a set value is called a:
Over‑voltage relay
Undervoltage relay
Earth‑fault relay
Surge arrester
Explanation - Over‑voltage relays monitor voltage levels and trip when they exceed a preset threshold, protecting equipment.
Correct answer is: Over‑voltage relay
Q.31 Which protective device is best suited for limiting the let‑through energy in a high‑capacity transformer protection scheme?
Current‑limiting fuse
Thermal overload relay
Residual‑current device (RCD)
Electronic circuit breaker
Explanation - Current‑limiting fuses are designed to open quickly, limiting the energy (I²t) that reaches the transformer during a fault.
Correct answer is: Current‑limiting fuse
Q.32 In a protective device coordination study, the term ‘margin of selectivity’ refers to:
The voltage difference between two devices
The time difference between the operation of upstream and downstream devices
The current difference between two devices
The physical distance between devices
Explanation - Margin of selectivity is the time interval that ensures the downstream device trips before the upstream one.
Correct answer is: The time difference between the operation of upstream and downstream devices
Q.33 Which of the following is a characteristic of a high‑rupture‑capacity (HRC) fuse?
Low breaking capacity
Ability to interrupt very high short‑circuit currents safely
Only for DC circuits
Slow‑blow operation only
Explanation - HRC fuses are built to handle high fault currents without exploding, making them suitable for industrial applications.
Correct answer is: Ability to interrupt very high short‑circuit currents safely
Q.34 What does the acronym ‘MCCB’ stand for?
Miniature circuit control breaker
Molded case circuit breaker
Motor control circuit breaker
Maximum current circuit breaker
Explanation - MCCBs are larger, higher‑rated breakers housed in a molded case, used for industrial over‑current protection.
Correct answer is: Molded case circuit breaker
Q.35 A circuit breaker that trips instantly when the fault current reaches 10 times its rated current is said to have an:
Instantaneous trip setting of 10× In
Time‑delay setting of 10× In
Long‑time setting of 10× In
Inverse‑time setting of 10× In
Explanation - The instantaneous setting defines the multiplier of rated current at which the breaker trips without intentional delay.
Correct answer is: Instantaneous trip setting of 10× In
Q.36 Which protective device is typically used to protect a photovoltaic (PV) array against over‑current?
Blade fuse
DC circuit breaker
RCD
Surge protector
Explanation - DC circuit breakers are specifically designed to handle the characteristics of photovoltaic systems, including DC fault currents.
Correct answer is: DC circuit breaker
Q.37 In a protective device, the term ‘pickup current’ refers to:
The minimum current required to reset the device
The current level at which the device begins to operate
The maximum current the device can carry
The rated current of the device
Explanation - Pickup current is the threshold that triggers the protective device to start its tripping process.
Correct answer is: The current level at which the device begins to operate
Q.38 Which of the following statements about a ‘thermal‑magnetic’ circuit breaker is true?
It only provides overload protection
It provides both overload and short‑circuit protection
It cannot be reset after tripping
It is used only for DC circuits
Explanation - Thermal‑magnetic breakers combine a thermal element for overload and a magnetic element for instantaneous short‑circuit tripping.
Correct answer is: It provides both overload and short‑circuit protection
Q.39 A fuse that must be replaced after it blows is called a:
Resettable fuse
Re‑armable fuse
One‑time (non‑reusable) fuse
Self‑healing fuse
Explanation - Standard fuses melt and cannot be reused; they must be replaced after a fault.
Correct answer is: One‑time (non‑reusable) fuse
Q.40 Which protective device is most suitable for protecting a low‑voltage lighting circuit from over‑current?
Miniature circuit breaker (MCB)
Residual‑current device (RCD)
High‑rupture‑capacity fuse
Current‑limiting relay
Explanation - MCBs are commonly used for lighting circuits due to their compact size and easy reset capability.
Correct answer is: Miniature circuit breaker (MCB)
Q.41 The ‘inverse‑definite‑time’ (IDT) characteristic of a circuit breaker means:
Trip time decreases with increasing current, reaching a definite minimum time
Trip time is constant regardless of current magnitude
Trip time increases with increasing current
Trip time is random
Explanation - IDT curves ensure faster tripping at higher fault currents, but with a set minimum time for lower overloads.
Correct answer is: Trip time decreases with increasing current, reaching a definite minimum time
Q.42 What is the primary advantage of a ‘re‑closeable’ circuit breaker in power systems?
It can interrupt higher currents than a normal breaker
It can automatically re‑close after a transient fault, reducing outage time
It does not require maintenance
It provides earth‑leakage protection
Explanation - Re‑closeable breakers attempt to restore service after temporary faults, improving reliability.
Correct answer is: It can automatically re‑close after a transient fault, reducing outage time
Q.43 Which rating on a fuse indicates the maximum continuous current it can safely carry without blowing?
Breaking capacity
Voltage rating
Current rating (In)
I²t rating
Explanation - The current rating (In) is the nominal current the fuse can carry indefinitely under normal conditions.
