Questions relating to the requirements for supplementary equipotential bonding are frequently asked; a very common one is where should it be installed. This article looks at the requirements for supplementary equipotential bonding in BS 7671:2008, where supplementary equipotential bonding should be installed and offers help on the process of evaluation.
TERMINOLOGY
Firstly, it is important that the terminology is correct. Earthing and bonding are two different concepts yet the terms are often used together. Once we have established that "earth-bonding"* is a nonsensical expression and should never be used, we can look at the requirements of supplementary equipotential bonding in BS 7671:2008.
*Earthing and bonding are two separate concepts
Earthing Connection of the exposedconductive- parts of an installation to the main earthing terminal of that installation.
An example of earthing is where the metallic outer-case of a class I appliance is connected by the circuit protective conductor to the means of earthing providing a safe path for fault or high leakage/high protective conductor currents.
Equipotential bonding
Electrical connection maintaining various exposed-conductive-parts and extraneous-conductiveparts at substantially the same potential. There are two types of equipotential bonding conductor:
Main protective bonding conductor Used to connect extraneous-conductive-parts, such as a metallic water pipe, to the main earthing terminal.
Supplementary equipotential bonding conductors
Used to supplement to fault protection by maintaining various exposed conductiveparts and extraneousconductive- parts at substantially the same potential, such as the connection of all exposedconductive - parts and extraneous-conductive-parts that can be touched by livestock in an agricultural installation.
HISTORY
The change from the 14th to the 15th Edition of the IEE Wiring Regulations in 1981 created a big upheaval in the electrical installation industry as many new concepts were introduced and many existing practices were expanded or enhanced - one of those being supplementary equipotential bonding. To a great extent, the 15th Edition was based on CENELEC harmonised documents (HDs) (more so now with the introduction of the 17th Edition).
HD 384.4.41 was the basis for Chapter 41 of the 15th Edition - Protection against electric shock, which had requirements for supplementary equipotential bonding in Regulation 413-7, reproduced here:
413-7 Within the zone formed by the main equipotential bonding, local supplementary equipotential bonding connections shall be made to metal parts, to maintain the equipotential zone, where those parts -
(i) are extraneous conductive parts, and
(ii) are simultaneously accessible with exposed conductive parts or other extraneous conductive parts, and
(iii) are not electrically connected to the main equipotential bonding by permanent and reliable metal-to-metal joints of negligible impedance. NOTE - Where local equipotential bonding is provided in accordance with Regulation 413-7, metalwork which may be required to be bonded includes baths and exposed metal pipes, sinks, taps, tanks, and radiators and, where practicable, accessible structural metalwork
Those far reaching changes, issued on 31st March 1981, are still being felt today with designers and specifiers still implementing the requirements for supplementary equipotential bonding from the 15th Edition. Much of the confusion can be attributed to the note of Regulation 413-7 which required the bonding of all metallic items, essentially, those within the designated equipotential zone. This led to the installation of supplementary equipotential bonding of general metallic items such as baths, ceiling grids, hand rails, kitchen sinks, radiators, pipework at boilers, etc. Thankfully, we have moved on from this general concept.
THE REQUIREMENTS FOR SUPPLEMENTARY EQUIPOTENTIAL BONDING
We'll look at the requirements for supplementary equipotential bonding then at instances where it would be required. Initially, the scene is set by Regulation 410.3.7 which requires that if the conditions of a protective measure cannot be met, supplementary provisions shall be applied to achieve the same degree of safety.
What is a protective measure?
There are four protective measures generally permitted by BS 7671:2008, given in Regulation 410.3.3:
(i) Automatic disconnection of supply (Section 411)
(ii) Double or reinforced insulation (Section 412)
(iii) Electrical separation for the supply to one item of current-using equipment (Section 413)
(iv) Extra-low voltage (SELV and PELV) (Section 414).
A note at the end of this Regulation acknowledges that, in electrical installations, the most commonly used protective measure is automatic disconnection of supply.
DISCONNECTION TIMES
Regulation 411.3.2.6 states that where automatic disconnection cannot be achieved in the required time, supplementary equipotential bonding shall be provided. In this instance, if disconnection will not occur in the required 0.4 s, for example, supplementary
equipotential bonding is used to hold various exposedconductive- parts and extraneous-conductive-parts at substantially the same potential to limit the risk of a dangerous electric shock - this clears the confusion created by Regulation 413-7 of the 15th Edition.
Do bear in mind that supplementary equipotential bonding need not be physically carried out by the installation of single-core green-and-yellow conductors in every instance. There may be a situation where, for example, two simultaneously accessible metallic parts are in reliable contact and the resistance between the two parts is sufficiently low.
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