City & Guilds 2393-10 Part P Building Regulations for Electrical Installations in Dwellings — Questions and Answers

Question 1: What is the minimum disconnection time for a circuit with a fault to earth in a TN system supplying a socket outlet circuit not exceeding 32 A?

0.4 seconds (Correct answer)
0.1 seconds
1 second
5 seconds

Correct answer: 0.4 seconds

BS 7671 Table 41.1 requires a maximum disconnection time of 0.4 seconds for socket outlet circuits up to 32 A in TN systems, to limit the duration of dangerous touch voltages in the event of a fault.

Question 2: What RCD protection is required for circuits supplying bathroom zones?

All circuits supplying equipment in bathroom zones must be protected by a 30 mA RCD (Correct answer)
Only circuits in Zone 0 require RCD protection
50 mA RCD is permitted for bathroom circuits
RCD protection is not required in bathrooms if the equipment has a high IP rating

Correct answer: All circuits supplying equipment in bathroom zones must be protected by a 30 mA RCD

BS 7671 requires all circuits that supply equipment within bathroom zones to be protected by a 30 mA RCD, providing additional protection against electric shock, recognising the increased shock risk in wet environments.

Question 3: Where a bare CPC is exposed at a termination point, how must it be identified according to BS 7671?

With solid green sleeving
With brown sleeving
With green and yellow sleeving (Correct answer)
With blue sleeving

Correct answer: With green and yellow sleeving

The bare CPC must be sleeved with green and yellow insulation wherever it is exposed at terminations, clearly identifying it as the protective conductor and preventing it being mistaken for a current-carrying conductor.

Question 4: What is the purpose of bonding a TV aerial (coaxial) cable screen to earth?

To improve signal quality
To prevent lightning striking the aerial
To discharge static and induced voltages on the aerial cable screen, preventing dangerous voltages being introduced into the installation or equipment (Correct answer)
To comply with planning requirements

Correct answer: To discharge static and induced voltages on the aerial cable screen, preventing dangerous voltages being introduced into the installation or equipment

The outer screen of a coaxial aerial cable should be earthed at the point of entry to the building to provide a safe discharge path for lightning-induced surges and static charges, preventing them from reaching equipment.

Question 5: What is measured by the R1+R2 test?

The resistance between phase and neutral
The combined resistance of the phase conductor (R1) and circuit protective conductor (R2) from the consumer unit to the furthest point of the circuit (Correct answer)
The resistance of the neutral conductor
The resistance of the phase conductor only

Correct answer: The combined resistance of the phase conductor (R1) and circuit protective conductor (R2) from the consumer unit to the furthest point of the circuit

The R1+R2 test (also called the conductor resistance test) measures the total resistance of the phase and CPC in a circuit, used to calculate the maximum Zs at the furthest point from the supply.

Question 6: What is the purpose of BS 7671?

To regulate the electricity supply industry
To certify electrical engineers
To provide requirements to protect persons, property and livestock against hazards from electrical installations (Correct answer)
To specify electrical product manufacturing standards

Correct answer: To provide requirements to protect persons, property and livestock against hazards from electrical installations

BS 7671 sets out the requirements for designing, erecting and verifying electrical installations to protect persons, property and livestock from dangers arising from those installations.

Question 7: After fitting a new consumer unit, which test must be performed to check the consumer unit itself?

Verification that all connections are secure, correct polarity, insulation resistance of busbars, and functional testing of all protective devices (RCD tests, MCB tests) (Correct answer)
Only an insulation resistance test of the whole installation
Only an RCD test
Only a visual inspection

Correct answer: Verification that all connections are secure, correct polarity, insulation resistance of busbars, and functional testing of all protective devices (RCD tests, MCB tests)

After installation, the new consumer unit should be verified: all connections checked for security and polarity, insulation resistance tested, and all RCDs tested for correct operation before the installation is handed back to the occupier.

Question 8: What is the purpose of a neutral isolation requirement in some consumer unit configurations?