Correct answer is: Current rating (In)
Q.44 A ‘dual‑rated’ fuse can be used in both AC and DC circuits. Which marking would you expect to see?
AC/DC
A‑C
D‑C
C/D
Explanation - Dual‑rated fuses are marked with ‘AC/DC’ to indicate suitability for both alternating and direct current.
Correct answer is: AC/DC
Q.45 Which protective device operates by detecting a magnetic field produced by fault current?
Thermal overload relay
Magnetic circuit breaker
Residual‑current device (RCD)
Electronic timer
Explanation - Magnetic breakers sense the magnetic field from high fault currents and trip instantaneously.
Correct answer is: Magnetic circuit breaker
Q.46 In a protective system, the term ‘backup protection’ refers to:
A device that trips before the primary device
A device that trips only if the primary device fails to operate
A redundant power supply
A device that monitors temperature
Explanation - Backup protection provides a secondary layer of protection, ensuring fault clearance if the primary fails.
Correct answer is: A device that trips only if the primary device fails to operate
Q.47 Which of the following is a key advantage of an electronic (solid‑state) circuit breaker over a traditional electromechanical breaker?
Higher breaking capacity
No moving parts, leading to faster and more precise operation
Lower cost
Larger physical size
Explanation - Solid‑state breakers use semiconductor devices, enabling rapid, accurate response without mechanical wear.
Correct answer is: No moving parts, leading to faster and more precise operation
Q.48 Which protective device is specifically designed to protect against over‑current caused by a phase‑to‑phase short circuit in a three‑phase system?
Phase‑to‑ground fault relay
Phase‑to‑phase over‑current relay
Residual‑current device (RCD)
Thermal fuse
Explanation - Phase‑to‑phase over‑current relays detect and clear faults between two phases in a three‑phase system.
Correct answer is: Phase‑to‑phase over‑current relay
Q.49 A fuse that must be replaced after a fault and is rated for 250 V is classified as:
Non‑re‑closing fuse
Resettable fuse
High‑rupture‑capacity fuse
Thermal fuse
Explanation - Standard one‑time fuses are non‑re‑closing; they melt and must be replaced after operation.
Correct answer is: Non‑re‑closing fuse
Q.50 What does the ‘kA’ unit represent when specifying a breaker’s breaking capacity?
Kilowatts
Kilo‑ampere (thousand amperes)
Kelvin‑ampere
Kilo‑ohm
Explanation - Breaking capacity is expressed in kilo‑amperes (kA), indicating the fault current the device can interrupt.
Correct answer is: Kilo‑ampere (thousand amperes)
Q.51 Which of the following devices provides protection against both over‑current and under‑voltage conditions?
Over‑current relay
Undervoltage relay
Combined over‑current/undervoltage relay
Ground‑fault relay
Explanation - A combined relay monitors both current magnitude and voltage level, tripping for over‑current or undervoltage.
Correct answer is: Combined over‑current/undervoltage relay
Q.52 The main function of a ‘current‑limiting’ fuse is to:
Increase the system voltage during a fault
Reduce the peak fault current that passes through the circuit
Delay the tripping time for overloads
Provide earth‑leakage protection
Explanation - Current‑limiting fuses open very quickly, limiting the let‑through fault current and protecting downstream equipment.
Correct answer is: Reduce the peak fault current that passes through the circuit
Q.53 Which type of protective device is commonly used as a ‘main disconnect’ in a residential panel?
Miniature circuit breaker (MCB)
Main circuit breaker (MCB)
Residual‑current device (RCD)
Fuse link
Explanation - The main breaker isolates the entire dwelling’s electrical supply and provides over‑current protection.
Correct answer is: Main circuit breaker (MCB)
Q.54 What does the ‘U‑type’ designation on a fuse indicate?
The fuse is a universal type for AC and DC
The fuse has a U‑shaped element for higher breaking capacity
The fuse is designed for underwater use
The fuse is a ‘U‑type’ (high‑speed) fuse for low‑energy circuits
Explanation - U‑type fuses employ a U‑shaped element that improves arc quenching, allowing higher breaking capacities.
Correct answer is: The fuse has a U‑shaped element for higher breaking capacity
Q.55 A ‘thermal overload relay’ is typically used in conjunction with which type of motor protection?
Electronic motor controller
Magnetic circuit breaker
Solid‑state drive
Thermal‑magnetic motor circuit breaker
Explanation - Thermal overload relays complement magnetic tripping for motor protection, handling overloads while the magnetic part clears short circuits.
Correct answer is: Thermal‑magnetic motor circuit breaker
Q.56 Which protective device would you select for a circuit where a maximum continuous current of 20 A is required, but occasional short‑circuit currents of 500 A may occur?
20 A fast‑acting fuse
20 A slow‑blow fuse with 500 A breaking capacity
30 A MCB with 1000 A breaking capacity
10 A MCB with 250 A breaking capacity
Explanation - A slow‑blow fuse handles the normal 20 A load and can safely interrupt a 500 A fault due to its higher breaking capacity.
