UK earthing systems explained: TN-S, TN-C-S (PME) and TT

Published 3 September 2026 · updated 3 September 2026 · SparkCerts guides for UK electricians

The earthing arrangement is the first box on every certificate, and everything after it depends on the answer: the bonding sizes, whether the installation can rely on MCBs for fault protection, the maximum Zs, and whether you are allowed to export that earth to the garage or the car charger. Get it wrong and the rest of the schedule is built on sand. Here is how the three UK systems work, how to recognise them at the cut-out, and what each one commits you to.

TN-S: separate earth from the substation

In a TN-S system the earth is a separate conductor all the way back to the substation, in practice the lead sheath or steel armour of the older paper-insulated supply cable. At the cut-out the earthing conductor is clamped to the sheath. Because the path back is a continuous metal conductor, Ze is low: the distribution network operators (DNOs) quote a typical maximum of 0.8 Ω, and measured values are often much less. TN-S was the standard arrangement on urban supplies laid up to the 1960s and 1970s. Its weakness is that the sheath can corrode or be interrupted by a repair, and a DNO may have converted the network to PME since the house was wired, so the label on the cut-out is not always the truth: measure.

TN-C-S: PME, the modern default

In a TN-C-S system the supply cable has a combined neutral and earth conductor, the PEN, which is earthed at the substation and at multiple points along the network. That is the protective multiple earthing, PME. At the cut-out the DNO separates the two: the neutral goes on to the meter and the earth for the installation is taken from a terminal on the neutral block. The multiple earth connections give a low Ze, typically quoted as a maximum of 0.35 Ω, so disconnection times are easy to meet with ordinary MCBs. Almost every new supply and most upgraded ones are TN-C-S.

The catch is the PEN conductor. If it breaks between the house and the substation, the installation's earth is connected to the neutral downstream of the break, and every earthed metal part in the house rises toward line voltage while the load still passes through it. That open-PEN fault is why PME earths come with two conditions. The main protective bonding must be larger, sized from the supply neutral under Table 54.8 rather than from the earthing conductor, so that the bonded pipework can carry the diverted current safely. And the PME earth must not be exported to places where a person could touch earthed metal and true earth at the same time, which is the reason behind the restrictions below.

TT: your own electrode

A TT installation has no earth from the supplier at all. The earthing conductor runs to an electrode driven into the ground, a rod, a plate or a tape. Rural overhead supplies are usually TT, and so is any installation where the PME earth cannot be used. Electrode resistance is measured in tens or hundreds of ohms rather than fractions, so an earth fault produces only a small current and an MCB would never see it. Fault protection therefore relies on RCDs. BS 7671 requires that the electrode resistance multiplied by the RCD rated residual current does not exceed 50 V; for a 30 mA device that allows up to 1667 Ω on paper, but a note to Regulation 411.5.3 warns that an electrode above 200 Ω may not be stable, and most electricians aim for well under 100 Ω. A TT installation typically has a 100 mA or 300 mA time-delayed RCD as the main switch, with 30 mA devices on the final circuits, so that a fault trips the circuit and not the whole house. The DNO design figure for Ze on a TT supply is 21 Ω, but the measured electrode resistance is what you record.

Identifying the system at the cut-out

Then measure Ze with the main earthing conductor disconnected from the installation, so parallel paths through the bonding do not flatter the reading. A TN-S supply reading 0.2 Ω may well have been converted to PME; a "PME" cut-out reading 15 Ω has lost its earth. The reading goes on the certificate as Ze, and Zs at the board should come out at about the same figure.

What each system means for bonding

On TN-S and TT the main protective bonding conductors are sized from the earthing conductor: at least half its cross-sectional area, with a 6 mm² minimum and a 25 mm² cap. On TN-C-S they are sized from the supply neutral under Table 54.8: 10 mm² copper for a neutral up to 35 mm², which covers the ordinary domestic supply, rising to 16 mm² and beyond for larger services. The earthing conductor itself comes from Table 54.7 in every case: 16 mm² for line conductors between 16 and 35 mm². The earthing and bonding size calculator applies both tables to your supply, and the full walk-through is in earthing and bonding conductor sizes.

Where the PME earth may not go

• Caravan pitches and marinas (Sections 708 and 709): the PME earthing facility must not be used. The pitch supplies are TT.
• EV charging equipment (Section 722): a PME earth may only be used with protection against the open-PEN condition, either a device that disconnects on a PEN fault, which most current chargers build in, or a separate earth electrode making the charger circuit TT.
• Agricultural and horticultural premises (Section 705) and other locations with livestock or extensive outdoor metalwork, where the risk assessment often points to TT.
• Outbuildings and garden supplies where extraneous conductive parts cannot be bonded: the usual practice is to convert the outbuilding to TT rather than export the PME earth.

Recording it on the certificate

The supply characteristics section of an EIC or EICR asks for the earthing arrangement, Ze, the prospective fault current and the type of supply. Choose TN-S, TN-C-S or TT from what you found and measured, not from the age of the house, and let the rest follow: bonding sizes from the right table, RCD protection where the system demands it, and a maximum Zs per circuit from Table 41.3 or from the RCD where that provides fault protection. SparkCerts checks the schedule against the earthing system you enter, so a TT installation without RCD protection or a PME supply with 6 mm² bonding is flagged before the report is signed.

Common questions

How do I tell if a supply is TN-S or TN-C-S?

Look at where the earth comes from at the cut-out. On TN-S the earthing conductor is clamped to the lead sheath or steel armour of the supply cable. On TN-C-S it comes from a terminal on the neutral side of the cut-out, and the DNO usually fits a PME label. Confirm by measuring Ze: TN-C-S is typically well under 0.35 ohm, TN-S under 0.8 ohm.

What is the maximum Ze for TN-C-S?

Distribution network operators declare a typical maximum external loop impedance of 0.35 ohm for TN-C-S (PME) supplies, 0.8 ohm for TN-S, and 21 ohm for TT. Measured values are usually lower. These are design figures; the installation is assessed on what you measure.

Why does a TT installation need an RCD?

With an earth electrode of tens or hundreds of ohms the earth fault current is far too small to trip an MCB in time, so fault protection relies on an RCD. The rule is that the electrode resistance times the RCD rated current must not exceed 50 V; for a 30 mA device that permits up to 1667 ohm, though BS 7671 notes an electrode above 200 ohm may not be stable.

Where can a PME earth not be used?

BS 7671 does not allow the PME earthing facility for caravan pitches and marinas, and it places conditions on its use for EV charging equipment, which needs open-PEN protection or a separate electrode, and for some agricultural and outdoor installations. In those cases the circuit is converted to TT with its own electrode.

What size main bonding does PME need?

On a TN-C-S supply the main protective bonding conductor is sized from BS 7671 Table 54.8 according to the supply neutral: 10 mm squared copper for a neutral up to 35 mm squared, which covers most domestic supplies. TN-S bonding is sized from the earthing conductor instead, with a 6 mm squared minimum.