Part P Cable Selection and Wiring Systems 1 — Questions and Answers
Question 1: What type of cable is most commonly used for fixed domestic wiring circuits in the UK?
- Steel Wire Armoured (SWA) cable
- Twin and Earth (T&E) flat cable (Correct answer)
- Mineral Insulated Copper Clad (MICC) cable
- Flexible cord (flex)
Correct answer: Twin and Earth (T&E) flat cable
Twin and Earth (T&E) flat cable is the standard cable used for most fixed domestic circuits including lighting, ring finals, and cooker circuits, routed under floors and behind plasterwork.
Question 2: What does the abbreviation 'T&E' stand for in UK electrical cable terminology?
- Tested and Earthed
- Twin and Earth (Correct answer)
- Triple and Earthed
- Terminated and Earthed
Correct answer: Twin and Earth
T&E stands for Twin and Earth, describing a cable containing two insulated conductors (line and neutral) plus a bare earth conductor within a flat PVC outer sheath.
Question 3: Following the 2004 harmonisation with European cable colour standards, what colour is the line conductor in UK fixed wiring?
- Red
- Blue
- Black
- Brown (Correct answer)
Correct answer: Brown
After the 2004 harmonisation, the line conductor in UK fixed wiring changed from red to brown, matching the rest of Europe and the existing colour of the live pin on UK plugs.
Question 4: What colour is the standard outer sheath of twin and earth cable used in UK domestic fixed wiring?
- White
- Black
- Grey (Correct answer)
- Yellow
Correct answer: Grey
Standard UK twin and earth cable has a grey PVC outer sheath, making it readily identifiable as fixed wiring cable in a domestic installation.
Question 5: What is the minimum cross-sectional area (CSA) of the line and neutral conductors in a standard 32A domestic ring final circuit?
- 1.0 mm²
- 1.5 mm²
- 2.5 mm² (Correct answer)
- 4.0 mm²
Correct answer: 2.5 mm²
BS 7671 specifies 2.5 mm² conductors (with a 1.5 mm² CPC) for a 32A ring final circuit supplying 13A socket outlets in a standard domestic installation.
Question 6: What is the primary purpose of the Circuit Protective Conductor (CPC) within a twin and earth cable?
- To carry the neutral return current under normal load
- To provide a low-impedance fault path so protective devices can operate quickly (Correct answer)
- To carry additional line current during heavy load
- To screen the cable from electromagnetic interference
Correct answer: To provide a low-impedance fault path so protective devices can operate quickly
The CPC provides a low-impedance return path for fault current back to the source, enabling the circuit protective device (fuse or MCB) to operate and disconnect the supply during an earth fault.
Question 7: Where a bare CPC is exposed at a termination point, how must it be identified according to BS 7671?
- With solid green sleeving
- With green and yellow sleeving (Correct answer)
- With brown sleeving
- 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.
Part P Cable Selection and Wiring Systems 1
What type of cable is most commonly used for fixed domestic wiring circuits in the UK?