It is easy to confuse earthing and bonding, because of the visual similarities between them. For example, green-and-yellow colour identification of the protective conductors is used for both, and both are associated with fault protection. However, earthing and bonding are quite distinct from each other in their purposes, their general arrangements, and in many of the requirements of BS 7671 that they have to satisfy.

Safety first

Both earthing and bonding must be provided in an installation, each meeting the respective safety requirements of BS 7671. Generally, conductors provided for earthing are required to be designed to carry earth fault current, whereas conductors provided for bonding are not. Additionally, earthing is intended to limit the duration of touch voltages, while bonding is intended to limit the magnitude of touch voltages.

Touch Voltage

The danger of electric shock due to an earth fault arises from the following voltages (sometimes called touch voltages) which may occur under such conditions in an installation:

a) voltages between exposed-conductive-parts1 and other exposed-conductive-parts
b) voltages between extraneous-conductive-parts2 and other extraneous-conductive-parts
c) voltages between exposed-conductive-parts and extraneous-conductive-parts
d) voltages between exposed-conductive-parts and Earth, or extraneous-conductive-parts and Earth.

Earthing

The purpose of earthing, where used for protective purposes within an installation, is to limit the duration of the voltages in (a), (b), (c) and (d) above. This is achieved by the operation of the relevant protective device (such as a fuse or circuit-breaker) under earth fault conditions. This removes the voltages by causing the automatic disconnection of the supply to the faulty circuit within the time specified in BS 7671.

Without an adequate earthing arrangement, the protective device could not operate as required under earth fault conditions.

Bonding

The purpose of bonding is to limit the magnitude of the voltages in (a), (b) and (c). This is achieved by electrically connecting together those conductive parts. The main safety benefit of adequate bonding is that the magnitude of voltages occurring between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts under earth fault conditions is insufficient to cause danger during the time taken for the relevant protective device to disconnect the supply to the faulty circuit.

A by-product of ‘protective equipotential’ (main and supplementary) bonding is that, under earth fault conditions, it may reduce the duration (not just magnitude) of the touch voltages in the installation. The reduction in touch voltage duration is related to the additional conductive paths that the bonding provides, which are in parallel with the earthing arrangement of the installation.

The parallel paths allow a greater magnitude of earth fault current to flow, which may reduce the time taken for the relevant protective device to automatically disconnect the supply to the faulty circuit, and consequently reduces the touch voltage duration. Even so, bonding must not be relied upon to satisfy the disconnection time requirements of BS 7671, which is the function of earthing.