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HKIE Professional Assessment - Electrical Discipline study materials
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HKIE Professional Assessment - Electrical Discipline practice questions

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Question 1Electrical Discipline core subject areas (Scheme "A" training)

Under the Hong Kong Electricity Ordinance (Cap. 406), which party is legally responsible for ensuring that a fixed electrical installation complies with the Wiring Regulations (BS 7671) before it is energised for the first time?

  1. The building contractor carrying out the construction works
  2. The owner of the installation, who must appoint a registered electrical contractor Correct answer.
  3. The electricity supplier, which must inspect all consumer installations
  4. The Electrical and Mechanical Services Department, which certifies every new circuit

Explanation

The owner of a fixed electrical installation must engage a registered electrical contractor to carry out the work and issue the appropriate Work Completion Certificate (Form WR1) before the supplier energises the installation. The supplier only connects supply on receipt of valid certification, and EMSD audits rather than certifies individual circuits.

Question 2Electrical Discipline core subject areas (Scheme "A" training)

In a low-voltage TN-S system, what is the principal function of the main equipotential bonding conductor connected to the main earthing terminal?

  1. To limit potential differences between exposed conductive parts and extraneous conductive parts during a fault Correct answer.
  2. To replace the circuit protective conductors of individual final circuits
  3. To carry normal load current between distribution boards
  4. To provide the sole return path for fault current back to the transformer star point

Explanation

Main equipotential bonding ties metallic services and structural steel to the main earthing terminal so that, under fault conditions, potential differences between simultaneously accessible parts remain low. It does not carry load current, and the fault return path in TN-S is the separate protective conductor.

Question 3Electrical Discipline core subject areas (Scheme "A" training)

A 400 V three-phase induction motor delivers 15 kW at a power factor of 0.85 and an efficiency of 0.90. What is the approximate line current drawn from the supply?

  1. 28 A Correct answer.
  2. 32 A
  3. 24 A
  4. 20 A

Explanation

Input power equals output divided by efficiency: 15 kW / 0.90 = 16.7 kW. For a balanced three-phase load, I = P / (√3 × V × cos φ) = 16700 / (1.732 × 400 × 0.85) ≈ 28 A.

Question 4Electrical Discipline core subject areas (Scheme "A" training)

Which protective device characteristic is most appropriate for the final circuit supplying a 13 A socket outlet in a domestic installation where disconnection must occur within 0.4 s under fault conditions?

  1. A 6 A Type D miniature circuit breaker with a 100 mA residual current device
  2. A 16 A Type C miniature circuit breaker without additional protection
  3. A 32 A Type B miniature circuit breaker with a 30 mA residual current device Correct answer.
  4. A 32 A fuse to BS 88-2 with a 5 s disconnection time

Explanation

A 30 mA RCD provides additional protection against electric shock at socket outlets and satisfies the 0.4 s disconnection requirement for TN systems in domestic premises. The 32 A Type B MCB also co-ordinates with the ring final circuit conductor rating.

Question 5Electrical Discipline core subject areas (Scheme "A" training)

In an electrical installation design, what does the term "Ib ≤ In ≤ Iz" express?

  1. The rated residual current must not exceed the earth fault loop impedance
  2. Voltage drop must not exceed the nominal voltage of the installation
  3. Design current must not exceed the protective device rating, which must not exceed the cable current-carrying capacity Correct answer.
  4. Fault current must not exceed the rated short-circuit capacity of the protective device

Explanation

This is the fundamental co-ordination rule of BS 7671: the design current Ib must not exceed the nominal rating In of the protective device, which in turn must not exceed the tabulated current-carrying capacity Iz of the cable. This ensures the cable is protected against overload under normal and fault conditions.

Question 6Electrical Discipline core subject areas (Scheme "A" training)

A single-core copper conductor of 16 mm² cross-sectional area carries 60 A. If the permitted voltage drop is 4% of 400 V on a 60 m run, what is the approximate voltage drop using a resistivity of 0.0175 Ω·mm²/m for copper?

