Every installation's safety depends on how it's earthed. In the UK you'll mostly meet three systems: TN-S, TN-C-S and TT. Learn what the letters mean and the rest falls into place.
What the letters mean
- First letter, the supply: T (from the French terre) means the supply is directly connected to earth at the source.
- Second letter, the installation: N means exposed metalwork is earthed through the supplier's network; T means it's earthed through the installation's own electrode.
- After the dash: S means separate neutral and protective conductors; C means combined.
The three systems side by side
| TN-S | TN-C-S (PME) | TT | |
|---|---|---|---|
| How it's earthed | Separate earth from the supplier, usually the cable sheath | Combined neutral and earth (PEN) in the supply, split at the cut-out | Your own earth electrode, such as an earth rod |
| How to spot it | Earthing conductor clamped to the supply cable's sheath | Earthing conductor connected to the neutral block or a terminal on the cut-out, PME label | No supplier earth; an earth rod and usually an RCD at the origin |
| Typical max Ze | 0.8 Ω | 0.35 Ω | Depends on the electrode (usually tens to hundreds of ohms) |
| Where you'll see it | Older urban properties | Most modern UK properties | Rural properties, overhead supplies, some outbuildings |
| Main protection | Circuit breakers or fuses, plus RCDs where required | Circuit breakers or fuses, plus RCDs where required | RCDs, because high impedance stops breakers tripping fast on earth faults |
TN-C-S: the special risk
PME is the most common system, but it carries one specific risk. If the combined PEN conductor breaks in the supply, earthed metalwork in the installation can rise towards line voltage. That's why main protective bonding matters so much on PME supplies, and why BS 7671 has extra requirements for things like EV charge points and some outdoor installations. Check the current regulations before connecting outside equipment to a PME earth.
TT: the electrode is everything
On a TT system, earth fault current returns through the ground, so the loop impedance is high. Breakers alone usually can't disconnect fast enough, which is why RCDs do the job. Test the electrode resistance and keep it as low and stable as possible.
Quick test yourself
- You find the earthing conductor clamped to the lead sheath of the supply cable. Which system? (TN-S)
- A farm building has an earth rod and no supplier earth. Which system, and what device provides fault protection? (TT; an RCD)
- Why is a broken PEN conductor dangerous? (Earthed metalwork can become live)
Learn it properly
Consumables Pro's earthing systems visualisation lets you switch between TN-S, TN-C-S and TT and watch the fault current path change. Start free for 14 days.
Revise it with the Level 2 Diploma course
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