接地系統概述（英文版）.doc

This article, which is based on IEE Guidance notes, is
intended to provide information that it is hoped will
prove helpful.
BS 7671 lists five types of earthing system:
TN-S, TN-C-S, TT, TN-C, and IT.
T = Earth (from the French word Terre)
N = Neutral
S = Separate
C = Combined
I = Isolated (The source of an IT system is either
connected to earth through a deliberately introduced
earthing impedance or is isolated from Earth. All
exposed-conductive-parts of an installation are
connected to an earth electrode.)
When designing an electrical installation, one of
the first things to determine is the type of earthing
system. The distributor will be able to provide this
information.
The system will either be TN-S, TN-C-S (PME) or TT
for a low voltage supply given in accordance with the
Electricity Safety, Quality and Continuity Regulations
2002. This is because TN-C requires an exemption
from the Electricity Safety, Quality and Continuity
Regulations, and an IT system is not permitted for a
low voltage public supply in the UK because the
source is not directly earthed. Therefore TN-C and IT
systems are both very uncommon in the UK.
EARTHING:www.ad176.com
YOUR QUESTIONS ANSWERED
By Geoff Cronshaw
What are earthed and unearthed systems?
What are the requirements of BS 7671?
What are the advantages and disadvantages
of the various types of earthing systems?

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