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发表于 2007-7-10 18:05:52
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来自： 中国河南南阳
仅供参考
Terra Neutral-Combined-Separate 8 i$ b ?! m) E. v Q3 x
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4 R" K( o6 a: G* k0 N1 V/ pInternational standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
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5 S0 f0 J$ k& _+ ]The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
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T : direct connection of a point with earth (French: terre); % l/ [* M/ G5 K# C1 z, J X
I : no point is connected with earth (isolation), except perhaps via a high impedance. . {8 b: s2 i5 _
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The second letter indicates the connection between earth and the electrical device being supplied:
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& u6 S. a4 a. ^" |T : direct connection with earth, independent of any other earth connection in the supply system;
3 p; j/ I9 ^. {: {4 ?N : connection to earth via the supply network
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The 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:
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5 Z) w' U' V& h9 C" b$ U" v: V# \TN‑S : PE and N are separate conductors that are connected together only near the power source. 7 M2 [8 j' D1 E2 b; @; n
TN‑C : A combined PEN conductor fulfils the functions of both a PE and an N conductor.
# b+ m1 C8 ^2 p6 W% h8 ~TN‑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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