Part P BS 7671 IET Wiring Regulations 4

Question 1

What colour is the phase (live) conductor in a single-phase installation under the current UK harmonised cable colour code?

Accepted Answer

Brown

Answer

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?

Accepted Answer

The second phase (L2) conductor

Answer

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)?

Accepted 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

Answer

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?

Accepted Answer

30 mA

Answer

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?

Accepted 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

Answer

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.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P BS 7671 IET Wiring Regulations 4