Earthing & equipotential bonding

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Work in Progress

- this is old material from the FAQ -

By Geoff Rowlands

(for a more recent treatment of the subject see Earthing and Bonding, and Earthing Types)

Why Earth?

One side of the electricity supply (the neutral) is firmly connected to earth at the substation to prevent the supply 'floating' relative to earth for safety reasons.

Many electrically operated devices (e.g. washing machines, heaters and some lighting fittings) have exposed metalwork which could become live if a fault occurred. Anyone touching it could then receive a shock or even be killed depending on the current flowing through them to earth. To prevent this, an earthing conductor should be provided to all socket outlets, lighting circuits and any fixed appliances to which exposed metal parts are then connected. The earth connection limits the voltage which can appear on the exposed metal parts under fault conditions to a safe value until the fuse blows or the MCB or RCD trips. Note that earthing does not necessarily prevent anyone receiving a shock, but together with the time/current characteristics of the protective device (fuse, MCB or RCD) it should ensure that it is not lethal. It is desirable to make the impedance (resistance) of the earth wiring a low as practicable. (1000A flowing through 0.1 ohm drops 100V!)

Note that exposed metalwork cannot be protected by connection to the neutral because current flowing will cause a voltage drop between the metalwork and true earth. Also, if the neutral connection breaks or the appliance is plugged into a socket with line and neutral reversed (!), the metalwork will be at full mains voltage.

Appliances with an earth connection are called Class I (one): Class II or 'double insulated' appliances incorporate additional insulation to prevent exposed metalwork becoming live, and do not require an earth connection. This means that a 2-core mains lead can be used and internal earth connections are not needed.

A fundamental principle of electrical safety is that no single fault condition should cause a hazardous situation. This is why some of the regulations may appear to be rather stringent: it is better to be safe than sorry.

Who Supplies the Earth?

The earth connection will usually be supplied by one of the following methods:

a). By the electricity company. Either through the armouring of the supply cable or through a combined neutral and earth conductor. The latter method is termed PME (protective multiple earthing) and requires some special attention (see below). There will usually be a label near the meter indicating a PME system.

b). Through an earth electrode; usually a rod or plate driven into the ground. This method is found where the electricity company cannot easily supply or guarantee an adequate earth conductor; for example, where the supply comes on a pair of overhead wires. The user is generally responsible for the adequacy of the earth electrode.

The method of earthing can normally be found out by tracing the wiring from the meter/consumer unit. It is usually fairly obvious. IMPORTANT! - It is no longer permitted to use a water or gas pipe for the main or only earthing connection. There may, however be earth bonding wires connected onto the water and gas pipes for 'equi-potential bonding' (see below). If there is no electricity company earth or dedicated separate earth electrode, then one must be provided. Contact the electricity company if in any doubt.

How Good Should the Earth Be?

This is a difficult question to answer; in general the impedance of the earth connection must be low enough to ensure that sufficient current can flow through the protective device so that it disconnects the supply quickly (< 0.4 second) and that voltage on the earth connection does not rise more than 50V. RCDs operate at much lower fault currents than fuses and so can provide much better protection against shock. RCDs are therefore recommended whatever the method of earthing but where the electricity company cannot supply an effective earth and a local earth electrode is used, an RCD (30 mA trip) must be used. Measuring the resistance of an earth electrode is not easy and is really outside the scope of most d-i-y'ers.

Earthing of Electrical Installation

Each circuit requires an earth conductor to accompany (but kept separate from) the line and neutral conductors throughout the distribution. Where the distribution is in the form of a ring, the earth connection must also complete the ring.

The bare tails of earth conductors must be insulated with green/yellow sleeving from the exit from the cable sheath to the earth terminal.

All metal boxes should be connected to the earth; either through a short tail covered with green/yellow sleeving to the socket earth terminal or directly by the earth conductor for a switch box.

Equi-potential Bonding

As mentioned elsewhere, a fault current flowing in the earth wiring will cause the voltage on that wiring to rise relative to true earth potential. This could cause a shock to someone touching, for instance, the case of a faulty washing machine and a water tap at the same time. In order to minimise this risk, an 'equi-potential zone' is created by connecting the services to the main earthing point. Such services are:

  • Water Pipes
  • Gas Pipes
  • Oil Pipes
  • Central Heating
  • Metallic Ventilation Trunking
  • Exposed Parts of Building Structure
  • Lightning Conductor
  • Any other Metallic Service

The equi-potential bonding reduces the voltage difference which could exist between the metalwork of these services if an earth fault occurred to any one of them. It does not necessarily reduce the voltage to true earth. For this reason, metal window frames or patio doors should not be included in the bonding system - it could lead, for instance, to a window cleaner receiving a shock if an earth fault occurred inside the building.