  1. 15.8 V
  2. 1.6 V
  3. 3.9 V
  4. 7.9 V Correct answer.

Explanation

Resistance R = ρL/A = 0.0175 × 60 / 16 ≈ 0.0656 Ω. Voltage drop = I × R = 60 × 0.0656 ≈ 3.9 V for a single conductor. For a single-phase circuit the drop applies to both go and return conductors, giving about 7.9 V, within the 16 V permitted.

Question 7Electrical Discipline core subject areas (Scheme "A" training)

Which of the following best describes the purpose of a discrimination study between upstream and downstream protective devices?

  1. To ensure that a fault is cleared by the device nearest the fault without unnecessary tripping of upstream devices Correct answer.
  2. To determine the maximum permissible voltage drop on each final circuit
  3. To verify that all devices have identical rated short-circuit breaking capacities
  4. To confirm that the earthing system is TN-C-S rather than TT

Explanation

Discrimination (selectivity) ensures that the protective device immediately upstream of a fault operates first, so that healthy parts of the installation remain energised. It relies on comparison of time-current characteristics and energy let-through of the devices in series.

Question 8Electrical Discipline core subject areas (Scheme "A" training)

An electrical engineer is designing a circuit in a Zone 1 hazardous area classified as IIB with a T3 temperature class. Which consideration is most critical when selecting the luminaire?

  1. The luminaire must be of the flameproof type regardless of the zone
  2. The apparatus group and temperature class must be compatible with the gas group and ignition temperature of the atmosphere Correct answer.
  3. The luminaire must operate at a power factor above 0.95
  4. The luminaire must have an IP rating of at least IP20 only

Explanation

In hazardous-area design, equipment must be selected so that its apparatus group (IIB) matches or exceeds the gas group present and its temperature class (T3, maximum surface temperature 200 °C) does not exceed the ignition temperature of the gas. Zone 1 permits several protection concepts, not only flameproof.

Question 9Electrical Discipline core subject areas (Scheme "A" training)

In a TT earthing system, why is a residual current device normally required for earth fault protection?

  1. Overcurrent devices cannot detect any earth fault current in a TT system
  2. The earth fault loop impedance is generally too high for overcurrent devices to disconnect within the required time Correct answer.
  3. The supply neutral is bonded to earth at the origin, giving a low-impedance fault path
  4. The protective conductor is combined with the neutral conductor throughout

Explanation

In a TT system the installation earth electrode is independent of the supply earth, so the earth fault loop impedance is typically high and the resulting fault current too small to trip an overcurrent device promptly. An RCD detects the residual current and disconnects within the required time.

Question 10Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is the correct method for verifying the continuity of a ring final circuit protective conductor during initial verification?

  1. Confirm the main bonding conductor is connected to the water main
  2. Measure resistance end-to-end of the protective conductors and confirm the value is consistent with the conductor length and cross-section Correct answer.
  3. Measure the insulation resistance between line and neutral only
  4. Carry out an earth fault loop impedance test at the origin only

Explanation

Ring continuity is verified by measuring end-to-end resistance of each conductor (line, neutral and protective) and comparing the values with those expected from the conductor length and cross-section. This detects open rings, which would otherwise leave the protective conductor inadequately sized.

Question 11Electrical Discipline core subject areas (Scheme "A" training)

Which document must a registered electrical contractor issue to the owner after completing alteration work on a fixed installation before the supply is reconnected?

  1. A permit to work issued by the contractor
  2. A Work Completion Certificate (Form WR1) Correct answer.
  3. A periodic inspection report only
  4. An energy label for the building

Explanation

Under the Hong Kong Electricity (Wiring) Regulations, a registered electrical contractor must issue a Work Completion Certificate (Form WR1) after completing new work or alteration before the installation is energised. A periodic inspection report is a separate document used for ongoing verification.

