# TN-S explained — the separate-earth supply, and what it means for V2L > TN-S is an earthing system where the electricity distributor brings a **separate** protective earth (historically the lead cable sheath) all the way from the substation, and the neutral and earth stay separate throughout — there is no combined PEN conductor, so there is no open-PEN-by-combination hazard of the PME (TN-C-S) kind. The catch is that an apparent TN-S is often actually PME, so treat a supply as TN-C-S unless the DNO confirms otherwise. For a V2L arrangement, knowing the supply is genuine TN-S removes the PME complication, but it does **not** answer the islanded-source earthing question: a floating V2L output still has no earth reference of its own, and that must be designed and proven by test by a competent person — the manufacturer does not sanction this use. **Safety frame.** This is education, not an instruction to carry out work. Work connecting V2L equipment to fixed wiring is safety-critical and may be notifiable under Part P. It must be designed, installed, inspected and tested by a competent person to the current edition of BS 7671. Vehicle manufacturers generally do not sanction back-feeding fixed wiring from V2L outlets; follow manufacturer instructions. ## In short - TN-S = a **separate** earth conductor from the substation, kept distinct from neutral the whole way — no combined PEN. - Historically the earth is the lead/steel **cable sheath**; the distributor's earth terminal is bonded to it at the cut-out. - Because there is no PEN, TN-S does **not** carry the open-PEN-by-combination hazard that drives the PME EV rules. - But an **apparent TN-S can actually be PME** — treat the supply as TN-C-S until the DNO confirms it is genuine TN-S. - For V2L: real TN-S removes the PME complication, but a **floating** V2L source still needs its own earth reference — designed and proven by test by a competent person; the car maker does not sanction this use. **Where this stops:** This explains what TN-S is and why it matters for a backup source. It is not a wiring recipe, and you should not assume your supply is TN-S — the supply type and earth-loop impedance must be confirmed by a competent person and, where needed, with the DNO. ## What TN-S actually is In a **TN-S** supply the electricity distributor provides a **separate** protective earth conductor that runs all the way from the substation to your service head. The neutral and the earth are kept distinct throughout the whole distribution network — they are never combined into one conductor. The letters say it: **T**erre (the source is earthed), **N**eutre, **S**éparé (separate). The supplier's earth terminal at the cut-out is bonded to that separate earth, historically the metallic sheath of a lead-covered cable. - **Supply earth:** a separate conductor (commonly the lead/steel **cable sheath**) from the substation — not derived from the neutral. - **Neutral and earth:** separate the entire way; there is **no PEN** (combined protective-earth-neutral) conductor. - **Earth terminal:** provided by the supplier at the cut-out, often a soldered connection to the cable sheath, with no neutral-to-earth link at the cut-out. - **Fault path:** earth-fault current returns to the substation along the dedicated earth conductor, giving a stable, low-impedance reference. **Figure: TN-S earthing system.** TN-S: the distributor provides a separate earth conductor all the way from the supply transformer — neutral (N) and earth (PE) are separate throughout. _A TN-S supply. At the supply transformer the star point is earthed. Three conductors reach the installation: line (L), neutral (N) and a separate protective earth (PE) — often the lead sheath or armour of the service cable. Neutral and earth are kept separate the whole way (the 'S' = separate). At the consumer unit, N goes to the neutral bar and PE to the main earthing terminal; they are not joined. Identify it on site by a separate earth conductor connected to the supply cable sheath, distinct from the neutral._ > **Chapter 54 (§542, §544)** (BS 7671, confidence: inference) — How the installation's earth is derived (§542) and how metalwork is bonded (§544) depend on the supply type. On TN-S the installation earth is taken from the supplier's separate earth terminal at the MET; the bonding rules still apply. _Reference only; standard text not reproduced._ ## Why no PEN means no open-PEN-by-combination hazard The defining danger of a **PME (TN-C-S)** supply is the broken PEN: because the neutral and earth share one conductor for part of the run, an open circuit in that combined conductor can push earthed metalwork up towards line voltage. TN-S does not combine the two, so that specific failure mode — the open-PEN-by-combination hazard that drives the special EV-charging earthing rules — does not arise in the same way. That is the genuine safety advantage of a real TN-S supply. TN-S has its own weakness, though: the separate earth is a single conductor, often an old cable sheath, and it can corrode or fracture. If it fails, the installation can lose its earth **silently** — no obvious symptom until a fault occurs. So TN-S is not 'no earthing worries', it is a *different* set of worries. > **TN-S vs PME, in one line** — PME risk is the combined conductor rising to line voltage if the PEN breaks. TN-S risk is the separate earth quietly going open-circuit through corrosion. Different failure, different mitigation. ## The catch: an apparent TN-S is often really PME Genuine TN-S is becoming rare. Distributors have widely converted old lead-sheathed supplies to PME by linking neutral and earth at the service head where the sheath is judged adequate — so a supply that *looks* like TN-S at a glance may actually be TN-C-S (PME). Getting this wrong is not cosmetic: the whole PME/open-PEN regime for backup sources and EV charging turns on it. > **Default to PME until proven otherwise** — Treat any supply as **TN-C-S (PME)** — and apply the PME earthing precautions — unless a competent person has positively confirmed it is genuine TN-S, with the DNO where there is any doubt. Do not assume TN-S from the look of the cut-out alone. See the supply-identification