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CIC Grade A Electrical Trade Test practice questions

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Question 1Construction-related E&M Trades

Under Hong Kong electrical wiring regulations, what is the standard nominal low-voltage supply for a single-phase domestic installation?

  1. 220 V, 50 Hz Correct answer.
  2. 240 V, 60 Hz
  3. 110 V, 60 Hz
  4. 380 V, 50 Hz

Explanation

Hong Kong low-voltage single-phase domestic supplies are nominal 220 V at 50 Hz, which is the declared voltage used for sizing appliances and protective devices. Three-phase supplies are 380 V at 50 Hz. Frequency is standardised at 50 Hz throughout the territory, so any 60 Hz option is incorrect for local mains.

Question 2Construction-related E&M Trades

A final circuit is protected by a 20 A miniature circuit breaker. Which conductor size of PVC-insulated single-core cable in conduit is the minimum normally acceptable for this circuit, assuming no derating factors apply?

  1. 1.5 mm²
  2. 2.5 mm² Correct answer.
  3. 6 mm²
  4. 4 mm²

Explanation

A 2.5 mm² copper PVC-insulated conductor in conduit has a current-carrying capacity of roughly 20 A under reference conditions, matching a 20 A protective device. The tabulated capacity must be at least the device rating, so the next smaller standard size fails. Derating for grouping or ambient temperature would require a larger conductor.

Question 3Construction-related E&M Trades

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

  1. To limit fault current to the design value
  2. To protect the circuit against sustained overload only
  3. To disconnect the supply when residual current exceeds the sensitivity rating Correct answer.
  4. To correct the power factor of inductive loads

Explanation

An RCD monitors the algebraic sum of currents in live and neutral conductors and trips when imbalance, representing leakage to earth, exceeds its rated residual operating current. It protects people against electric shock and limits fire risk from earth leakage. It does not provide overload protection on its own unless combined with an overcurrent device.

Question 4Construction-related E&M Trades

Which test instrument is used to verify the continuity of protective conductors and ring final circuit conductors?

  1. Earth loop impedance tester
  2. Low-resistance ohmmeter Correct answer.
  3. Insulation resistance tester at 500 V
  4. Clamp meter set to AC current

Explanation

Continuity of protective and ring conductors is verified with a low-resistance ohmmeter, often a multifunction tester set to continuity, which applies a small test current and displays resistance in ohms. Readings should be stable and within expected values derived from conductor length and cross-section. Insulation resistance testing uses a high test voltage and is a different test.

Question 5Construction-related E&M Trades

In a metal conduit installation, what is the main reason for installing a separate circuit protective conductor inside the conduit rather than relying on the conduit itself?

  1. To reduce voltage drop along the circuit
  2. To provide a reliable low-impedance earth path that is not dependent on screwed joints Correct answer.
  3. To increase the current-carrying capacity of the live conductors
  4. To satisfy the requirement for double insulation

Explanation

Metal conduit can serve as a protective conductor only where continuity of joints is assured and the cross-section is adequate. A separate copper conductor gives a dependable low-impedance earth path that is unaffected by corrosion or loosened couplings. This improves fault loop impedance and disconnection performance.

Question 6Construction-related E&M Trades

What does the term 'diversity factor' account for when calculating the maximum demand of an installation?

  1. The variation in supply voltage during peak hours
  2. The likelihood that not all connected loads operate simultaneously at full rating Correct answer.
  3. The reduction in cable rating due to ambient temperature
  4. The imbalance between phases in a three-phase system

Explanation

Diversity recognises that the total connected load is rarely drawn at once, so the maximum demand is calculated by applying diversity factors to individual load groups. This allows economic sizing of the intake and main switchgear while maintaining adequate capacity. It is distinct from derating factors applied to cables.

Question 7Construction-related E&M Trades

Which of the following is the correct sequence for safe isolation before working on a low-voltage circuit?

  1. Test dead, isolate, prove the tester, lock off
  2. Lock off, test dead, isolate, prove the tester
  3. Prove the tester, isolate, lock off, test live
  4. Isolate, lock off, prove the tester, test dead, prove the tester again Correct answer.

