TN-C-S (PME) Earthing Explained: PEN Conductor Risks, Gas Pipe Bonding and EV Charger Restrictions

Quick Answer: TN-C-S — known as PME (Protective Multiple Earthing) — is the most common UK domestic supply system. The Distribution Network Operator (DNO) combines neutral and earth into a single PEN (Protective Earth and Neutral) conductor, which is split into separate earth and neutral terminals at your cut-out. Main protective bonding conductors must be a minimum 10mm² (Regulation 544.1.1), rising to 16mm² where the supply neutral exceeds 100A. PME earth must generally not be used for outdoor EV charge points (Regulation 722) because a broken PEN conductor can make exposed metalwork live.

Summary

Almost every modern UK home is fed on a PME (TN-C-S) supply. It is cheap, robust and gives a low earth fault loop impedance, which is exactly what you want for protective devices to operate quickly. The catch is in the name: the earth and the neutral share one conductor — the PEN — all the way back through the network, and your installation's earth is derived from that combined conductor at the cut-out. As long as the PEN is intact, this is safe and effective. If the PEN conductor breaks somewhere in the supply network, the consequences can be serious: the entire earth and bonded metalwork of the property can rise to a dangerous voltage relative to true earth.

This single risk drives a series of BS 7671 requirements that every electrician must understand: larger main protective bonding conductors than TN-S, careful treatment of incoming gas and water services, restrictions on exporting the PME earth to outbuildings and caravans, and — the big one in recent years — the prohibition on using the PME earth for outdoor EV charge points unless specific open-PEN protection is provided. Most domestic supplies are PME, so getting these rules right is everyday work, not an edge case.

This article explains what PME/TN-C-S actually is, the broken-PEN hazard in plain terms, how to size and install main protective bonding, the gas/water/structural bonding requirements, and why outdoor EV chargers need a TT earth island or an approved PEN-fault detection device. For the EV-specific detail and approved devices, also read pme earthing ev charging.

Summary of the systems

• TN-C-S (PME) — combined PEN in the network, split to separate earth/neutral at the property. Most UK domestic supplies.
• TN-S — separate earth conductor all the way back (historically the lead cable sheath). Increasingly rare.
• TT — no DNO earth provided; the installation has its own earth electrode (rod). Common rural, caravans, and the basis of the "TT island" used for EV chargers on PME supplies.

Key Facts

• PEN conductor — "Protective Earth and Neutral": one conductor doing both jobs in the supply network. Split into separate earth and neutral at the consumer's cut-out under PME.
• PME — Protective Multiple Earthing: the network neutral is earthed at multiple points along its length, which is what keeps the earth potential close to true earth in normal conditions.
• Most common UK supply — the majority of domestic properties are PME (TN-C-S).
• Low Ze — typical external earth fault loop impedance (Ze) for PME is around 0.10–0.35Ω, lower than TN-S (~0.40–0.80Ω). Low Ze means fast disconnection. (Cross-check current values in earthing bonding.)
• Main protective bonding minimum — 10mm² for PME under Regulation 544.1.1, rising to 16mm² where the supply neutral (PEN) conductor cross-section requires it — typically supplies above 100A.
• Main earthing conductor — minimum 16mm² for PME (sized to the supply per Regulation 542 / Table 54.8).
• Broken-PEN hazard — if the PEN fails in the network, neutral current returns via the earth path, lifting the property's earth and all bonded metalwork to a dangerous voltage relative to true earth.
• Gas bonding — incoming metallic gas service bonded within 600mm of the meter outlet, on the consumer side, using a BS 951 clamp labelled "Safety Electrical Earth — Do Not Remove".
• Water bonding — incoming metallic water service bonded within 600mm of entry / before the first branch. Plastic incoming main may not require bonding (assess downstream metalwork).
• Structural/other services — structural steel, oil lines, central-heating/air pipework where extraneous-conductive-parts apply must be assessed and bonded as required.
• EV charge points outdoors — Regulation 722 restricts using the PME earth for an outdoor EV charge point. Use a TT island (dedicated earth electrode for the charge point) or a charger/device providing open-PEN (PEN-fault) detection that disconnects all live conductors including earth.
• Caravans / boats / outbuildings — exporting the PME earth to a caravan pitch or boat is generally prohibited; similar restrictions apply to outdoor locations with a bath/shower.
• Disconnection times — 0.4s for final circuits ≤32A, 5s for distribution circuits (Regulation 411.3.2 family). PME's low Ze helps achieve these.
• Bonding clamp — BS 951 earth clamp, correct conductor, sleeved green/yellow, with the durable warning label.
• Identifying PME — at the cut-out the earth terminal is linked to the supply neutral; the DNO can confirm. No local earth rod is present (unlike TT).

