Part P BS 7671 IET Wiring Regulations 4 — Questions and Answers
Question 1: What colour is the phase (live) conductor in a single-phase installation under the current UK harmonised cable colour code?
- Brown (Correct answer)
- Red
- Blue
- Grey
Correct answer: Brown
Under the harmonised cable colour code adopted in the UK (from 2004 for new installations), the phase conductor in single-phase installations is brown, with blue for neutral and green/yellow for earth.
Question 2: What does the cable colour 'grey' represent in a three-phase installation under the harmonised colour code?
- The second phase (L2) conductor (Correct answer)
- The neutral conductor
- The earth conductor
- The third phase conductor
Correct answer: The second phase (L2) conductor
In the harmonised three-phase colour code: L1 is brown, L2 is grey, L3 is black, neutral is blue and earth is green/yellow.
Question 3: What is the purpose of a residual current device (RCD)?
- To detect a difference between the currents in the live and neutral conductors indicating leakage to earth, and disconnect within milliseconds to prevent electrocution (Correct answer)
- To protect cables from overload
- To prevent short circuits
- To measure current consumption
Correct answer: To detect a difference between the currents in the live and neutral conductors indicating leakage to earth, and disconnect within milliseconds to prevent electrocution
An RCD continuously monitors the balance of current flowing in the live and neutral conductors; any difference (residual current) flowing to earth through a person or fault causes it to trip within 25–40 ms, protecting against electrocution.
Question 4: What is the standard operating current threshold for a Type AC RCD used for additional protection?
- 30 mA (Correct answer)
- 100 mA
- 500 mA
- 10 mA
Correct answer: 30 mA
A 30 mA RCD is the standard for additional protection in domestic and similar installations; 30 mA is below the threshold of ventricular fibrillation (~50–100 mA), providing effective protection against electrocution.
Question 5: What does 'additional protection' by RCD mean in BS 7671?
- RCD protection provided as a supplementary measure in addition to basic and fault protection, required for socket outlets up to 20 A, circuits supplying mobile equipment outdoors, and in bathrooms (Correct answer)
- The only protection required
- Protection instead of fuses or MCBs
- An optional enhancement only in commercial buildings
Correct answer: RCD protection provided as a supplementary measure in addition to basic and fault protection, required for socket outlets up to 20 A, circuits supplying mobile equipment outdoors, and in bathrooms
Additional protection by 30 mA RCD is required by BS 7671 for specific circumstances (socket outlets ≤20 A, outdoor mobile equipment, cable in walls/floors, bathroom zones) to protect against residual risks not covered by basic measures.
Question 6: Under BS 7671, where must cables be installed in walls to avoid additional mechanical protection?
- Within 150 mm of the top of a wall or partition, within 150 mm of an angle in the wall, or in a zone from a switch or socket to a point directly above or below (Correct answer)
- Anywhere in a wall without restriction
- Only in conduit regardless of depth
- Only at 450 mm from floor level
Correct answer: Within 150 mm of the top of a wall or partition, within 150 mm of an angle in the wall, or in a zone from a switch or socket to a point directly above or below
BS 7671 Regulation 522.6.6 permits cables in walls without additional protection only if installed in prescribed zones (within 150 mm of the top or an angle of the wall, or within a defined zone from an accessory), otherwise mechanical protection is required.
Part P BS 7671 IET Wiring Regulations 4
What colour is the phase (live) conductor in a single-phase installation under the current UK harmonised cable colour code?