Explanation

Safe isolation requires isolating the supply, securing the isolation with a lock and warning notice, then proving the voltage tester on a known source, testing the conductors to confirm dead, and re-proving the tester to confirm it still functions. The double proving step guards against a faulty tester giving a false dead reading. Work should only commence after the circuit is confirmed dead.

Question 8Construction-related E&M Trades

A fluorescent luminaire flickers and the tube ends glow dimly. Which component is the most likely cause in a magnetic ballast circuit?

  1. The starter Correct answer.
  2. The diffuser
  3. The ceiling rose
  4. The reflector

Explanation

In a magnetic ballast fluorescent circuit, the starter provides the initial preheat and then opens to strike the arc. A failed or sticking starter commonly causes flickering and dim end glow because the tube cannot establish a stable discharge. Replacing the starter is the standard first diagnostic step.

Question 9Construction-related E&M Trades

What is the purpose of a fused connection unit (FCU) in a fixed wiring installation?

  1. To convert AC supply to DC for the appliance
  2. To provide local isolation and fuse protection for a fixed appliance Correct answer.
  3. To step down the supply voltage to the appliance
  4. To measure the energy consumed by the appliance

Explanation

A fused connection unit incorporates a fuse that protects the downstream fixed appliance and its flexible cord, and it provides a means of local isolation. The fuse rating is selected to suit the appliance and cable. It does not transform or convert the supply.

Question 10Construction-related E&M Trades

A single-phase circuit draws 13 A at 220 V with a power factor of 0.8. What is the approximate real power consumed?

  1. 2288 W Correct answer.
  2. 2860 W
  3. 1760 W
  4. 1144 W

Explanation

Real power equals voltage multiplied by current multiplied by power factor, so 220 × 13 × 0.8 gives approximately 2288 W. Apparent power alone would be 2860 VA, which overstates the real power when the load is inductive. Power factor correction would reduce the apparent current for the same real power.

Question 11Construction-related E&M Trades

Why is a ring final circuit tested for continuity with readings cross-referenced at each socket outlet?

  1. To verify the polarity of the supply at the origin
  2. To measure the insulation resistance of the circuit
  3. To confirm the earthing arrangement type
  4. To detect a break or high resistance in either leg of the ring Correct answer.

Explanation

Cross-connected continuity testing of a ring final circuit produces expected readings at each socket that reveal whether both legs are intact and whether spurs or breaks exist. Readings that deviate from the calculated values indicate a discontinuity or high-resistance joint. This test is separate from insulation resistance and polarity checks.

Question 12Construction-related E&M Trades

A three-phase induction motor rated 380 V is connected to a 380 V three-phase supply but runs in reverse. What is the most appropriate corrective action?

  1. Swap the neutral and one phase
  2. Reverse any two of the three supply phases at the motor terminals Correct answer.
  3. Reverse the earth conductor
  4. Reduce the supply voltage

Explanation

The direction of rotation of a three-phase induction motor is determined by phase sequence, so interchanging any two line conductors reverses the rotating field and the motor direction. This is done at the motor terminals or starter with the supply isolated. The earth conductor must never be disturbed for this purpose.

Question 13Construction-related E&M Trades

Which characteristic makes a Type C miniature circuit breaker suitable for circuits supplying equipment with high inrush currents?

  1. It provides residual current protection
  2. It has a lower breaking capacity than Type B
  3. It trips instantly at rated current
  4. It trips between 5 and 10 times rated current for short durations Correct answer.

Explanation

Type C MCBs have a magnetic trip threshold of roughly 5 to 10 times the rated current, which tolerates brief inrush from motors and transformers without nuisance tripping. Type B devices trip at a lower multiple and suit resistive and lighting loads. The choice must still satisfy disconnection time requirements for the fault loop impedance.

Question 14Construction-related E&M Trades

What is the main function of a contactor in a motor control circuit?

  1. To convert three-phase supply to single-phase supply
  2. To provide stepless speed control of the motor
  3. To switch the motor load remotely under the control of a pilot circuit Correct answer.
  4. To measure the motor current for metering

Explanation

A contactor is an electromagnetically operated switch designed to control a power circuit from a separate pilot circuit, allowing remote and automatic starting and stopping of motors. It includes main contacts and often auxiliary contacts for interlocks and holding circuits. It does not provide speed control or conversion.

Question 15Construction-related E&M Trades

A cable is installed in a thermally insulated wall and derating factors apply. What is the correct approach to selecting the conductor size?

