Part P Earthing & Bonding 1 — Questions and Answers
Question 1: What is the primary purpose of earthing in an electrical installation?
- To provide a low-impedance return path for fault current so that protective devices operate quickly, limiting the duration of dangerous touch voltages (Correct answer)
- To improve power factor
- To prevent voltage surges
- To provide a reference for measuring current
Correct answer: To provide a low-impedance return path for fault current so that protective devices operate quickly, limiting the duration of dangerous touch voltages
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)?
- The central connection point in the installation where the earthing conductor, main bonding conductors and circuit protective conductors are all connected (Correct answer)
- The earth rod in the ground
- The earth terminal in the consumer unit only
- The earthing point at the electricity meter
Correct answer: The central connection point in the installation where the earthing conductor, main bonding conductors and circuit protective conductors are all connected
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?
- The conductor connecting the main earthing terminal to the earthing arrangement (e.g., earth terminal at the supply, earth electrode or PME neutral) (Correct answer)
- The earth conductor within each circuit cable
- The bonding conductor to gas pipes
- The earth terminal in a socket outlet
Correct answer: The conductor connecting the main earthing terminal to the earthing arrangement (e.g., earth terminal at the supply, earth electrode or PME neutral)
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?
- 16 mm² if unprotected, or 10 mm² if protected (buried or in conduit) (Correct answer)
- 2.5 mm² copper
- 4 mm² copper
- 6 mm² copper
Correct answer: 16 mm² if unprotected, or 10 mm² if protected (buried or in conduit)
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?
- 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) (Correct answer)
- Any metal part of an electrical accessory
- The armour of a cable
- The neutral conductor
Correct 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)
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.
Question 6: What is the main bonding conductor's connection point on a metallic gas installation pipe?
- As close as practicable to the point of entry into the building, upstream of any meter or stop valve (Correct answer)
- At any convenient point along the pipe
- At the gas appliance
- At the meter only
Correct answer: As close as practicable to the point of entry into the building, upstream of any meter or stop valve
BS 7671 Regulation 544.1.2 requires main bonding to be made as close as practicable to the point of entry of the service, upstream of any meter, stop valve or shut-off, to ensure all pipework within the building is at earth potential.
Part P Earthing & Bonding 1
What is the primary purpose of earthing in an electrical installation?