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صفحه اصلی سایت Technical Information Power And Control Cables Power Appendixes Guide to the Wiring Regulations Earthing arrangements

صفحه Earthing arrangements

Earthing arrangements

IEC terminology : International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
The frst letter indicates the connection between earth and the power-supply equipment(generator or transformer):
T : Direct connection of a point with earth ;
I : No point is connected with earth (isolation), except perhaps via a high impedance.
The second letter indicates the connection between earth and the electrical device being supplied:
T : Direct connection of a point with earth
N : Direct connection to neutral at the origin of installation, which is connected to the earth
TN networks :
In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer
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 :
TN−S: PE and N are separate conductors that are connected together only near the power source.
TN−C : A combined PEN conductor fulflls the functions of both a PE and an N conductor (Rarely used) .
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, and separated in the service head.
TT network : In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.
The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that beneft from the
interference-free earthing. Also, TT does not have the risk of a broken neutral.
IT network : In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance. For safety reasons this network is not accepted under European norms