# TT explained — and the TT-island method for outbuildings

> TT means the installation makes its **own** earth with a local electrode; the supply provides only line and neutral, and protection leans on an **RCD** plus an electrode resistance (Ra) that is low and stable. That is exactly what a **separate outbuilding** needs to host a V2L island: drive a local electrode, take all the protective conductors to a continuous earth bar, do **not** export the dwelling's PME earth (broken-PEN risk), make a single V2L neutral-earth bond through a KM2 auxiliary contact so it exists only on V2L, and protect with a Type A RCD. It only works if no metal pipe, structure or cable armour bridges the two earth systems — and it must be designed and proven by test by a competent person.

**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

- **TT** = the installation provides its own earth electrode; the supply gives only line (L) and neutral (N), no earth.
- Because the fault loop runs through soil, the loop impedance is high — an **RCD is essential**, and overcurrent devices cannot be relied on for disconnection.
- The electrode resistance (**Ra**) must be low and **stable**; a high electrode resistance tends to drift with the seasons (BS 7671 Table 41.5; BS 7430 for electrode design).
- **TT-island method (outbuilding):** local electrode → earth bar → all CPCs continuous; the dwelling's **PME earth is never exported** (broken-PEN risk); one V2L neutral-earth bond via a KM2 aux contact; additional RCD protection.
- It only works if **no metal pipe, structure or cable armour bridges the two earth systems** — and it is designed and **proven by test** by a competent person.

**Where this stops:** This explains what TT is and why the TT-island method works. It is not a wiring recipe — the electrode design, the bond arrangement and the testing are for a competent person, and the result must be proven by test.

## What a TT earthing system actually is

In a **TT** system the distributor provides only line (L) and neutral (N) — **no earth at all**. The installation must make its own earth with a local **earth electrode** (one or more rods driven into the ground), and the main earthing terminal connects to that electrode, not to the supply. There is no metallic earth return to the substation: an earth-fault current has to travel from your electrode, through the general mass of earth, back to the source's earthed point.

That soil path has a relatively **high** impedance, so earth-fault currents are often too small to operate fuses or MCBs within the required disconnection time. The consequence is the defining feature of TT: protection leans on a **residual current device (RCD)**, not on overcurrent devices, and the consumer unit carries a 'TT System — RCD required' label.

> **Where TT shows up** — Rural overhead-line supplies, caravan parks, marinas, filling stations and construction sites are commonly TT — and a local TT electrode is one of the sanctioned ways to earth an outdoor EV charge point on a PME network. The same principle is what lets a separate outbuilding host a V2L island.

**Figure: TT earthing system.** TT: the installation makes its own earth with a local electrode; the supply provides only line and neutral. Protection relies on an RCD plus a low, stable electrode resistance.

_A TT supply. The distributor provides only line (L) and neutral (N); the installation provides its OWN earth via a local earth electrode (an earth rod) driven into the ground. The main earthing terminal connects to that electrode, not to the supply. Because the earth-fault loop runs through soil, fault-loop impedance is relatively high, so protection depends on a residual current device (RCD) plus an electrode resistance (Ra) low and stable enough to operate it (BS 7671 Table 41.5; BS 7430 for electrode design). Identify it on site by an earth rod and an RCD as the main protective device, with no earth taken from the supply cable._

> **Chapter 41 (§411) + Table 41.5** (BS 7671, confidence: inference) — On TT the earth-fault loop impedance is high, so disconnection is achieved by an RCD rather than an overcurrent device. Table 41.5 frames the maximum electrode resistance compatible with reliable RCD operation. _Reference only; standard text not reproduced._

## Why the electrode resistance (Ra) has to be low — and stable

An RCD only protects if the electrode resistance is low enough for the fault loop to pass a residual current the device will trip on, **and** that resistance stays where you measured it. The trap is seasonal: soil dries in summer and freezes in winter, so a marginal electrode that measured fine in May can drift high in August or January. The fix is the electrode designer's job — pick the right rod type, drive it **deep enough to reach moist soil**, and use multiple spaced rods where one will not hit the target.

> **Confidence: inference** — A high electrode resistance (Ra) tends to be unstable across the seasons, so it should be kept as low as practicable. (Stated in the BS 7430 card (read with BS 7671 §542.2.4 and Table 41.5): a high electrode resistance tends to drift with drying and freezing, so the electrode should be 'as low as practicable' and deep enough to reach moist soil. For a V2L island this is the difference between an island that trips safely on a fault and one that does not.)

