An earth electrode on a TT system must have a resistance low enough that RA × IΔn does not exceed 50 V (Regulation 411.5.3 of BS 7671), where RA is the resistance of the electrode and its protective conductor and IΔn is the rated residual current of the RCD giving fault protection. For a 30 mA RCD that calculates to 1667 Ω, but IET Guidance Note 3 and the On-Site Guide advise that values above 200 Ω are liable to be unstable, so 200 Ω is the working maximum. The resistance is measured with a three-terminal earth electrode tester by the fall-of-potential method or, for RCD-protected installations, with a loop impedance tester at the origin with the earthing conductor disconnected.
- On TT the installation's earth is a local electrode; every earth fault current flows through the ground.
- The rule is RA × IΔn ≤ 50 V. The practical limit is 200 Ω for stability; aim well below it.
- Fall of potential with a three-terminal tester measures the electrode directly. The loop-tester method is acceptable for RCD-protected installations and reads slightly high.
- RCDs are the fault protection on TT. A time-delayed 100 or 300 mA device at the origin with 30 mA devices downstream is the usual arrangement.
- TT is the right choice where the distributor provides no earth or where PME cannot be exported safely: EV charging under Section 722, caravan pitches under Section 708, many outbuildings.
Why does a TT installation need an earth electrode?
In a TT system the supply neutral is earthed at the transformer and the installation is earthed separately through its own electrode, with no metallic earth path between the two; the distributor's earth terminal is either not provided, as on many rural overhead supplies, or deliberately not used. Every earth fault returns through the mass of earth via the electrode, so the loop impedance is tens or hundreds of ohms rather than fractions of an ohm (see What is Ze? External earth fault loop impedance). On a TN-C-S supply with Ze of 0.35 Ω a line-to-earth fault draws several hundred amps and a 32 A Type B MCB trips in milliseconds; on a TT supply with a 100 Ω electrode the same fault draws about 2.3 A and exposed metalwork sits near 230 V until something else disconnects it. That is why Regulation 411.5.3 makes RCDs the normal fault protection on TT systems.
What resistance is acceptable?
| Reference | Requirement | Meaning in practice |
|---|---|---|
| Regulation 411.5.3 | RA × IΔn ≤ 50 V | The touch voltage on exposed metalwork during a fault must not exceed 50 V before the RCD trips |
| Table 41.5 | Maximum Zs for RCDs on TT: 1667 Ω (30 mA), 500 Ω (100 mA), 167 Ω (300 mA), 100 Ω (500 mA) | The arithmetic of 50 V / IΔn |
| IET Guidance Note 3 and On-Site Guide | Values above 200 Ω may not be stable | 200 Ω is the working maximum; lower is better |
The 200 Ω figure is guidance rather than a regulation, but it is the number a scheme assessor will look for. Soil resistivity rises as ground dries or freezes, so an electrode reading 180 Ω in March can read 400 Ω in August. Where the reading is above about 100 Ω, add a second rod in parallel or a longer coupled rod before signing the certificate.
RA × IΔn uses the RCD providing fault protection for the circuit concerned. With a 100 mA time-delayed RCD at the origin and 30 mA devices downstream, design the distribution part to the 100 mA device (500 Ω) and expect the 200 Ω stability limit to govern anyway. Where the origin device is 300 mA, Table 41.5 gives 167 Ω, which becomes the limit.
How do I test an earth electrode?
There are two accepted methods. Both need the installation isolated, because the electrode must be disconnected to be measured on its own, and disconnecting the earthing conductor on a live TT installation removes the only earth path.
| Method | Instrument | What it measures | When to use it |
|---|---|---|---|
| Fall of potential (three-terminal) | Earth electrode tester with two auxiliary spikes | The electrode resistance directly, independent of the supply | New installations, EICs, larger electrodes, where an accurate value is wanted |
| Loop impedance at the origin | Earth fault loop impedance tester | The whole loop: electrode, ground path, transformer earth and supply | Periodic inspection of RCD-protected installations (GN3) |
Fall-of-potential method step by step
- Isolate the installation and disconnect the earthing conductor from the electrode. Confirm the supply is dead; there is no earth while the conductor is off.
- Connect the instrument's E (or C1/P1) terminals to the electrode.
- Drive the current spike (C2) into the ground in a straight line away from the electrode, typically 30 to 50 m for a single rod.
