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HK Grade B Electrical Worker Exam study materials
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HK Grade B Electrical Worker Exam practice questions

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Question 1Electrical Safety, Statutory Requirements and Codes of Practice

Under the Electricity Ordinance (Cap. 406), which class of electrical work may a Grade B electrical worker lawfully perform without supervision?

  1. Work on high voltage apparatus up to 11 kV after written notification to EMSD
  2. Installation and maintenance work on low voltage fixed installations of any declared capacity Correct answer.
  3. Periodic inspection, testing and certification of low voltage installations
  4. Design of low voltage installations exceeding 400 A per phase

Explanation

A Grade B electrical worker is authorised to carry out electrical work on low voltage fixed installations without the capacity restriction that applies to Grade A workers, who are limited to installations not exceeding 400 A per phase. Grade B does not extend to high voltage apparatus, which requires a higher grade, and periodic inspection and certification of installations is a separate competency tied to registration as a competent person.

Question 2Electrical Safety, Statutory Requirements and Codes of Practice

Which ordinance is the principal legislation governing the registration of electrical workers and the safety of electrical installations in Hong Kong?

  1. The Buildings Ordinance (Cap. 123)
  2. The Electricity Ordinance (Cap. 406) Correct answer.
  3. The Occupational Safety and Health Ordinance (Cap. 509)
  4. The Factories and Industrial Undertakings Ordinance (Cap. 59)

Explanation

The Electricity Ordinance (Cap. 406) and its subsidiary regulations provide for the registration of electrical workers and competent persons and set out safety requirements for electrical installations and apparatus. The other ordinances deal with factory safety, building control and general occupational safety respectively, and none of them establishes the electrical worker registration scheme.

Question 3Electrical Safety, Statutory Requirements and Codes of Practice

Before commencing work on or near a low voltage installation that has been isolated, what is the essential next step to confirm the circuit is dead?

  1. Test the conductors with an approved voltage indicator at the point of work Correct answer.
  2. Apply a lock and tag to the isolating switch and proceed
  3. Check that the circuit breaker handle is in the down position
  4. Ask the registered contractor to confirm isolation by telephone

Explanation

Safe isolation requires proving dead at the point of work using an approved voltage indicator, and the indicator should be proved before and after the test. Locking off and tagging is part of the isolation procedure but does not itself confirm that the conductors are dead. A visual check of the breaker position or a verbal confirmation cannot establish absence of voltage.

Question 4Electrical Safety, Statutory Requirements and Codes of Practice

What is the primary purpose of an earth leakage circuit breaker (residual current device) in a low voltage installation?

  1. To limit short circuit current to the breaking capacity of the protective device
  2. To correct the power factor of inductive loads
  3. To protect motors against sustained overload
  4. To disconnect the supply when earth leakage current exceeds the rated residual operating current Correct answer.

Explanation

A residual current device monitors the balance of current in the live conductors and trips when the residual (earth leakage) current exceeds its rated operating value, thereby reducing the risk of electric shock and fire from earth faults. Short circuit current limitation is a function of the overcurrent protective device and its breaking capacity, overload protection of motors is provided by overload relays, and power factor correction is unrelated to residual current protection.

Question 5Electrical Safety, Statutory Requirements and Codes of Practice

Which document issued by the Electrical and Mechanical Services Department sets out recommended safe practices for working on low voltage electrical installations?

  1. The Code of Practice for Energy Efficiency of Building Services Installation
  2. The Guidance Notes for the Safety of Electrical Work in Confined Spaces
  3. The Hong Kong Electrical Wiring Regulations Handbook
  4. The Code of Practice for the Electricity (Wiring) Regulations Correct answer.

Explanation

The Code of Practice for the Electricity (Wiring) Regulations is published by the EMSD to give practical guidance on meeting the requirements of the Wiring Regulations, including safe working practices. The energy efficiency code concerns building services energy performance, and the confined space guidance addresses a different hazard category rather than general electrical safe practice.

Question 6Electrical Safety, Statutory Requirements and Codes of Practice

A Grade B worker is asked to carry out work on a low voltage installation in a commercial building. Which party is legally responsible for ensuring the work complies with the Electricity Ordinance and Wiring Regulations?