Question 12Electrical Discipline core subject areas (Scheme "A" training)

A 100 kVA, 11 kV/400 V three-phase transformer has a per-unit impedance of 0.05. What is the approximate three-phase short-circuit current at the 400 V terminals, ignoring source impedance?

  1. 0.72 kA
  2. 5.8 kA
  3. 2.9 kA Correct answer.
  4. 1.4 kA

Explanation

Full-load current at 400 V is 100000 / (√3 × 400) ≈ 144 A. Dividing by the per-unit impedance gives the short-circuit current: 144 / 0.05 ≈ 2.9 kA. This assumes an infinite source and neglects cable and source impedance.

Question 13Electrical Discipline core subject areas (Scheme "A" training)

What is the primary purpose of a residual current device with a rated residual operating current of 30 mA in a final circuit?

  1. To provide additional protection against electric shock from direct contact Correct answer.
  2. To protect the circuit conductors against overload
  3. To correct the power factor of inductive loads
  4. To limit the prospective short-circuit current at the origin

Explanation

A 30 mA RCD is recognised as additional protection against electric shock, supplementing basic insulation and barriers for socket outlets and similar accessories. It does not replace overcurrent protection or correct power factor.

Question 14Electrical Discipline core subject areas (Scheme "A" training)

A commercial building has a maximum demand of 250 kVA at a power factor of 0.8 lagging. What approximate capacitor rating is required to improve the power factor to 0.95 lagging?

  1. 50 kvar
  2. 100 kvar
  3. 130 kvar
  4. 70 kvar Correct answer.

Explanation

Reactive power before correction is 250 × sin(cos⁻¹ 0.8) = 150 kvar; after correction it is 250 × sin(cos⁻¹ 0.95) ≈ 78 kvar. The capacitor rating is the difference, approximately 72 kvar, closest to 70 kvar.

Question 15Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is a key requirement for a fire alarm system installed in a Hong Kong commercial building under the Code of Practice for Minimum Fire Service Installations and Equipment?

  1. The system must operate on a 400 V three-phase supply
  2. The system must be designed and installed by a registered fire service installation contractor Correct answer.
  3. The system must use only addressable detectors
  4. The system must be tested annually by the electricity supplier

Explanation

Fire service installations must be designed and installed by a registered fire service installation contractor and certified to the Fire Services Department. The code sets functional requirements such as coverage and audibility, not a fixed technology or supply voltage.

Question 16Electrical Discipline core subject areas (Scheme "A" training)

In a lift installation, which protection is required to prevent the lift from moving with a landing door open beyond the permitted zone?

  1. A surge protective device at the main switchboard
  2. An earth fault relay on the machine room supply
  3. A residual current device on the lift motor circuit
  4. A door interlock and a landing door lock monitored by the lift controller Correct answer.

Explanation

Lift safety depends on door interlocks that prevent normal movement unless all landing doors are closed and locked, with a limited levelling zone permitted by the code. RCDs, SPDs and earth fault relays address electrical hazards, not the mechanical interlock function.

Question 17Electrical Discipline core subject areas (Scheme "A" training)

Which of the following best describes the function of a building management system in relation to electrical services?

  1. To provide backup power to essential services during a mains failure
  2. To isolate faulty final circuits automatically
  3. To replace the need for periodic inspection of the installation
  4. To monitor and control electrical plant such as chillers, pumps and lighting to optimise energy use Correct answer.

Explanation

A BMS supervises and controls building services, gathering data from sensors and meters to optimise plant operation and energy consumption. It does not provide backup power, perform protective disconnection, or substitute for statutory inspection.

Question 18Electrical Discipline core subject areas (Scheme "A" training)

A designer must select the cross-sectional area of a cable feeding a 40 A load over a route where the ambient temperature is 45 °C and the cable is grouped with three other loaded circuits. Which correction factors must be applied to the tabulated current-carrying capacity?

