What are earthing systems: TN-S, TN-C-S and TT?

Glossary · Wiring, circuits and faults

How earthing systems work

Every installation needs a path for fault current to return safely to the source, so the protective device switches off quickly. The earthing system describes where that path comes from. The IET says the supply will be TN-S, TN-C-S (PME) or TT, and that the distributor can tell you which.^{[1]IET, Earthing: your questions answered} In the names, T means earth (from the French terre), N means neutral, S means separate and C means combined.^{[1]IET, Earthing: your questions answered}

The three systems compared

The IET explains each. In TN-S the neutral of the source is earthed at one point only, near the source, and the consumer’s earth terminal is typically connected to the metallic sheath or armour of the distributor’s service cable.^{[1]IET, Earthing: your questions answered} In TN-C-S the supply neutral is connected to earth at the source and at intervals along its run, usually called protective multiple earthing (PME). The distributor’s neutral also returns earth fault current to the source, and the distributor provides a consumer’s earth terminal linked to the incoming neutral.^{[1]IET, Earthing: your questions answered} In TT there is no earth facility from the distributor, so the consumer must provide an earth electrode.^{[1]IET, Earthing: your questions answered}

Earth rods and TT

A TT earthing facility is installed when the distributor does not provide a TN system or when one cannot be used.^{[3]IET, TT earthing considerations} The electrode can be an earth rod or pipe, tape, plate or buried metalwork, and the IET says a rod is a simple and usually the most cost-effective choice.^{[3]IET, TT earthing considerations}

A TT system has a higher external loop impedance, which does not give enough fault current to operate fuses or circuit breakers within the required time, so an RCD must be used for shock protection.^{[3]IET, TT earthing considerations} See our guide to earth fault loop impedance. Soil conditions matter. The IET says freezing greatly raises resistance, so a TT electrode needs periodic inspection and testing, and ideally at different times of year.^{[3]IET, TT earthing considerations}

The PME risk and the supply fault

In PME, the shared neutral can be broken. The IET warns that if the distributor’s combined neutral and earth conductor breaks, diverted neutral currents and dangerous touch voltages can appear on metalwork connected to the home’s earth terminal. Outdoor taps and Class I equipment are examples, and the risk of shock is greater for people outdoors who are in contact with the ground.^{[2]IET, Broken PEN} That is why bonding on PME homes is more onerous, as covered under protective bonding, and why the 2020 amendment to BS 7671 added methods of protection against an open-circuit supply neutral for EV charging. See PEN fault and open PEN devices.^{[2]IET, Broken PEN}

The IET also says PME is not provided for places such as caravans, boats and fuel filling stations, and that TT is a good solution when PME risks are unacceptable.^{[2]IET, Broken PEN}

Typical values

The IET quotes typical external earth fault loop impedances declared by the distributor: 0.8 ohm for TN-S, 0.35 ohm for TN-C-S and 21 ohm for TT. It stresses these are typical only and will differ when tested on site.^{[4]IET, BS 7671 FAQs - Earthing and Bonding}

Example

Sandeep is adding an outside socket at his house in Cookridge. His electrician first identifies the earthing system, because the answer affects the protection needed outdoors. The electrician checks the supply arrangement and tests before quoting. Sandeep learns that the earthing type is part of the design, not a detail.

What people get wrong

Many people assume a separate earth wire at the meter means TN-S. The IET says an installer should presume a supply with a separate protective conductor is TN-C-S unless the distributor confirms TN-S in writing.

That guidance comes from the IET’s article on broken supply neutrals.^{[2]IET, Broken PEN} The second mistake is thinking TT is a lesser system. The IET describes it as safe and effective when designed and tested properly.^{[3]IET, TT earthing considerations}

The bottom line

A UK home is earthed in one of three ways: a separate cable earth (TN-S), the supplier’s neutral (TN-C-S or PME) or its own earth rod (TT). The system sets the protection and bonding the home needs. Ask the network operator or a registered electrician which you have, and leave any change to an earth electrode to them.