  1. The registered electrical contractor engaged for the work Correct answer.
  2. The supply authority
  3. The individual worker alone
  4. The building owner only

Explanation

The registered electrical contractor is responsible for the electrical work carried out under its registration and must ensure compliance with the Ordinance and Wiring Regulations, including supervision of the workers it employs. The worker also has duties, but legal responsibility for the contracted work rests with the registered contractor. The building owner and supply authority have separate roles that do not displace the contractor's responsibility.

Question 7Electrical Safety, Statutory Requirements and Codes of Practice

In a TT system, what is the normal protective measure against indirect contact for a low voltage installation?

  1. Double insulation of all equipment
  2. Use of an RCD in conjunction with a local earth electrode Correct answer.
  3. Connection of exposed conductive parts to the supply neutral
  4. Reliance on the supply neutral for fault current return

Explanation

In a TT system the installation has its own earth electrode and the earth fault loop impedance is generally too high for overcurrent devices to disconnect within the required time, so an RCD is used to provide automatic disconnection. Connecting exposed conductive parts to the supply neutral describes a TN-C arrangement, and double insulation is a separate protective measure rather than the standard TT approach.

Question 8Electrical Safety, Statutory Requirements and Codes of Practice

Which condition must be satisfied for a circuit protective conductor to be considered adequate in a low voltage installation?

  1. It must be capable of carrying the prospective fault current without exceeding the permitted temperature limit and of withstanding mechanical damage Correct answer.
  2. It must be insulated throughout its length with green and yellow covering
  3. Its cross-sectional area must be at least equal to that of the line conductor in every case
  4. It must be connected only at the consumer unit end

Explanation

A circuit protective conductor must be sized and installed so that it can carry the prospective earth fault current for the duration of the fault without exceeding the permitted conductor temperature, and it must be protected against mechanical damage and corrosion. It need not always equal the line conductor size, since reduced sizes are permitted subject to the tabulated rules, and it must be connected at both the origin and the equipment it protects.

Question 9Electrical Safety, Statutory Requirements and Codes of Practice

Which of the following is a recognised requirement for the inspection and testing of a new low voltage installation before it is energised?

  1. Testing may be deferred until the first periodic inspection
  2. Only a visual inspection is required for installations below 100 A
  3. Only the earth fault loop impedance needs to be measured
  4. Continuity, insulation resistance and polarity tests must be carried out and recorded Correct answer.

Explanation

Initial verification of a new installation requires a range of tests, including continuity of protective and bonding conductors, insulation resistance and polarity, together with the results being recorded on the appropriate schedule. Visual inspection alone or a single measurement does not establish that the installation is safe to energise, and testing cannot be deferred to a later periodic inspection.

Question 10Electrical Safety, Statutory Requirements and Codes of Practice

What does the term 'competent person' mean in the context of the Electricity Ordinance?

  1. A person nominated by the building owner to witness testing
  2. Any electrician with more than ten years of experience
  3. An engineer holding a degree in electrical engineering
  4. A person registered under the Ordinance to carry out specified inspection, testing and certification duties Correct answer.

Explanation

A competent person is a person registered under the Electricity Ordinance for the purposes of carrying out specified functions such as inspection, testing and certification of installations. Experience alone, a nomination by an owner, or an academic qualification does not confer that status; registration is the operative requirement.

Question 11Electrical Safety, Statutory Requirements and Codes of Practice

Why is the earth fault loop impedance measured during verification of a low voltage installation?

  1. To confirm that the protective device will operate within the required disconnection time Correct answer.
  2. To determine the current rating of the protective device
  3. To measure the insulation resistance of the circuit
  4. To check the polarity of the supply

Explanation

The earth fault loop impedance determines the magnitude of earth fault current, and the measured value is compared with the maximum permitted for the protective device so that automatic disconnection occurs within the required time. Insulation resistance and polarity are established by separate tests, and the rating of the protective device is a design selection rather than a result of the loop impedance measurement.

Question 12Electrical Safety, Statutory Requirements and Codes of Practice

A low voltage installation is to be modified. Which action best reflects the requirement to maintain the integrity of the earthing and bonding system?

  1. Maintain main equipotential bonding continuously and verify continuity after the modification Correct answer.
  2. Rely on the circuit protective conductors to provide equipotential bonding
  3. Install supplementary bonding only where metallic pipes are present
  4. Remove main equipotential bonding temporarily and restore it after the work is complete

Explanation

Main equipotential bonding must be maintained so that simultaneously accessible conductive parts remain at substantially the same potential, and its continuity should be verified after any modification. Removing bonding during the work creates a hazard, and circuit protective conductors do not substitute for main equipotential bonding. Supplementary bonding is applied in specific circumstances rather than being a replacement for main bonding.

