Quick answer: TN-S is an earthing arrangement where the property's earth is provided by a separate conductor from the supply company, usually the metal sheath of the incoming cable, all the way back to the transformer. The live, neutral and earth stay separate throughout. It is one of three common UK arrangements alongside TN-C-S (PME) and TT, and you identify it by an earth cable clamped to the lead sheath of the service cable at the cutout.
Every UK installation has an earthing arrangement, and which one you have changes how bonding, RCDs and shock protection are designed. TN-S is the older of the two "TN" types. Here is what it means and how to tell it apart from the others.
What TN-S means
The letters describe where the earth comes from. T means the supply transformer's star point is connected to earth. N means your installation's exposed metalwork is connected to that supply earth (rather than its own electrode). The final S means the neutral and the protective earth are separate conductors all the way from the transformer to your property.
In practice the earth path is the metallic sheath or armour of the supply company's underground cable. Your main earthing terminal is connected to it at the service head.
TN-S vs TN-C-S vs TT at a glance
| Arrangement | Earth source | How you spot it |
|---|---|---|
| TN-S | Separate earth conductor (cable sheath) back to transformer | Earth clamped to the lead/steel sheath of the service cable |
| TN-C-S (PME) | Earth derived from the supply neutral at the cutout | Earth tail taken from the neutral block; "PME" label common |
| TT | Your own earth rod in the ground | Earth cable runs to an electrode outside; RCD essential |
How to identify TN-S at the cutout
- Find the service head (the supplier's fuse) near the meter.
- Look for the main earth conductor. On TN-S it is clamped to the metallic sheath of the incoming cable, not to the neutral.
- If the earth instead comes off the neutral terminal block, you have TN-C-S (PME).
- If the earth runs out to a rod in the ground, you have TT.
- When in doubt, an electrician confirms it with an earth-loop impedance (Ze) test at the EICR.
Why the arrangement matters
The earthing type sets the earth-fault loop impedance, which decides how fast a protective device disconnects under fault, and it drives the main bonding and RCD strategy under BS 7671. TN-S typically gives a low, stable Ze. It avoids the broken-neutral risk that makes PME unsuitable for some outdoor and special locations, which is why caravans, EV chargers and some agricultural setups are treated carefully on PME supplies. None of this is DIY territory: it is assessed and tested by a qualified electrician.
What it means for your installation
Whatever the arrangement, the fixed parts have to be right: correctly rated main bonding to gas and water, RCD protection on the circuits that need it, and accessories in good order. If yours are cracked or scorched, replace like for like with quality UK stock from our British General range. Landlords: the inspector records the earthing arrangement and Ze on the EICR, so it is worth knowing which you have.
Frequently asked questions
How do I know if I have TN-S or TN-C-S?
Look at where the main earth connects at the service head. TN-S takes earth from the metal sheath of the incoming cable; TN-C-S (PME) takes it from the supply neutral. An electrician confirms it with a Ze test.
Is TN-S better than PME?
TN-S avoids the broken-neutral hazard that PME carries, which makes it preferable for some outdoor and special installations. For ordinary indoor circuits both are safe when designed to BS 7671. You generally cannot choose; it depends on the supply you are given.
Can TN-S become TN-C-S?
Yes. When supply companies replace old cables, a former TN-S supply is often converted to TN-C-S (PME). After any such change the installation's bonding should be reassessed.
Does TN-S need an earth rod?
No. TN-S takes its earth from the supply cable sheath, so no electrode is required. Only TT installations rely on an earth rod.
Keep your installation compliant
- BG sockets and switches — like-for-like replacements for damaged accessories
- Fused connection units for fixed appliances
- Emergency lighting for HMO common areas
Read the EICR landlord checklist or the BS 7671 bathroom zones guide for more on UK wiring compliance. Managing several properties? Open a trade account for trade pricing. This guide is general information, not a substitute for advice from a qualified electrician.