Part P Earthing & Bonding

Question 1

What is the primary purpose of earthing in an electrical installation?

Accepted Answer

To provide a low-impedance return path for fault current so that protective devices operate quickly, limiting the duration of dangerous touch voltages

Answer

Earthing ensures that if a fault occurs (e.g., live conductor touching a metal casing), the fault current flows to earth through a low-impedance path, causing the protective device to trip quickly and limiting the touch voltage duration.

Question 2

What is the main earthing terminal (MET)?

Accepted Answer

The central connection point in the installation where the earthing conductor, main bonding conductors and circuit protective conductors are all connected

Answer

The MET is the main hub of the earthing system; all earthing conductors converge here — the earthing conductor to the supply earth, main bonding conductors to services, and the CPCs from the consumer unit.

Question 3

What is an earthing conductor?

Accepted Answer

The conductor connecting the main earthing terminal to the earthing arrangement (e.g., earth terminal at the supply, earth electrode or PME neutral)

Answer

The earthing conductor connects the installation's main earthing terminal to the supply's earth point (PME neutral, cable sheath terminal, or local earth electrode), forming the backbone of the earthing system.

Question 4

What minimum size is required for an earthing conductor in a typical domestic installation connected to a PME (TN-C-S) supply?

Accepted Answer

16 mm² if unprotected, or 10 mm² if protected (buried or in conduit)

Answer

For a PME supply, BS 7671 Table 54.8 requires the main earthing conductor to be at least half the size of the phase conductor, with minimum sizes specified; commonly 16 mm² unprotected or 10 mm² when protected.

Question 5

What is extraneous conductive part?

Accepted Answer

A conductive part that is not part of the electrical installation but can introduce a potential (e.g., metallic water pipes, gas pipes, structural steelwork)

Answer

An extraneous conductive part is a metallic component not part of the electrical installation (e.g., gas pipes, water pipes, structural steel) that could be at a dangerous potential if a fault occurs and must be bonded to earth.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Earthing & Bonding