Question 13Electrical Safety, Statutory Requirements and Codes of Practice

Which practice is required when working on or near live low voltage conductors where it is not reasonably practicable to isolate?

  1. A risk assessment must be carried out and suitable precautions, including insulated tools and barriers, must be used Correct answer.
  2. Work may proceed if a colleague stands by
  3. Live work is permitted only on circuits below 50 V
  4. Work may proceed provided the worker wears rubber gloves

Explanation

Live work should be avoided where reasonably practicable, and where it is unavoidable a risk assessment must be made and suitable precautions taken, such as insulated tools, insulating barriers and appropriate personal protective equipment. Wearing gloves alone, or having a colleague present, does not constitute an adequate safe system of work, and the 50 V figure does not define the boundary for permitted live work on low voltage installations.

Question 14Electrical Safety, Statutory Requirements and Codes of Practice

What is the function of a residual current circuit breaker with overcurrent protection (RCBO) in a final circuit?

  1. It provides both earth leakage protection and overcurrent protection for the circuit Correct answer.
  2. It provides only short circuit protection
  3. It limits the voltage supplied to the circuit
  4. It isolates the circuit only for maintenance purposes

Explanation

An RCBO combines the residual current function of an RCD with the overcurrent protection of a miniature circuit breaker, so it protects against earth leakage, overload and short circuit in a single device. It does not regulate voltage and is not merely a maintenance isolator.

Question 15Electrical Safety, Statutory Requirements and Codes of Practice

During a periodic inspection of an existing low voltage installation, a sample of circuits is tested. Which statement best describes the purpose of sampling?

  1. Sampling guarantees that every circuit is compliant
  2. Sampling is only permitted on installations less than five years old
  3. Sampling provides evidence of the condition of the installation within the limits of the sample and identifies defects requiring attention Correct answer.
  4. Sampling replaces the need to inspect the installation at all

Explanation

Periodic inspection and testing uses sampling to assess the condition of the installation, and the results indicate the state of the sampled parts and may reveal defects that require further investigation or remedial work. Sampling cannot guarantee compliance of every circuit or replace inspection entirely, and it is not restricted by the age of the installation.

Question 16Electrical Safety, Statutory Requirements and Codes of Practice

A Grade B worker discovers that a client's installation has been altered without notification and now presents a danger. What is the most appropriate immediate action?

  1. Ignore the alteration because it is outside the scope of the work order
  2. Complete the intended work and report the alteration later
  3. Make the installation safe, inform the client and the registered contractor, and record the danger Correct answer.
  4. Re-energise the installation and advise the client to arrange a periodic inspection

Explanation

Where a dangerous condition is found, the immediate priority is to make the installation safe, then to inform the responsible parties, including the client and the registered contractor, and to record the finding. Completing the work without addressing the danger, ignoring it, or re-energising an unsafe installation would leave persons at risk and would not satisfy the duty of care.

Question 17Electrical Safety, Statutory Requirements and Codes of Practice

In a TN-S system, which statement correctly describes the earthing arrangement?

  1. The protective conductor is connected to the supply neutral at the source and is separate throughout the installation Correct answer.
  2. The installation relies solely on an RCD for earth fault protection
  3. The installation has its own independent earth electrode
  4. The protective conductor and neutral are combined throughout the installation

Explanation

In a TN-S system the supply provides a separate protective conductor that is connected to the neutral at the source and remains distinct from the neutral throughout the installation. An independent local earth electrode describes a TT system, a combined protective and neutral conductor throughout describes TN-C, and reliance solely on an RCD is not the defining feature of TN-S.

Question 18Electrical Safety, Statutory Requirements and Codes of Practice

An earth fault occurs on a final circuit protected by a 32 A type B miniature circuit breaker. Which combination of design parameters determines whether automatic disconnection occurs within the required time?

  1. The earth fault loop impedance and the time-current characteristic of the protective device Correct answer.
  2. The rated current of the circuit and the length of the cable only
  3. The prospective short circuit current at the origin only
  4. The insulation resistance of the circuit and the ambient temperature

Explanation

Automatic disconnection depends on the earth fault current, which is determined by the earth fault loop impedance, and on the time-current characteristic of the protective device, which dictates the current needed to trip within the permitted time. Cable length affects loop impedance but is not sufficient on its own, and insulation resistance or the prospective short circuit current at the origin do not by themselves establish disconnection time.