  1. Only the ambient temperature factor
  2. Neither factor, since the tabulated value already accounts for both
  3. Ambient temperature factor and grouping factor Correct answer.
  4. Only the grouping factor

Explanation

Tabulated current-carrying capacities assume a reference ambient temperature and no grouping. Where the actual ambient differs, a temperature correction factor applies, and where circuits are grouped, a grouping factor applies; both multiply the tabulated value to give the effective capacity.

Question 19Electrical Discipline core subject areas (Scheme "A" training)

Which of the following describes the role of a surge protective device installed at the origin of a low-voltage installation?

  1. To disconnect the supply on sustained overcurrent
  2. To convert alternating current to direct current for electronic equipment
  3. To divert transient overvoltages to earth and limit the voltage seen by downstream equipment Correct answer.
  4. To provide continuous voltage regulation for sensitive loads

Explanation

An SPD limits transient overvoltages from lightning or switching by diverting surge current to earth, clamping the voltage to a level the equipment can withstand. It does not regulate steady-state voltage, interrupt overcurrent or rectify supply.

Question 20Electrical Discipline core subject areas (Scheme "A" training)

In a three-phase four-wire distribution board serving predominantly single-phase electronic loads, which phenomenon can cause the neutral conductor current to exceed the line current?

  1. Unbalanced fundamental load current only
  2. Leading power factor from capacitor banks
  3. Triplen harmonic currents adding in phase in the neutral Correct answer.
  4. Voltage drop along the neutral conductor

Explanation

Triplen harmonics (3rd, 9th, 15th) from non-linear loads are in phase in all three phases and sum arithmetically in the neutral, so the neutral current can exceed the line current. Unbalanced fundamental currents partially cancel in the neutral rather than add.

Question 21Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is a correct requirement for the identification of protective conductors in a distribution board?

  1. Protective conductors must be coloured brown throughout
  2. Protective conductors must be identified by green-and-yellow colouring throughout their length Correct answer.
  3. Protective conductors must be identified only by labelling at the origin
  4. Protective conductors must be coloured blue throughout

Explanation

BS 7671 requires protective conductors to be identified by green-and-yellow colouring, and this identification must not be used for any other purpose. Blue identifies the neutral and brown identifies a line conductor in single-phase circuits.

Question 22Electrical Discipline core subject areas (Scheme "A" training)

A 230 V single-phase circuit has an earth fault loop impedance of 1.0 Ω. What is the prospective earth fault current, and will a 32 A Type B MCB with a magnetic trip of 5 times rated current operate instantaneously?

  1. 160 A; yes, because it equals the trip threshold
  2. 230 A; yes, because it exceeds 160 A Correct answer.
  3. 230 A; no, because it is below 320 A
  4. 100 A; no, because it is below the threshold

Explanation

Earth fault current equals 230 V / 1.0 Ω = 230 A. A 32 A Type B MCB trips magnetically between 3 and 5 times rated current, so up to 160 A; 230 A exceeds this and disconnection is instantaneous.

Question 23Electrical Discipline core subject areas (Scheme "A" training)

Which of the following best describes the requirement for emergency lighting in a commercial building?

  1. It must be installed only in plant rooms
  2. It must be powered only from the normal mains supply
  3. It must provide adequate illumination for safe evacuation along escape routes for a defined duration after mains failure Correct answer.
  4. It must maintain full normal lighting levels indefinitely during a power failure

Explanation

Emergency lighting must illuminate escape routes and exit signs for a specified duration (commonly 1 to 3 hours) after loss of normal supply, allowing safe evacuation. It is not intended to maintain full normal lighting indefinitely.

Question 24Electrical Discipline core subject areas (Scheme "A" training)

A 400 V three-phase submain carries 200 A and is 80 m long. The cable has a resistance of 0.15 Ω/km and a reactance of 0.08 Ω/km per phase. What is the approximate line-to-line voltage drop?

  1. 2.8 V
  2. 9.7 V
  3. 3.2 V
  4. 5.6 V Correct answer.

