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.
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.
Some details below depend on sources still being verified against the published standard, so we mark them Not confirmed rather than guess:
- BS 7430:2026 precise clause numbers (e.g. the rod-resistance formulae, 2011 ed. 9.5.3/9.5.4) — the 2026 edition is < 1 month old; numbering Not confirmed until checked against the published text.
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.
What the diagram shows: 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. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
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.
What the diagram shows: 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. Legend (stated in words, not colour alone): L = line/live conductor; N = neutral; E/CPC = earth / circuit protective conductor.
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
- 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)
Electrode design, installation and the Ra measurement are competent-person work. The island must be designed and proven by test, never a recipe.
- 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
Confidence: Inference rolled up across the clauses cited above (the strictest state wins).
Frequently asked questions
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.
- Last reviewed
- 14 June 2026
- Written against
- BS 7671:2018 + A4:2026
- Reviewed by
- Martin (qualified UK electrician)
- Next review due
- 14 December 2026
General information, not project-specific design advice. Standards are cited by reference only and never reproduced. How we source this.
References & sources (2)
- BS 7430:2026 — Protective earthing of electrical installations. Code of practice (BSI) — cited by reference only; 2026 clause numbers Not confirmed
- BS 7671:2018+A4:2026 Reg 542.2.2 / 542.2.4 / Table 41.5 — IET/BSI — cited by clause only