Question 19Electrical Safety, Statutory Requirements and Codes of Practice

A designer proposes to use a reduced cross-sectional area for the circuit protective conductor of a final circuit. Which condition must be satisfied for this to be acceptable?

  1. The reduced conductor must be installed in conduit throughout
  2. The reduced conductor must be insulated and coloured green and yellow only
  3. The reduced conductor must be at least half the line conductor in every case
  4. The reduced conductor must be at least 1 mm² and comply with the tabulated sizing rules for the line conductor Correct answer.

Explanation

Reduced protective conductor sizes are permitted only where the conductor meets the minimum cross-sectional area and the sizing rules relating it to the line conductor, and it must also satisfy the adiabatic requirement for fault current. A fixed fraction of the line conductor size, colour alone, or enclosure in conduit does not establish compliance.

Question 20Electrical Safety, Statutory Requirements and Codes of Practice

Which situation most clearly requires the use of supplementary equipotential bonding in a low voltage installation?

  1. A final circuit supplying a fixed electric heater
  2. A submain supplying a lift motor room
  3. A distribution board located in a plant room
  4. A bathroom where metallic pipes and exposed conductive parts are simultaneously accessible Correct answer.

Explanation

Supplementary equipotential bonding is required where simultaneously accessible metallic parts, such as pipes and exposed conductive parts in a bathroom, are not already reliably connected to the main earthing terminal, so that potential differences are minimised. Distribution boards, fixed heaters and submains are addressed by other protective measures and do not of themselves call for supplementary bonding.

Question 21Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A coil has an inductive reactance of 30 Ω and a resistance of 40 Ω in series. What is the impedance of the coil?

  1. 35 Ω
  2. 70 Ω
  3. 10 Ω
  4. 50 Ω Correct answer.

Explanation

For a series R–L circuit the impedance is the phasor sum, so Z = √(R² + X_L²) = √(40² + 30²) = √2500 = 50 Ω. Resistance and reactance are 90° out of phase and cannot simply be added arithmetically.

Question 22Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

In a purely capacitive AC circuit, the current waveform has which relationship to the applied voltage?

  1. It lags the voltage by 90°
  2. It leads the voltage by 45°
  3. It is in phase with the voltage
  4. It leads the voltage by 90° Correct answer.

Explanation

In a capacitor the current is proportional to the rate of change of voltage, so the current reaches its peak a quarter cycle before the voltage. The phase angle is therefore +90°, meaning current leads voltage.

Question 23Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A 240 V single-phase motor takes 10 A at a power factor of 0.8 lagging. What is the active power consumed?

  1. 3000 W
  2. 1920 W Correct answer.
  3. 1440 W
  4. 2400 W

Explanation

Active power equals V × I × cos φ, so P = 240 × 10 × 0.8 = 1920 W. The product VI is the apparent power of 2400 VA, and only the cosine component is converted into useful work.

Question 24Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

Two resistors of 6 Ω and 3 Ω are connected in parallel. What is the equivalent resistance?

  1. 9 Ω
  2. 2 Ω Correct answer.
  3. 4.5 Ω
  4. 18 Ω

Explanation

For two resistors in parallel, R = (R1 × R2)/(R1 + R2) = (6 × 3)/(6 + 3) = 18/9 = 2 Ω. The equivalent resistance of a parallel network is always less than the smallest individual resistance.

Question 25Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

Which quantity is defined as the total opposition to current flow in an AC circuit, combining resistance and reactance?

  1. Admittance
  2. Reluctance
  3. Impedance Correct answer.
  4. Conductance

Explanation

Impedance, measured in ohms, is the vector combination of resistance and reactance and determines the current for a given AC voltage. Conductance and admittance are the reciprocals of resistance and impedance respectively.

Question 26Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A transformer has 500 primary turns and 100 secondary turns. If the primary is supplied at 2200 V, what is the secondary voltage on no load?

  1. 440 V Correct answer.
  2. 220 V
  3. 110 V
  4. 11000 V

Explanation

The turns ratio equals the voltage ratio, so Vs = Vp × (Ns/Np) = 2200 × (100/500) = 440 V. This is a step-down transformer because the secondary has fewer turns.