Because neutral conductors carry no current
Only for three-phase consumer units
To ensure that when maintenance is carried out on a circuit, the neutral is also isolated (preventing shock from the neutral conductor, which may be at a hazardous voltage in some fault conditions) (Correct answer)
Neutral isolation is never required in domestic consumer units

Correct answer: To ensure that when maintenance is carried out on a circuit, the neutral is also isolated (preventing shock from the neutral conductor, which may be at a hazardous voltage in some fault conditions)

Although neutral is normally at or near earth potential, in some fault conditions (e.g., broken PEN in PME) the neutral can rise to dangerous voltage; double-pole isolation (at circuit level via RCBOs) ensures complete isolation for maintenance.

Question 9: What colour is the protective conductor (earth) identified by in the harmonised cable colour code used in the UK?

Green and yellow (Correct answer)
Brown
Blue
Grey

Correct answer: Green and yellow

The protective conductor (earth/CPC) is identified by green and yellow striped insulation under the harmonised colour code (BS 7671, Table 51). Line conductors are brown (single phase) or brown/black/grey (three phase), and the neutral is blue. These colours have been mandatory for new installations since 2006.

Question 10: How does the zone apply in a bathroom where the ceiling height is less than 2.25 m?

Zones are not required in bathrooms with low ceilings
The zones are proportionally reduced
Zone 1 extends to the ceiling regardless of whether the ceiling height is below 2.25 m (Correct answer)
The zone extends to 2.25 m only regardless of ceiling height

Correct answer: Zone 1 extends to the ceiling regardless of whether the ceiling height is below 2.25 m

BS 7671 specifies that Zone 1 extends vertically from the bath/shower to 2.25 m or to the ceiling if the ceiling is lower than 2.25 m; the zone continues to the ceiling to reflect that the ceiling surface can be wet.

Question 11: When performing a continuity test on a protective conductor, what type of instrument is used?

A clamp meter
A voltage tester
An insulation resistance tester
A low-resistance ohmmeter (typically using a test current of at least 200mA DC) (Correct answer)

Correct answer: A low-resistance ohmmeter (typically using a test current of at least 200mA DC)

Continuity testing uses a low-resistance ohmmeter that delivers a test current of at least 200mA DC. This ensures that the measurement can detect poor connections and resistive joints that would not be revealed by a high-impedance instrument. The instrument must be calibrated and the test lead resistance nulled (zeroed) before taking measurements.

Question 12: What is a Type B circuit breaker and what is it typically used for?

A current-limiting circuit breaker
A circuit breaker that trips at 3–5 times its rated current for magnetic operation — used for resistive loads such as lighting and socket outlets (Correct answer)
A circuit breaker for high inrush current loads
A circuit breaker for motor loads

Correct answer: A circuit breaker that trips at 3–5 times its rated current for magnetic operation — used for resistive loads such as lighting and socket outlets

A Type B MCB operates magnetically between 3–5 times its rated current; it is suitable for circuits with little or no inrush current such as domestic socket outlets, lighting and heating circuits.

Question 13: According to BS 7671, what is the correct sequence of tests for initial verification of an electrical installation?

RCD operation first, then all other tests in any order
Earth fault loop impedance, RCD, continuity, insulation resistance
Continuity of protective conductors, continuity of ring final circuits, insulation resistance, polarity, earth fault loop impedance, RCD operation (Correct answer)
Insulation resistance, continuity, polarity, earth fault loop impedance, RCD

Correct answer: Continuity of protective conductors, continuity of ring final circuits, insulation resistance, polarity, earth fault loop impedance, RCD operation

BS 7671 Chapter 64 specifies that dead tests must be completed before live tests for safety. The correct sequence is: (1) continuity of protective conductors, (2) continuity of ring final circuits, (3) insulation resistance, (4) polarity, (5) earth fault loop impedance (live test), (6) prospective fault current, (7) RCD operation, and (8) functional testing.

Question 14: What does 'RCBO' stand for and what is its advantage over using a separate RCD and MCB?

