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A TT earthing system is one where there is no metallic connection at all between the customer's earth and the neutral terminal of the transformer in the supplier's network.
The main earth terminal of the customer's installation is connected to earth through an electrode (usually a copper rod or mesh) buried in the ground. Earth fault current must flow via this earth electrode through the soil to the earth electrode at the substation, which is in turn connected to the neutral terminal of the transformer.
Pros
You have complete control. The actions of other customers cannot put dangerous voltages on the earth system.
Cons
Earth loop impedance 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 impedance means that a RCCB must be used on the whole installation. In many cases this is a time delayed type with a trip point of at least 100mA to provide discrimination with more sensitive RCCBs installed downstream on higher risk circuits.
Due to the high earth loop impedance there will be a significant voltage on the earthing system until the RCCB disconnects the fault. Bonding of pipework to the earthing system is important to eliminate a potential difference between the earthing system and the pipework. However due to the fact that high earth impedence leads to low currents this bonding does not need to be very big (6mm2 in the UK).
Other earthing systems
TN-C-S earth and neutral combined in the supply network but separated in customer installation.
TN-S earth and neutral run back separately to the transformer neutral terminal.
TN-C earth and neutral are combined right throughout the installation.
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