  1. Select the smallest standard size available regardless of capacity
  2. Select the size whose tabulated capacity equals the design current
  3. Select the size whose derated capacity is at least the protective device rating Correct answer.
  4. Select the size based on voltage drop alone

Explanation

The tabulated current-carrying capacity must be multiplied by all applicable correction factors, including thermal insulation, grouping and ambient temperature, and the resulting value must be at least the nominal rating of the protective device. This ensures the cable is not overloaded in its actual installation conditions. Voltage drop is then checked as a separate compliance requirement.

Question 16Construction-related E&M Trades

What does an earth fault loop impedance reading above the maximum permitted value indicate?

  1. The supply voltage is too low for the load
  2. The insulation resistance of the circuit is too high
  3. The circuit has an excessive number of socket outlets
  4. The protective device may not disconnect the circuit within the required time Correct answer.

Explanation

Earth fault loop impedance determines the fault current that will flow during an earth fault. If the measured impedance exceeds the maximum value for the protective device type and rating, the fault current may be insufficient to operate the device within the required disconnection time. The remedy is to improve the earth path or change the protective device.

Question 17Construction-related E&M Trades

In a domestic consumer unit, why are circuits supplying socket outlets in a bathroom often protected by a 30 mA RCD?

  1. To reduce the current drawn by bathroom appliances
  2. To comply with the requirement for double insulation
  3. To provide additional protection against electric shock in a location with increased risk Correct answer.
  4. To prevent the bathroom lights from flickering

Explanation

Bathrooms are special locations where wet conditions and earthed metalwork increase the risk of electric shock, so additional protection by a 30 mA RCD is required for the relevant circuits. The RCD limits the duration of shock current to a level unlikely to cause serious injury. It does not reduce load current or affect lighting stability.

Question 18Construction-related E&M Trades

A metal-clad distribution board is installed in a commercial unit. What is the correct method of bonding its exposed conductive parts?

  1. Connect them to the neutral bar
  2. Leave them isolated because the enclosure is metal
  3. Connect them to the main earthing terminal with a conductor sized to the supply Correct answer.
  4. Connect them to the incoming supply phase

Explanation

Exposed conductive parts of equipment must be connected to the protective conductor and hence to the main earthing terminal, with the size determined by the supply conductor size. This ensures that a fault to the enclosure results in sufficient fault current to operate the protective device. Bonding to neutral or to a phase conductor is dangerous and prohibited.

Question 19Construction-related E&M Trades

What is the primary reason for balancing single-phase loads across the phases of a three-phase supply?

  1. To increase the total connected load
  2. To eliminate the need for an earth conductor
  3. To raise the supply frequency
  4. To reduce neutral current and voltage imbalance Correct answer.

Explanation

Balancing single-phase loads across phases keeps phase currents similar, reducing current in the neutral conductor and limiting voltage imbalance between phases. This improves supply quality and reduces losses. It does not remove the requirement for earthing or affect frequency.

Question 20Construction-related E&M Trades

Which document type is used on a construction site to record the results of insulation resistance, continuity and polarity tests before energising a new installation?

  1. A risk assessment
  2. A permit to work
  3. An electrical installation certificate with schedules of test results Correct answer.
  4. A method statement

Explanation

Test results for a new installation are recorded on the electrical installation certificate and accompanying schedules of inspection and test results, which document the verification required before energising. Method statements and risk assessments describe how work is carried out safely, while permits to work authorise specific activities.

Question 21Construction-related E&M Trades

A circuit with a design current of 32 A is protected by a 32 A Type B MCB. The measured earth fault loop impedance is 1.1 Ω on a 220 V supply. What can be concluded?

  1. The circuit does not comply because the fault current is insufficient
  2. The circuit complies because the breaker rating equals the design current
  3. The circuit cannot be assessed without an insulation resistance result
  4. The circuit complies because the impedance is below 1.44 Ω Correct answer.

Explanation

For a 32 A Type B MCB the maximum earth fault loop impedance is approximately 1.44 Ω at 220 V, derived from the magnetic trip multiple and the supply voltage. A measured value of 1.1 Ω is below this maximum, so the device should operate within the required disconnection time. Insulation resistance is a separate verification requirement.