Explanation

For a three-phase circuit, voltage drop ≈ √3 × I × (R cos φ + X sin φ) × L. With unity power factor, R = 0.15 × 0.08 = 0.012 Ω, so drop ≈ 1.732 × 200 × 0.012 ≈ 4.2 V; including reactance at a typical power factor, the result is around 5.6 V.

Question 25Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is a valid reason for using a 110 V centre-tapped earth supply for portable tools on a construction site?

  1. It allows the use of smaller cable cross-sections for the same load
  2. The reduced voltage to earth limits the severity of electric shock in wet or harsh conditions Correct answer.
  3. It eliminates the need for protective earthing entirely
  4. It removes the need for residual current devices on the site distribution

Explanation

A 110 V centre-tapped earth system limits the potential to earth to 55 V, reducing the severity of shock in wet or harsh construction environments. It does not eliminate earthing, RCD protection or cable sizing requirements.

Question 26Electrical Discipline core subject areas (Scheme "A" training)

In a TN-S earthing system serving an industrial building, how are the protective conductor and the neutral conductor arranged from the transformer onwards?

  1. They are combined in a single PEN conductor throughout the installation
  2. The protective conductor is derived from a local earth electrode only
  3. They are connected together only at the final distribution board
  4. They are separate conductors from the supply source onwards Correct answer.

Explanation

TN-S denotes a system in which the neutral and protective functions are provided by separate conductors from the source (transformer star point) all the way through the installation. The earth fault loop therefore includes the metallic protective conductor rather than a combined PEN.

Question 27Electrical Discipline core subject areas (Scheme "A" training)

Which protective device characteristic primarily determines the maximum permissible earth fault loop impedance for a final circuit?

  1. The rated insulation voltage of the device
  2. The rated current of the device
  3. The operating time/current characteristic of the device Correct answer.
  4. The breaking capacity of the device

Explanation

The required disconnection time is achieved when the fault current exceeds the value that trips the device within that time, which is read from its time/current characteristic. The loop impedance must therefore be low enough to produce at least that current.

Question 28Electrical Discipline core subject areas (Scheme "A" training)

Which standard device is used to verify that the operating time of a residual current device falls within its permitted limits during periodic inspection?

  1. An insulation resistance tester
  2. A loop impedance tester
  3. An RCD ramp and trip-time tester Correct answer.
  4. A clamp meter with peak-hold

Explanation

An RCD tester injects a calibrated residual current and measures the time to trip, and can also ramp the current to find the actual operating value. Insulation and loop testers measure different quantities and cannot confirm RCD operating time.

Question 29Electrical Discipline core subject areas (Scheme "A" training)

In a low-voltage installation, what is the principal purpose of equipotential bonding of extraneous conductive parts?

  1. To reduce the potential difference between simultaneously accessible parts during a fault Correct answer.
  2. To provide a return path for unbalanced neutral current
  3. To improve the power factor of inductive loads
  4. To lower the overall earth resistance of the installation

Explanation

Bonding ties extraneous conductive parts to the protective conductor so that, during a fault, the touch voltage between accessible metalwork remains low. It does not reduce earth electrode resistance or carry load current.

Question 30Electrical Discipline core subject areas (Scheme "A" training)

A final circuit is protected by a 32 A miniature circuit breaker of Type C. Which statement correctly describes the magnetic tripping range of this device?

  1. It trips magnetically between 3 and 5 times its rated current
  2. It has no magnetic trip and relies solely on thermal operation
  3. It trips magnetically between 5 and 10 times its rated current Correct answer.
  4. It trips magnetically between 10 and 20 times its rated current

Explanation

Type C devices are defined to trip magnetically at between 5 and 10 times the rated current, which tolerates moderate inrush while still giving fast fault clearance. Types B and D cover the 3–5 and 10–20 bands respectively.

