Where the electricity distributor does not provide a means of earthing for an installation, or where required for regulatory reasons, an earth electrode should be installed and the installation should satisfy the requirements of BS 7671 for a TT system.
Suitable types of earth electrodes are listed in Regulation 542.2.1 of BS 7671 and this article looks at some of the types recognised.
When selecting the appropriate type of electrode the installation designer should take into account the type and composition of the ground, any external influences likely to be encountered including the effects of soil drying or freezing (Regulation 542.2.4 refers) and the risk of mechanical damage.
Earth rods or pipes
The driven rod or pipe earth electrode is often preferred as it is suitable for many, if not most, earthing applications. A range of standard lengths of rod of various diameters are readily available, including rods made of copper, copper-clad steel (bonded steel-cored), stainless steel or galvanized steel.
Due to the resistivity of the soil, the reliability of the connection of the rod to the general mass of Earth can vary significantly from one location to the next. To confirm that the connection is sufficiently low enough to function reliably, BS 7671 requires that the electrode resistance to Earth (RA) is measured and the value is recorded on the relevant part of the certificate or report (Regulation 612.7).
Where a measured value of the resistance of the electrode to Earth (RA) is not practicable, the measured value of external earth fault loop impedance (Ze) may be used as an approximate value. It should be noted that a value greater than 200 Ω may not be reliable to provide earth fault protection (Note 2 of Table 41.5 of BS 7671 refers).
Common methods used to reduce the resistance value of a rod-type earth electrode include; extending the length of the electrode or installing additional rods. Depending on the soil resistivity it may be beneficial to use an extension rod to increase the length of the electrode, but in some locations, due to the composition of the subsoil, extending the earth electrode beyond a certain depth may not only be time consuming but also require specialist equipment.
Furthermore, it may achieve very little reduction in earth resistance, especially where the deeper soil layers are formed of materials having higher values of resistivity. Where sufficient space is available, the installation of additional rods, connected in parallel may be more appropriate.
However, where this is the case, the resistance areas of the electrodes should not overlap. Generally, this is achieved by ensuring that the rods are separated by a distance that is not less than their driven depth, as shown in Fig 1. For electrodes connected in parallel the combined resistance of all the rods is approximately proportional to the reciprocal of the number of rods employed (clause 9.5.3 of BS 7430: 2011 + A1: 2015 Code of practice for protective earthing of electrical installations refers).
For example, where one rod has a measured earth resistance of 300 Ω, then connecting 2 additional rods in parallel should reduce the total earth resistance value to approximately:
300/3 = 100 Ω.
Lead sheaths and other metal coverings of cables
Regulation 542.2.5 of BS 7671, permits the lead sheath or other metal covering of a cable to serve as an earth electrode, provided that adequate precautions are taken to prevent excessive deterioration by corrosion, such as may be caused where two dissimilar metals are in close proximity.
The sheath or covering must have effective contact with Earth, which cannot be achieved if the cable has a polymeric oversheath. The consent of the owner of the cable must be obtained, and arrangements should be put in place to ensure the owner of the electrical installation is warned of any changes to the cable which may affect its resistance.
Underground structural metalwork embedded in foundations
As an alternative to the installation of earth rods it may be possible to utilise a building’s steel-reinforced foundations or steel stanchions encased in concrete as a means of earthing, provided suitable connections can be made to the structure.
The total electrode area formed by the combined underground metalwork of a large structure can provide an overall earth resistance much lower than obtainable by other methods, sometimes achieving values below 1 Ω. However, careful consideration should be given to the suitability of using such foundation metalwork as an earth electrode.
Corrosion of the metalwork and cracking of the surrounding concrete may result where continuous DC circulating earth currents flow, such as where the foundation metalwork is incompatible with other buried metalwork (clause 9.5.8.6 of BS 7430 refers).
Information on methods of calculation and measurement to determine the resistance to Earth of a foundation arrangement or number of such arrangements is given in BS 7430: 2011 + A1: 2015.
Other suitable underground metalwork
Except as prohibited in Regulation 543.2.3, an extraneous-conductive-part may be used as a protective conductor as long as it satisfies all the following requirements:
- Electrical continuity shall be assured, either by construction or by suitable connection, in such a way as to be protected against mechanical, chemical or electrochemical deterioration.
- The cross-sectional area shall be at least equal to that resulting from the application of Regulation 543.1.1.
- Unless compensatory measures are provided, precautions shall be taken against its removal.
- It has been considered for such use and, if necessary, suitably adapted.
However, metallic pipes used to supply gas or flammable liquids should not be used as an earth electrode (Regulation 542.2.6).
Guidance on Earth electrode testing is given in the NICIEC and ELECSA publication; Inspection, Testing and Certification. This guide is currently being updated to the 18th Edition of BS 7671.