Correct answer is: 20 A slow‑blow fuse with 500 A breaking capacity
Q.57 What is the primary purpose of a ‘surge protective device’ (SPD) in an electrical installation?
To interrupt overload currents
To protect against transient over‑voltage spikes
To provide earth‑leakage protection
To limit short‑circuit currents
Explanation - SPDs clamp voltage spikes caused by lightning or switching, safeguarding equipment.
Correct answer is: To protect against transient over‑voltage spikes
Q.58 In a protection scheme, a device with a ‘U‑type’ characteristic curve is typically:
Very slow to operate
Designed for high‑energy, high‑current faults
Used only for DC circuits
A type of fuse with low breaking capacity
Explanation - U‑type devices have fast operation and high breaking capacity, suitable for high‑energy faults.
Correct answer is: Designed for high‑energy, high‑current faults
Q.59 Which protective device would be most appropriate for protecting a small solar inverter (max 5 kW) from over‑current?
DC circuit breaker rated at 30 A
AC miniature circuit breaker rated at 10 A
High‑rupture‑capacity fuse rated at 5 A
Residual‑current device (RCD)
Explanation - DC circuit breakers are suited for the inverter’s DC side, handling the possible fault currents.
Correct answer is: DC circuit breaker rated at 30 A
Q.60 A protective device with a ‘short‑time rating’ of 10 kA for 1 second indicates that:
It can carry 10 kA continuously for 1 second without tripping
It can interrupt a 10 kA fault within 1 second
It will trip instantly when 10 kA flows
It can withstand 10 kA for 1 second before it may be damaged
Explanation - Short‑time rating defines the temporary fault current the device can endure without permanent damage.
Correct answer is: It can withstand 10 kA for 1 second before it may be damaged
Q.61 Which type of fuse is commonly used in low‑voltage switchgear for industrial applications?
Cartridge fuse
Blade fuse
Thermal fuse
High‑rupture‑capacity (HRC) fuse
Explanation - HRC fuses are standard in industrial switchgear due to their high breaking capacity and reliability.
Correct answer is: High‑rupture‑capacity (HRC) fuse
Q.62 A protective device that trips when the current exceeds its rated value for a specified period is called a:
Instantaneous trip device
Inverse‑time over‑current relay
Thermal overload relay
Voltage‑sensing relay
Explanation - Inverse‑time relays provide a time delay that decreases as the overload current increases.
Correct answer is: Inverse‑time over‑current relay
Q.63 In a three‑phase four‑wire system, which protective device protects the neutral conductor from overload?
Neutral overload relay
Phase‑to‑ground fault relay
Residual‑current device (RCD)
Current‑limiting fuse
Explanation - Neutral overload relays monitor neutral current and trip if it exceeds the rating, preventing overheating.
Correct answer is: Neutral overload relay
Q.64 Which of the following is NOT a typical cause for a fuse to blow?
Short‑circuit fault
Overload condition
Ambient temperature rise
Undervoltage condition
Explanation - Fuses respond to over‑current; undervoltage does not cause them to melt.
Correct answer is: Undervoltage condition
Q.65 A fuse with a ‘B’ rating in IEC classification is best suited for:
Protecting motor circuits
Protecting lighting circuits with low inrush
Protecting transformer windings
Protecting high‑inrush equipment
Explanation - IEC B‑type fuses blow between 3–5× rated current, ideal for low‑inrush applications like lighting.
Correct answer is: Protecting lighting circuits with low inrush
Q.66 Which protective device is designed to disconnect a circuit when the line voltage exceeds a preset value for a specific duration?
Over‑voltage relay
Undervoltage relay
Current‑limiting relay
Thermal overload relay
Explanation - Over‑voltage relays monitor voltage levels and operate when the voltage exceeds a set threshold for a defined time.
Correct answer is: Over‑voltage relay
Q.67 The primary function of a ‘ground‑fault circuit interrupter’ (GFCI) is to:
Protect against overloads
Detect and interrupt earth‑leakage currents
Limit short‑circuit currents
Regulate voltage
Explanation - GFCIs monitor the difference between live and neutral currents and trip when leakage exceeds a few milliamps.
Correct answer is: Detect and interrupt earth‑leakage currents
Q.68 A fuse is rated ‘250 V 3 A F’. What does the ‘F’ signify?
Fast‑acting (quick‑blow) characteristic
Fusible link
Full‑wave rectifier
Flame‑proof construction
Explanation - ‘F’ indicates a fast‑acting fuse, which clears faults quickly with minimal delay.
Correct answer is: Fast‑acting (quick‑blow) characteristic
Q.69 Which protective device is typically employed to protect a transformer’s secondary winding from overload?
Thermal overload relay
Residual‑current device (RCD)
Magnetic circuit breaker
Surge arrester
Explanation - Thermal overload relays monitor current heating and trip to protect transformer windings from prolonged overloads.
