Part P Earthing and Bonding — Questions and Answers
Question 1: What is the minimum cross-sectional area of the main protective bonding conductor for a domestic installation with a 25mm² line conductor?
- 4mm²
- 6mm²
- 10mm² (Correct answer)
- 16mm²
Correct answer: 10mm²
Table 54.8 of BS 7671 states that the main protective bonding conductor must be not less than half the cross-sectional area of the earthing conductor, with a minimum of 6mm² (10mm² where PME conditions apply). For a typical domestic PME supply with a 25mm² meter tail, the main bonding conductor must be at least 10mm².
Question 2: Which of the following services must be connected to the main protective bonding?
- Only the water pipe
- All metallic services entering the building: water, gas, oil, and any other metallic service pipes (Correct answer)
- Only the gas pipe and water pipe
- Plastic pipes do not require bonding, and neither do metal ones
Correct answer: All metallic services entering the building: water, gas, oil, and any other metallic service pipes
Main protective bonding must be applied to all metallic services entering the building from outside, including water, gas, oil pipes, structural steelwork, and any other extraneous-conductive-part. The bonding must be made as close as practicable to the point of entry. If services are entirely plastic (e.g., plastic water mains), bonding of that service is not required.
Question 3: In a TT earthing system, what provides the earth fault return path?
- The supply neutral conductor (PEN)
- The general mass of earth via a local earth electrode (Correct answer)
- A metallic sheath on the supply cable
- The water pipe network
Correct answer: The general mass of earth via a local earth electrode
In a TT system, there is no earth connection provided by the supply company. The installation must have its own earth electrode (typically a driven rod or buried tape) through which fault current returns via the general mass of earth. Due to the higher earth fault loop impedance, TT systems always require RCD protection as the primary means of fault protection.
Question 4: What is the purpose of equipotential bonding in a domestic installation?
- To provide a low-impedance path for fault current to flow back to the source
- To ensure all simultaneously accessible metallic parts are at the same potential, eliminating dangerous touch voltages (Correct answer)
- To increase the current-carrying capacity of the cables
- To improve the power factor of the installation
Correct answer: To ensure all simultaneously accessible metallic parts are at the same potential, eliminating dangerous touch voltages
Equipotential bonding connects all extraneous-conductive-parts (metallic pipes, structural steel) to the main earthing terminal, ensuring they are all at the same electrical potential. If a fault occurs, the voltage between any two accessible metallic parts remains low enough to be safe, preventing dangerous electric shock even before the protective device disconnects.
Question 5: Where should the main earthing terminal be located in a domestic installation?
- In the garage
- At or near the consumer unit (origin of the installation) (Correct answer)
- At the earth electrode
- In the bathroom
Correct answer: At or near the consumer unit (origin of the installation)
The main earthing terminal (MET) should be located at or near the consumer unit at the origin of the installation. All protective conductors, bonding conductors, and the earthing conductor are connected to this central point. It must be accessible for inspection, testing, and disconnection (using a removable link for testing purposes as required by Regulation 542.4.2).
Question 6: What is the maximum resistance of an earth electrode in a TT installation protected by a 100mA RCD?
- 50Ω
- 100Ω
- 200Ω
- 500Ω (Correct answer)
Correct answer: 500Ω
For a TT system, the product of the earth electrode resistance (RA) and the operating current of the RCD (IΔn) must not exceed 50V: RA × IΔn ≤ 50V. For a 100mA RCD: RA ≤ 50/0.1 = 500Ω. For a 30mA RCD: RA ≤ 50/0.03 = 1667Ω. However, lower electrode resistance values are always preferred for reliable operation.
Part P Earthing and Bonding
What is the minimum cross-sectional area of the main protective bonding conductor for a domestic installation with a 25mm² line conductor?