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 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 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 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.