> **Earth-electrode design + keeping Ra low and stable** (BS 7430, confidence: inference) — The UK code of practice for selecting and sizing the electrode (rod/plate type, depth, spacing, soil resistivity) and for the Ra calculation a designer makes before measuring. Cited alongside BS 7671 §542.2.2 / §542.2.4; precise 2026 clause numbers are Not confirmed. _Reference only; standard text not reproduced._

> **Not confirmed:** The exact BS 7430:2026 clause numbers (e.g. the rod-resistance formulae that sat at 9.5.3 / 9.5.4 in the 2011+A1:2015 edition). The 2026 edition is recent and may renumber — confirm against the published text before citing a clause number.

## The TT-island method for a separate outbuilding

A V2L output backing up a board is a **switched-alternative source**: when you are on the car, the grid's earth reference is not the one you want to rely on, and a floating V2L output has no reference of its own. For a genuinely **separate outbuilding**, the clean answer is to make the outbuilding **TT** — give it its own local earth — and reference the V2L source to that. Indoors this approach does not work, because a single TT sub-board inside a PME-earthed dwelling creates a simultaneous-reach hazard; that case uses an isolation transformer instead. The TT-island method below is for the outbuilding.

1. **Drive a local earth electrode** and take it to the outbuilding's earth bar. All the outbuilding's circuit protective conductors (CPCs) connect to that bar **continuously** — the earth is never switched.
2. **Do not export the dwelling's PME earth** to the outbuilding. Only line (L) and neutral (N) run over. Exporting a PME earth carries the broken-PEN risk into the outbuilding, which is exactly what the TT island is there to avoid.
3. **Make one V2L neutral-earth bond** to the earth bar through a **KM2 auxiliary contact**, so the single source bond exists **only when the load is on V2L** — never two bonds in parallel, never a bond left in on the grid.
4. **Protect with additional RCD protection.** On grid the board is TT referenced to the local electrode; on V2L it is referenced through the switched neutral-earth bond. The RCD must be proven to operate in **both** states.
5. **Confirm nothing bridges the two earth systems** — no metal pipe, structural steel or cable armour linking the outbuilding's local earth back to the dwelling's PME earth, or it silently re-imports the very risk you separated out.

**Figure: TT-island earthing — bond and aux-contact detail.** For an outbuilding made TT: a local electrode feeds the earth bar and all CPCs continuously; the V2L neutral-earth bond is made through a KM2 auxiliary contact, so it exists only on V2L.

_The TT-island arrangement. A local earth electrode connects to the outbuilding earth bar; all circuit protective conductors (E/CPC) connect to that bar continuously and are never switched. The PME (distributor's) earth is NOT exported to the outbuilding — only line (L) and neutral (N) run over. The V2L neutral-earth (N–E) bond is made through an auxiliary contact on the V2L contactor (KM2), so the single bond exists only while on V2L. An RCD protects the island. The point: one continuous local earth, one switched source bond, no exported PME earth._

> **§722.411.4.1** (BS 7671, confidence: inference) — On a PME (TN-C-S) supply you must not simply rely on the distributor's earth for the EV side — an open PEN can raise metalwork to a dangerous voltage. This is why the outbuilding gets a local TT electrode rather than the dwelling's PME earth, and why the floating-on-PME approach is contested and must be proven by test. _Reference only; standard text not reproduced._

> **§551.4.3.2.1** (BS 7671, confidence: inference) — A switched-alternative source needs an independent means of earthing — it must not rely solely on the distributor's earth, which may be disconnected during network maintenance. The local TT electrode supplies that independent reference. _Reference only; standard text not reproduced._

## The whole outbuilding scheme on one page

Put the power, control and earthing together and the outbuilding scheme is one consistent picture: an interlocked two-pole contactor changeover (KM1 grid / KM2 V2L) feeds a small essential board; a scheduled smart relay commands the changeover, fail-safe to grid; the local electrode feeds the earth bar and all CPCs continuously; the PME earth is not exported; the V2L neutral-earth bond is switched in via a KM2 auxiliary contact; and a Type A RCD protects the board. The load is dropped, the source swapped and re-energised — **never bridged** — and the installation is earthed in **every** state.

**Figure: Outbuilding TT — full sheet (power + control + earthing).** The complete outbuilding scheme on one page: interlocked contactor changeover, scheduled control, TT electrode and switched N-E bond — everything ends earthed.

