Earthing Systems
The British Wiring Regulations BS 7671 lists five types of main earthing system, the three most common are detailed below:
TN-S, TN-C-S and TT
T = Earth (from the French word Terre)
N = Neutral
S = Separate
C = Combined
Overview of earthing systems
TN-S system earthing
A TN-S system, shown below, has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source, and the consumer's earthing terminal is typically connected to the metallic sheath or armour of the distributor's service cable into the premises.
A TN-C-S system, shown below, has the supply neutral conductor of a distribution main connected with earth at source and at intervals along its run. This is usually referred to as protective multiple earthing (PME). With this arrangement the distributor's neutral conductor is also used to return earth fault currents arising in the consumer's installation safely to the source. To achieve this, the distributor will provide a consumer's earthing terminal which is linked to the incoming neutral conductor.
A TT system, shown below, has the neutral of the source of energy connected as for TN-S, but no facility is provided by the distributor for the consumer's earthing. . The circumstances in which a distributor will not provide a means of earthing for the consumer are usually where the distributor cannot guarantee the earth connection back to the source, e.g. a low voltage overhead supply, where there is the likelihood of the earth wire either becoming somehow disconnected or even stolen. With TT, the consumer must provide their own connection to earth, i.e. by installing a suitable earth electrode local to the installation. This type of earthing arrangement is commonly found in rural areas.
BS 7671 recognises a wide variety of types of earth electrode including earth rods, earth plates and underground structural metal work. The soil resistivity of the ground is probably the single most important factor in the determination of the type of earth electrode. Rods can only be as effective as the contact they make with the surrounding material. Thus, they should be driven into virgin ground, not disturbed (backfilled) ground. Where it is necessary to drive two or more rods and connect them together to achieve a satisfactory result, the separation between rods should be at least equal to their combined driven depth to obtain maximum advantage from each rod. In some locations low soil resistivity is found to be concentrated in the topsoil layer, beneath which there may be rock or other impervious strata which prevents the deep driving of rods, or a deep layer of high resistivity. Only a test or known information about the ground can reveal this kind of information. In such circumstances, the installation of copper earth tapes, or pipes or plates, would be most likely to provide a satisfactory earth electrode resistance value. Whatever form an earth electrode takes, the possibility of soil drying and freezing, and of corrosion, must be taken into account. Preferably, testing of an earth electrode should be carried out under the least favourable conditions, i.e. after prolonged dry weather.
Earthing systems provide the fundamental basis of protection for the electrical installation and therefore it is imperative that it is designed and installed correctly and thereafter inspected and tested at regular intervals so as to ensure its efficiency.
London Electrical Inspection & Testing – Leading electrical engineers and expert electricians serving London