Many existing electrical installations require one or more additions or alterations during their lifetime. An addition or alteration may be minor such as the addition of a point to a final circuit, or major such as the addition of new final circuits, distribution circuits, or consumer units.
Where an addition or alteration (whether temporary or permanent) is to be made to an installation, the relevant requirements of BS 7671:2018 have to be met such that both the addition or alteration is safe, and the safety of the existing installation is not impaired.
In order to meet the requirements of BS 7671, the following must be undertaken:
- Assessment of the existing installation to establish that the parts upon which the addition or alteration will rely for safety are both present and suitable for the altered circumstances.
- Design and construction of the addition or alteration is in accordance with the requirements of BS 7671.
- Inspection and testing of any upgrading of the existing installation and of the addition or alteration before being put into service, as required by Chapter 64 of BS 7671.
- Certification of any upgrading of the existing installation, and of the addition or alteration, as required by Section 644 of BS 7671.
Items 3. and 4. are beyond the scope of this article.
Assessment of an existing installation
Regulations 132.16 requires an assessment to be made of the relevant parts of an existing installation which is to be added to or altered and of the supply (see Fig 1).
The adequacy of the existing installation is therefore required to be assessed in three areas:
- existing equipment (including that of the supply) which may have to carry increased load;
- earthing and bonding arrangements;
- certain additional considerations such as grouping and isolation and switching.
Existing equipment which may have to carry increased load
It is necessary to establish that the rating and condition of all appropriate equipment within an installation such as overcurrent protective devices, final and distribution circuit conductors, switchgear and distribution boards is capable of supporting the planned addition or alteration. The condition of the existing equipment should be assessed for its ability to carry any increased load and for defects, omissions, damage and minor deterioration. Any inadequacies in the existing installation that would result in a reduced level of safety in the new work, that is a level of safety less than afforded by compliance with BS 7671, must be corrected before the new work is put into service.
Existing earthing and bonding arrangements
Assessing the adequacy and suitability of the existing earthing and bonding arrangements includes:
- Establishing the suitability of the means of earthing in respect of its type such as a PME earthing facility or an earth rod and its condition (for example – is it corroded or damaged or inaccessible?)
- Verification of the means of earthing by determination of the external earth fault loop impedance Ze(see Regulation 313.1). If Ze is determined by enquiry or calculation, it may also be necessary to obtain a measured value to verify that the intended means of earthing is both present and of the expected value.
- Verifying that the earthing conductor is of adequate cross-sectional area.
- Establishing, determining or confirming the presence and adequacy of circuit protective conductors for both the final circuit(s) and any distribution circuit(s) forming part of the addition or alteration.
- Where the protective measure against electric shock is Automatic Disconnection of Supply (ADS), the adequacy of the existing bonding must be established. It is an essential requirement for safety that main protective bonding is provided (see Fig 2).
Upgrading of an existing installation, where necessary
When an existing installation is not adequate to support a proposed addition or alteration safely the work must not proceed, and the client must be advised immediately and preferably in writing. The necessary upgrading work should be recommended to the client, and when consent has been given, the upgrading work should be completed before the addition or alteration is put into service.
Upgrading work necessary to enable an installation to safely accommodate the addition or alteration may include:
• replacement of any equipment or cabling having defects, omissions, damage or minor deterioration
• replacement or upgrading of existing equipment in the installation where its rating is not adequate
• addition to, or upgrading of, main protective bonding in an installation relying on the protective measure ADS for protection against electric shock. If the cross-sectional area of the existing bonding conductors is found to be less than that required by BS 7671, the designer of the addition or alteration will need to make a judgment as to the suitability of those conductors and whether the new work may proceed.
Design and construction of the addition or alteration
The addition or alteration is required to meet all relevant requirements of BS 7671, including, where appropriate, the special requirements contained in Part 7, Special Installations or Locations. Furthermore, the addition or alteration must not impair the safety of the existing installation (Regulation 641.5 refers).