Earthing Systems: TN-S, TN-C-S and TT Explained
The earthing system decides where your circuit's fault current goes home — a separate earth conductor on TN-S, a combined neutral-and-earth on TN-C-S/PME, or your own electrode in the ground on TT — and it changes your Ze, your RCD requirements and your bonding.
Every installation needs a path back to the source for fault current, and the earthing system is how that path is provided. It's the first thing you establish on any job, because it sets your external earth fault loop impedance (Ze), determines whether you can rely on the supply for your earth at all, and drives decisions about RCDs and bonding downstream.
This guide explains the three arrangements you'll meet in UK domestic and commercial work — TN-S, TN-C-S (PME) and TT — how each provides the earth, the typical Ze you'd expect, and what each means for the rest of the installation.
Key takeaways
- TN-S has a separate earth conductor all the way back to the source — often the sheath of the old lead supply cable.
- TN-C-S (PME) combines neutral and earth in a single PEN conductor in the supply, split into separate N and E at the intake.
- TT has no earth from the supply — you provide an earth electrode in the ground and rely on RCDs for fault protection.
- Typical Ze: TN-S up to about 0.8Ω, TN-C-S up to about 0.35Ω, TT often several ohms to tens of ohms.
- TT installations require RCD protection because the loop impedance is too high for overcurrent devices to disconnect fast enough.
Why the earthing system matters
The letters describe the earthing arrangement: the first letter is how the supply is earthed (T = directly to earth), the second is how the installation's exposed-conductive-parts are earthed (N = to the supply's earth/neutral, T = to a local earth electrode). The system fixes where fault current returns and how low the loop impedance is, which is why identifying it is step one of any Ze measurement and any design.
It matters in three practical ways: it sets your Ze at the origin and therefore every circuit's achievable Zs; it decides whether you can lean on the supply for your earth or must make your own; and it drives your protective measures — most critically whether RCDs are essential rather than merely required for additional protection.
TN-S: a separate earth back to source
On TN-S the supply provides a separate protective earth conductor that runs all the way back to the source's earth, alongside the line and neutral. Classically the earth is the metallic sheath or armour of the older lead-covered service cable, connected to earth at the transformer. The installation's earth terminal connects to this dedicated conductor.
Because the earth path is a continuous metallic conductor back to source, Ze is low and stable — typically up to around 0.8Ω. TN-S is a straightforward arrangement to design around: your loop impedance is good, fault currents are healthy, and overcurrent devices can meet disconnection times on their own, though RCDs are still used for additional protection where required.
TN-C-S (PME): a combined PEN conductor
TN-C-S is the most common modern UK supply. The distributor combines neutral and earth into a single conductor — the PEN (protective earthed neutral) — in the supply network, and splits it into separate neutral and earth at your intake. This arrangement is called Protective Multiple Earthing (PME) because the PEN is earthed at multiple points along the network. Your earth terminal is provided at the cut-out by the distributor.
The combined conductor gives an even lower Ze — typically up to about 0.35Ω — so fault currents are high and disconnection is easy. The catch is the PEN conductor itself: if it becomes open somewhere in the network, the neutral current can raise the potential of every earthed part in the installation. That risk is why PME supplies demand robust main protective bonding (often larger than on other systems) and why PME earths are prohibited in some situations — notably certain caravan, boat and EV charging installations where a broken PEN could put people in contact with a rising earth potential.
The broken PEN risk
On PME, a lost neutral in the network can make your earthed metalwork live. It's why main bonding is heavier on PME and why a PME earth isn't allowed for some EV, caravan and marina installs.
TT: your own electrode in the ground
On TT the supply gives you no earth at all — the second T means the installation is earthed to its own local electrode. You drive an earth rod (or use a foundation earth), connect the installation's earth to it, and the fault path returns through the ground to the source's earth. Common where the supply is overhead and rural, and increasingly on some newer supplies where the distributor won't offer a PME earth.
The ground is a poor conductor, so Ze on TT is high — commonly several ohms and often into the tens of ohms, far too high for an overcurrent device to disconnect fast enough on an earth fault. This is the defining consequence: TT installations must rely on RCDs for fault protection, because only a residual current device will detect and clear an earth fault through such a high-impedance path within the required time. Every circuit on a TT system needs to sit behind suitable RCD protection.
TT means RCDs, not optional
On TT the loop impedance is too high for MCBs to clear an earth fault in time. RCDs aren't additional protection here — they're the fault protection. No working RCD, no safe TT circuit.
Identifying the system and its consequences
Identify the system before you design or test: look at the incoming supply and cut-out, check how the earth is provided (separate earth conductor, an earth terminal at the cut-out, or an earth rod), and confirm with a Ze measurement at the origin. A very low Ze suggests PME, a moderate one TN-S, a high one TT — but confirm it visually rather than inferring from the number alone.
The consequences ripple through the whole job. TN-S and TN-C-S give you low Ze and let overcurrent devices meet disconnection times, with RCDs for additional protection. PME adds bonding demands and some prohibited applications. TT forces RCD-based fault protection on every circuit and needs the electrode's resistance kept low and verified. Get the system right first and the rest of the design falls into place; get it wrong and your Zs figures and protection strategy are built on a false premise.
Frequently asked questions
What's the difference between TN-S, TN-C-S and TT?
TN-S has a separate earth conductor from the supply back to source. TN-C-S (PME) combines neutral and earth into one PEN conductor in the supply, split at the intake. TT has no supply earth at all — you provide your own earth electrode and rely on RCDs.
What is a PME (TN-C-S) supply?
Protective Multiple Earthing is a TN-C-S arrangement where neutral and earth are combined into a PEN conductor that's earthed at multiple points on the network. It gives a very low Ze but carries a broken-PEN risk, which is why it demands robust main bonding and is prohibited for some EV and caravan installations.
Why does a TT system need an RCD?
On TT the earth path goes through the ground via an electrode, giving a high loop impedance — often several ohms or more. That's too high for an overcurrent device to disconnect an earth fault in time, so an RCD is required to provide fault protection on every circuit.
What Ze should I expect on each system?
As a guide, TN-S is up to about 0.8Ω, TN-C-S up to about 0.35Ω, and TT is typically several ohms to tens of ohms. Always measure Ze at the origin rather than assuming — these are maximums and indicators, not guarantees.
From guidance to action
Related guides
Main Protective Bonding Explained
What main protective bonding does, conductor sizes, and where bonding is required to BS 7671.
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
RCD Types: AC, A, F and B Explained
The difference between Type AC, A, F and B RCDs and when each one is required.