Correct answer is: Thermal overload relay
Q.70 In a protection scheme, a device with a higher ‘pick‑up current’ than its upstream neighbour will:
Trip before the upstream device
Trip after the upstream device
Never trip
Trip only on earth faults
Explanation - A higher pick‑up current means the device requires a larger fault current to operate, causing it to trip later.
Correct answer is: Trip after the upstream device
Q.71 Which protective device uses a gas‑filled tube to extinguish the arc during interruption?
Air‑magnetic circuit breaker
Vacuum circuit breaker
SF₆ circuit breaker
Thermal fuse
Explanation - Sulfur hexafluoride (SF₆) gas has excellent dielectric properties and is used in high‑voltage breakers to quench arcs.
Correct answer is: SF₆ circuit breaker
Q.72 The term ‘rated short‑circuit breaking capacity’ (Ics) of a circuit breaker refers to:
The maximum continuous current rating
The maximum fault current the breaker can safely interrupt under specific conditions
The minimum voltage required for operation
The time it takes to reset after a fault
Explanation - Ics is the rated breaking capacity under defined test conditions, indicating the highest fault current the breaker can handle.
Correct answer is: The maximum fault current the breaker can safely interrupt under specific conditions
Q.73 Which protective device is most appropriate for protecting a circuit that supplies a motor with a high inrush current?
Fast‑acting fuse
Time‑delay (slow‑blow) fuse
Residual‑current device (RCD)
Electronic over‑current relay
Explanation - Slow‑blow fuses tolerate the brief high inrush current of motor start‑up without nuisance trips.
Correct answer is: Time‑delay (slow‑blow) fuse
Q.74 A protective device that combines both over‑current and earth‑leakage detection in one unit is called:
Miniature circuit breaker (MCB)
Residual‑current circuit breaker (RCCB)
Thermal overload relay
Surge protective device (SPD)
Explanation - RCCBs integrate over‑current protection (often as an MCB) with residual‑current detection.
Correct answer is: Residual‑current circuit breaker (RCCB)
Q.75 What is the main purpose of a ‘current transformer’ (CT) in protective relaying?
To step down voltage for the relay
To provide isolation between the power circuit and the relay
To measure current and provide a scaled replica for the relay
To limit fault current
Explanation - CTs generate a proportional current that can be safely used by protective relays for measurement and tripping decisions.
Correct answer is: To measure current and provide a scaled replica for the relay
Q.76 Which of the following protective devices is designed to operate solely on the principle of magnetic repulsion?
Thermal overload relay
Magnetic circuit breaker
Residual‑current device (RCD)
Electronic timer
Explanation - Magnetic circuit breakers trip instantly when the magnetic field generated by high fault currents causes a repulsive force that opens the contacts.
Correct answer is: Magnetic circuit breaker
Q.77 In a protection coordination study, the term ‘selectivity margin’ is measured in:
Amperes
Seconds
Volts
Degrees
Explanation - Selectivity margin is the time difference between the operation of two coordinated protective devices.
Correct answer is: Seconds
Q.78 A fuse is marked ‘500 V 1 A D’. The ‘D’ class indicates:
Fast‑acting characteristic
Slow‑blow characteristic
Very high rupture capacity
Delayed (time‑delay) characteristic for high inrush currents
Explanation - ‘D’ class fuses are designed for circuits with high inrush currents, such as motor start‑up, providing a time‑delay before blowing.
Correct answer is: Delayed (time‑delay) characteristic for high inrush currents
Q.79 Which protective device is typically used to protect against over‑current in low‑voltage (≤ 1000 V) DC power distribution systems?
Thermal magnetic circuit breaker
DC circuit breaker
Residual‑current device (RCD)
Vacuum circuit breaker
Explanation - DC circuit breakers are specifically engineered to handle the arcing characteristics of direct current.
Correct answer is: DC circuit breaker
Q.80 A protective device rated for 240 V AC, 10 A, ‘C‑type’ is most appropriate for:
Lighting circuits
Motor circuits with moderate inrush
Highly sensitive electronic equipment
High‑energy transformer protection
Explanation - C‑type fuses blow between 5–10× rated current, suitable for moderate inrush currents typical of many motor loads.
Correct answer is: Motor circuits with moderate inrush
Q.81 Which protective device is designed to protect against both over‑current and over‑temperature conditions using a bimetallic strip?
Magnetic circuit breaker
Thermal overload relay
Residual‑current device (RCD)
Electronic over‑current relay
Explanation - Thermal overload relays use a bimetallic strip that bends with temperature rise due to overload, opening the circuit.
Correct answer is: Thermal overload relay
Q.82 In a three‑phase system, a protective device that operates when any two phases experience a fault is called a:
Phase‑to‑ground relay
Phase‑to‑phase over‑current relay
Residual‑current device (RCD)
Neutral overload relay
Explanation - Phase‑to‑phase relays detect faults between any two phases, providing protection for phase‑to‑phase short circuits.
Correct answer is: Phase‑to‑phase over‑current relay
Q.83 Which of the following is a key advantage of using a ‘magnetic‑blow’ fuse over a traditional slow‑blow fuse?
