Every electrical installation has to answer one deceptively simple question: when something goes wrong and current escapes to exposed metal, how does it get back to the source fast enough to trip the protection? The internationally used answer is a family of "earthing systems" with cryptic names — TN-S, TN-C-S, TT, IT — defined in the IEC's wiring standards and adopted, with local variations, across much of the world.
This site has covered the practical side in installing protective grounding and grounding electrodes for private houses; this article gives you the map those articles fit into.
Decoding the letters
- First letter — the source: T (from French terre) means the transformer's neutral point is connected directly to earth. I means it is isolated from earth, or earthed only through a high impedance.
- Second letter — your installation: N means exposed metalwork is connected back to the source's earthed neutral point via a conductor. T means it is connected to a local earth electrode of its own.
- Suffixes: S means the neutral (N) and protective earth (PE) are Separate conductors; C means they are Combined into one conductor (PEN).
TN-S: separate neutral and earth all the way
The supply provides five conductors in three-phase form (or three in single-phase): phases, neutral, and a dedicated protective earth running back to the transformer. A fault to metalwork sees a low-impedance metallic loop, drives a large fault current, and the circuit breaker trips quickly.
Strengths: clean, predictable, no fault current flowing through combined conductors. Weaknesses: an extra conductor to run, and the earth path depends entirely on that conductor's integrity.
TN-C and TN-C-S: the combined PEN conductor
In TN-C the neutral and protective earth are one conductor (PEN) throughout — rare inside modern buildings. Far more common is TN-C-S: combined out in the distribution network, then split into separate N and PE at the service entrance. In the UK this arrangement is known as PME (protective multiple earthing).
Strengths: economical for the network, low-impedance fault path. The known weakness: everything hangs on the PEN conductor. If it breaks in the network, exposed metalwork in the installation can rise to dangerous voltage — which is why regulations hedge TN-C-S with extra bonding requirements and restrict it for special situations such as some outdoor and vehicle-supply installations. Those detailed rules are jurisdiction-specific; this is firmly electrician territory.
TT: your own earth electrode
The source is earthed, and your installation has its own earth electrode — rods, tapes or a grounding loop — with no metallic earth conductor from the supply. Fault current must return through the general mass of earth, so it is far smaller than in TN systems; often too small to trip an ordinary breaker.
That is why TT installations lean on residual current devices: an RCD detects a few tens of milliamps of leakage regardless of how weak the earth path is. If your installation is TT, working RCDs aren't an upgrade — they're the design. (See our comparison of RCD, RCBO and GFCI protection.)
Typical use: rural supplies, overhead networks, countries where the distributor doesn't export an earth.
IT: isolated from earth on purpose
In an IT system the source is unearthed or earthed through a deliberately high impedance. A single fault to earth produces almost no fault current — the system keeps running, an insulation-monitoring device raises an alarm, and maintenance finds the fault before a second one develops. That continuity is why IT systems serve hospitals' operating theatres, some industrial processes and other places where an unplanned trip is worse than a fault.
The price is complexity: insulation monitoring, trained staff and strict rules about clearing the first fault promptly.
Comparison at a glance
| System | Earth path for a fault | Main protection | Typical setting |
|---|---|---|---|
| TN-S | Dedicated PE conductor to source | Overcurrent devices (fast trip) | Urban networks, newer installations |
| TN-C-S (PME) | PEN conductor, split at entry | Overcurrent devices + bonding rules | Very common in modern distribution |
| TT | Local electrode through soil | RCDs (mandatory in practice) | Rural/overhead supplies |
| IT | Almost none on first fault | Insulation monitoring + planned maintenance | Hospitals, critical industry |
Why you should care which one you have
The earthing system decides what "safe" looks like for everything downstream: whether RCDs are optional or essential, how bonding must be done, what happens during a network fault, and how additions like EV chargers or outbuilding supplies must be designed. It's determined by your electricity network and verified at your service entrance — not something to guess from an article. If you don't know what your installation uses, that's a good question for your next electrical inspection.
Safety note: earthing terminology here follows IEC usage; national standards implement it with local rules and exceptions that change over time. Nothing in this overview replaces the judgement of a licensed electrician working to your local regulations.