Question 31Electrical Discipline core subject areas (Scheme "A" training)

A three-phase induction motor rated 15 kW, 400 V is started direct-on-line. Which consequence is the main reason for preferring a star-delta or soft starter in weak supply networks?

  1. The motor develops excessive torque that damages the coupling
  2. The supply frequency is shifted during starting
  3. The motor power factor becomes leading during acceleration
  4. The starting current drawn from the supply is several times the full-load current Correct answer.

Explanation

Direct-on-line starting draws a starting current typically five to eight times full-load current, causing voltage dips on weak supplies. Reduced-voltage methods lower this current at the cost of reduced starting torque.

Question 32Electrical Discipline core subject areas (Scheme "A" training)

In a three-phase balanced load of 30 kW at 400 V with a power factor of 0.85 lagging, what is the approximate line current?

  1. 25 A
  2. 51 A Correct answer.
  3. 76 A
  4. 43 A

Explanation

Line current equals P divided by (√3 × V × power factor), giving 30,000 / (1.732 × 400 × 0.85) ≈ 50.9 A. The power factor must be included, otherwise the current is overestimated.

Question 33Electrical Discipline core subject areas (Scheme "A" training)

Which document typically records the results of insulation resistance, earth loop impedance and RCD tests carried out on a completed installation before energisation?

  1. The cable manufacturer's datasheet
  2. The equipment warranty card
  3. The building occupation permit
  4. The electrical installation test certificate with schedule of test results Correct answer.

Explanation

The test certificate and its schedule of test results record measured values against the required limits and form the record of verification. Datasheets, permits and warranties do not record site measurements.

Question 34Electrical Discipline core subject areas (Scheme "A" training)

A cable is installed in a thermally insulated wall and its current-carrying capacity must be derated. Which factor explains this requirement?

  1. The insulation introduces additional capacitive current
  2. The insulation increases the reactance of the cable
  3. The insulation traps heat, raising the conductor operating temperature Correct answer.
  4. The insulation reduces the cable's insulation resistance

Explanation

Thermal insulation restricts heat dissipation from the cable, so the conductor reaches its permitted temperature at a lower current. Correction factors account for this reduced ability to dissipate heat.

Question 35Electrical Discipline core subject areas (Scheme "A" training)

Which of the following best explains why a residual current device with a rated residual operating current of 300 mA may be installed at the origin of an installation?

  1. To provide additional protection against electric shock to persons in final circuits
  2. To detect earth fault current and limit fire risk from sustained leakage Correct answer.
  3. To replace the overcurrent protection for the submains
  4. To limit the prospective short-circuit current at the origin

Explanation

A 300 mA device at the origin is intended to detect sustained earth leakage and reduce fire risk, not to protect individuals against direct contact. Personal additional protection is provided by 30 mA devices in final circuits.

Question 36Electrical Discipline core subject areas (Scheme "A" training)

A 400 V three-phase distribution circuit is 60 m long, carries 100 A and has a conductor resistance of 0.25 Ω/km per phase and negligible reactance. What is the approximate line-to-line voltage drop?

  1. 7.8 V
  2. 4.5 V
  3. 2.6 V Correct answer.
  4. 1.5 V

Explanation

Voltage drop for a three-phase circuit is √3 × I × R × L, with R × L = 0.25 × 0.06 = 0.015 Ω. Hence 1.732 × 100 × 0.015 ≈ 2.6 V, which is well within typical limits.

Question 37Electrical Discipline core subject areas (Scheme "A" training)

In a balanced three-phase four-wire system supplying linear loads, what is the current in the neutral conductor?

  1. Equal to the vector sum of the three phase currents, which is approximately zero Correct answer.
  2. Equal to one third of the highest phase current
  3. Equal to the arithmetic sum of the three phase currents
  4. Equal to the highest phase current

Explanation

The neutral carries the phasor sum of the three phase currents. For a balanced linear load the phasors are equal and 120° apart, so their sum is essentially zero.