Higher voltage rating
Faster clearing of high‑fault currents
Lower cost
Adjustable trip setting
Explanation - Magnetic‑blow fuses incorporate a magnetic element that speeds up the interruption of high fault currents.
Correct answer is: Faster clearing of high‑fault currents
Q.84 A device that trips when the earth‑leakage current exceeds 30 mA is typically used for:
Industrial motor protection
Personal protection against electric shock
Transformer over‑current protection
Surge protection
Explanation - A 30 mA trip threshold is standard for human safety, preventing dangerous shock levels.
Correct answer is: Personal protection against electric shock
Q.85 The term ‘instantaneous trip’ in a circuit breaker specification refers to:
The time delay before the breaker can be reset
The maximum time the breaker can stay closed under overload
The fault current level at which the breaker trips without intentional delay
The voltage rating of the breaker
Explanation - Instantaneous trip settings define the current threshold for immediate operation, bypassing any thermal delay.
Correct answer is: The fault current level at which the breaker trips without intentional delay
Q.86 Which protective device is commonly used in data‑center UPS systems to protect against short‑circuit currents?
High‑rupture‑capacity (HRC) fuse
Blade fuse
Thermal overload relay
Ground‑fault circuit interrupter (GFCI)
Explanation - HRC fuses provide high breaking capacity needed for UPS short‑circuit protection.
Correct answer is: High‑rupture‑capacity (HRC) fuse
Q.87 A protective device rated ‘250 V 6 A S’ will:
Trip instantly at 6 A
Trip after a short delay when current exceeds 6 A
Trip only on short‑circuit currents
Trip at 6 A without any time delay
Explanation - The ‘S’ indicates a slow‑blow (time‑delay) characteristic, allowing temporary overloads before tripping.
Correct answer is: Trip after a short delay when current exceeds 6 A
Q.88 In a protection scheme, the term ‘fault clearing time’ is:
The time taken for a protective device to detect a fault
The total time from fault inception to device operation and isolation
The time a circuit remains energized after a fault
The time required to reset a device after a fault
Explanation - Fault clearing time includes detection, decision, and mechanical opening of the protective device.
Correct answer is: The total time from fault inception to device operation and isolation
Q.89 Which protective device is most appropriate for a circuit that must be disconnected when the current falls below a minimum value (under‑current)?
Over‑current relay
Undervoltage relay
Under‑current (reverse‑power) relay
Thermal overload relay
Explanation - Under‑current relays monitor for insufficient current flow and can open the circuit when a minimum threshold is not met.
Correct answer is: Under‑current (reverse‑power) relay
Q.90 The primary function of a ‘breaker’s arcing chute’ is to:
Increase the breaking capacity
Cool the contacts after opening
Quench the electric arc during interruption
Provide mechanical support
Explanation - Arcing chutes divide and cool the arc, allowing safe interruption of high fault currents.
Correct answer is: Quench the electric arc during interruption
Q.91 Which type of fuse is commonly used in low‑voltage distribution boards for general purpose protection?
Cartridge fuse
Blade fuse
Thermal fuse
High‑rupture‑capacity (HRC) fuse
Explanation - HRC fuses are standard in LV distribution boards due to their reliability and high breaking capacity.
Correct answer is: High‑rupture‑capacity (HRC) fuse
Q.92 A protective device that provides both over‑current and earth‑leakage protection with a rating of 30 mA is known as:
Miniature circuit breaker (MCB)
Residual‑current circuit breaker (RCCB)
Magnetic circuit breaker
Thermal overload relay
Explanation - RCCBs combine MCB functionality with a residual‑current detection set at a typical 30 mA for personal protection.
Correct answer is: Residual‑current circuit breaker (RCCB)
Q.93 The ‘inverse‑characteristic’ of a protective relay means that:
Trip time increases as fault current increases
Trip time is constant regardless of fault current
Trip time decreases as fault current increases
Trip time is random
Explanation - Inverse characteristics ensure faster operation for higher fault currents, providing effective protection.
Correct answer is: Trip time decreases as fault current increases
Q.94 Which protective device is specifically required by electrical codes for bathrooms and kitchens to protect against electric shock?
Miniature circuit breaker (MCB)
Ground‑fault circuit interrupter (GFCI)
Thermal overload relay
Surge protective device (SPD)
Explanation - GFCIs are mandated in wet locations to detect and interrupt leakage currents that could cause shock.
Correct answer is: Ground‑fault circuit interrupter (GFCI)
Q.95 A protective device with a ‘breaking capacity’ of 25 kA is capable of:
Carrying 25 kA continuously
Interrupting fault currents up to 25 kA safely
Operating only at 25 kA fault level
Limiting fault current to 25 kA
Explanation - Breaking capacity indicates the maximum fault current that can be safely cleared without damage.
Correct answer is: Interrupting fault currents up to 25 kA safely
Q.96 Which protective device is typically used to protect a low‑voltage (≤ 400 V) three‑phase motor from short‑circuit and overload conditions?