Quick Reference Table

Detailed Guidance

What the PEN conductor is and why PME works

In a PME (TN-C-S) network the DNO runs a single conductor that is both neutral and earth — the PEN. To keep its potential close to true earth, the network neutral is bonded to earth electrodes at multiple points (hence "Protective Multiple Earthing"). At your property the cut-out splits the PEN into a separate neutral terminal and a separate earth terminal; from the earth terminal the main earthing conductor runs to your Main Earthing Terminal (MET), and from the MET the main protective bonding reaches the incoming services.

The benefit is a very low earth fault loop impedance. When a fault to earth occurs, the loop back to the source is short and low-resistance, so the protective device sees a large fault current and disconnects fast — comfortably inside the 0.4s requirement for socket and final circuits.

The broken-PEN hazard — in plain terms

Everything depends on the PEN staying intact. Imagine the PEN conductor in the street fractures somewhere between your house and the substation. Now the neutral current from your home — and possibly neighbouring homes — has no clean route back. It looks for another path, and that path is through your earthing system, your main protective bonding, and out via the incoming gas and water pipes into the ground.

Two things follow. First, all the bonded metalwork in the house — taps, radiators, the EV charger casing, exposed-conductive-parts — can rise to a substantial voltage relative to the true earth outside. Second, a person standing on the ground touching that metalwork (especially outdoors, barefoot, or touching a car body) is across that voltage. This is the reason PME earth must not casually be exported to outdoor locations.

Inside an equipotential zone the danger is reduced because everything rises together — there is no large potential difference to drive current through a person. The danger is greatest where someone bridges the inside (bonded, raised) and the outside (true earth): outdoor sockets, EV chargers, caravans, garden taps and metal outbuildings.

Main protective bonding — sizing and installation

Main protective bonding ties the incoming metallic services to the MET so the whole installation rises and falls together. Under PME the conductors are larger than under TN-S because the bonding may have to carry diverted neutral current under a PEN fault.

• Minimum 10mm² for PME (Regulation 544.1.1).
• 16mm² where the supply neutral conductor cross-sectional area requires it — in practice for larger supplies, typically those above 100A.
• Conductor is green/yellow, run to each service, terminated with a BS 951 clamp carrying the label "Safety Electrical Earth — Do Not Remove".

MAIN BONDING SIZING (PME)
=========================
What is the supply / PEN neutral size?
  |
  +-- Up to ~100A supply --> 10mm2 main protective bonding
  |
  +-- Above ~100A supply --> 16mm2 main protective bonding
        (size to neutral per Reg 544.1.1 table)

Then for EACH incoming metallic service:
  gas / water / oil / structural steel
  --> bond within 600mm of entry, BS 951 clamp,
      labelled, green/yellow, to the MET.

Gas, water and structural bonding

• Gas: bond the consumer's pipework (not the meter, which can be swapped) within 600mm of the meter outlet, before the first branch/tee. Plastic-bodied meters and plastic incoming services change the assessment — bond the first metallic section that is an extraneous-conductive-part.
• Water: bond the incoming metallic main within 600mm of entry. A blue MDPE plastic main may not need bonding at entry, but assess metallic pipework downstream that could introduce earth potential.
• Structural steel and other services: any part liable to introduce a potential (an extraneous-conductive-part under Regulation 411.3.1.2) must be bonded. Internal pipework that is not extraneous (no path to true earth) does not require main bonding.

Confirm continuity of bonding by test (the bonding conductor and clamp should be electrically sound). Supplementary bonding in bathrooms is a separate question covered in bathroom zones and is generally not required where main bonding is confirmed and circuits are RCD-protected.

EV chargers and the PME prohibition

This is the most common modern PME pitfall. An outdoor EV charge point on a PME supply is the textbook broken-PEN danger scenario: the person is outdoors, standing on real earth, touching a car connected to the charger's earth. If the PEN fails, the car body could be live.

