Why the earthing system comes first
When a live conductor touches a metal case, the fault current has to get back to the source fast enough for a protective device to disconnect. The route it takes is set by the earthing system — also called the earthing arrangement or system of earthing.
Everything downstream follows from it: whether a breaker alone can clear a fault, whether an RCD is mandatory, how N and PE are arranged in the board. Get the system wrong and you design protection for a fault current that never flows.
Reading the letters
The codes look cryptic but follow one rule — each letter answers a question.
- First letter: how is the source earthed? T (from French *terre*) means one point of the supply, normally the transformer star point, is connected directly to earth. I means the source is isolated from earth, or connected through a high impedance.
- Second letter: how are the exposed metal parts earthed? T means through a local electrode at the installation. N means through a protective conductor back to the earthed source.
- Following letters: how are N and PE arranged? S (separate) means two distinct conductors the whole way. C (combined) means one shared PEN conductor.
So TN-C-S reads as: source earthed, casings earthed via the protective conductor, combined for part of the route and separate after that.
TN-S
PE and N are separate all the way from the transformer to the socket. A line-to-case fault produces hundreds of amps, and the circuit breaker clears it in a fraction of a second.
In the board it is the simple case: PE to the earth bar, N to the neutral bar, never linked. In the UK a TN-S supply typically arrives with the earth taken from the sheath of the service cable.
TN-C
One PEN conductor does both jobs, neutral and protective. It saves a conductor and was common for decades.
The problem is what happens when it breaks: everything earthed downstream of the break loses its reference and can rise to mains voltage through connected appliances. Hence the modern rules:
- A PEN conductor is only permitted at generous cross-sections — commonly 10 mm² copper or 16 mm² aluminium and above.
- PEN is not permitted in final circuits. Combining N and PE at a socket outlet belongs to history.
- No PEN downstream of an RCD: the RCD would either trip immediately or not work at all.
TN-C therefore survives in distribution networks and in old installations, not in new final circuits.
TN-C-S, also known as PME
The common arrangement in much of Europe and the UK. A PEN conductor runs up to the building, where it is split into N and PE at the service head or main earthing terminal. From that point on, the installation is effectively TN-S. In the UK this supply is called PME — protective multiple earthing.
Two rules carry the whole thing:
- Split once, as early as possible, at the main earthing terminal — never inside a downstream consumer unit.
- After the split, N and PE must never be reconnected. Any later bridge sends load current down the protective conductor and blinds every RCD in the installation.
One practical caveat worth knowing: because a broken PEN raises the voltage of every earthed part, PME supplies get special treatment for outdoor equipment such as EV charge points. Local rules either require an earth electrode or a device that disconnects on an open PEN, so check what your national standard says before wiring a charger outdoors.
TT
No usable PE arrives from the supply. Earthing is provided by a local electrode — a rod or foundation earth. The fault current returns through soil, which limits it to a few amps: far below what a breaker needs to trip.
The consequence is simple. In a TT system an RCD is not optional, it is the disconnecting device, and the electrode resistance must be measured, not assumed. TT is common for rural properties, outbuildings and older detached houses.
IT
The source is unearthed or earthed through a high impedance. A first fault drives almost no current and disconnects nothing — an insulation monitor raises an alarm instead. That is the point: in an operating theatre or certain industrial processes, the supply must survive the first fault. The second fault behaves like a short circuit and must be cleared. IT does not appear in ordinary housing.
How to identify your system
Guessing is the dangerous part, because a socket without an earth pin proves nothing on its own.
- Look at the service head and main earthing terminal: a visible split of a combined conductor into N and PE means TN-C-S.
- Ask the network operator — they know how the supply is arranged.
- Have an electrician measure it, especially the electrode resistance if TT is suspected.
If no separate protective conductor arrives, "it is probably TN-C-S" is the classic and risky assumption: splitting a PEN inside a dwelling board is prohibited, and assuming TT presumes an electrode that may not be there.
What it means for your board
- Separate bars for N and PE downstream of the split, with no link between them anywhere.
- One neutral per circuit. Borrowed neutrals shared between circuits are the most common cause of unexplained RCD tripping.
- In TT, an RCD is part of the basic protection, not an extra. In TN systems it is still required for socket and lighting circuits by most national rules.
- The earthing system also decides which surge protection arrangement applies.
What PlanMyPanel does
The wizard asks for the earthing system and then uses it throughout the checks: in TT it requires an RCD at the incomer, in TN-C-S it verifies that no PEN split is planned inside the dwelling board. When the system is unknown, the project is flagged and calculated for the worst case — TT — until an electrician confirms it.