In electricity supply a TN-C-S earthing system is one where where earth and neutral are combined in the supply wiring but are separate in the installation.

Pros

• Very low earth loop impedance.
• Cheaper for the supplier to provide than TN-S.

Cons

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

Correction.

In the event of a loss of neutral on the incoming supply, the bonding within a premises can rise from zero to 230volts. In actual practice, lower limit voltages are more often realised but which, could cause a fatality to livestock.Hence the necessity to bond all metalwork and to isolate the bonding where it leaves a premise.

pros

neutral conductor, which is used as the return fault path, is self-monitoring for open circuit events through faults/damage to cable.

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-S 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 in a concentric arrangement to mimimise the risk of a lost neutral.

The main feed should be rodded down to earth at frequent intervals. This is done to minimise the risks posed by a broken neutral in the supply network and is effective because all the rods after any break should keep the neutral pretty close to the voltage level of the body of the earth.

Protective multiple earthing

TN-C-S is also known as protective multiple earthing (PME) because of the procedure of rodding down the combined neutral and earth conductor in the supplier's network to earth at regular intervals.

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 earth and neutral are combined right through the installation.