It would appear that many people, including some whose work requires involvement with electrical installations, are not aware of the differences between earthing and bonding. This article outlines their different, but equally important, functions within an installation.
In general, virtually all electrical installations will require both earthing and bonding in order to meet the requirements of BS 7671. However, some installations, or parts thereof, may not require bonding where other protective measures have been put in place, and some of the less commonly used methods of protection will require the installation of bonding but not generally earthing. As such, an understanding of the purpose of, and the differences between, earthing and bonding is essential.
In order to explain the differences between earthing and bonding, it is first necessary to consider touch voltage.
What is a touch voltage?
Touch voltage is the voltage present between simultaneously accessible conductive parts; that is, any combination of exposed-conductive-parts and/or extraneous-conductive-parts, or between accessible conductive parts and Earth, under Earth fault conditions.
It should be noted that the value of actual (or effective) touch voltage may differ significantly from that of the calculated (prospective) touch voltage as a result of the difference in impedance of the person, or even livestock, making the contact.
In general, the touch voltage limit for installations is taken as 50 V AC or 120 V DC (415.2.2). It should be noted, however, that the touch voltage in medical locations of Group 1 and 2 should not exceed 25 V AC or 60 V DC (710.418.104.22.168 and 710.415.2.2). BS 7671 also contains requirements for measures to limit touch current in locations intended to be accessible to livestock (705.415.2.1 and 740.415.2.1).
Earthing is a process that is intended to limit the duration of exposure to touch voltages. Disconnection occurs when the relevant protective device, such as a circuit-breaker or fuse, operates under Earth fault conditions, disconnecting the supply to the circuit(s) in question within the time required by BS 7671.
Such protective devices would not be able to operate within the required time without adequate earthing arrangements being in place.
Bonding, on the other hand – or, more correctly, protective equipotential bonding (422.214.171.124) – is a process that is intended to keep the magnitude of a touch voltage (as described earlier) below the conventional touch voltage limit – that is, the touch voltage which is permitted to be present indefinitely without constituting a danger to persons or livestock.
The danger of electric shock due to Earth fault conditions arises from the voltages, which may occur in an installation between:
- exposed-conductive-parts and other exposed-conductive-parts;
- extraneous-conductive-parts and other extraneous-conductive-parts;
- exposed-conductive-parts and extraneous-conductive-parts; and
- exposed-conductive-parts and Earth; or
- extraneous-conductive-parts and Earth.
The presence of bonding, while primarily intended to limit the magnitude of touch voltage, will also typically help to reduce the time that such a touch voltage is present. This is because bonding provides a number of parallel paths, reducing Earth fault loop impedance and hence increasing the fault current. This in turn results in the more rapid operation of the relevant protective device.
There are two sub-sets of protective equipotential bonding:
- Main protective bonding; and
- Supplementary bonding.
It should be noted that, while earthing and protective equipotential bonding are both constituent parts of fault protection, where the protective measure of automatic disconnection of supply is employed (411.1), supplementary bonding:
- is considered to be an addition that can be used to supplement fault protection; and
- may be applied to all, or part, of an electrical installation, or to a location, or to items of equipment (415.2).
Use of supplementary bonding where automatic disconnection is not feasible
Where it is not feasible for an overcurrent protective device to provide automatic disconnection in the event of a fault, or where the use of an RCD for this purpose is not appropriate, regulation 419.3 calls for the provision of supplementary protective equipotential bonding such that the voltage between simultaneously accessible exposed-conductive-parts and/or extraneous-conductive-parts shall not exceed 50 V AC or 120 V DC.
Earth-free local bonding
Where Earth-free local bonding is employed as a protective measure in an installation controlled by, or under the supervision of, an electrically skilled or instructed person, protective bonding conductors must be installed interconnecting all simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts (418.2.2).
However, there must be no connection between the bonded exposed-conductive-parts and extraneous-conductive-parts and Earth (418.2.3).
It is essential to the effectiveness of this protective measure that persons entering or leaving the location cannot be exposed to a dangerous potential difference, especially where a conductive floor isolated from Earth is connected to the earth-free bonding (418.2.4).
For this reason, although not called for specifically in BS 7671, measures such as the provision of electrical safety matting complying with an appropriate standard, (such as BS EN 61111: 2009 Live working – electrical insulating matting) are required outside the location.
Bonding is quite distinct from earthing in its purpose, its general arrangement and in many of the requirements of BS 7671 that it has to satisfy. Earthing is intended to limit the duration of touch voltages, while bonding is intended to limit the difference in potential between two accessible conductive parts (touch voltage).
A by-product of equipotential (main and supplementary) bonding is that, under Earth fault conditions, it may reduce the duration (not just the 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 reduces the time taken for the relevant protective device to automatically disconnect the supply to the faulty circuit and consequently reduces the touch voltage duration.
Where Earth-free local bonding is employed as a protective measure, the installation, or part thereof in which this protective measure is employed, must be controlled by, or under the supervision of, an electrically skilled or instructed person. This is necessary to ensure that that no changes can be made that would impair the effectiveness of the protective measure.
Get more details about NICEIC membership and benefits here