A guide to the correct use of earth electrodes

Jun 21, 2023

Earth electrodes are to be found in a wide range of electrical installations including domestic premises. Typically, an earth electrode will be installed for TT earthing systems, IT earthing systems and generating sets.

Where an earth electrode is used as part of the measures for the protection of persons, livestock and property against dangers and damage which may arise in the reasonable use of an electrical installation, the connection of the earthing conductor to the earth electrode is vital for safety and has to meet the requirements of BS 7671.

Where, for example, the use of an earth electrode is part of the means of protection against electric shock by Automatic Disconnection of Supply (ADS) for an electrical installation forming part of a TT (or IT) system, the effective protection against electric shock is dependent upon the integrity of the installation earth electrode, the earthing conductor and the connection of the earthing conductor to the earth electrode.

It is necessary for the connection to be electrically and mechanically sound, labelled and protected against the external influences likely to be encountered at its location. The external influences to be protected against may include: the weather, corrosion caused by substances such as farm chemicals, animal urine etc, corrosion that can result if dissimilar metals are connected together in a damp or wet environment and physical damage from farm machinery, vehicles, gardening tools, animals, excavations, vandalism etc.

BS 7430: 2011 + A1: 2015 Code of practice for protective earthing of electrical installations gives guidance applicable to such a connection including the recommendations given later in this article.

General requirements applicable to all electrical connections BS 7671 places general requirements including the need for proper construction, protection against external influences and accessibility which are applicable to every electrical connection, (with certain permitted exceptions).

Every electrical joint and connection is to be of proper construction as regards conductance, insulation, mechanical strength and protection (Regulation 134.1.4 of BS 7671).

External influences

The installation designer should consider, and where necessary protect against, all external influences likely to be encountered at the location of the joint or connection. Such external influences may include:

• ambient temperature (Regulation Group 522.1 refers),

• the presence of water or high humidity (Regulation Group 522.3 refers),

• the presence of corrosive or polluting substances such as farm chemicals or animal urine (Regulation Group 522.5 refers),

• the impact such as physical damage from farm machinery, vehicles, excavations, vandalism, (Regulation Group 522.6 refers),

• the presence of flora (such as mould) (Regulation Group 522.9 refers), and

• the presence of fauna (such as livestock)(Regulation Group 522.10 refers).

Every electrical connection and joint, including that at an earth electrode, is required, with certain permitted exceptions, to be accessible for inspection, testing and maintenance. (Regulation 526.3 refers).

BS 7671 gives additional requirements applicable to the connection of an earthing conductor to an earth electrode and Regulation 542.3.2 relates specifically to such a connection. The requirements embodied in Regulation 542.3.2 are that the connection must be:

• soundly made and electrically and mechanically satisfactory,

• labelled in accordance with Regulation 514.13.1, and

• suitably protected against corrosion.

The meaning of the phrase ‘soundly made and electrically and mechanically satisfactory’ is further explained in BS 7671.

Every connection between conductors and between conductors and equipment is to provide durable electrical continuity and adequate mechanical strength and protection. The selection of the means of connection is to take account of parameters such as the material of the conductor, the number and shape of the wires, their cross-sectional area and, where required, the need for adequate locking arrangements (Regulation 526.2).

Regulation 514.13.1 requires that a permanent label to BS 951 with the words ‘Safety Electrical Connection – Do Not Remove' (see Fig 1) is permanently fixed in a visible position at or near the point of connection of every earthing conductor to an earth electrode.

Corrosion during the life of an electrical installation may have a detrimental effect on the electrical connection to the earth electrode and therefore the likelihood of corrosion developing should be minimised. Where possible, such connections should be located away from corrosive influences. If this is not possible, materials resistant to corrosion should be used. BS 7430 Table 11 recommends materials to be used for earthing components. Dissimilar metals should not be used where these are likely to be in contact with each other in the presence of moisture. This effectively precludes the use of aluminium for final connections to electrodes. When connecting to a copper rod or a copper plate electrode, it is preferable to use copper conductors.

Connections that may suffer corrosion should be given a protective coating of inhibitor paste or bitumastic paint, or be wrapped in self-amalgamating tape to protect against ingress of moisture. For further details, reference should be made to BS 7430 Section 9.9.4 Jointing Conductors.

BS 7430, the code of practice for protective earthing, gives further guidance on all aspects of earthing practice. Section 9.7 of BS 7430 (Selection of an earthing conductor and connection to an electrode) gives recommendations which include the following:

• Galvanic corrosion is to be avoided both for an earth electrode and any earthing conductors installed in the ground.

• Aluminium or copper-clad aluminium conductors should not be used in contact with soil and should not, under any circumstances, be used to make the final connection to an earth electrode.

• Connectors should be mechanically strong and securely fastened.

• All earth clamps should be inherently corrosion resistant.

• Where bolted, bolts should be tightened to a torque of at least 20 Newton-metres.

Any disconnection arrangements at the earth electrode to permit earth electrode testing must also meet the above requirements.

Fig. 2 gives an example of the connection of an earthing conductor to a rod-type earth electrode. The connection will, in most cases, be bolted, and will need to be accessible. A PVC or cast concrete box or concrete or brick enclosure may be used to enclose and protect an earth electrode connection and provide means of access.

Table 54.1 of BS 7671 gives details on the minimum size of earthing conductor. It should be noted that green and yellow insulated copper conductor is not considered to be mechanically protected as it is not sheathed and, therefore, under such circumstances, the minimum size of conductor should be 16 mm2. Where the earthing conductor is given mechanical protection, such as enclosed in PVC conduit, then the minimum size of conductor could be 2.5 mm2.

Inspection and testing

Where an installation uses an earth electrode for protection against electrical hazards, the connection between the earthing conductor and the earth electrode must be verified both upon completion of the construction of the installation and during subsequent periodic inspections.

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