EARTHING SYSTEM

The electrical installation does not exist on its own; the supply is part of the overall system. Although
Electricity Supply Companies will often provide an earth terminal, they are under no legal obligation to do
so. As far as earthing types are concerned, letter classifications are used.
The first letter indicates the type of supply earthing.
T - indicates that one or more points of the Supply are directly earthed (for example, the earthed neutral
at the transformer).
I - indicates either that the supply system is not earthed at all, or that the earthing includes a deliberatelyinserted
impedance, the purpose of which is to limit fault current. This method is not used for public
supplies in the UK.
The second letter indicates the earthing arrangement in the installation.
T - all exposed conductive metalwork is connected directly to earth.
N - all exposed conductive metalwork is connected directly to an earthed supply conductor provided by
the Electricity Supply Company.
The third and fourth letters indicate the arrangement of the earthed supply conductor system.
S - neutral and earth conductor systems are quite separate.
C - neutral and earth are combined into a single conductor.
A number of possible combinations of earthing systems in common use is indicated in the following
subsections.
Protective conductor systems against lightning need to be connected to the installation earthing system to
prevent dangerous potential differences. Where a functional earthing system is in use, the protective
requirements of the earthing will take precedence over the functional requirements.

TT systems

This arrangement covers installations not provided with an earth terminal by the Electricity Supply
Company. Thus it is the method employed by most (usually rural) installations fed by an overhead supply.
Neutral and earth (protective) conductors must be kept quite separate throughout the installation, with the
final earth terminal connected to an earth electrode by means of an earthing conductor.
Effective earth connection is sometimes difficult. Because of this, socket outlet circuits must be protected
by a residual current device (RCD) with an operating current of 30 mA.

TN-S system
This is probably the most usual earthing system in the UK, with the Electricity Supply Company providing
an earth terminal at the incoming mains position. This earth terminal is connected by the supply protective
conductor (PE) back to the star point (neutral) of the secondary winding of the supply transformer, which
is also connected at that point to an earth electrode. The earth conductor usually takes the form of the

armour and sheath (if applicable) of the underground supply cable.

TN-C-S system
In this system, the installation is TN-S, with separate neutral and protective conductors. The supply,
however, uses a common conductor for both the neutral and the earth. This combined earth and neutral
system is sometimes called the 'protective and neutral conductor' (PEN) or the 'combined neutral and
earth' conductor (CNE). The system, which is shown diagrammatically in {Fig 5.6}, is most usually called the protective multiple earth (PME) system,

TN-C system

This installation is unusual, because combined neutral and earth wiring is used in both the supply and
within the installation itself. Where used, the installation will usually be the earthed concentric system, which can only be installed under the special conditions.

IT system

The installation arrangements in the IT system are the same for those of the TT system .However,
the supply earthing is totally different. The IT system can have an unearthed supply, or one which is not solidly earthed but is connected to earth through a current limiting impedance.
The total lack of earth in some cases, or the introduction of current limiting into the earth path, means that the usual methods of protection will not be effective. For this reason, IT systems are not allowed in the public supply system in the UK. An exception is in medical situations such as hospitals. Here it is recommended that an IT system is used for circuits supplying medical equipment that is intended to be used for life-support of patients. The method is also sometimes used where a supply for special purposes is taken from a private generator.