The equi-potential bonding connections for incoming services should be made close to where the service enters the building on the consumer's side of the meter, stop cock etc. It is convenient to use purpose-made bonding clips (obtainable from most d-i-y stores) which include a label "SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE". The connections must, of course, be made to metal pipes - not plastic. The bonding conductors back to the main earthing block should be 6 sq mm minimum with green/yellow insulation (but see 'PME.' if applicable).

Bathrooms require special attention: The aim is to create a local equi-potential zone, so all extraneous metalwork should be bonded together. This could include:

  • Hot & Cold water pipes to sink, bath, shower
  • Waste pipes (metal)
  • Central heating pipes, radiator, towel rail
  • Electric towel rail, radiant or convector heater

Shaver sockets incorporate special isolating transformers which provide an earth-free output. The primary (input) side requires an earth which is connected internally to the transformer core.

Protective Multiple Earthing (PME.)

With PME. the neutral and earth conductors of the supply are combined. The supply company connects the neutral solidly to earth frequently throughout the distribution network. At the customer's connection point the company supplies an 'earth' (which is actually connected to the neutral) to which all the installation earths and equi-potential bonding are connected. Note that within the installation, the earth and equi-potential bonding are kept separate from the neutral in the usual way.

With PME. there is a potential danger in that if the combined neutral/earth conductor of the supply became broken (very unlikely but nevertheless possible), the voltage on the earth conductors could rise towards the full supply voltage. It is most important therefore that equi-potential bonding is rigorously applied in installations supplied by PME. The minimum size of main bonding conductor is 10 sq mm but may need to be up to 25 sq mm depending on the size of the incoming neutral/earth conductor: the supply company will advise you.

Electricity System Earthing Arrangements

By [ Andrew Gabriel] 27/4/1998

Mains electricity systems are categorised in the UK according to how the earthing is implemented. The common ones are TN-S, TN-C-S and TT. You will sometimes see these referred to in questions and answers about mains wiring.

Note that in these descriptions, 'system' includes both the supply and the installation, and 'live parts' includes the neutral conductor.

First letter:

T The live parts in the system have one or more direct connections to earth.

I The live parts in the system have no connection to earth, or are connected only through a high impedance.

Second letter:

T All exposed conductive parts are connected via your earth conductors to a local ground connection.

N All exposed conductive parts are connected via your earth conductors to the earth provided by the supplier.

Remaining letter(s):

C Combined neutral and protective earth functions (same conductor).

S Separate neutral and protective earth functions (separate conductors).

Valid system types in the 16th Edition IEE regulations:

TN-C No separate earth conductors anywhere - neutral used as earth throughout supply and installation (never seen this).

TN-S Probably most common, with supplier providing a separate earth conductor back to the substation.

TN-C-S [Protective Multiple Earthing] Supply combines neutral and earth, but they are separated out in the installation.

TT No earth provided by supplier; installation requires own earth rod (common with overhead supply lines).

IT Supply is e.g. portable generator with no earth connection, installation supplies own earth rod.

Inside or nearby your consumer unit (fuse box) will be your main earthing terminal where all the earth conductors from your final sub-circuits and service bonding are joined. This is then connected via the 'earthing conductor' to a real earth somehow...

TN-S The earthing conductor is connected to separate earth provided by the electricity supplier. This is most commonly done by having an earthing clamp connected to the sheath of the supply cable.

TN-C-S The earthing conductor is connected to the supplier's neutral. This shows up as the earthing conductor going onto the connection block with the neutral conductor of the supplier's meter tails. Often you will see a label warning about "Protective Multiple Earthing Installation - Do Not Interfere with Earth Connections" but this is not always present.

TT The earthing conductor goes to (one or more) earth rods, one of them possibly via an old Voltage Operated ELCB (which are no longer used on new supplies).

There are probably other arrangements for these systems too. Also, a system may have been converted, e.g. an old TT system might have been converted to TN-S or TN-C-S but the old earth rod was not disconnected.