Thermal magnetic motor circuit breaker (MCB)
Residual‑current device (RCD)
High‑rupture‑capacity fuse
Surge arrester
Explanation - Motor circuit breakers combine thermal overload protection with magnetic short‑circuit tripping for motor applications.
Correct answer is: Thermal magnetic motor circuit breaker (MCB)
Q.97 What does the ‘U’ designation in IEC fuse classes (e.g., ‘U‑type’) indicate?
Ultra‑fast blow
Ultra‑high rupture capacity
Universal (AC/DC) use
Under‑voltage protection
Explanation - U‑type fuses are designed for very high fault energy, providing superior arc‑quenching capability.
Correct answer is: Ultra‑high rupture capacity
Q.98 Which protective device is used to detect and isolate a fault when the phase‑to‑ground voltage exceeds a preset limit?
Phase‑to‑ground over‑voltage relay
Phase‑to‑phase over‑current relay
Residual‑current device (RCD)
Thermal overload relay
Explanation - Phase‑to‑ground over‑voltage relays monitor voltage between a phase and ground and trip if it exceeds a set threshold.
Correct answer is: Phase‑to‑ground over‑voltage relay
Q.99 In a protection system, a device that operates only after a predetermined time delay to allow coordination with downstream devices is called:
Instantaneous relay
Time‑delay (inverse‑time) relay
Under‑current relay
Voltage‑sensing relay
Explanation - Time‑delay relays provide intentional delay, enabling coordination with faster‑acting downstream devices.
Correct answer is: Time‑delay (inverse‑time) relay
Q.100 Which protective device is commonly used in low‑voltage switchgear to protect against high fault currents while providing a resettable operation?
Miniature circuit breaker (MCB)
High‑rupture‑capacity fuse
Thermal overload relay
Residual‑current device (RCD)
Explanation - MCBs are resettable, have high breaking capacity, and are widely used in LV switchgear for over‑current protection.
Correct answer is: Miniature circuit breaker (MCB)
Q.101 A protective device with a ‘trip unit’ that can be adjusted for different current settings is known as:
Fixed‑setting relay
Adjustable‑setting relay
Non‑adjustable breaker
Static fuse
Explanation - Adjustable‑setting relays allow the user to set the pickup current and time delay to match system requirements.
Correct answer is: Adjustable‑setting relay
Q.102 Which protective device uses a sacrificial element that vaporizes during a fault to interrupt current flow?
Magnetic circuit breaker
Thermal overload relay
Solid‑state circuit breaker
Fuse
Explanation - During a fault, the fuse element melts (or vaporizes) creating an open circuit, stopping current flow.
Correct answer is: Fuse
Q.103 What is the main benefit of using a ‘digital’ protective relay over an electromechanical one?
Higher breaking capacity
Faster tripping speed
Ability to implement complex logic and communications
Lower cost
Explanation - Digital relays can process multiple inputs, communicate via protocols, and execute sophisticated protection algorithms.
Correct answer is: Ability to implement complex logic and communications
Q.104 Which protective device is primarily used to protect against over‑temperature conditions in a motor winding?
Thermal overload relay
Residual‑current device (RCD)
Magnetic circuit breaker
Surge arrester
Explanation - Thermal overload relays monitor temperature rise (via current) and disconnect the motor when overheating is detected.
Correct answer is: Thermal overload relay
Q.105 A protective device that must be manually reset after tripping is known as:
Auto‑resetting breaker
Self‑resetting fuse
Non‑automatic (manual‑reset) device
Electronic relay
Explanation - Manual‑reset devices require a person to reset them after a fault, ensuring inspection before re‑energizing.
Correct answer is: Non‑automatic (manual‑reset) device
Q.106 Which of the following is a typical characteristic of a ‘C‑type’ fuse?
Blows at 1–2× rated current
Blows at 3–5× rated current
Blows at 5–10× rated current
Blows at 10–20× rated current
Explanation - C‑type fuses are designed to operate at medium fault currents, making them suitable for many general‑purpose circuits.
Correct answer is: Blows at 5–10× rated current
Q.107 In protective device coordination, the upstream device is said to be ‘backup’ to the downstream device because it:
Trips before the downstream device
Trips only if the downstream device fails to operate
Operates at a lower current level
Has a lower breaking capacity
Explanation - Backup protection ensures system safety when the primary (downstream) protective device does not clear the fault.
Correct answer is: Trips only if the downstream device fails to operate
Q.108 Which protective device is most appropriate for a circuit that must be protected from both over‑current and under‑voltage conditions?
Combined over‑current/under‑voltage relay
Thermal overload relay
Residual‑current device (RCD)
Surge protective device (SPD)
Explanation - Such relays monitor both current magnitude and voltage, tripping when either parameter exceeds its limit.
Correct answer is: Combined over‑current/under‑voltage relay
Q.109 A protective device that operates when the line current drops below a set value (under‑current) is typically used in:
Motor start‑up circuits
Generator protection
Battery charging circuits
Lighting circuits
Explanation - Under‑current relays protect generators from loss of load conditions which could cause overspeed.
