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TT earthing system
A TT earthing system is one where there is no connection at all between the customers earth and the neutral terminal of the transformer in the suppliers network.
Pros
You have complete control. The actions of other customers cannot put dangerous voltages on the earth system.
Cons
Earth loop impedence is very high so very little current can flow in a fault to earth.
Mitigation of specific risks
In most cases the high earth loop impedence means that a RCCB must be used on the whole install. In many cases this is a time delayed type to provide decent discrimination with rcds downstream on higher risk cuircuits.
Bonding of pipework to the earthing system is important because of the rasie in earth potential during a fault but because of the low currents involved this bonding doesn't need to be very big (6mm^2 in the UK).
Other earthing systems
TN-C-S earth and neutral joined in the supply network but seperated in install.
TN-S earth and neutral run back seperately to the transformer neutral terminal.
TN-C earth and neutral are combined right through the install.
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You may copy and modify it as long as the entire work (including additions) remains under this license.
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To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/TT_earthing_system
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