- Drive the potential spike (P2) between them at about 62 per cent of that distance, in line with both.
- Take a reading, then move the potential spike about 10 per cent of the distance towards the electrode and read again, then 10 per cent away and read again.
- If the three readings agree within a few per cent, the middle reading is the electrode resistance. If not, the spikes are inside the electrode's resistance area; increase the distance and repeat.
- Reconnect the earthing conductor, confirm the connection is sound and re-energise. Record the value, the method and the soil conditions.
Loop-tester method step by step
- Isolate the installation at the main switch and disconnect the earthing conductor from the main earthing terminal so the bonded metalwork is not in parallel with the electrode.
- Perform a Ze test between the incoming line and the electrode side of the disconnected earthing conductor, using the no-trip setting if any RCD is in the path.
- Record the value as the electrode resistance, noting the method. It includes the supply source resistance and is slightly higher than RA, which errs on the safe side.
- Reconnect, check the connection and re-energise.
Guidance Note 3 accepts this method where fault protection is by RCD; the Ze and Zs procedures are at Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained. A Zs reading at the far end of a final circuit is not an electrode test: it passes the 1667 Ω limit comfortably but says nothing about stability or corrosion. Test the electrode, then the RCDs as at RCD Testing Explained: Types, Test Currents and Trip Times.
What types of electrode are permitted?
Section 542 lists the permitted types:
| Electrode type | Notes |
|---|---|
| Copper-bonded steel rod | The domestic standard; 1.2 m sections coupled to depth, or rods in parallel spaced at least the driven depth apart |
| Copper tape or bare conductor buried horizontally | Where rock prevents driving rods; long trench runs |
| Foundation earth (structural steel or a conductor cast in concrete) | Excellent low-resistance electrode in new buildings, planned at the design stage |
| Metal water pipes | Not permitted as the electrode; metal gas pipes and other services are also excluded |
The connection must be accessible for inspection and test, sound and protected against corrosion, which usually means an inspection pit with a lid and a clamp to BS 951. Regulation 514.13.1 requires a durable label reading "Safety Electrical Connection – Do Not Remove" at the connection. On the EIC and EICR, tick TT as the earthing arrangement and complete the electrode particulars (type, location, measured resistance), Ze and the origin RCD rating and type, noting the test method; see Electrical Installation Certificate (EIC) Explained. On an EICR, a corroded clamp or a resistance above 200 Ω is an observation; an electrode that cannot be found is FI.
How is fault protection arranged on a TT installation?
Because RCDs are the fault protection, their arrangement is a design decision. The common domestic arrangement is a 100 mA time-delayed (S-type) RCD at the origin with 30 mA RCBOs on every final circuit. The delayed device protects the tails, main switch and distribution circuits, and its delay means a fault on one final circuit trips only its own 30 mA device. The main switch should be double-pole, because with a high loop impedance the neutral cannot be assumed to be near earth potential, and a fault to the metal case of a consumer unit upstream of the RCD has no protection except the distributor's fuse, which is why an upstream time-delayed RCD in its own enclosure is common with a metal board under Regulation 421.1.201. See What is an RCD? Types AC, A, F, B and RCBOs explained.
When is TT the right choice?
TT is mandatory where the distributor provides no earth terminal, and is chosen where a PME (protective multiple earthing) earth cannot safely be exported, because under an open-circuit PEN conductor fault the PME earth can rise towards line voltage and every bonded metal part goes with it.
| Situation | Position under BS 7671 |
|---|---|
| Outdoor EV charging equipment | Section 722: a PME earth may only be used if one of the specified conditions is met (open-PEN detection device, an earth electrode with a limited touch voltage, or equivalent). Many installers TT the charging point |
| Caravan pitches and marinas | Section 708 and Section 709 prohibit the PME earth for the pitch supplies; TT with local electrode is the norm |
| Detached outbuildings, hot tubs, swimming pools | Not prohibited, but a TT island on the outbuilding avoids exporting the PME potential to metalwork in contact with the ground |
Where part of an installation is TT and part TN, keep the two earths separate: the outbuilding's electrode and metalwork must not be in contact with anything bonded to the PME earth, including an SWA armour, which must be isolated at the outbuilding end. Record the arrangement clearly; it is a common FI on later reports.
How Certio helps
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