ELECSA consistently receives enquiries relating to supplies to detached outbuildings of domestic premises, such as garages, sheds or home-offices.
This article addresses the questions which typically arise where such a supply is provided by an SWA cable buried in the ground, in particular:
- Can the armouring be used as a circuit protective conductor?
- What are the equipotential bonding requirements?
- What are the particular requirements where PME conditions apply?
Armouring for use as a circuit protective conductor
Unless provided with adequate mechanical protection, cables buried directly in the ground should incorporate an earthed armour or metal sheath or both which is suitable for use as a protective conductor (Regulation 522.8.10).
In view of this, a SWA cable is often specified for the supply to an outbuilding with the armouring serving as the cpc. However, where this is the case, the relevant disconnection times of Chapter 41 must be met.
For a distribution circuit connected to a TN system a maximum disconnection time of 5 s is permitted (Regulation 411.3.2.3) whilst for a TT system the maximum disconnection time is 1 s (Regulation 411.3.2.4). To confirm the adequacy of the armouring, the size of the cpc may be determined by either of the following methods:
- selection in accordance with Regulation 543.1.4 (Table 54.7 of BS 7671) or
- calculation in accordance with Regulation 543.1.3 (adiabatic equation).
Where the csa of the protective conductor is determined using Table 54.7, reference should be made to the column highlighted in Table 1.
For most cable sizes the armouring will generally be suitable for use as a cpc nonetheless the designer should ensure compliance with BS 7671.
Using the armouring as a protective equipotential bonding conductor (where PME conditions do not apply)
Where the outbuilding contains extraneous-conductive-parts, such as metallic water, gas or oil pipework, the designer may decide to use the armouring of the SWA cable to also perform the function of an equipotential bonding conductor.
In such circumstances, the armouring should afford the ‘equivalent conductance’ to that of copper (Regulation 544.1.1). It should be noted that the resistivity of steel is approximately 8 times higher than that of copper (8:1 ratio), so in order to achieve the ‘equivalent conductance’, the size of the steel armouring will need to be 8 times larger than that required for copper.
For a TN-S or TT earthing arrangement, the csa of a copper protective bonding conductor should be not less than half the size of the protective earthing conductor for the installation and not less than 6 mm². This would require the armouring of the cable to have a minimum csa of 48 mm2, which should be confirmed by reference to the particular cable manufacturer’s data.
Using the armouring as a protective equipotential bonding conductor (where PME conditions apply)
Where the outbuilding is to be connected to a TN-C-S (PME) earthing system the csa of the protective bonding conductor(s) should satisfy the requirements of Table 54.8. As a minimum, this requires the armouring to have the equivalent conductance of a 10mm² copper conductor and sized in accordance with the largest PEN conductor of the supply.
As previously stated, this should be confirmed by referencing the particular cable manufacturer’s data. However, where the installation is connected to a PME terminal, ELECSA recommends that the outbuilding is provided with a separate means of protective bonding.
TT earthing arrangement
Where the PME terminal is extended to an outbuilding, there is an increased risk that in the event of loss of the supply neutral, say as a result of an open circuit fault on the distributor’s PEN conductor, the cable armouring and all conductive parts connected to the MET are likely to be raised to the supply potential (230V).
In order to minimise this risk, it is recommended that the armouring is connected to the MET at the origin but the outbuilding is divorced (separated) from the PME terminal. Normally, this is achieved by terminating the protective conductor (armouring) into a fully insulated (Class II) enclosure Fig 1, and connecting the outbuilding to an earth electrode to form a TT earthing arrangement.
The earth electrode should be a type listed in Regulation 542.2.2 and where practicable, it should be located close to the outbuilding. it is not permitted to use a metallic utility water supply pipe or a pipe containing gas or flammable liquid as an earth electrode (Regulation 542.2.6). Other metallic water supply pipes may be used, but suitable precautions must be taken to prevent their removal, which in practice is difficult to implement and maintain.
Cable installation
Cables should be sufficiently buried to avoid being damaged by any reasonably foreseeable disturbance of the ground (Regulation 522.8.10). Although BS 7671 doesn’t specify a particular depth for buried cables, ELECSA recommends a minimum depth of 600mm, but where there is a possibility of the ground being disturbed such as by planting for example, the depth should be increased appropriately or additional mechanical protection should be provided for the cable.
Conclusion
While it is permitted for the metallic sheath or armouring of a SWA cable to serve as both a circuit protective conductor and a protective equipotential bonding conductor, as outlined in this article, the relevant requirements of BS 7671 must be satisfied.
