Why it matters
When a live conductor touches a metal case, the fault current must return to the source fast enough to trip a breaker or an RCD. The path it takes is defined by the earthing system — and each system needs different protection.
The three residential systems
- TN-S. A separate protective conductor (PE) comes in with the supply. Fault currents are large, breakers trip fast. The panel just connects PE to the PE busbar.
- TN-C-S. The supply arrives with a combined PEN conductor which is split into N and PE before your dwelling panel — at the building service entrance. Splitting PEN inside an apartment panel is prohibited: a broken PEN upstream would put mains voltage on every earthed case.
- TT. There is no usable PE from the supply; a local earth electrode provides the earth. The fault current is limited by the electrode resistance (tens of ohms) and is far too small to trip a breaker — an RCD at the panel input is mandatory, and the electrode resistance must be confirmed by measurement.
No PE conductor? Do not guess
If no separate green-yellow PE enters your home, the system cannot be deduced from the wall: guessing TN-C-S means an illegal PEN split, guessing TT assumes an electrode that may not exist. The earthing system must be designed and verified by a licensed electrician. PlanMyPanel marks such projects explicitly and assumes the worst case (TT) until then.