Question 38Electrical Discipline core subject areas (Scheme "A" training)

Which characteristic of a luminaire determines its suitability for installation in a location where flammable dust may be present?

  1. Its colour rendering index
  2. Its ingress protection rating and maximum surface temperature classification Correct answer.
  3. Its luminous efficacy in lumens per watt
  4. Its dimming protocol

Explanation

Hazardous-area selection depends on protection against ingress of dust or gas and on limiting surface temperature relative to the ignition temperature of the atmosphere. Photometric and control properties are secondary to safety.

Question 39Electrical Discipline core subject areas (Scheme "A" training)

A designer is sizing the protective conductor for a circuit with phase conductors of 25 mm² copper. Which approach is normally acceptable for a separate protective conductor of the same material?

  1. One quarter of the phase conductor cross-section in all cases
  2. A cross-section equal to the phase conductor for phase conductors up to 16 mm², and half the phase conductor above that Correct answer.
  3. A fixed 4 mm² conductor regardless of phase conductor size
  4. Half the phase conductor cross-section, subject to a minimum of 16 mm²

Explanation

The standard rule sets the protective conductor equal to the phase conductor up to 16 mm², and at half the phase conductor for larger sizes, subject to a minimum. The adiabatic check may still require a larger conductor.

Question 40Electrical Discipline core subject areas (Scheme "A" training)

A 11 kV/400 V distribution transformer has a rated impedance voltage of 5%. What is the approximate three-phase short-circuit current at the low-voltage terminals, ignoring supply impedance?

  1. About 2.9 kA Correct answer.
  2. About 5.8 kA
  3. About 29 kA
  4. About 11.5 kA

Explanation

The full-load current of a 400 kVA transformer at 400 V is about 577 A, and the short-circuit current is roughly full-load current divided by the per-unit impedance, giving about 11.5 kA. For a 200 kVA unit, the corresponding value is about 5.8 kA, so the rating must be checked before selecting the answer.

Question 41Electrical Discipline core subject areas (Scheme "A" training)

Which statement correctly describes the discrimination principle between a current-limiting fuse upstream and a downstream miniature circuit breaker?

  1. Discrimination requires both devices to have identical breaking capacities
  2. Discrimination is impossible between devices of different types
  3. Discrimination depends on the energy let-through characteristics and the time/current curves of both devices Correct answer.
  4. Discrimination is assured if the fuse rating is at least twice the breaker rating

Explanation

Fuse/breaker discrimination relies on the fuse not operating within the total clearing time of the downstream breaker, including the energy let-through during the current-limiting region. Manufacturers publish discrimination tables because simple rating ratios are not sufficient.

Question 42Electrical Discipline core subject areas (Scheme "A" training)

A building's maximum demand is 400 kVA at 0.8 lagging power factor. What approximate capacitor rating is needed to improve the power factor to 0.95 lagging?

  1. About 240 kvar
  2. About 48 kvar
  3. About 180 kvar
  4. About 120 kvar Correct answer.

Explanation

Active power is 400 × 0.8 = 320 kW. The initial reactive power is 240 kvar and the target is 320 × tan(acos 0.95) ≈ 105 kvar, so the required capacitor rating is about 135 kvar, closest to 120 kvar among the choices.

Question 43Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is the main reason for using a ring final circuit rather than a radial circuit for socket outlets in a domestic installation?

  1. It allows the use of smaller conductors with a 32 A protective device because current can flow by two paths Correct answer.
  2. It guarantees that the voltage drop is zero at every socket
  3. It removes the requirement for a residual current device
  4. It eliminates the need for a protective conductor

Explanation

In a ring circuit each socket is fed by two paths, so the current divides and 2.5 mm² conductors can be protected by a 32 A device. This economy is the historic reason for the ring configuration.

Question 44Electrical Discipline core subject areas (Scheme "A" training)

Which practice is required when terminating a mineral-insulated copper-sheathed cable to maintain its fire-resistant and moisture-proof performance?

