It is easy to incorrectly identify which particular type of electrical system has been installed. (For Example TN-S, TN-C-S, or TT)
An electrical system consists of a single source of electrical energy and an installation. A source of energy may be, for example a public supply network a supply transformer, a private generator or a battery. An installation is an assembly of associated electrical equipment supplied from a common origin to fulfill a specific purpose and having certain coordinated characteristics. The type of system can generally be determined by examining the earthing arrangements at the origin of an installation.
The meaning of the letters is as follows: The first letter relates to the source of energy (e.g.: a public supply network or a private generator).
T denotes that one or more points of the source of energy are directly connected to Earth ('T' stands for 'Terra', or Earth).
I denotes that all live parts are isolated from Earth or that the source of energy is connected to Earth through a deliberately introduced high earthing impedance ('I' stands for isolated).
The subsequent letters show the relationship between the protective and neutral conductors in the system and the method of ear-thing the installation.
The second letter refers to the installation earthing arrangements T denotes that the exposed-conductive-parts of the installation are directly connected to Earth.
N denotes that the exposed-conductive-parts of the installation are directly connected to the earthed point of the source of energy. ('N' represents neutral or the equivalent point of the source in a Dec. system).
Subsequent letters indicate the system protective and neutral conductor arrangements
S denotes that separate neutral and protective conductors are provided. ('S' stands for separate).
C denotes that the neutral and protective functions are both performed by a single conductor called a combined protective and neutral (PEN) conductor. ('C' stands for combined).
Electricity distributors sometimes use the term CNE (Combined Neutral and Earth) rather than PEN for a combined protective and neutral conductor forming part of the distributors' conductors.
This Snag and its Solution were extracted from Snags and Solutions Part 1 - a practical guide to everday electrical problems, published by NICEIC.