5.5.2 - Earth electrode types

Acceptable electrodes are rods, pipes, mats, tapes, wires, plates and structural steelwork buried or driven into the ground. The pipes of other services such as gas and water must not be used as earth electrodes although they must be bonded to earth as described in {5.4.3}. The sheath and armour of a buried cable may be used with the approval of its owner and provided that arrangements can be made for the person responsible for the installation to be told if the cable is changed, for example, for a type without a metal sheath.

The effectiveness of an earth electrode in making good contact with the general mass of earth depends on factors such as soil type, moisture content, and so on. A permanently-wet situation may provide good contact with earth, but may also limit the life of the electrode since corrosion is likely to be greater. If the ground in which the electrode is placed freezes, there is likely to be an increase in earth resistance. In most parts of the UK an earth electrode resistance in the range 1 Ohm to 5 Ohms is considered to be acceptable.

The method of measuring the resistance of the earth electrode will be considered in {8.6.1}; the resistance to earth should be no greater than 220 Ohms. The earthing conductor and its connection to the earth electrode must be protected from mechanical damage and from corrosion. Accidental disconnection must be avoided by fixing a permanent label as shown in {Fig 5.17} which reads:

Fig 5.17 Connection of earthing conductor to earth electrode