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发表于 2007-7-10 18:05:52
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Terra Neutral-Combined-Separate
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/ P! X1 m# b4 E9 I0 X. O: _" zInternational standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.1 ]- x, ^7 ]9 `0 c/ W
9 U W7 N8 a; ?' xThe first letter indicates the connection between earth and the power-supply equipment (generator or transformer):! _% N5 q( w7 x! i2 i, k2 S6 |
# Z+ r$ r6 U7 T- k" A# A5 V* eT : direct connection of a point with earth (French: terre);
2 g. k3 _; E- S4 vI : no point is connected with earth (isolation), except perhaps via a high impedance. 7 S9 X# _/ b& o5 ?
5 U6 Y, ^2 U+ M8 C$ C8 e0 {% t1 g& HThe second letter indicates the connection between earth and the electrical device being supplied:5 ^! R2 w6 B) k5 C) c
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T : direct connection with earth, independent of any other earth connection in the supply system;
: G6 j9 m: Q9 ]+ AN : connection to earth via the supply network ; [6 s6 P& h4 x2 r
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( p& Q" S4 E1 A; _4 HThe 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: @5 C7 B' M& H! |5 P
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TN‑S : PE and N are separate conductors that are connected together only near the power source. / b& E( V5 D; t: y. u" y9 a- j
TN‑C : A combined PEN conductor fulfils the functions of both a PE and an N conductor. ! q9 i; t$ Y9 k
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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