In electricity supply a TN-C earthing system is one where where earth and neutral are combined in the supply wiring and right through the install.

Pros

• Very low earth loop impedance.
• Cheaper for the supplier to provide than TN-S.
• Less copper in the installation wiring.

Cons

In the event of a lost neutral in the supplier's network, the house's earth system can float up away from earth.

The same thing can happen with the earths of individual circuits.

You can get voltages between the earths of different equipment which, while small, could pose a danger in some situations.

The system is BANNED in some countries (notably the UK) because of the risks it involves.

Mitigation of specific risks

Bonding of pipework to the earthing system is ABSOLUTELY ESSENTIAL because of the possibility of a lost neutral in the supplier's network. This bonding has to have quite a large size, especially if the combined neutral and earth core in the supply wiring is large, because it may well end up carrying circulating network currents.

Sometimes outdoor wiring from a TN-C will be made into a separate TT system, as a raised up earth poses a much greater danger outside the equipotential zone, but this is not always done.

In most cases the supply cable to the house from the main feed will have the live completely surrounded by the neutral to mimimise the risk of a lost neutral.

The main feed should be rodded down to earth at frequent intervals.

The installation cable like that of the main feed should have the neutral completely surrounded by the earth, but unfortunately such cables tend to get expensive at small sizes.

Other earthing systems

TT - earth is rodded down in the consumer's installation and has no direct connection to the neutral of the supply transformer.

TN-S - earth and neutral run back separately to the transformer neutral terminal.

TN-C-S - earth and neutral joined in the supply network but separated in installation.