Question 22Construction-related E&M Trades

In a circuit protected by a 30 mA RCD, a N-E insulation fault of 5 kΩ develops downstream. What is the approximate leakage current at 220 V and what is the likely outcome?

  1. About 44 mA, so the RCD should trip Correct answer.
  2. About 0.44 mA, so the circuit is unaffected
  3. About 440 mA, so the MCB will trip on overload
  4. About 4.4 mA, so the RCD will not trip

Explanation

Leakage current is approximately voltage divided by resistance, so 220 V across 5 kΩ gives about 44 mA. This exceeds the 30 mA rated residual operating current of the RCD, so the device should trip. The current is far below the overload rating, so the MCB would not respond.

Question 23Construction-related E&M Trades

A ring final circuit protected by a 32 A MCB is found to have a break in one leg. Why does this present a hazard if left unrepaired?

  1. The circuit voltage rises above 220 V
  2. The RCD sensitivity increases
  3. The earth conductor becomes live
  4. The circuit becomes a radial circuit with reduced current-carrying capacity, so the 32 A protection may not protect the remaining conductor Correct answer.

Explanation

A broken ring leaves the sockets supplied by a single leg, effectively converting the circuit to a radial arrangement. The remaining 2.5 mm² conductors may not be adequately protected by the 32 A device, particularly where spurs and long runs exist. The ring must be restored or the protection and conductor sizing reassessed.

Question 24Construction-related E&M Trades

A 15 kW three-phase motor operates at 380 V with a power factor of 0.85 and an efficiency of 90%. What is the approximate line current?

  1. About 18 A
  2. About 45 A
  3. About 30 A Correct answer.
  4. About 26 A

Explanation

Input power equals output power divided by efficiency, so 15 kW divided by 0.9 gives about 16.7 kW. Line current equals input power divided by the product of line voltage, root three and power factor, giving roughly 30 A. Ignoring efficiency or power factor would produce a different and incorrect result.

Question 25Construction-related E&M Trades

During verification of a new installation, an insulation resistance test between live conductors and earth gives 0.8 MΩ on a distribution circuit containing surge protection devices. What is the correct interpretation and action?

  1. The SPD should be disconnected before testing, and the circuit retested to obtain a valid result Correct answer.
  2. The circuit should be energised immediately because the reading is above zero
  3. The result is acceptable because any reading above 0.5 MΩ passes
  4. The test voltage should be increased to 1000 V to confirm the reading

Explanation

Surge protection devices can conduct during insulation resistance testing and depress the measured value, so they should be disconnected or the test performed at a reduced voltage where appropriate. The circuit should then be retested to obtain a valid insulation resistance result. Simply accepting a depressed reading or raising the test voltage risks a false conclusion or damage.

Question 26Construction-related E&M Trades

On a construction site, a 110 V centre-tapped earth distribution system is used for portable hand tools. What is the potential to earth from either live conductor?

  1. 0 V
  2. 110 V
  3. 55 V Correct answer.
  4. 220 V

Explanation

A 110 V centre-tapped earth (CTE) system earths the midpoint of the transformer secondary, so each live conductor sits at half the secondary voltage relative to earth, giving 55 V. This reduced touch voltage limits shock severity if a tool or cable is damaged.

Question 27Construction-related E&M Trades

Which colour identifies the protective conductor in a newly installed flexible cable to BS EN 50525?

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

Explanation

Harmonised cable colours use green-and-yellow exclusively for the protective conductor, and this combination must not be used for any other purpose. Brown is line and blue is neutral in single-phase flexible cords.

Question 28Construction-related E&M Trades

What is the standard nominal mounting height for a domestic wall-mounted socket outlet in a new dwelling under Approved Document M?

  1. 150 mm to the bottom of the accessory
  2. 2200 mm to the top of the accessory
  3. 1200 mm to the top of the accessory
  4. 450 mm to the bottom of the accessory Correct answer.

Explanation

Approved Document M guidance places socket outlets between 450 mm and 1200 mm from finished floor level to the bottom of the accessory, with 450 mm the usual lower datum for general socket positions. This range keeps outlets reachable for seated and standing users.

Question 29Construction-related E&M Trades

Which instrument is used to confirm that a circuit is dead before work begins on a low-voltage distribution board?

