# Earth electrodes & resistance (Ra) for a TT island — BS 7430 in practice > When a V2L output backs up a separate outbuilding as a TT island, the island's safety rests on a local earth electrode whose resistance (**Ra**) is low and **stable** — because protection leans on an RCD plus that electrode. **BS 7430:2026** is the UK code of practice an electrician uses to select and size the electrode (rod, plate or tape), account for soil resistivity, and verify Ra by measurement. Read it with **BS 7671** 542.2.2 / 542.2.4 and Table 41.5. The 2026 edition is very new, so we cite its principles but treat precise 2026 clause numbers as **Not confirmed**. **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 - A TT island's protection leans on an RCD **plus** an electrode resistance (Ra) low and stable enough to disconnect reliably. - **BS 7430:2026** (published 26 May 2026) is the UK code of practice for designing and proving that electrode; it supports BS 7671. - Electrode type, length/depth, soil resistivity and parallel-rod spacing decide whether one rod suffices. - At a high electrode resistance, the value tends to drift with the seasons — keep it as low as practicable and deep enough to reach moist soil. - The 2026 edition is < 1 month old at our check — precise 2026 clause numbers are **Not confirmed**; Ra must be **proven by measurement**. **Where this stops:** This explains the principles BS 7430 sets out. Electrode design, sizing and the Ra measurement that proves it are competent-person work — the result must be proven by test. ## Why Ra is the whole game on a TT island On a TT island the earth-fault loop runs through soil, so the loop impedance is relatively high and overcurrent devices cannot be relied on to disconnect. Protection leans on a **Type A RCD plus** an electrode resistance low and stable enough to make it operate. If Ra drifts up in a dry or frozen season, the protection you assumed may not be there. **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._ ## What BS 7430 helps you decide BS 7430 covers electrode selection and arrangement (rods, plates, tapes), the effect of soil type and resistivity, methods to estimate and measure electrode resistance, and the discipline around generator/UPS changeover earthing — all the background to sizing a V2L island electrode. For most domestic islands the question is whether a single rod is adequate or whether you need multiple, spaced rods to hit a target Ra. **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._ > **Not confirmed:** BS 7430:2026 was published on 26 May 2026 and was less than a month old at our last check. Most third-party guidance still cites the 2011+A1:2015 clause numbers, so we cite the 2026 edition's principles but treat precise 2026 clause numbers as Not confirmed until verified against the published text. ### How this is made and proven compliant **What governs it:** BS 7430:2026 (protective earthing code of practice) supporting BS 7671; BS 7671 Reg 542.2.2 / 542.2.4 and Table 41.5 (TT electrode + disconnection) **Who may do it:** Electrode design, installation and the Ra measurement are competent-person work. The island must be designed and proven by test, never a recipe. **How compliance is demonstrated:** Estimate Ra from electrode type, soil resistivity and rod count/spacing; Confirm Ra by measurement (the 'proven by test' step) — low and stable; Confirm Type A RCD operation by test on the island; no metallic bridge to the PME earth ## FAQ ### What Ra do I need for a TT island? Low enough that the Type A RCD disconnects reliably (per BS 7671 Table 41.5) and stable across seasons. As a rule of thumb, at a high electrode resistance the value tends to drift, so keep it as low as practicable — and prove it by measurement. ### Can I just hammer in one earth rod? Maybe, maybe not — it depends on soil resistivity and depth. BS 7430 is how a competent person decides whether one rod suffices or several spaced rods are needed, and the result is confirmed by measuring Ra. ### Why do you flag the 2026 clause numbers? BS 7430:2026 is a brand-new edition that may renumber clauses from the 2011 version. We cite its principles but mark precise 2026 clause numbers Not confirmed rather than state numbers we have not verified against the published text. --- _Author: Martin — qualified UK electrician (BEng Mech Eng; vehicle mechanic)._ _Last reviewed: 14 June 2026. Written against: BS 7671:2018 + A4:2026._ _Status: published. General information, not project-specific design advice._ _[How we source this](/methodology) — evidence hierarchy, confidence flags and source policy._ ## Sources 1. BS 7430:2026 — Protective earthing of electrical installations. Code of practice (BSI) — https://knowledge.bsigroup.com/products/code-of-practice-for-protective-earthing-of-electrical-installations (cited by reference only; 2026 clause numbers Not confirmed) 2. BS 7671:2018+A4:2026 Reg 542.2.2 / 542.2.4 / Table 41.5 — IET/BSI (cited by clause only)