In a Schéma III installation, appliances are earthed through the neutral conductor instead of a separate protective earth conductor. This was standard practice until the 1980s. Since 1985 it is no longer considered state of the art — and for the past few years, it has changed how often your OIBT inspection (the Swiss Low-Voltage Installations Ordinance) is due.
There are no half measures. As soon as any part of the installation is wired in Schéma III, the inspection cycle of the entire installation falls to five years, even if everything else is TN-S.
Many apartments renovated after 1985 are already TN-S, supply included. Conversely, a building with a cosmetic refurbishment may have kept its old wiring. Only the inspection settles it.
Renovating an apartment while leaving the supply in Schéma III means closing the walls over the problem. The cheapest moment to rewire is when they are already open.
The facts, with their source. Nothing more.
Since 1 January 2018, parts of an installation wired in Schéma III are inspected every 5 years. Since 1 July 2021, this 5-year cycle applies to the entire installation as soon as it contains any Schéma III part. That is section 4.2 of ESTI directive no. 225 (ESTI = Swiss Federal Inspectorate for Heavy Current Installations).
If there is no defect, we issue the safety report, with a five-year period. It is the last one: at the next inspection, the installation will have to be TN-S. Those five years are the time you have to organise the work.
ESTI notes that these installations no longer correspond to the state of the art and present a significantly higher risk. It follows that they will have to be remediated and converted to the TN-S system. The five-year period cannot be renewed indefinitely: it leaves time to organise the work, not to postpone it.
The owner is responsible for the safety of their installation and must have defects rectified without delay (art. 5 para. 3 OIBT). If they knowingly leave an installation in Schéma III, they bear the civil and criminal consequences in the event of an accident.
Holders of an inspection authorisation are required to draw owners' attention to this responsibility and to recommend remediation measures. That is why we tell you in writing, dated, with the report — and not only verbally on the doorstep.
Sources: ESTI directive no. 225, version 0824, in force since 1 August 2024 · Ordinance on Low-Voltage Electrical Installations (OIBT, SR 734.27) · Ordinance on Heavy Current Installations (SR 734.2).
Two exceptions the directive names, and that people often forget to tell you.
The connection between the consumer's main overcurrent protection and the panel does not fall within the scope of the directive. It is therefore not what moves your installation to 5 years.
Installations wired in the TN system whose conductors have a cross-section of at least 10 mm² in copper (16 mm² in aluminium) are not covered either.
A properly built Schéma III installation does protect. The problem is what happens next.
A broken neutral, a phase and neutral swapped in a junction box, a forgotten bridge in a socket: in all three cases, the metal casing of connected appliances can become live. And all three situations do happen, because these installations have been modified for fifty years, often by non-professionals.
You cannot simply add a residual current device in the panel to make up for it. Sockets with a built-in RCD protect the socket, not the wiring, not the light points, not permanently connected appliances. And fitting an RCD (FI) on SH3 wiring is prohibited, as it cuts the protection when it trips.
The insulation measurement, which is what allows the condition of an installation to be judged, can only be carried out with disproportionate effort in a Schéma III installation. So we measure poorly what we cannot see.
Solar PV, battery, EV charging station, smart meter: this equipment copes badly with a Schéma III installation, and many of these old circuits still have cotton sheathing, which ages badly, and are sized for 6 A at most… One kettle and your fuse blows!
Five years is not an eternity. During that period, every alteration — a kitchen, a bathroom, a partial renovation — requires an installation notification and an acceptance inspection. That many visits, that many costs, on an installation that will have to be redone anyway.
The reverse reasoning is often the right one: if work is planned, use the opportunity to redo the panel and the wiring. The extra cost at the time is almost always lower than the sum of the visits you avoid.
We have nothing to sell in this. We do not carry out the work, and we earn nothing on someone else's. That is precisely what makes the advice usable.
A safety report is issued per part of the installation: in a condominium (PPE), one report for the common areas and one per apartment. A unit wired in Schéma III moves that unit to 5 years — not necessarily the whole building. Knowing which ones are affected changes how you budget.
The inspection tells you for certain. One reliable clue in the meantime: if the inspection cycle of your installation has already been reduced to 5 years, some parts are still Schéma III. A yellow neutral conductor can be another clue.
No, not immediately. If there is no defect, your installation can be validated once more, for five years at most. The switch to TN-S will be required at the next inspection. Those five years are therefore the time you have to organise it — ideally by making it coincide with work that is already planned.
Yes, if there is no defect that prevents safety from being guaranteed. The report then carries a five-year period. If safety is not assured, no report is issued until the defects have been rectified.
No, not in a Schéma III installation. Residual current protection, RCD / FI, is not really achievable there: sockets with a built-in RCD cover neither the wiring, nor the lighting, nor permanently connected appliances. Moreover, fitting an RCD on SH3 wiring is prohibited, as it cuts the protection when it trips.
The switch to TN-S will be required for the installation to be validated again. That is why it is better to put this work in the budget now rather than discover it on the day of the inspection.
One inspection, a report the same day when the installation is compliant, and a clear answer on Schéma III. Flat rates announced in advance, with no mileage surcharge.
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