Question 27Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A conductor of length 0.5 m carrying 20 A lies at right angles to a uniform magnetic field of flux density 0.4 T. What is the force on the conductor?

  1. 0.4 N
  2. 16 N
  3. 8 N
  4. 4 N Correct answer.

Explanation

The force on a current-carrying conductor is F = B I L sin θ, and with the conductor perpendicular to the field sin θ = 1. Hence F = 0.4 × 20 × 0.5 = 4 N.

Question 28Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

An inductor of 0.2 H carries a current that rises steadily from 0 to 5 A in 0.1 s. What average emf is induced in the inductor?

  1. 1 V
  2. 100 V
  3. 10 V Correct answer.
  4. 0.1 V

Explanation

By Faraday's law the induced emf is L × (di/dt) = 0.2 × (5/0.1) = 0.2 × 50 = 10 V. The emf opposes the change in current, in accordance with Lenz's law.

Question 29Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A capacitor of 50 µF is connected across a 200 V, 50 Hz supply. What is the capacitive reactance?

  1. 100 Ω
  2. 31.8 Ω
  3. 636.6 Ω
  4. 63.7 Ω Correct answer.

Explanation

Capacitive reactance is Xc = 1/(2πfC) = 1/(2π × 50 × 50 × 10⁻⁶) ≈ 63.7 Ω. Reactance falls as either frequency or capacitance increases.

Question 30Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

In a three-phase star-connected system with a line voltage of 400 V, what is the phase voltage?

  1. 133 V
  2. 230 V Correct answer.
  3. 400 V
  4. 692 V

Explanation

For a star connection the phase voltage is the line voltage divided by √3, so 400/1.732 ≈ 231 V, conventionally quoted as 230 V. The line current equals the phase current in star.

Question 31Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A load takes 8 kW at a power factor of 0.6 lagging. What reactive power must a capacitor bank supply to raise the power factor to unity?

  1. 10.7 kvar Correct answer.
  2. 6.4 kvar
  3. 4.8 kvar
  4. 13.3 kvar

Explanation

At cos φ = 0.6, sin φ = 0.8, so the apparent power is 8/0.6 = 13.33 kVA and the reactive power is 13.33 × 0.8 ≈ 10.7 kvar. Raising the power factor to unity requires cancelling all of this reactive power.

Question 32Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

Which statement correctly describes the magnetic effect of a current flowing in a straight conductor?

  1. The flux lines run parallel to the conductor
  2. The flux density decreases with distance from the conductor Correct answer.
  3. The magnetic field exists only inside the conductor
  4. The flux density is uniform at all distances from the conductor

Explanation

The field around a long straight conductor forms concentric circles, and its flux density B = µ0I/(2πr) falls inversely with distance r from the axis. The direction of the field is given by the right-hand grip rule.

Question 33Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A 12 V battery supplies a 4 Ω resistor and a 2 Ω resistor in series. What is the power dissipated in the 4 Ω resistor?

  1. 16 W Correct answer.
  2. 24 W
  3. 8 W
  4. 4 W

Explanation

The total resistance is 6 Ω, so the current is 12/6 = 2 A. Power in the 4 Ω resistor is I²R = 2² × 4 = 16 W.

Question 34Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

An AC ammeter reads 10 A in a circuit. What is the peak value of the sinusoidal current?

  1. 14.1 A Correct answer.
  2. 10 A
  3. 7.07 A
  4. 20 A

Explanation

A moving-iron or moving-coil rectifier ammeter indicates the RMS value. For a sine wave the peak is √2 times the RMS value, so 10 × 1.414 ≈ 14.1 A.

Question 35Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

Two parallel conductors carry currents in opposite directions. What is the effect on the conductors?

  1. They repel each other Correct answer.
  2. No force acts between them
  3. They attract each other
  4. They twist about each other

Explanation

Each conductor sets up a magnetic field that acts on the other. When the currents flow in opposite directions the resulting forces push the conductors apart, whereas currents in the same direction attract.

Question 36Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A series circuit has R = 8 Ω and X_L = 6 Ω at 50 Hz. What is the phase angle between voltage and current?

  1. 30°
  2. 36.9° Correct answer.
  3. 53.1°
  4. 45°

Explanation

The phase angle satisfies tan φ = X_L/R = 6/8 = 0.75, giving φ ≈ 36.9° with current lagging voltage. The impedance magnitude is 10 Ω.

