Published: 15 April 2013
Category: Technical articles
A suitable additional conductor, 'an earthing tail', may be required between an accessory and a flush-mounted metal back box to ensure that the metal back box is reliably earthed.
Where a metal back box has adjustable lugs, the reliability of the connection may be compromised.
During the lifetime of the installation, effects such as corrosion may result in a poor connection.
Under earth fault conditions, the circuit over-current protective device may fail to operate in the required time resulting in overheating and, under certain conditions, a risk of electric shock.
Whether an earthing tail is required depends upon whether one or both of the lugs on the back box are adjustable (to permit the socket-outlet to be levelled) and upon the earthing strap and eyelet arrangement of the socket-outlet.
Should the installation designer decide that the reliability of the connection may not be assured throughout the lifetime of the installation, and earthing tail should be provided. An earthing tail consists of an additional protective conductor consisting of a length of suitably-sized and identified protective conductor connecting the earthing terminal of the socket-outlet with the earthing terminal of the metal black box.
During the lifetime of the installation, effects such as corrosion may result in a poor connection.
Under earth fault conditions, the circuit over-current protective device may fail to operate in the required time resulting in overheating and, under certain conditions, a risk of electric shock.
Solution
A metal back box for a surface-mounted accessory such as a socket-outlet is an exposed-conductive-part and a metal back box for a flush-mounted socket-outlet is deemed to be an exposed-conductive-part (even though it may not be able to be touched). Therefore, such back boxes, no less than every other exposed-conductive-part, are required to be earthed in accordance with Regulations 411.4.2 (TN systems) and 411.5.1 (TT systems).Whether an earthing tail is required depends upon whether one or both of the lugs on the back box are adjustable (to permit the socket-outlet to be levelled) and upon the earthing strap and eyelet arrangement of the socket-outlet.
Should the installation designer decide that the reliability of the connection may not be assured throughout the lifetime of the installation, and earthing tail should be provided. An earthing tail consists of an additional protective conductor consisting of a length of suitably-sized and identified protective conductor connecting the earthing terminal of the socket-outlet with the earthing terminal of the metal black box.
Regulation 411.4.2 (TN Systems)(Part of)
Each exposed-conductive-part of the installation shall be connected by a protective conductor to the main earthing terminal of the installation, which shall be connected to the earthed point of the power supply system.Regulation 411.5.1 (TT Systems)
Every exposed-conductive-part which is to be protected by a single device shall be connected, via the main earthing terminal, to a common earth electrode. However if two or more protective devices are in a series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device.This article is extracted from Snags and Solutions Part 1 published by the: NICEIC Group