Part P Earthing and Bonding 2 — Questions and Answers
Question 1: A domestic property has a PME (TN-C-S) supply. The electrician notices the gas pipe entering the building is plastic. Is main bonding of the gas supply still required?
- Yes — all gas supplies must always be bonded regardless of material
- No — bonding is only required for metallic services that could introduce an earth potential (Correct answer)
- Only if the gas meter is metallic
- Bonding must be applied to the plastic pipe using special clamps
Correct answer: No — bonding is only required for metallic services that could introduce an earth potential
If the gas service pipe entering the building is entirely plastic (non-conductive), it is not an extraneous-conductive-part and does not require main protective bonding. However, if there are metallic gas pipes within the installation connected to a metallic meter, these may still need bonding. The key question is whether the metallic parts are connected to the general mass of earth via the external service.
Question 2: What is the minimum cross-sectional area of the earthing conductor connecting the MET to the earth electrode in a TT system?
- 2.5mm² copper
- 6mm² copper if mechanically protected, 16mm² if not (Correct answer)
- 10mm² copper
- 25mm² copper minimum
Correct answer: 6mm² copper if mechanically protected, 16mm² if not
Table 54.1 of BS 7671 specifies minimum sizes for earthing conductors. For copper conductors to an earth electrode: 2.5mm² if mechanically protected and corrosion-protected (e.g., enclosed in conduit), 4mm² if corrosion-protected but not mechanically protected, and 16mm² if buried and not protected against corrosion. In practice, 6mm² mechanically protected or 16mm² bare is common.
Question 3: 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 4: In a bathroom, supplementary equipotential bonding connects which parts?
- Only the bath and shower tray
- All simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts within the location (Correct answer)
- Only the light switch and shaver socket
- The toilet cistern and towel rail only
Correct answer: All simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts within the location
Supplementary bonding in a bathroom (if required) must connect all simultaneously accessible exposed-conductive-parts (metallic parts of Class I equipment) and extraneous-conductive-parts (metallic pipes, radiators, metallic baths/shower trays, structural metalwork) to each other and to the protective conductors of all circuits in the location, using conductors of at least 4mm² (2.5mm² if mechanically protected).
Question 5: What is the consequence of a broken PEN (combined protective earth and neutral) conductor in a PME (TN-C-S) supply?
- The lighting dims slightly
- Dangerous voltages can appear on all earthed metalwork in the installation, including bonded pipework (Correct answer)
- The RCD will trip immediately
- The electricity supply will be cut off automatically
Correct answer: Dangerous voltages can appear on all earthed metalwork in the installation, including bonded pipework
A broken PEN conductor is the most dangerous fault in a PME system. The installation's earthing is lost, and neutral current from the installation (and potentially other properties) must return through the earth electrode and bonded metalwork. This can raise the potential of all earthed metalwork to dangerous levels (potentially up to 230V). This is why PME earthing has specific restrictions for some external installations.
Question 6: What size bonding conductor is required for supplementary bonding between two extraneous-conductive-parts (e.g., two metal pipes)?
- 1.0mm²
- 2.5mm² if mechanically protected, 4mm² if not (Correct answer)
- 6mm² minimum
- 10mm² minimum
Correct answer: 2.5mm² if mechanically protected, 4mm² if not
Regulation 544.2 of BS 7671 requires supplementary bonding conductors between two extraneous-conductive-parts to be at least 2.5mm² copper if mechanically protected (e.g., enclosed in conduit or run along a pipe) or 4mm² if not mechanically protected. If connecting an exposed-conductive-part to an extraneous-conductive-part, the conductor must be at least half the CPC of the circuit.
Part P Earthing and Bonding 2
A domestic property has a PME (TN-C-S) supply.
The electrician notices the gas pipe entering the building is plastic.
Is main bonding of the gas supply still required?