_A single-sheet assembly of the outbuilding TT scheme. Power: grid line/neutral (L/N) and V2L L/N feed an interlocked two-pole contactor changeover (KM1 grid / KM2 V2L) into a small essential board. Control: a scheduled smart relay drives the interlocked coils, fail-safe to grid. Earthing: a local electrode feeds the outbuilding earth bar and all CPCs continuously; the PME earth is not exported; the V2L neutral-earth bond is switched in via a KM2 auxiliary contact; an RCD protects the board. The single key point: the load is dropped, the source swapped, and re-energised — never bridged — and the installation is earthed in every state._

> **A floating output passes a socket tester but offers no RCD protection** — Until the single neutral-earth bond is made, the V2L output is floating: a plug-in socket tester reads 'fine', yet an RCD has no reference to operate against. The bond is the source's earth reference — like the substation transformer's earthed star point — not an optional extra. Make exactly one, on the source side, in circuit only on V2L.

> **Contested method — design and prove by test** — Earthing a floating V2L output relative to a PME network is contested, not a settled recipe. It must be designed and proven by test by a competent person, and the vehicle manufacturer does not sanction back-feeding fixed wiring from a V2L outlet.

### How this is made and proven compliant

**What governs it:** BS 7671 Chapter 54 (§542 earthing arrangements / electrode) and Chapter 41 (§411 ADS, Table 41.5); BS 7671 §722.411.4.1 (no reliance on a PME earth for the EV side) and §551.4.3.2.1 (independent earthing for a switched-alternative source); BS 7430 (electrode selection, soil resistivity, Ra calculation and measurement) — supports BS 7671, not a substitute for it

**Who may do it:** Design, installation, inspection and testing by a competent person. Adding a V2L inlet circuit, changeover or sub-board to an outbuilding is normally notifiable under Part P (England; Wales, Scotland and NI differ).

**How compliance is demonstrated:** Electrode resistance (Ra) calculated to BS 7430, then measured low and stable enough for the RCD to disconnect (the 'proven by test' step); RCD operation confirmed by test in **both** grid and V2L modes (initial verification to BS 7671 Part 6, with an Electrical Installation Certificate); Confirmed by inspection that no metal pipe, structure or cable armour bridges the local TT earth and the dwelling's PME earth; Confirmed the actual V2L adapter's neutral-earth behaviour on the bench before relying on the single source bond

## FAQ

### What is a TT earthing system in plain terms?

The supply gives you only line and neutral, no earth, so **you** provide the earth with a local rod (electrode). Because the fault path runs through soil, the loop impedance is high — so an **RCD does the disconnecting**, not the fuses or MCBs, and the board is labelled 'TT System — RCD required'.

### Why does the electrode resistance (Ra) have to be so low and stable?

Because the RCD can only clear a fault if enough residual current flows, and that depends on Ra. A high electrode resistance tends to drift with the seasons (drying, freezing). Keep it as low as practicable and deep enough to reach moist soil; design it to **BS 7430** and confirm it **by measurement**.

### Why not just run the dwelling's earth out to the outbuilding?

Because that exports the dwelling's **PME earth**, carrying the broken-PEN risk into the outbuilding. Run only line and neutral over, and give the outbuilding its **own** local TT earth. BS 7671 §722.411.4.1 is the reason you do not lean on a PME earth for the EV side.

### Why is the V2L neutral-earth bond switched, and made only on V2L?

Every source needs exactly **one** earth reference. The grid's is at the substation; the floating V2L has none, so you supply one — through a **KM2 auxiliary contact**, so the single bond is in circuit only while on the car. Two bonds in parallel, or a bond left in on the grid, is wrong.

### Can I use this TT-island method for an indoor sub-board?

No. A single TT sub-board inside a PME-earthed dwelling creates a **simultaneous-reach hazard** with the rest of the installation. TT-island is for a genuinely **separate outbuilding**; indoors you keep the one PME/MET earth and use an **isolation transformer** instead.

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_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. BS 7430:2026 — Protective earthing of electrical installations. Code of practice (BSI, committee GEL/600) — https://knowledge.bsigroup.com/products/code-of-practice-for-protective-earthing-of-electrical-installations (cited by reference only; 2026 clause numbers Not confirmed against the published text)
3. IET Wiring Matters — TT earthing considerations — https://electrical.theiet.org/wiring-matters/years/2021/87-september-2021/tt-earthing-considerations/
4. V2L Workshop technical reference and UK earthing-systems explainer (internal) — verified design facts and confidence flags