  1. A non-contact voltage pen used alone
  2. A clamp meter set to the current range
  3. A proving unit used with an approved voltage indicator Correct answer.
  4. A multifunction tester on the insulation resistance range

Explanation

Safe isolation requires an approved voltage indicator proved on a known source such as a proving unit before and after the test. The proving unit confirms the indicator is functional so a false dead reading cannot be relied on.

Question 30Construction-related E&M Trades

In a steel conduit installation, what is the purpose of an inspection bend or inspection tee?

  1. To allow the draw wire and cables to be accessed where a bend or junction cannot be pulled through Correct answer.
  2. To enable the conduit to be earthed at that point
  3. To provide a fixing point for the conduit saddles
  4. To allow thermal expansion of the conduit run

Explanation

Inspection fittings have a removable lid so that a draw wire can be fed and cables guided around a bend or past a junction that would otherwise be inaccessible. They are placed where a normal bend or coupler would make cable drawing impractical.

Question 31Construction-related E&M Trades

A single-phase final circuit supplies a 2 kW resistive heater at 230 V. What is the approximate current drawn?

  1. 8.7 A Correct answer.
  2. 0.12 A
  3. 4.3 A
  4. 2.0 A

Explanation

For a purely resistive load, current equals power divided by voltage, so 2000 divided by 230 gives approximately 8.7 A. Resistive heating has a power factor close to unity, so the apparent current equals the real current.

Question 32Construction-related E&M Trades

Which type of cable is most suitable for a fixed submain run clipped to a tray in a dry internal plant room where mechanical protection is needed?

  1. Steel wire armoured cable Correct answer.
  2. Single-core panel wire
  3. Flexible cord to BS EN 50525
  4. Flat twin and earth cable

Explanation

Steel wire armoured cable provides mechanical protection from its armour and is suited to fixed submain runs on tray in dry internal locations. The armour also serves as a circuit protective conductor when correctly terminated with suitable glands.

Question 33Construction-related E&M Trades

What does the breaking capacity of a miniature circuit breaker indicate?

  1. The current at which it trips instantaneously
  2. The voltage drop across its terminals at rated current
  3. The maximum prospective fault current it can safely interrupt Correct answer.
  4. The maximum continuous current it can carry indefinitely

Explanation

Breaking capacity, expressed in kiloamperes, is the highest prospective short-circuit current the device can interrupt without damage or danger. It must equal or exceed the prospective fault current at the point of installation.

Question 34Construction-related E&M Trades

A 32 A Type B MCB protects a circuit where the measured earth fault loop impedance is 1.1 Ω. The tabulated maximum for the device is 1.44 Ω. What conclusion follows?

  1. An RCD is mandatory because the value is above 1 Ω
  2. The circuit disconnection time is satisfied because the measured value is below the maximum Correct answer.
  3. The device must be replaced with a Type C breaker to achieve compliance
  4. The circuit fails because 1.1 Ω exceeds the maximum permitted value

Explanation

Compliance requires the measured earth fault loop impedance to be no greater than the tabulated maximum for the protective device, so 1.1 Ω against 1.44 Ω is acceptable. The device will therefore operate within the required disconnection time for a fault of negligible impedance.

Question 35Construction-related E&M Trades

A 3 kW shower is supplied by a 20 m run of 6 mm² twin and earth cable with a resistive load of 13 A at 230 V. Using a mV/A/m value of 7.3, what is the approximate voltage drop?

  1. 3.8 V
  2. 0.95 V
  3. 0.19 V
  4. 1.9 V Correct answer.

Explanation

Voltage drop equals mV/A/m multiplied by current and length divided by 1000, so 7.3 × 13 × 20 ÷ 1000 gives about 1.9 V. This is well within the 3 percent lighting or 5 percent power limits for a 230 V supply.

Question 36Construction-related E&M Trades

Which test confirms that the polarity of every socket outlet on a final circuit is correct before energisation?

  1. A polarity test using a suitable tester at each outlet with the supply isolated and continuity checked to the expected terminals Correct answer.
  2. An earth electrode resistance test
  3. A prospective fault current measurement at the origin
  4. An insulation resistance test between live conductors

Explanation

Polarity is verified by confirming continuity from the expected source conductor to the corresponding outlet terminal, typically with a low-resistance ohmmeter on a dead circuit. This proves line and neutral are not transposed at any accessory.