Regulation 722 therefore restricts using the PME earth for outdoor EV charge equipment. The two compliant approaches are:

1. TT island — provide a dedicated earth electrode (rod) for the EV circuit only, isolated from the PME earth, so the charger's earth references true earth and an RCD/RCBO clears faults. The rest of the house stays on PME.
2. Open-PEN (PEN-fault) detection device — a charger or in-line device that monitors for the broken-PEN condition (typically by detecting an out-of-range voltage) and disconnects all live conductors including the protective earth before the metalwork can become dangerous. Many "PME-compatible" EV chargers have this built in.

Full detail, including which devices are accepted and how the TT island is implemented, is in pme earthing ev charging and ev charger.

Caravans, outbuildings and other outdoor risks

The same logic restricts exporting PME earth to caravan/boat pitches and, in many cases, to outbuildings containing a bath or shower or accessible outdoor metalwork. The usual solution is a local TT arrangement for the outbuilding/pitch. Always assess the specific location against the relevant section of BS 7671 rather than assuming the house PME earth can simply be carried out.

Frequently Asked Questions

How do I tell if a property is on PME?

At the cut-out, the earth terminal is connected (linked) to the supply neutral, and there is no local earth rod. The main earthing conductor runs from that earth terminal to the MET. If you see a separate lead-sheathed earth it is TN-S; if there is a local rod and no DNO earth it is TT. When in doubt, ask the DNO to confirm the supply type.

Why is PME bonding 10mm² when TN-S is only 6mm²?

Because under a PEN fault the main bonding can carry diverted neutral current, the conductor must be larger to handle it safely without overheating. Regulation 544.1.1 sets 10mm² minimum for PME, increasing to 16mm² for larger supplies, against 6mm² for TN-S.

Can I run an outdoor socket or EV charger straight off the PME earth?

For a general outdoor socket, RCD protection is required and the PME earth is normally acceptable within the building's equipotential zone — but assess the location. For an outdoor EV charge point the answer under Regulation 722 is generally no: you must provide a TT island or use an approved open-PEN detection device. Do not simply extend the PME earth to the charger casing outdoors.

What happens to the house if the PEN actually breaks?

Bonded metalwork can rise to a dangerous voltage relative to true earth, and neutral current diverts through the earthing/bonding system. Inside the equipotential zone the shock risk is reduced because everything rises together; the acute danger is at the boundary — outdoor metalwork, EV chargers, garden taps, caravans — where a person bridges raised metalwork and true ground.

Does internal copper pipework still need bonding?

Only if it is an extraneous-conductive-part — i.e. it can introduce a potential from outside (true earth) into the installation. With all-plastic incoming services and internal copper that has no external earth path, internal pipework is often no longer extraneous and does not require main bonding. Assess each case; the incoming metallic service is what matters most.

Regulations & Standards

• BS 7671:2018+A2:2022 Regulation 544.1.1 — sizing of main protective bonding conductors (10mm² PME, up to 16mm² for larger supplies).

• BS 7671:2018+A2:2022 Regulation 411.3.1.1 / 411.3.1.2 — protective earthing, protective equipotential bonding and extraneous-conductive-parts.

• BS 7671:2018+A2:2022 Regulation 411.3.2 — automatic disconnection times (0.4s / 5s).

• BS 7671:2018+A2:2022 Section 722 — EV charging installations, including PME/open-PEN requirements for outdoor charge points.

• BS 7671:2018+A2:2022 Regulation 542 / Table 54.8 — earthing conductor sizing.

• BS 951 — clamps for earthing and bonding (the labelled pipe clamp).

• IET Code of Practice for Electric Vehicle Charging Equipment Installation — practical PME/EV guidance.

• Building Regulations Approved Document P — notifiable work in dwellings.

• IET / BSI — BS 7671:2018+A2:2022 Requirements for Electrical Installations — the Wiring Regulations and IET guidance notes.

• Energy Networks Association — Engineering Recommendations — DNO guidance on PME supplies and PEN integrity.

• Electrical Safety First — Best Practice Guides — earthing, bonding and EV guidance for installers.

• HSE — Electrical safety — duties and shock-risk context.

• earthing bonding — full main and supplementary bonding requirements across PME/TN-S/TT.

• pme earthing ev charging — PEN-fault detail, TT island and approved EV protection devices.

• ev charger — domestic EV charge point installation overview.

• bathroom zones — supplementary bonding and equipotential zones in bathrooms.