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发表于 2007-7-10 18:05:52
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Terra Neutral-Combined-Separate ! {+ u6 q+ O7 r' n
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; d" B x+ }. I: t# E* D ~- n% _International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT./ d* R: h# N8 U$ I
! F8 n: R* J" h- zThe first letter indicates the connection between earth and the power-supply equipment (generator or transformer):# E5 R( |1 b* B$ P
s* T4 F) s& G" U# [% L+ pT : direct connection of a point with earth (French: terre); + Q* P) _/ G5 w
I : no point is connected with earth (isolation), except perhaps via a high impedance.
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The second letter indicates the connection between earth and the electrical device being supplied:5 w) V+ S. s \7 s8 C$ o; Y5 K
1 T$ B% ?6 o. q+ cT : direct connection with earth, independent of any other earth connection in the supply system; ! O0 S2 U( y; }# I. i4 {) B" b0 i& d
N : connection to earth via the supply network # {- W ^# A+ Q5 f
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8 q5 ?' Z2 o% b H ?0 ZThe conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished:8 Y, b) I) |% t2 j
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TN‑S : PE and N are separate conductors that are connected together only near the power source. / s7 S- O2 E2 E! X4 U0 v! d6 O
TN‑C : A combined PEN conductor fulfils the functions of both a PE and an N conductor.
" Z0 M6 Y9 W, P6 v: c6 dTN‑C‑S : Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN). |
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