Page 134 - Electrical Safety of Low Voltage Systems

CHAPTER7

TN Grounding

System

Meglio agitarsi nel dubbio che riposare
nell’errore.
Better to seethe with doubt than to rest with
mistake.
alessandro manzoni (1785–1873)

7.1 Introduction
Users in industrial facilities may receive their power supply from the
local utility in medium or high tension, and therefore, install, and
own, front-end substations. In this case, within the facility, customers
employ TN grounding system (e.g., Terre Neutral), even if the outside
low-voltage earthing system is TT.
The user’s substation may contain one or more transformers,
whose windings are typically wound as a delta at the primary side and
as a wye at the secondary side. The transformers are necessary in order
to step down the medium/high tension to low-voltage levels suitable
for the customer. The user must solidly ground the center of the trans-
former’s wye and directly connect all the exposed-conductive-parts
(ECPs) to it via protective conductors. The neutral conductor may be
carried in order to provide power to single-phase loads.
If separate neutral and protective conductors are used throughout
the facility, the system is defined as TN-S (Sec. 1.2.22). If in the electrical
system, or a part of it, neutral and protective functions are combined
in a single conductor, referred to as PEN conductor, and the system is
defined TN-C or TN-C-S (Secs. 1.2.20 and 1.2.21).
In TN systems, the ground-fault current will return to the trans-
former through the protective conductor and, unlike in TT systems,
will not circulate through the earth (Fig. 7.1).

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