Remote Controlled Breaker Output — operated remotely
Residual Current Breaker with Overcurrent — combines RCD and MCB in a single device protecting a single circuit (Correct answer)
Residual Circuit Breaker Output — a type of main switch
Rapid Circuit Breaking Output — faster than a standard MCB

Correct answer: Residual Current Breaker with Overcurrent — combines RCD and MCB in a single device protecting a single circuit

An RCBO combines residual current (earth leakage) and overcurrent (overload and short circuit) protection in a single device for a single circuit, eliminating the problem of a single RCD tripping multiple circuits.

Question 15: What does BS 7671 18th Edition require all new domestic consumer units to be made from?

Non-combustible material (metal enclosures) (Correct answer)
Plastic (to prevent shock hazard)
Any material if the consumer unit is less than 100 A
Any material rated to 850°C

Correct answer: Non-combustible material (metal enclosures)

Following Amendment 3 to the 17th Edition (and carried into the 18th), domestic consumer units must have enclosures manufactured from non-combustible material (metal), to prevent fires from arcing faults inside the unit spreading to plastic.

Question 16: What is the recommended maximum time between periodic inspections for a domestic owner-occupied property?

Every year
Every 5 years
Every 10 years or on change of occupancy (Correct answer)
Every 25 years

Correct answer: Every 10 years or on change of occupancy

For domestic owner-occupied properties, BS 7671 guidance recommends an EICR every 10 years or at change of ownership; private rented properties require an EICR every 5 years under the 2020 Regulations.

Question 17: What is the permitted maximum touch voltage in AC installations under BS 7671?

25 V always
230 V
110 V
50 V AC (or 120 V DC ripple-free) — above this level rapid disconnection or other measures are required (Correct answer)

Correct answer: 50 V AC (or 120 V DC ripple-free) — above this level rapid disconnection or other measures are required

BS 7671 (and IEC 60364) sets the conventional touch voltage limit at 50 V AC (UL); above this voltage, protective measures must ensure disconnection within the required time to prevent prolonged dangerous shock current.

Question 18: What is 'voltage indicator' and why must it be approved to GS38?

A voltage testing device (test lamp or two-pole tester); GS38 specifies requirements for test probes and leads to prevent injuries from arcing faults during live testing (Correct answer)
A clamp meter for measuring current
A device for measuring insulation resistance
A device for testing RCD sensitivity

Correct answer: A voltage testing device (test lamp or two-pole tester); GS38 specifies requirements for test probes and leads to prevent injuries from arcing faults during live testing

GS38 (HSE guidance) specifies requirements for electrical test probes and leads used by electricians, including insulated probe tips, shrouded crocodile clips and limited exposed metal, to reduce the risk of injury from accidental short circuits during live working.

Question 19: What type of cable gland is used when a steel-armoured supply cable enters a metal consumer unit?

Any gland of sufficient size
No cable gland is needed for armoured cables
A standard PVC cable gland
An armoured cable gland that clamps the cable armour, providing both strain relief and continuity of the metal enclosure's protection (Correct answer)

Correct answer: An armoured cable gland that clamps the cable armour, providing both strain relief and continuity of the metal enclosure's protection

Armoured cables (e.g., SWA — steel wire armour) entering a metal consumer unit must use a purpose-made armoured cable gland that clamps the armour, providing mechanical protection, strain relief, and earthing of the armour through the gland.

Question 20: How is a cable's current-carrying capacity (Iz) selected for circuit design?

Iz must be greater than or equal to the design current (Ib) and the selected protective device's nominal current (In), applying correction factors for temperature, grouping and thermal insulation (Correct answer)
Iz only needs to exceed the design current
Iz is selected based on the cable length only
Iz is selected only based on the MCB rating

Correct answer: Iz must be greater than or equal to the design current (Ib) and the selected protective device's nominal current (In), applying correction factors for temperature, grouping and thermal insulation

Cable selection requires: Iz ≥ In ≥ Ib (after applying all derating factors for temperature, grouping, thermal insulation), and the protective device's effective operating current must not exceed 1.45 × Iz of the cable.

Question 21: What test is used to measure the resistance of an earth electrode?