page. > **Confidence: inference** — A supply that appears to be TN-S should be treated as PME until confirmed. (Drawn from the internal UK-earthing reference: distributors convert legacy TN-S to PME by bonding N and E at the service head, so visual identification is unreliable. The DNO has a duty under ESQCR to declare the supply type — confirm rather than infer.) ## What this means for a V2L arrangement If — and only if — the supply is confirmed genuine TN-S, you avoid the contested PME/open-PEN complication that dominates the floating-V2L-on-PME problem. That is helpful. But it does **not** make the islanded-source earthing question go away. When the changeover puts the essential board on the car, the grid's earth reference (at the substation) is no longer the source reference for that island; a **floating** V2L output has no neutral-earth bond of its own, so an RCD has nothing to operate against until one is supplied. > **§551.4.3.2.1** (BS 7671, confidence: inference) — A source supplying in island mode or as a switched alternative needs an independent means of earthing and must not rely solely on the distributor's earth. This holds on TN-S as much as on PME — the V2L island needs its own earth reference. _Reference only; standard text not reproduced._ So the earthing of the V2L island is still a design-and-test problem in its own right, whatever the grid supply type. On a genuine TN-S dwelling you still keep one continuous earth to the main earthing terminal and provide the island its single neutral-earth reference (for example via an isolation transformer on the V2L feed), made in circuit only on V2L. The contested floating-V2L approaches must be **designed and proven by test by a competent person; the manufacturer does not sanction this use.** > **Not confirmed (safety-critical):** Whether Amendment 4:2026 changed anything for §722 / V2X / PME / open-PEN is Not confirmed — the published change-lists do not name these. Confirm against the licensed standard before relying on any A4-specific V2X/PME claim. ### How this is made and proven compliant **What governs it:** BS 7671 Chapter 54 (§542 earthing arrangements, §544 protective bonding) — how the installation earth is derived and bonded; BS 7671 §551 island/switched-alternative source provisions (§551.4.3.2.1 independent means of earthing) — the V2L island still needs its own earth reference; The Electricity Safety, Quality and Continuity Regulations (ESQCR) — the DNO's duty to declare the supply type and earth-fault loop impedance **Who may do it:** Confirming the supply type, measuring Ze and designing any backup-source earthing are for a competent person; where the supply type is uncertain it must be confirmed with the DNO. Alterations that add an inlet, changeover or earthing arrangement are normally notifiable under Part P (England; Wales/Scotland/NI differ). **How compliance is demonstrated:** Supply type confirmed by inspection of the cut-out and, where uncertain, by the DNO — never assumed from appearance; External earth-fault loop impedance (Ze) measured at the MET with main bonding disconnected; For any V2L island, RCD operation proven by test in **both** grid and V2L modes; The actual V2L adapter's neutral-earth behaviour confirmed on the bench before it is relied on ## FAQ ### What is the difference between TN-S and TN-C-S (PME)? In **TN-S** the distributor brings a separate earth conductor from the substation and keeps neutral and earth apart the whole way. In **TN-C-S (PME)** the neutral and earth are combined into one PEN conductor for part of the run and split at the service head. The combined conductor is what creates the broken-PEN hazard, so TN-S does not share it. ### Is my supply TN-S? Probably not — genuine TN-S is now rare, and many old TN-S supplies have been converted to PME. Do not assume it from the look of the cut-out. **Treat the supply as TN-C-S (PME) until a competent person confirms otherwise**, with the DNO where there is any doubt. The DNO has a duty under the ESQCR to declare the supply type and earth-loop impedance. ### Is TN-S safer than PME? It avoids the open-PEN-by-combination hazard, which is a real advantage. But TN-S has its own failure mode: the separate earth — often an old cable sheath — can corrode and go open-circuit silently. Neither is 'safe by default'; both depend on correct design, bonding and testing. ### If my supply is genuine TN-S, is V2L earthing simpler? It removes the PME complication, but not the core problem. A **floating** V2L output has no earth reference of its own, so the islanded source still needs a single neutral-earth bond supplied at changeover (for example via an isolation transformer), and an RCD proven by test in both grid and V2L modes. It must be designed and proven by test by a competent person; the car maker does not sanction this use. ### How does an electrician tell TN-S from PME? By inspecting the cut-out (a separate earth bonded to the cable sheath with no neutral-earth link suggests TN-S; a combined block or 'PME' label indicates TN-C-S) and by measuring the external earth-fault loop impedance (Ze). The Ze figures are indicative, not definitive — where it matters, the supply type is confirmed with the DNO, not assumed. --- _Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._ _Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._ _Status: reviewed. General information, not project-specific design advice._ _[How we source this](/methodology) — evidence hierarchy, confidence flags and source policy._ ## Sources 1. BS 7671:2018+A4:2026 — Requirements for Electrical Installations (IET/BSI) — https://electrical.theiet.org/bs-7671-18th-edition-wiring-regulations/about-bs-7671/ (cited by clause only; standard text not reproduced) 2. IET — Amendment 4 updates to the 18th Edition — https://electrical.theiet.org/amendment-4-updates-to-18th-edition 3. The Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 (as amended) — DNO duty to declare supply type and earth-fault loop impedance — https://www.legislation.gov.uk/uksi/2002/2665/contents/made 4. V2L Workshop UK earthing systems reference (internal) — TN-S/TN-C-S/TT descriptions and indicative Ze values (working summary, verify against primary sources)