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Electrician's Guide To Earthing Systems

Let’s Start at the Very Beginning…

…A very good place to start, since earthing is one of the first things to consider when designing an electrical installation. Here we provide an electrician’s guide to earthing systems and the BS 7671 regulations relating to them.

What type of earthing systems are there?

There are five types listed in BS 7671 Wiring Regulations:

1. TN–S
2. TN–C–S
3. TT
4. TN–C
5. IT

What do these codes relate to?

The first letter relates to the connection between earth and the source of power supply, i.e. generator or transformer; and the second relates to the connection between earth and the electrical device the power is supplying; with each letter meaning as follows:

• T = Earth. Direction connection with Earth (From the French word for ground: Terre).
• N = Neutral. Connection to earth via the supply network.
• S = Separate.
• C = Combined.
• I = Isolated. No connection with earth, unless through a high impedance.

The three systems used in the UK are: TN–S, TN–C–S (PME) or TT. However, because TT systems are not directly earthed they are prohibited for low voltage public supply in the UK.

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Which type of earthing system should be used?

It is imperative that the right type of electrical earthing system is chosen since it will affect both safety and the electromagnetic compatibility of the power supply.

Providing a Protective Earth (PE) connection will ensure that all exposed conductive surfaces have the same electrical potential as the earth’s surface, thereby preventing anyone touching the device in the case of a short circuit from suffering an electric shock. A short circuit will also cause a PE connection to give rise to a greatly increased current flow, triggering fuses and circuit breakers and disconnecting the power supply.

A Function Earth (FE) connection is used for devices such as antennas, and may carry a current during normal operation.

The three common types of earthing systems operate as follows:

TN–S

This type of electrical earthing system connects the neutral source of energy with the earth at one point only (or as near as practically possible), and with the consumer’s earthing terminal commonly connected to the metallic sheath or armour of the distributor’s service cable into the premises. With a TN–S the PE connection and N are separate conductors that are connected together only near the power source.

TN–C–S (PME)

A TN–C–S earthing system, typically known as Protective Multiple Earthing (PME), connects the supply neutral conductor of a distribution main with earth at course and at intervals along its stretch. The neutral conductor is also used to return earth faults currents safely to source by the provision of a consumer’s earthing terminal linked to the incoming neutral conductor.

TT

The neutral of the energy source is connected as with the TN–S system, however there is no provision for the consumer’s earthing, therefore they must provide their own connection to earth.

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Points to consider in BS 7671 regulations

• Earth electrodes

  BS 7671 lists a wide range of earth electrodes recognised by wiring regulations, including earth rods, earth plates and underground structural metalwork. The single most importance deciding factor in which type of electrode to use is resistance capacity of the soil in the ground. Ideally it should virgin, undisturbed ground, and the effects of soil drying, freezing, and the potential for corrosion, should also be considered, with tests carried out in the worst weather conditions.

• Sizing of Circuit Protective Conductors

  Several factors must be considered when working out the required size of circuit protective conductor. A minimum cross–sectional area of 2.5mm2 copper is necessary for any separate circuit protective conductor, meaning one which is not part of a cable or created by/contained inside a wiring enclosure.

• Earthing Conductors

  Earthing conductors defined by BS 7671 as a protective conductor connecting the main earthing terminal of an installation to an earth electrode must be sized appropriately, especially if partially buried. They must be made from suitable material and protected against corrosion and mechanical damage. The appropriate size is determined in the same way as for a circuit protective conductor, except for with buried earthing conductors, in which case check BS 7671 for further guidelines. In addition, earthing conductors for a TN–C–S supply should not be smaller than the main bonding conductors.

• Special Locations

  These are locations where extra precautions need to be taken. For example PMEs cannot be used for caravans or boats as the combined neutral and protective conductor is not allowed to be connected electrically to any metalwork in them, whilst a TT system is recommended for hazardous areas such as petrol stations, and should be accompanied by the supply of a separate electrode and circuit breaker such as an RCD, in order to ensure that the earthing in the petrol filling area and the PMP earth of the distribution network are separated.

Earthing is not a subject to be taken lightly and all factors should be carefully considered, making reference and adhering strictly to all relevant electrical and safety guidelines. Failure to do so could result in potentially fatal accidents, with the electrician responsible facing prosecution.

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