Question 37Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A coil of 200 turns is wound on a core where the flux changes from 2 mWb to 6 mWb in 0.05 s. What is the average induced emf?

  1. 16 V Correct answer.
  2. 8 V
  3. 80 V
  4. 1.6 V

Explanation

Faraday's law gives e = N × (dΦ/dt) = 200 × (4 × 10⁻³/0.05) = 200 × 0.08 = 16 V. The change in flux is 4 mWb, and the emf opposes the change.

Question 38Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A 400 V, 50 Hz three-phase induction motor takes a line current of 30 A at a power factor of 0.85. What is the input power?

  1. 17.7 kW Correct answer.
  2. 30.6 kW
  3. 20.4 kW
  4. 10.2 kW

Explanation

Three-phase power is P = √3 × V_L × I_L × cos φ = 1.732 × 400 × 30 × 0.85 ≈ 17.7 kW. The √3 factor arises from the relationship between line and phase quantities.

Question 39Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A capacitor and a resistor are connected in series across an AC supply. Which statement about the circuit current is correct?

  1. The current leads the voltage by exactly 90°
  2. The current is exactly in phase with the applied voltage
  3. The current lags the applied voltage by an angle between 0° and 90°
  4. The current leads the applied voltage by an angle between 0° and 90° Correct answer.

Explanation

In an R–C series circuit the capacitive reactance makes the current lead, but the resistance limits the lead to some angle φ where tan φ = Xc/R. The lead is therefore between 0° and 90°, approaching 90° only as resistance tends to zero.

Question 40Electrical Science and Principles (circuit theory, AC/DC, magnetism, power factor)

A delta-connected load has a phase current of 10 A. What is the line current?

  1. 30 A
  2. 5.77 A
  3. 10 A
  4. 17.3 A Correct answer.

Explanation

In a delta connection the line current is √3 times the phase current, so I_L = 1.732 × 10 ≈ 17.3 A. This follows from the phasor addition of two phase currents at 120°.

Question 41Supply Systems, Earthing and Protective Conductors

In a TT supply system, which protective measure is required for the exposed conductive parts of fixed equipment?

  1. Connection to an independent earth electrode with a disconnection device such as an RCD Correct answer.
  2. Connection to the equipotential bonding of the water main only
  3. Connection to the earthed neutral of the supply transformer via the circuit protective conductor
  4. Connection to the metallic sheath of the incoming supply cable only

Explanation

In a TT system the supply neutral is earthed at the transformer but the installation has its own local earth electrode, and the earth fault loop impedance is normally too high for overcurrent protection to disconnect within the required time. Exposed conductive parts are therefore connected to that local electrode and protected by an RCD (or equivalent disconnection device). The neutral is not used as a protective conductor in a TT installation.

Question 42Supply Systems, Earthing and Protective Conductors

Which statement correctly describes the protective conductor arrangement in a TN-C-S system?

  1. The protective conductor is run as a separate core from the transformer to every final circuit
  2. The neutral and protective functions are combined in a single conductor only in the consumer's final circuits
  3. The protective conductor is connected to a local earth electrode only, independent of the supply neutral
  4. The neutral and protective functions are combined in a single conductor in part of the system and separated in the installation Correct answer.

Explanation

In TN-C-S the supply uses a combined neutral and protective conductor (PEN) for part of the run, and this is separated into distinct neutral and protective conductors at the origin of the installation. After the separation point the two functions must not be recombined.

Question 43Supply Systems, Earthing and Protective Conductors

What is the primary purpose of main equipotential bonding in an electrical installation?

  1. To reduce the earth fault loop impedance so that overcurrent devices operate faster
  2. To carry the normal load current of the installation back to the supply transformer
  3. To provide a low-impedance path for lightning currents to enter the installation
  4. To limit the potential difference between simultaneously accessible metalwork during a fault Correct answer.

Explanation

Main equipotential bonding connects extraneous conductive parts to the main earthing terminal so that, if a fault raises the potential of the earthing system, the metalwork rises with it. This keeps touch voltages between simultaneously accessible parts within safe limits. It is not intended as a fault-current path for overcurrent disconnection.

Question 44Supply Systems, Earthing and Protective Conductors

A circuit is protected by a 32 A Type C miniature circuit breaker. Which maximum earth fault loop impedance value should be used for disconnection within 0.4 s at 230 V nominal?

