2015-10-13
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TT network
First of all, 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 strength of current depends on both the voltage and on the resistance in the circuit. A current of 50 mA is dangerous for a man and a current of 100 mA and higher is lethal.
The big advantage of the TT earthing system is that it is clear of high and low frequency noises that come through the neutral wire from connected equipment.TT has always been preferable for special applications like
telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral. Safe voltage for circuits used in dry atmosphere is under 36 V.
In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution conductor be fractured by, say, a fallen tree or branch.
In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.
The TT earthing system is used throughout Japan, with RCD units in most
industrial settings. This can impose added requirements on variable frequency drives and switched-mode power supplies which often have substantial filters passing high frequency noise to the ground conductor.
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