Earthing · 21 July 2026 · For contractors and project engineers
Whether a circuit disconnects safely in a fault, what protective device you need, and how much bonding the job requires all depend on one thing decided before you draw a single cable: the earthing system. BS 7671 describes it with a short letter code — TN-S, TN-C-S, TT, IT — and getting it right is the foundation of a safe design. Here's what each code means, how to tell which you have, and why it matters.
The letters come from IEC 60364 and read left to right:
Neutral (N) and protective earth (PE) are separate conductors all the way from the transformer to the installation. The earth path is traditionally the metallic sheath or armour of the service cable. It gives a reliable, low-impedance earth and avoids the combined-conductor risk of PME. Common on older urban supplies.
The supply brings a single PEN (combined protective-earth-and-neutral) conductor to the property, and it is split into separate N and PE at the origin. This is Protective Multiple Earthing (PME), the standard modern UK arrangement. It usually has the lowest external loop impedance (Ze), so faults clear fast — but it carries the open-PEN risk: if the combined conductor breaks out in the network, the installation's earthed metalwork can rise toward mains voltage. That single risk drives the strict main-bonding requirements on PME supplies, and the special rules for outdoor EV charging.
The DNO provides no earth; the installation makes its own connection to earth with a local electrode (usually one or more rods). Common in rural areas and where the supply is overhead. Because an electrode's resistance is high compared with a metallic supply earth, the earth-fault loop impedance is high — too high for an overcurrent device to disconnect in time — so a TT system needs an RCD for automatic disconnection, sized so that Ra × IΔn ≤ 50 V.
The supply is unearthed or earthed only through a high impedance, and exposed metalwork is earthed locally. A first fault doesn't cause a large fault current, so supply continuity is maintained and an alarm is raised — which is why IT is used in specialist settings such as some medical, industrial process and safety-critical installations. It's uncommon in general UK distribution.
| System | Installation earth | Typical max Ze | RCD for ADS? |
|---|---|---|---|
| TN-S | Supply earth (separate) | ≈ 0.8 Ω | Often not required |
| TN-C-S (PME) | Supply earth (PEN, split) | ≈ 0.35 Ω | Often not required |
| TT | Own electrode | ≈ 21 Ω (declared) | Yes |
| IT | Own electrode | — | Special (fault monitoring) |
Ze figures are the DNO's typical declared maxima — always use the value measured or declared for the actual supply.
The earthing system sets the earth-fault loop impedance you start from, which decides whether an overcurrent device disconnects in time or whether you need an RCD; it sets the main protective bonding requirements (heaviest on PME); and it constrains specific applications such as EV charging (Section 722) and swimming pools. It also feeds straight into cable sizing — the fault level and the Zs checks in a cable calculation depend on it. On HV sites the earthing arrangement is a study in its own right; see earth potential rise and our earthing study design service.
Look at the supply intake: a TN-C-S (PME) supply has the main earth taken from the incoming neutral/PEN at the cut-out; a TN-S supply has a separate earth terminal, traditionally the lead sheath of the service cable; a TT supply has no earth from the DNO and relies on a local earth electrode. If in doubt, measure the external loop impedance (Ze) and ask the DNO — do not assume.
In TN-S the neutral and protective earth are separate all the way from the source. In TN-C-S the supply combines them into one PEN conductor and they are only separated at the origin of the installation. TN-C-S usually has the lowest earth loop impedance but carries the open-PEN (broken combined conductor) risk.
On a TN-C-S (PME) supply the neutral and earth share one conductor up to the property. If that PEN conductor breaks in the network, the installation's earth can rise toward mains voltage, making earthed metalwork live. This is why PME systems require robust main protective bonding, and why outdoor EV charge points on PME need open-PEN protection or a TT arrangement.
Almost always, yes. A TT earth electrode gives a high earth-fault loop impedance, so an overcurrent device usually cannot disconnect fast enough. BS 7671 requires an RCD for automatic disconnection, sized so that Ra × IΔn ≤ 50 V (the electrode resistance times the RCD rated residual current stays below the 50 V touch-voltage limit).
Yes, but not by simply bonding to the PME earth outdoors. BS 7671 Section 722 requires either a charge point with built-in open-PEN detection, or a dedicated TT earth electrode for the charger, to remove the shock risk if the PEN conductor fails.
For the authoritative requirements, see BS 7671 Chapter 41 and the IET's Wiring Matters series.