Part P Earthing and Bonding

Question 1

What is the minimum cross-sectional area of the main protective bonding conductor for a domestic installation with a 25mm² line conductor?

Accepted Answer

10mm²

Answer

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?

Accepted Answer

All metallic services entering the building: water, gas, oil, and any other metallic service pipes

Answer

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?

Accepted Answer

The general mass of earth via a local earth electrode

Answer

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?

Accepted Answer

To ensure all simultaneously accessible metallic parts are at the same potential, eliminating dangerous touch voltages

Answer

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?

Accepted Answer

At or near the consumer unit (origin of the installation)

Answer

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).

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Earthing and Bonding