Part P Earthing & Bonding 3 — Questions and Answers
Question 1: What test is performed to verify the continuity of a circuit protective conductor?
- R1 + R2 test: measuring the combined resistance of phase and CPC conductors from the consumer unit to the furthest point of the circuit (Correct answer)
- Insulation resistance test
- Earth fault loop impedance test only
- Visual inspection only
Correct answer: R1 + R2 test: measuring the combined resistance of phase and CPC conductors from the consumer unit to the furthest point of the circuit
The R1 + R2 test connects the phase and CPC at the far end of the circuit and measures their combined resistance from the CU; this confirms the CPC is continuous and correctly sized throughout the circuit.
Question 2: What is the purpose of bonding a TV aerial (coaxial) cable screen to earth?
- To discharge static and induced voltages on the aerial cable screen, preventing dangerous voltages being introduced into the installation or equipment (Correct answer)
- To improve signal quality
- To prevent lightning striking the aerial
- 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 3: What is the minimum cross-sectional area of a supplementary bonding conductor between two exposed conductive parts?
- 2.5 mm² if mechanically protected, 4 mm² if not mechanically protected (Correct answer)
- 0.5 mm²
- 10 mm²
- 16 mm²
Correct answer: 2.5 mm² if mechanically protected, 4 mm² if not mechanically protected
BS 7671 Regulation 544.2 specifies minimum sizes for supplementary bonding conductors: 2.5 mm² when mechanically protected (e.g., in conduit) and 4 mm² when not protected against mechanical damage.
Question 4: What is a PME (Protective Multiple Earthing) warning label and where is it required?
- A warning label fixed at the origin of a PME installation advising that additional bonding may be required, as required by the electricity supplier's distribution code (Correct answer)
- A label on the gas meter
- A label on all socket outlets in a PME installation
- A label on the electricity meter only
Correct answer: A warning label fixed at the origin of a PME installation advising that additional bonding may be required, as required by the electricity supplier's distribution code
Where a PME supply is used, the electricity supplier's conditions include a requirement to label the installation at the origin with a PME warning, noting that the neutral is used as the earthing conductor and that disconnection of the PEN conductor would energise metalwork.
Question 5: What risk is associated with using PME earth terminals for caravan or boat hook-up?
- If the PEN conductor breaks in the supply, the caravan or boat metalwork could rise to near line voltage — PME earth must not be used for caravans, boats or temporarily connected equipment (Correct answer)
- PME earth is preferred for caravan hook-ups
- PME earth terminals always provide a better earth than a rod
- There is no risk associated with PME for caravans
Correct answer: If the PEN conductor breaks in the supply, the caravan or boat metalwork could rise to near line voltage — PME earth must not be used for caravans, boats or temporarily connected equipment
A broken PEN conductor (common in ageing networks) would cause any PME-earthed metalwork to rise to dangerous voltages. For this reason, BS 7671 prohibits using a PME terminal as the earth for caravans, boats or temporary structures.
Question 6: What is the purpose of testing continuity of main protective bonding conductors during an installation inspection?
- To confirm the bonding conductors are connected and have a low resistance, ensuring they can carry fault current and limit potential differences (Correct answer)
- To test the insulation of the bonding conductors
- To measure the earth fault loop impedance of the bonding conductors
- To verify the bonding conductors are the correct colour
Correct answer: To confirm the bonding conductors are connected and have a low resistance, ensuring they can carry fault current and limit potential differences
Continuity testing of bonding conductors confirms they are correctly connected at both ends and have a resistance low enough to carry fault current and maintain equipotential conditions between all bonded metalwork.
Part P Earthing & Bonding 3
What test is performed to verify the continuity of a circuit protective conductor?