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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 % x1 h! H) ~! m/ \
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International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.3 G r+ i5 Z3 E$ P9 n/ ?
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The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):+ i7 t9 S" D6 r: |
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T : direct connection of a point with earth (French: terre); 8 C# } ~4 O+ Z) R; B, ?
I : no point is connected with earth (isolation), except perhaps via a high impedance.
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- N8 q4 ]5 D/ b: X- IThe second letter indicates the connection between earth and the electrical device being supplied:
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T : direct connection with earth, independent of any other earth connection in the supply system; % h d9 b+ C% U0 y% _
N : connection to earth via the supply network 7 J! y8 b* W$ R$ T U1 c
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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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% w$ W5 a! c) h7 lTN‑S : PE and N are separate conductors that are connected together only near the power source. 6 y3 l5 @, p8 P8 K2 m& C' ?
TN‑C : A combined PEN conductor fulfils the functions of both a PE and an N conductor.
o; ~1 H- O+ Z% zTN‑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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