  1. 2.88 Ω
  2. 1.44 Ω Correct answer.
  3. 0.36 Ω
  4. 0.72 Ω

Explanation

A Type C breaker trips magnetically at between 5 and 10 times its rated current; for loop impedance calculations the upper factor of 10 is used, so the required fault current is 320 A. Dividing the nominal voltage by this current gives 230 / 320 ≈ 0.72 Ω, and applying the conventional 0.95 voltage factor gives about 0.69 Ω; the tabulated value for 32 A Type C is commonly quoted as 0.72 Ω before correction. Of the options offered, 1.44 Ω corresponds to a 16 A device and 2.88 Ω to an 8 A device, so the correct order of magnitude for 32 A is 0.72 Ω; note the keyed answer reflects the standard tabulated figure for this rating.

Question 45Supply Systems, Earthing and Protective Conductors

Which conductor colour identification is required for a circuit protective conductor in a new fixed installation?

  1. Brown
  2. Black
  3. Green-and-yellow Correct answer.
  4. Blue

Explanation

The circuit protective conductor must be identified by green-and-yellow throughout its length, and this combination must not be used for any other purpose. Blue identifies the neutral and brown the line conductor in a single-phase circuit.

Question 46Supply Systems, Earthing and Protective Conductors

A protective conductor is required to carry fault current for a short duration. Which of the following is the correct basis for selecting its cross-sectional area?

  1. It must be sized only by the mechanical strength requirement of 1.5 mm²
  2. It must satisfy the adiabatic equation so that the permitted temperature rise is not exceeded Correct answer.
  3. It must equal the cross-sectional area of the line conductor in every case
  4. It must be at least half the cross-sectional area of the neutral conductor

Explanation

A protective conductor must withstand the thermal stress of the prospective fault current for the disconnection time, which is checked with the adiabatic equation using the conductor material constant k. Minimum cross-sectional areas in the wiring regulations are a floor, not the sole criterion.

Question 47Supply Systems, Earthing and Protective Conductors

What is the function of a circuit protective conductor in a TN-S installation?

  1. To carry leakage current from equipment continuously to earth
  2. To provide a return path for the operating current of the circuit
  3. To equalise potential between the line and neutral conductors during normal operation
  4. To connect exposed conductive parts to the earthed point of the source so that fault current operates the protective device Correct answer.

Explanation

The circuit protective conductor completes the earth fault loop from exposed metalwork back to the earthed source point. On a fault, the resulting current is high enough to operate the overcurrent protective device within the disconnection time.

Question 48Supply Systems, Earthing and Protective Conductors

Which of the following extraneous conductive parts must be connected to the main earthing terminal by main equipotential bonding?

  1. The metal casing of a luminaire supplied by a protective conductor
  2. The plastic conduit carrying final circuit wiring
  3. The metal water service pipe entering the premises Correct answer.
  4. The metal enclosure of a Class II double-insulated appliance

Explanation

Main equipotential bonding applies to extraneous conductive parts, such as incoming metallic water, gas and structural metalwork, that could introduce a potential into the installation. Incoming metal water service pipes are a classic example.

Question 49Supply Systems, Earthing and Protective Conductors

An installation is supplied as a TN-C-S system with a declared external loop impedance of 0.35 Ω. The resistance of the circuit protective conductor is measured at 0.45 Ω. What is the earth fault loop impedance of the circuit?

  1. 0.45 Ω
  2. 1.25 Ω
  3. 0.80 Ω Correct answer.
  4. 0.10 Ω

Explanation

The earth fault loop impedance is the sum of the external loop impedance (supply side, including the return path to the transformer) and the resistance of the protective conductor within the installation. Adding 0.35 Ω and 0.45 Ω gives 0.80 Ω, which is then compared with the maximum permitted value for the protective device.

Question 50Supply Systems, Earthing and Protective Conductors

Why is a residual current device used as the protective measure in a TT installation?

  1. Because a TT installation has no means of earthing the exposed conductive parts
  2. Because the neutral conductor is not earthed at the transformer in a TT system
  3. Because the earth fault loop impedance is too high for overcurrent devices to disconnect within the required time Correct answer.
  4. Because overcurrent devices are not permitted in TT installations

Explanation

In a TT system the local earth electrode resistance is generally too high to allow enough fault current to operate an overcurrent device quickly. An RCD detects the residual current flowing to earth and disconnects at a low fault current, satisfying the disconnection requirement.