Electrical Systems

It is easy to incorrectly identify which particular type of electrical system has been installed. (For example TN-S, TN-C-S, or TT).

Solution

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 fulfil a specific purpose and having certain co-ordinated 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 (eg: a public supply network or a private generator).

• T denotes that one or more points of the source of energy are directly connected to the 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 earthing 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 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 d.c. 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.

Types of system are identified by the letter code which indicates the relationship to Earth of the source of energy and of the exposed-conductive-parts of the installation as follows:-

TN System

A system having one or more points of the source of energy directly connected with Earth and having the exposed-conductive-parts of the installation connected to that point by protective conductors.

There are three types of TN System:

• The TN-S System, having separate neutral and protective conductors throughout the system.
  
  A TN-S system will most likely be met in practice in the form of an earth terminal provided by the electricity distributor which is connected to the cable sheath or to a separate overhead conductor.
• The TN-C-S System, in which the neutral and protective functions are combined in a single conductor, the CNE (Combined Neutral and Earth) conductor or PEN conductor in part of the system.
  
  The more common variant of TN-C-S, known as 'TN-C-S (PME)'. This type of distribution is known as Protective Multiple Earthing (PME), and has been used by electricity distributors for most of the new low voltage supplies they have been installing since, the mid - 1970s. The Combined Neutral and Earth (CNE) conductor of the distributor's lines is connected to Earth at several points in accordance with Regulations 8.(3).(b) and 9(2) of The Electricity Safety, Quality and Continuity Regulations 2002, thereby providing low resistance connections between all parts of that conductor and Earth.
  
  The other variant of TN-C-S,known as 'TN-C-S(PNB)'. This type of system may be used only where a single consumer is supplied from a distribution transformer (or other source such as a generating set). The CNE (or PEN) conductor is connected to Earth at one point only; this may be at the installation or closer to the source.
• The TN-C system, in which the neutral and protective functions are combined in a single conductor throughout the system (ie both the source and the installation). TN-C systems are not permitted in consumer's installations.

TT System

A system having one point of the source of energy directly connected with earth and having the exposed-conductive-parts of the installation connected to one ore more installation earth electrodes electrically independent of the source earth electrode. This will be met in practice where the distributor does not provide an earth terminal or where, for one reason or another, it is not appropriate to earth the installation to such a terminal. The consumer must provide an earth terminal.

IT System

A system having no direct connection between the source of energy and earth but having the exposed-conductive-parts of the electrical installation being earthed.

Definitions from BS 7671

System: An electrical system consisting of a single source or multiple sources running in parallel of electrical energy and an installation. For certain purposes of the Regulations, types of system are identified as follows, depending upon the relationship of the source, and of exposed-conductive-parts of the installation, to Earth:

• TN system, a system having one or more points of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to that point by protective conductors,
• TN-C system, a system in which neutral and protective functions are combined in a single conductor throughout the system
• TN-S system, a system having separate neutral and protective conductors throughout the system.
• TN-C-S system, a system in which neutral and protective functions are combined in a single conductor in part of the system,
• TT system, a system having one point of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source.
• IT system, a system having no direct connection between live parts and earth, the exposed-conductive-parts of the electrical installation being earthed.

This article is extracted from Snags and Solutions Part 1 published by the: NICEIC Group