Part P Earthing and Bonding 2

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?

Accepted Answer

No — bonding is only required for metallic services that could introduce an earth potential

Answer

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?

Accepted Answer

6mm² copper if mechanically protected, 16mm² if not

Answer

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?

Accepted Answer

Earth electrode resistance test using the fall-of-potential method

Answer

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?

Accepted Answer

All simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts within the location

Answer

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?

Accepted Answer

Dangerous voltages can appear on all earthed metalwork in the installation, including bonded pipework

Answer

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.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Earthing and Bonding 2