Insulation resistance test
Earth electrode resistance test using the fall-of-potential method (Correct answer)
Continuity test with a low-resistance ohmmeter
Loop impedance test

Correct answer: Earth electrode resistance test using the fall-of-potential method

The fall-of-potential method uses a dedicated earth electrode resistance tester with two auxiliary electrodes (current and potential) driven into the ground in a straight line away from the electrode under test. The potential electrode is moved between the current electrode and the test electrode to find the plateau reading, which represents the true electrode resistance.

Question 22: What is the typical rated current for the main switch in a single-phase domestic consumer unit?

63 A
230 A
32 A
100 A (Correct answer)

Correct answer: 100 A

The standard main switch in a domestic consumer unit is rated at 100 A, matching the standard domestic supply fuse rating; this allows for the maximum domestic load without nuisance operation of the main switch.

Question 23: Why must domestic consumer units have metal enclosures (since 2016 amendment)?

Because metal units provide better electromagnetic shielding
To reduce the risk of fire spreading from an arcing fault within the consumer unit to plastic and combustible materials (Correct answer)
Because metal units are cheaper
Because plastic degrades over time

Correct answer: To reduce the risk of fire spreading from an arcing fault within the consumer unit to plastic and combustible materials

Following a 2016 amendment to BS 7671, domestic consumer units must have enclosures made of non-combustible material (metal), as arc faults in plastic units had been identified as a cause of fires.

Question 24: What is a 'periodic inspection report' now called?

Periodic Inspection Certificate
Building Regulations compliance certificate
Electrical Installation Condition Report (EICR) (Correct answer)
Annual safety record

Correct answer: Electrical Installation Condition Report (EICR)

What was previously called a Periodic Inspection Report (PIR) was renamed the Electrical Installation Condition Report (EICR) following amendments to BS 7671, to more clearly describe its purpose.

Question 25: What must be checked about the existing earthing arrangement before replacing a consumer unit?

Earthing does not need to be checked during consumer unit replacement
The type of earthing system (TN-S, TN-C-S, TT), the condition and size of the earthing conductor, and compliance with the current requirements of BS 7671 (Correct answer)
Only the earthing system type
Only the visual condition of the earthing conductor

Correct answer: The type of earthing system (TN-S, TN-C-S, TT), the condition and size of the earthing conductor, and compliance with the current requirements of BS 7671

The earthing system type determines permissible protections (e.g., TT requires RCDs); the earthing conductor must be verified as correctly sized and connected; deficiencies must be remedied before the new consumer unit is commissioned.

Question 26: What is Regulation 110.1 of BS 7671 concerned with?

Consumer unit specifications
Cable sizing calculations
The purpose, scope and fundamental principles of the regulations — protecting against shock, fire and burns (Correct answer)
Testing procedures

Correct answer: The purpose, scope and fundamental principles of the regulations — protecting against shock, fire and burns

Part 1 of BS 7671 (including Regulation 110.1) sets out the scope, object and fundamental principles of the regulations, stating the purpose of providing protection against electric shock, fire, burns and other hazards.

Question 27: What is the correct procedure before working on a live circuit in an existing installation?

Pull the fuse
Simply switch off the MCB
Isolate the circuit at the consumer unit, lock off (or secure) the MCB, prove dead using an approved voltage indicator, and post a notice; only work live if absolutely necessary with written risk assessment and permit to work (Correct answer)
Ask the occupier to turn off at the mains

Correct answer: Isolate the circuit at the consumer unit, lock off (or secure) the MCB, prove dead using an approved voltage indicator, and post a notice; only work live if absolutely necessary with written risk assessment and permit to work

Safe isolation requires switching off, locking off (securing the MCB or removing the fuse), proving dead with an approved voltage indicator, fitting a warning notice, and only re-energising after work is complete and safe.

Question 28: What does 'dead testing' mean in electrical installation testing?

Tests performed on decommissioned circuits only
Tests using low-voltage probes only
Tests for the presence of voltage
Tests performed with the installation isolated from the supply (de-energised), such as continuity and insulation resistance tests (Correct answer)

Correct answer: Tests performed with the installation isolated from the supply (de-energised), such as continuity and insulation resistance tests

Dead testing refers to tests carried out with no electrical supply connected (de-energised), including continuity (CPC, bonding, ring), insulation resistance, and polarity (initial check) — all performed safely with no live parts.

