5.2.1 - System classification
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
The first letter indicates
the type of supply earthing.
- indicates that one or more points of the Supply
are directly earthed (for example, the earthed neutral at
I - indicates
either that the supply system is not earthed at all, or
that the earthing includes a deliberately-inserted 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
S - neutral and earth conductor systems are
C - neutral and earth are combined into a single
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.