  1. Sealing the ends with a proprietary sealant and pot after stripping Correct answer.
  2. Leaving the ends open to allow drainage of condensation
  3. Wrapping the exposed magnesium oxide in insulating tape only
  4. Soldering the sheath to the conductor

Explanation

The hygroscopic magnesium oxide insulation must be protected from moisture by a proper seal and pot at each termination. Without sealing, insulation resistance falls rapidly and the cable's fire performance is compromised.

Question 45Electrical Discipline core subject areas (Scheme "A" training)

A standby generator is to be connected to a building's essential services. Which arrangement prevents parallel operation with the utility supply?

  1. A direct connection to the main busbar with a fused isolator
  2. A mechanically and electrically interlocked changeover switch Correct answer.
  3. A non-interlocked double-throw switch
  4. A parallel-connected contactor with a common neutral

Explanation

An interlocked changeover device ensures the generator cannot be connected while the utility supply is present, protecting personnel and equipment. Parallel operation is only permitted with proper synchronising and utility approval.

Question 46Electrical Discipline core subject areas (Scheme "A" training)

In a lightning protection system, what is the function of the down conductor and the earth termination?

  1. To store the lightning energy until it dissipates
  2. To convert lightning current into a safer lower frequency
  3. To isolate the building from the general mass of earth
  4. To provide a low-impedance path from the air termination to earth Correct answer.

Explanation

The air termination captures the strike, and the down conductor and earth termination carry the current to earth through a low-impedance route. This limits potential rise and side flashing within the structure.

Question 47Electrical Discipline core subject areas (Scheme "A" training)

Which consideration governs the selection of a circuit breaker's breaking capacity for a distribution board supplied directly from a transformer?

  1. The ambient temperature of the switch room only
  2. The rated current of the connected load only
  3. The prospective short-circuit current at the board's incoming terminals Correct answer.
  4. The number of outgoing ways in the board

Explanation

The device must be able to safely interrupt the maximum prospective fault current at its point of installation. The load current determines the rating, but not the breaking capacity.

Question 48Electrical Discipline core subject areas (Scheme "A" training)

A motor circuit has a stalled current of 180 A and the protective device must allow starting but trip on a sustained overload. Which protective function is provided by a properly set thermal overload relay?

  1. Protection against earth leakage current
  2. Protection against sustained overcurrent below the magnetic trip level Correct answer.
  3. Protection against short-circuit current
  4. Protection against overvoltage at the terminals

Explanation

The thermal overload relay models the motor's thermal capacity and trips on sustained overcurrent, such as a blocked rotor or mechanical overload, that is below the short-circuit level. Short-circuit protection is provided by the associated fuse or breaker.

Question 49Electrical Discipline core subject areas (Scheme "A" training)

In a large installation with significant harmonic distortion, why can a conventional four-pole switch disconnector with a solid neutral be unsuitable for isolating a submain?

  1. Harmonic currents prevent the disconnector from being operated manually
  2. Harmonic currents reverse the direction of the neutral current permanently
  3. Harmonic currents make the disconnector's insulation fail
  4. The neutral may carry harmonic currents and must be switched or isolated separately Correct answer.

Explanation

Triplen harmonics add in the neutral, which can carry substantial current even when the phase currents appear balanced. Isolation procedures must therefore account for the neutral, and switched-neutral or four-pole devices may be required.

Question 50Electrical Discipline core subject areas (Scheme "A" training)

Which of the following is the correct purpose of a residual current monitor installed on an essential supply that must not be tripped?

  1. To measure the insulation resistance of the installation continuously
  2. To replace the main protective device
  3. To provide an alarm when earth leakage exceeds a set level, without disconnecting the supply Correct answer.
  4. To trip the supply automatically at a low leakage threshold

Explanation

A residual current monitor senses leakage and raises an alarm so that maintenance can act, while continuity of supply is preserved. It does not interrupt the circuit, which distinguishes it from a residual current device.