Question 29: What hazard can arise from a broken CPC (earth conductor) in a circuit?

The circuit will trip immediately
If a fault causes a live conductor to contact exposed metalwork, the metalwork will remain live at dangerous voltage as there is no path for fault current to operate the protective device (Correct answer)
The neutral conductor will carry excess current
There will be no hazard as the fuse will blow

Correct answer: If a fault causes a live conductor to contact exposed metalwork, the metalwork will remain live at dangerous voltage as there is no path for fault current to operate the protective device

A broken CPC means exposed metalwork is not earthed; a fault energising the metalwork creates a dangerous live surface that will not trip the protective device (no fault current path), creating a sustained shock risk.

Question 30: What is the maximum floor area that can be served by a single 32 A ring main circuit in a domestic dwelling?

200 m²
100 m² (any floor, or individual floors above 100 m² require additional circuits) (Correct answer)
No limit specified in BS 7671
50 m²

Correct answer: 100 m² (any floor, or individual floors above 100 m² require additional circuits)

BS 7671 Appendix 15 recommends that a single 32 A ring main circuit should not serve more than 100 m² of floor area, to limit the number of socket outlets and connected load.

Question 31: Why is the use of BS 3036 semi-enclosed fuses discouraged in modern installations?

They trip too quickly for modern loads
They have unpredictable characteristics, can be incorrectly rewired with wrong-gauge wire, and have a low breaking capacity compared to modern MCBs or cartridge fuses (Correct answer)
They are too large for modern consumer units
They are too expensive

Correct answer: They have unpredictable characteristics, can be incorrectly rewired with wrong-gauge wire, and have a low breaking capacity compared to modern MCBs or cartridge fuses

BS 3036 rewirable fuses are prone to being rewired with incorrect gauge wire (either too small causing nuisance blowing, or too large causing fire risk), and their characteristics are imprecise compared to MCBs.

Question 32: What information must be provided on a circuit chart fixed inside a consumer unit?

The installer's name and date of installation only
The purpose of each circuit, its protective device rating, cable size, and RCD assignment (Correct answer)
Only the circuit numbers
Only the fuse ratings

Correct answer: The purpose of each circuit, its protective device rating, cable size, and RCD assignment

BS 7671 Regulation 514.9 requires a circuit chart (schedule of circuits) to be fixed inside or adjacent to the consumer unit, showing each circuit's purpose, cable size, protective device type and rating, and RCD assignment.

Question 33: How is the maximum Zs of a circuit calculated from the R1+R2 test result?

Zs = Ze only
Zs = Ze + (R1+R2), where Ze is the external impedance measured at the origin (Correct answer)
Zs = R1+R2 minus Ze
Zs = R1+R2 alone

Correct answer: Zs = Ze + (R1+R2), where Ze is the external impedance measured at the origin

The total earth fault loop impedance Zs for any circuit is the sum of the external impedance Ze (from the supply) and the circuit's R1+R2 resistance; the result must not exceed the maximum Zs for the protective device.

Question 34: How does splitting circuits between two RCDs in a consumer unit reduce the impact of nuisance tripping?

It doubles the tripping sensitivity of each RCD
It limits the number of circuits on each RCD so leakage currents don't accumulate as easily, and limits how many circuits are lost when one RCD trips (Correct answer)
It prevents overloads
It increases the rated current of each circuit

Correct answer: It limits the number of circuits on each RCD so leakage currents don't accumulate as easily, and limits how many circuits are lost when one RCD trips

With circuits split across two RCDs, fewer circuits are on each RCD (limiting cumulative leakage), and if one RCD does trip, only half the circuits are affected rather than all circuits, maintaining supply to critical loads.

Question 35: What is an 'approved contractor' under Part P?

