TNS System

If there is a broken neutral anywhere on any earthing network, then expect some single phase loads to go phut, Uncle. That is why the electricians do a special check on new installations before energising them, and periodic re-inspection.

By the way, did you read the articles about earthing systems before posting here?

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 or transformer.

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.

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 TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an overcurrent circuit-breaker or fuse and disconnect the L conductors. With TT systems, the earth fault loop impedance can be too high to do this, or too high to do it quickly, so an RCD (or formerly ELCB) is usually employed. The provision of a Residual-current device (RCD) or ELCB to ensure safe disconnection makes these installations EEBAD (Earthed Equipotential Bonding and Automatic Disconnection). Earlier TT installations may lack this important safety feature, allowing the CPC (Circuit Protective Conductor) to become energized for extended periods under fault conditions, which is a real danger.

http://en.wikipedia.org/wiki/Earthing_system

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