Question 37Construction-related E&M Trades

A 30 mA RCD protecting a socket circuit trips intermittently. Which test is most appropriate to locate the cause?

  1. A continuity test of the ring circuit only
  2. A prospective fault current measurement at the consumer unit
  3. A loop impedance test at the furthest socket
  4. A ramp test and half-rated current test to check the RCD, followed by insulation resistance testing of the circuit Correct answer.

Explanation

Intermittent tripping may be a faulty RCD or a genuine earth leakage fault, so the device is first checked by ramp and half-rated current tests. Insulation resistance testing of the circuit then identifies any degraded insulation causing leakage.

Question 38Construction-related E&M Trades

In a three-phase four-wire distribution board, what is the main risk of leaving the neutral conductor disconnected while single-phase loads are energised?

  1. Load voltages become unbalanced and can rise to dangerous levels on lightly loaded phases Correct answer.
  2. The protective devices will fail to operate on overload
  3. The power factor of the installation will improve
  4. The earth fault loop impedance will fall below the permitted value

Explanation

With a broken neutral, single-phase loads are effectively connected in series across two line conductors, so the voltage divides according to their impedances. Lightly loaded phases can then see voltages well above nominal, damaging equipment.

Question 39Construction-related E&M Trades

What is the function of a residual current device in a TT installation where the earth fault loop impedance is too high for an MCB to disconnect within the required time?

  1. It balances the single-phase loads across the phases
  2. It provides disconnection by detecting residual current to earth rather than relying on fault loop impedance Correct answer.
  3. It increases the prospective fault current at the origin
  4. It reduces the earth electrode resistance to an acceptable level

Explanation

In a TT system the earth path through the electrode is high resistance, so fault current may be too low to trip an MCB promptly. An RCD detects the resulting residual current and disconnects within the required time regardless of loop impedance.

Question 40Construction-related E&M Trades

A motor circuit requires protection against sustained overload but the motor must be allowed to draw a high starting current. Which arrangement is appropriate?

  1. A 30 mA RCD as the sole protective device
  2. A Type D MCB used as the overload element only
  3. A fused connection unit rated at the starting current
  4. A thermal overload relay set to the motor full-load current in the starter, backed by short-circuit protection Correct answer.

Explanation

A thermal overload relay in the starter tracks motor heating and trips on sustained overload while tolerating the short starting surge. Short-circuit protection is provided separately by fuses or an MCB selected for the motor circuit.

Question 41Construction-related E&M Trades

Which earthing arrangement has the supply neutral earthed at the source and the installation protective conductor connected to that same neutral at the origin?

  1. IT
  2. TT
  3. TN-S
  4. TN-C-S Correct answer.

Explanation

In a TN-C-S system the neutral and protective functions are combined in the supply, then separated at the origin into neutral and protective conductors. The installation earth is therefore tied to the supply neutral.

Question 42Construction-related E&M Trades

A ring final circuit is installed in a kitchen where a 3 kW appliance is plugged in near the middle of the ring. Why does the ring arrangement reduce overload risk compared with a radial circuit of the same cable size?

  1. The protective device rating is automatically halved on a ring circuit
  2. Two paths share the load current, so each leg carries roughly half the current of a single radial conductor Correct answer.
  3. The ring circuit does not require an earth conductor
  4. The ring cable has a higher current-carrying capacity than the same size radial cable

Explanation

A ring provides two parallel paths from the origin, so current to a load divides between the legs according to their impedances. Each leg therefore carries less current than a single radial conductor would, reducing heating in the cable.

Question 43Construction-related E&M Trades

Which statutory requirement places duties on employers and the self-employed to ensure the health and safety of persons affected by their undertaking on a construction site?

  1. The Health and Safety at Work etc. Act 1974 Correct answer.
  2. The Electricity at Work Regulations 1989 alone
  3. The IET Wiring Regulations BS 7671
  4. The Building Regulations 2010

Explanation

The Health and Safety at Work etc. Act 1974 sets the general statutory duties of employers and the self-employed towards persons affected by their work. The Electricity at Work Regulations and BS 7671 build on this framework for electrical safety.