Correct answer is: Generator protection
Q.110 What does the term ‘coordination interval’ refer to in protection schemes?
The voltage difference between two devices
The time difference between the operation of two coordinated devices
The current rating gap between devices
The physical spacing between devices
Explanation - Coordination interval ensures the downstream device clears the fault before the upstream one, preventing unnecessary outages.
Correct answer is: The time difference between the operation of two coordinated devices
Q.111 Which protective device is specifically designed to protect against high‑energy short‑circuit currents in high‑voltage transmission systems?
Vacuum circuit breaker
Miniature circuit breaker (MCB)
Blade fuse
Thermal overload relay
Explanation - Vacuum circuit breakers are used in high‑voltage applications due to their excellent arc‑quenching capabilities.
Correct answer is: Vacuum circuit breaker
Q.112 A fuse marked ‘250 V 2 A D’ will:
Trip instantly at 2 A
Allow a short‑time overload before tripping
Only protect against short‑circuit currents
Trip at 2 A with no time delay
Explanation - The ‘D’ designation indicates a delayed (time‑delay) characteristic, suitable for circuits with temporary overloads.
Correct answer is: Allow a short‑time overload before tripping
Q.113 Which protective device uses a magnetic field to detect the direction of fault current?
Residual‑current device (RCD)
Phase‑sequence relay
Magnetic circuit breaker
Thermal overload relay
Explanation - Phase‑sequence relays detect the direction of fault current using magnetic fields, ensuring correct phase rotation before tripping.
Correct answer is: Phase‑sequence relay
Q.114 In a protective fuse, the ‘I²t’ value is used to:
Specify the breaking capacity
Define the energy let‑through during a fault
Indicate the voltage rating
Set the time delay
Explanation - I²t represents the energy absorbed by the fuse during the fault, influencing its coordination with downstream devices.
Correct answer is: Define the energy let‑through during a fault
Q.115 Which protective device is typically employed to protect a photovoltaic (PV) inverter from over‑current on its DC side?
DC circuit breaker
Residual‑current device (RCD)
Thermal overload relay
Surge protector
Explanation - DC circuit breakers are designed for the high fault currents and arcing characteristics of PV systems.
Correct answer is: DC circuit breaker
Q.116 A protective device that provides automatic reclosing after a fault has been cleared is called:
Auto‑resetting breaker
Self‑healing fuse
Re‑closeable circuit breaker
Instantaneous relay
Explanation - Re‑closeable breakers attempt to restore service after temporary faults, enhancing system reliability.
Correct answer is: Re‑closeable circuit breaker
Q.117 Which type of protective device typically has a ‘tripping curve’ that is shaped like a reverse‑S on a log‑log plot?
Inverse‑time over‑current relay
Instantaneous relay
Thermal overload relay
Residual‑current device (RCD)
Explanation - Inverse‑time relays exhibit a reverse‑S curve, indicating faster operation at higher fault currents.
Correct answer is: Inverse‑time over‑current relay
Q.118 Which protective device is most appropriate for a circuit that must be protected from both over‑current and ground‑fault currents simultaneously?
Miniature circuit breaker (MCB)
Residual‑current circuit breaker (RCCB)
Thermal overload relay
Surge protective device (SPD)
Explanation - RCCBs combine over‑current protection with residual‑current detection, safeguarding against both fault types.
Correct answer is: Residual‑current circuit breaker (RCCB)
Q.119 The main advantage of a ‘solid‑state’ circuit breaker over an electromechanical breaker is:
Higher breaking capacity
Zero moving parts, leading to faster and more reliable operation
Lower cost
Higher voltage rating
Explanation - Solid‑state breakers use semiconductor devices, eliminating mechanical wear and enabling rapid response.
Correct answer is: Zero moving parts, leading to faster and more reliable operation
Q.120 A protective device that operates when the current exceeds 1.13 times its rated current within 0.1 seconds is an example of:
Fast‑acting fuse
Slow‑blow fuse
Instantaneous magnetic trip
Thermal overload relay
Explanation - Instantaneous magnetic trips respond extremely quickly to currents just above the rated value, without thermal delay.
Correct answer is: Instantaneous magnetic trip
Q.121 Which protective device is specifically designed to protect against voltage surges caused by lightning strikes?
Surge protective device (SPD)
Residual‑current device (RCD)
Thermal overload relay
Magnetic circuit breaker
Explanation - SPDs clamp transient over‑voltages, protecting equipment from lightning‑induced spikes.
Correct answer is: Surge protective device (SPD)
Q.122 A fuse with a ‘D‑type’ characteristic is best suited for:
Protecting lighting circuits
Protecting motor circuits with high inrush current
Protecting sensitive electronic equipment
Protecting transformer windings
Explanation - D‑type fuses have a higher time‑delay, allowing them to handle the large inrush currents typical of motor starts.
Correct answer is: Protecting motor circuits with high inrush current
Q.123 Which protective device would you select for a circuit requiring a breaking capacity of 30 kA, a voltage rating of 415 V, and a rated current of 63 A?
