Looking at the principles of protective multiple earthing (PME) and the issues to be considered when PME electrical supplies are used outdoors or to supply external buildings.
This article looks at the widely used Protective Multiple Earthing (PME) system and Protective Earth- Neutral (PEN) conductors. PME provides a robust and reliable means of distributing electricity but, under certain fault-conditions, a potential can develop between the conductive parts connected to the PME earthing terminal and the general mass of Earth.
First, let’s look at the definitions of the terms:
■ CNE – Combined neutral and earth;
■ DNO – Distribution network operator;
■ Earth – Capital ‘E’ to imply the general mass of Earth, i.e. true Earth, e.g. the ground that you walk on;
■ earth – Lower-case ‘e’ to imply the earth of the electrical installation;
■ PEN conductor – A conductor combining the functions of both protective conductor and neutral conductor;
■ PME (Protective multiple earthing) – An earthing arrangement, found in TN‑C-S systems, in which the supply neutral conductor is used to connect the earthing conductor of an installation with Earth, in accordance with the Electricity Safety, Quality and Continuity Regulations 2002 (as amended)
■ 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-S – A system in which neutral and protective functions are combined in a single conductor in part of the system
■ TN-C – A system in which neutral and protective functions are combined in a single conductor throughout the system. Where: T – Terre (from the French, meaning ‘Earth’), N – Neutral, C – Combined and S – Separate.
How do the terms fit together?
On the low-voltage distribution network, the earthing and neutral functions are combined in
the same conductor of the supply cable; this is known as a PEN conductor and the distribution arrangement is TN-C; note that DNOs can refer to the PEN conductor as CNE. Along the length of the low-voltage distribution cable the PEN conductor is earthed, using earth electrodes at regular intervals. To supply the electrical installation, the neutral and earthing functions of the PEN conductor are separated out to create neutral and earth provisions (see fig 1, p34).
Combining the neutral and earth functions in one conductor means that the cable costs are reduced and any fault between conductors will be line-to-line or lineneutral so that, owing to the low value of earth fault loop impedance, the protective device will operate very
quickly to remove the fault. Compare this to a TN-S distribution arrangement where a neutral-to-earth fault could exist for a long period.
Continued...
To view the full article from the IET's Wiring Matters Winter 2012, download the PDF attachment at the end of this article.
If you would like to have a full free copy of the Winter 2012 Issue of Wiring Matters, be sure to visit the IET website here.
Winter 2012 (Issue 45) Features:
EV charging
Floor and ceiling heating systems
Heritage buildings
LED advances
Marinas and jetties
PME
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