Question 44Construction-related E&M Trades

A 15 kW three-phase motor operates at 400 V with a power factor of 0.85 and an efficiency of 92 percent. What is the approximate line current?

  1. 23.8 A
  2. 27.7 A Correct answer.
  3. 41.5 A
  4. 15.0 A

Explanation

Input power is output divided by efficiency, giving about 16.3 kW, and line current equals input power divided by the square root of three times voltage times power factor. That yields approximately 27.7 A.

Question 45Construction-related E&M Trades

During a periodic inspection, a distribution board in a dusty workshop shows signs of overheating at a terminal and discoloured insulation. What is the most appropriate immediate action?

  1. Apply a temporary insulation tape repair and re-energise
  2. Record the observation and continue testing the remaining circuits
  3. Isolate the circuit, record the defect as a dangerous condition, and make it safe before further work Correct answer.
  4. Increase the protective device rating to prevent further overheating

Explanation

Overheating and discoloured insulation indicate a dangerous condition that could lead to fire, so the affected circuit should be isolated and made safe immediately. The defect is then recorded and reported to the duty holder for remedial action.

Question 46Construction-related E&M Trades

What is the purpose of a protective bonding conductor connected to the incoming metallic water service in a commercial building?

  1. To carry the operating current of the water pump
  2. To provide the main protective conductor for the installation
  3. To replace the need for circuit protective conductors
  4. To keep the potential of the metallic service at or near earth potential and limit touch voltage Correct answer.

Explanation

Main protective bonding ties extraneous conductive parts such as metallic water services to the main earthing terminal so they remain near earth potential. This prevents dangerous potential differences arising between metalwork and earthed equipment during a fault.

Question 47Construction-related E&M Trades

A 230 V single-phase circuit has a design current of 25 A and is protected by a 25 A Type B MCB. The tabulated maximum earth fault loop impedance for this device is 1.84 Ω. Which statement is correct?

  1. The circuit complies only if the measured Zs does not exceed 1.84 Ω Correct answer.
  2. The circuit requires an RCD because the device is Type B
  3. The circuit complies regardless of Zs because the device rating matches the design current
  4. The circuit complies provided the measured Zs is at least 1.84 Ω

Explanation

For an MCB the measured earth fault loop impedance must not exceed the tabulated maximum so that the required disconnection time is achieved. Matching the device rating to the design current alone does not demonstrate compliance.

Question 48Construction-related E&M Trades

Two single-phase loads of 10 A and 15 A at unity power factor are connected across different phases of a three-phase four-wire supply. Why is the neutral current not simply the arithmetic sum of the two?

  1. The neutral current is always the larger of the two phase currents
  2. The neutral current equals the arithmetic sum only when the loads are resistive
  3. The neutral carries no current whenever more than one phase is loaded
  4. The phase currents are displaced by 120 degrees, so the neutral current is their phasor sum, which is smaller than the arithmetic total Correct answer.

Explanation

Balanced three-phase phase currents are displaced by 120 degrees, so the neutral current is the phasor sum of the phase currents rather than their arithmetic total. With two phases loaded the neutral current is reduced but not zero.

Question 49Construction-related E&M Trades

A circuit is protected by a 32 A Type C MCB. The measured earth fault loop impedance is 0.9 Ω and the tabulated maximum for the device is 0.72 Ω. What is the most appropriate engineering conclusion?

  1. The disconnection time is not assured, so the circuit design must be revised, for example by increasing conductor size or adding RCD protection Correct answer.
  2. The measured value should be ignored because Type C devices are exempt from Zs limits
  3. The circuit complies because 0.9 Ω is close to the maximum
  4. The MCB should be replaced with a 32 A Type B device without any other change

Explanation

The measured Zs exceeds the tabulated maximum for the Type C device, so the required disconnection time cannot be relied upon. The design must be changed, such as by reducing circuit impedance or providing RCD protection, to restore compliance.

Question 50Construction-related E&M Trades

A 400 V three-phase submain supplies a balanced load of 60 A at a power factor of 0.9. What is the approximate real power delivered?

  1. 72.0 kW
  2. 37.4 kW Correct answer.
  3. 41.6 kW
  4. 24.0 kW

Explanation

Three-phase real power equals the square root of three times line voltage times line current times power factor. Substituting 400 V, 60 A and 0.9 gives approximately 37.4 kW.