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Intermediate10 min readUpdated 2024

Earthing Systems — TN-S, TN-C-S and TT Explained

How the supply earth is arranged, how to tell which one you have, and what each means for protection.

BS 7671:2018+A2:2022 Section 312IET On-Site Guide
The earthing system decides how a fault current gets back to the source, how low the earth-fault loop impedance is, and therefore what protection you can rely on. The first letter is the source earth, the second is the installation's exposed metalwork. Get the arrangement wrong in your head and every Zs limit and RCD decision that follows will be wrong too.

The three you'll meet

EARTHING ARRANGEMENTS · SUPPLY → INSTALLATIONSOURCELNPECUTN-SSeparate PEback to sourceSOURCELPENCUMETTN-C-S (PME)PEN split atoriginSOURCELNCUTTLocal earthelectrode

The letters describe the earthing at the source (first letter) and at the installation (second letter). T = a direct connection to earth; N = connected to the supply's earthed point via the network.

TN-S — separate earth back to source

The protective conductor is separate from the neutral all the way back to the transformer, usually via the metallic sheath of the supply cable. Fault current has a dedicated low-impedance metallic path, so earth-fault loop impedance (Ze) is typically low.

Typical Ze≤ 0.8 Ω (0.35 Ω is a common design figure)
Earth sourceSupply cable sheath / separate PE

TN-C-S (PME) — combined then split

The supply uses a combined neutral-and-earth conductor (PEN) that is split into separate N and PE at the origin of the installation (the main earth terminal). This is Protective Multiple Earthing (PME) and it is now the most common arrangement for new domestic supplies.

Typical Ze≤ 0.35 Ω
Earth sourceSupplier's PEN, split at the MET
PME has a catch
If the supplier's PEN conductor goes open-circuit, exposed metalwork can rise to a dangerous potential. That's why PME earths are prohibited or restricted in some situations — notably caravans, boats and (unless specific conditions are met) EV charging points, where an open-PEN detection device or a TT island is used instead.

TT — your own electrode

There is no metallic earth back to the source. The installation makes its own connection to earth through a local electrode (a rod). The earth-fault loop is completed through the ground itself, so the loop impedance is high and often variable with weather.

Typical ZeOften 20–200 Ω+ (soil dependent)
ProtectionRCD is essentially mandatory — the loop is too high for overcurrent devices to disconnect in time
Why TT relies on RCDs
With a high loop impedance, a fault won't draw enough current to trip an MCB quickly. A 30 mA (or 100 mA S-type upstream) RCD detects the leakage directly and disconnects regardless of loop impedance.

How to identify the system on site

  • Look at the supply head: a separate earth from the cut-out sheath suggests TN-S; an earth link from the neutral block suggests TN-C-S/PME.
  • An earth electrode (rod) with an earthing conductor to the MET, and no supplier earth, points to TT.
  • PME supplies are usually labelled at the cut-out. When in doubt, ask the DNO — do not guess.
  • Confirm by measuring Ze at the origin with the main switch off and the earthing conductor disconnected (safe-isolation applies).
Reference material for qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.

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The correct sequence for proving a circuit dead before you work on it. Non-negotiable.