Any electrician who has passed a Part P exam
Any City & Guilds qualified electrician
A contractor registered with a government-approved Competent Person scheme for electrical work in dwellings, able to self-certify compliance with Part P (Correct answer)
Any electrician with public liability insurance

Correct answer: A contractor registered with a government-approved Competent Person scheme for electrical work in dwellings, able to self-certify compliance with Part P

An approved (or registered) contractor is one enrolled in an approved Competent Person scheme (e.g., NICEIC, NAPIT, STROMA) that allows them to self-certify Part P notifiable work without separately notifying Building Control.

Question 36: What is the purpose of Appendix 3 of BS 7671?

To provide certification forms
To provide time/current characteristics of overcurrent protective devices for use in calculations (Correct answer)
To list cable types
To specify earthing system requirements

Correct answer: To provide time/current characteristics of overcurrent protective devices for use in calculations

Appendix 3 of BS 7671 provides time/current characteristics graphs for fuses and circuit breakers, which are used to verify that protective devices will disconnect within the required time under fault conditions.

Question 37: Which type of MCB (circuit breaker) is most commonly used in domestic installations?

Type D — trips at 10–20 times rated current
Type C — trips at 5–10 times rated current
Type A — trips at 2–3 times rated current
Type B — trips at 3–5 times rated current (Correct answer)

Correct answer: Type B — trips at 3–5 times rated current

Type B MCBs are standard in domestic installations as they provide adequate protection for resistive and lightly inductive loads (lighting, heating, socket outlets). Their magnetic trip range of 3–5 times rated current ensures fast disconnection under fault conditions without nuisance tripping from normal load switching surges. Type C is used for motor circuits with higher inrush currents.

Question 38: What is the NICEIC and what role does it play in the UK electrical industry?

National Inspection Council for Electrical Installation Contracting — a government-approved operator of the Part P Competent Person scheme and industry quality assurance body (Correct answer)
The statutory regulator for all electrical work in the UK
National Institute for Certification of Electrical Installers and Contractors
The organisation that publishes BS 7671

Correct answer: National Inspection Council for Electrical Installation Contracting — a government-approved operator of the Part P Competent Person scheme and industry quality assurance body

NICEIC is a leading industry body and government-approved Competent Person scheme operator; it assesses and registers electrical contractors, administers Part P self-certification, and carries out ongoing technical audits of member work.

Question 39: What is a 'split-load' consumer unit?

A consumer unit where lights and sockets are on separate MCBs
A consumer unit split into two separate enclosures
A consumer unit with two RCDs, each protecting a separate group of circuits, so a fault tripping one RCD does not affect the other group (Correct answer)
A consumer unit where the main fuse is split between two phases

Correct answer: A consumer unit with two RCDs, each protecting a separate group of circuits, so a fault tripping one RCD does not affect the other group

A split-load consumer unit has two RCDs in the main unit; circuits are divided between the two RCDs so that if one trips (due to fault or nuisance trip), only half the circuits lose supply, maintaining some services.

Question 40: What is the maximum earth fault loop impedance (Zs) for a 16 A Type B MCB?

0.5 Ω
2.87 Ω (as per BS 7671 Table 41.3 for TN systems at 0.4 s disconnection) (Correct answer)
1 Ω
10 Ω

Correct answer: 2.87 Ω (as per BS 7671 Table 41.3 for TN systems at 0.4 s disconnection)

BS 7671 Table 41.3 specifies maximum Zs values for each device; for a 16 A Type B MCB on a socket circuit (0.4 s disconnection), the maximum Zs is 2.87 Ω at the standard table temperature of 20°C.

City & Guilds 2393-10 Part P Building Regulations for Electrical Installations in Dwellings

The Part P (C&G 2393-10) certifies electricians to carry out domestic electrical installation work in compliance with UK Building Regulations Part P, covering BS 7671 wiring regulations, circuit protection, consumer unit installation, and inspection & testing procedures.

Questions

80 min

Time Limit

60%

To Pass

Checking for saved progress...

Exam Rules

You can skip questions and return to them later
Flag questions for review before submitting
No feedback shown until you submit the entire exam
Unanswered questions count as wrong — answer everything
10 pretest questions are mixed in and don't affect your score
Timer auto-submits when time runs out
Your progress is auto-saved every 30 seconds