High‑rupture‑capacity (HRC) fuse
Miniature circuit breaker (MCB) rated 63 A, 30 kA
Residual‑current device (RCD) 30 mA
Thermal overload relay
Explanation - An MCB with the specified current and breaking capacity meets the requirements for a 415 V LV circuit.
Correct answer is: Miniature circuit breaker (MCB) rated 63 A, 30 kA
Q.124 The primary purpose of a ‘ground‑fault relay’ is to:
Detect over‑current conditions
Detect earth‑leakage currents and trip the circuit
Limit voltage spikes
Provide thermal overload protection
Explanation - Ground‑fault relays sense imbalance between phase and neutral currents, indicating a leakage to ground.
Correct answer is: Detect earth‑leakage currents and trip the circuit
Q.125 Which protective device is commonly used as a main disconnect in industrial low‑voltage switchgear?
Molded case circuit breaker (MCCB)
Miniature circuit breaker (MCB)
Residual‑current device (RCD)
Blade fuse
Explanation - MCCBs handle higher currents and provide robust interruption, making them suitable for main disconnect applications.
Correct answer is: Molded case circuit breaker (MCCB)
Q.126 A protective device that operates when the voltage falls below a preset value for a set duration is called a:
Over‑voltage relay
Undervoltage relay
Phase‑sequence relay
Thermal overload relay
Explanation - Undervoltage relays monitor voltage and trip when it drops below the set threshold for a defined time.
Correct answer is: Undervoltage relay
Q.127 Which protective device is designed to protect against over‑current in a direct‑current (DC) distribution system?
Vacuum circuit breaker
DC circuit breaker
Miniature circuit breaker (MCB)
Residual‑current device (RCD)
Explanation - DC circuit breakers are engineered for the arcing characteristics of DC currents, making them suitable for DC distribution.
Correct answer is: DC circuit breaker
Q.128 The term ‘short‑time withstand rating’ of a circuit breaker specifies:
The maximum fault current it can interrupt
The time it can carry a specified overload without damage
The voltage at which it operates
The minimum current needed to trip
Explanation - Short‑time withstand rating defines the duration a breaker can endure a higher than rated current before being damaged.
Correct answer is: The time it can carry a specified overload without damage
Q.129 Which protective device is commonly used to protect the neutral conductor in a four‑wire system?
Neutral overload relay
Phase‑to‑ground fault relay
Residual‑current device (RCD)
Surge arrester
Explanation - Neutral overload relays monitor the neutral current and trip if it exceeds the set limit, preventing overheating.
Correct answer is: Neutral overload relay
Q.130 A protective device with a rating of ‘63 A 30 kA U‑type’ provides:
Slow‑blow operation for motor circuits
Ultra‑high rupture capacity for high fault energy
Under‑voltage protection
Universal (AC/DC) usage
Explanation - U‑type devices are designed for very high fault energies, ensuring safe interruption of large short‑circuit currents.
Correct answer is: Ultra‑high rupture capacity for high fault energy
Q.131 Which protective device is specifically designed to protect against over‑current caused by a short circuit in a low‑voltage three‑phase system?
Miniature circuit breaker (MCB)
Residual‑current device (RCD)
Surge protective device (SPD)
Thermal overload relay
Explanation - MCBs provide rapid over‑current protection for low‑voltage three‑phase circuits, clearing short circuits efficiently.
Correct answer is: Miniature circuit breaker (MCB)
Q.132 The primary function of a ‘protective earth (PE) conductor’ in a protective device circuit is to:
Carry normal load current
Provide a low‑impedance path for fault currents to ground
Regulate voltage levels
Limit short‑circuit currents
Explanation - PE conductors ensure fault currents flow safely to earth, enabling protective devices to detect and clear faults.
Correct answer is: Provide a low‑impedance path for fault currents to ground
Q.133 Which protective device is most commonly used to protect a distribution board against over‑current and provide a resettable function?
High‑rupture‑capacity (HRC) fuse
Miniature circuit breaker (MCB)
Thermal fuse
Blade fuse
Explanation - MCBs are resettable, compact, and widely used for over‑current protection in distribution boards.
Correct answer is: Miniature circuit breaker (MCB)
Q.134 A protective device with a ‘trip unit’ that can be set for both instantaneous and time‑delay operation is known as a:
Thermal overload relay
Magnetic circuit breaker
Thermal‑magnetic circuit breaker
Residual‑current device (RCD)
Explanation - Thermal‑magnetic breakers combine a thermal element for overload and a magnetic element for instantaneous fault clearing.
Correct answer is: Thermal‑magnetic circuit breaker
Q.135 Which protective device is specifically designed to interrupt high fault currents in medium‑voltage (10–36 kV) networks?
Vacuum circuit breaker
Miniature circuit breaker (MCB)
Blade fuse
Thermal overload relay
Explanation - Vacuum circuit breakers are widely used in medium‑voltage systems for their effective arc quenching and high breaking capacity.
Correct answer is: Vacuum circuit breaker