Determining the earthing system on an unknown installation — TN-S, TN-C-S, TT or IT?

The §411 earthing guide explains what the five earthing systems (TN-S, TN-C, TN-C-S, TT, IT) are and how they differ. This article covers the question that precedes that in practice: how do you establish, on an existing, undocumented installation, which system you are actually dealing with — for example in an older industrial building, a rented property without as-built drawings, or an agricultural site that has been rebuilt over the years.

Why this step cannot be skipped

The earthing system directly determines which protective measure is in effect and how it must be assessed: a loop-impedance measurement (see the loop-impedance (Zs) guide) is meaningful in a TN system, but means something entirely different in a TT system (there, the earth-electrode resistance of the site's own electrode is the determining factor, see the earth-resistance measurement guide). An RCD in a TT system is not optional but mandatory. Without first establishing the system, no follow-up measurement can be correctly interpreted.

Step 1 — Look at the main supply and the number of conductors

Count the number of conductors entering the meter cabinet from the grid operator:

Four conductors on a 3-phase supply (three phases + PEN), or two conductors on a single-phase supply (phase + PEN), points to a TN-C supply that is split into separate N and PE at the meter cabinet — this is TN-C-S (often called PME in the Netherlands), by far the most common system in Dutch homes and many smaller commercial buildings.
Five conductors on a 3-phase supply (three phases + separate N + separate PE), or three conductors on a single-phase supply (phase
- separate N + separate PE), points to TN-S from the origin — more common in newer industrial installations or a building with its own medium-voltage transformer.
If a direct PEN/PE supply from the grid operator is entirely absent, and instead there is a dedicated earth electrode at the building (earth rod, foundation earth electrode) that is not connected to a supplied earth — this points to TT.

Step 2 — Locate the PEN split point

In a TN-C-S installation there is a point, before or inside the meter cabinet, where the PEN conductor is split into a separate N busbar and a separate PE busbar (main earthing terminal). This split point is usually clearly recognisable: a thicker incoming conductor connecting to both the neutral busbar and the earth busbar. If no such split point can be found and N and PE run fully separate from the origin, this points to TN-S.

Step 3 — Measure between N and PE (in case of doubt)

In case of doubt — for example when the meter cabinet is wired in a confusing way — a measurement between the N busbar and the PE busbar, at a point after the split point and with the installation under normal load, gives an additional indication:

A negligible, stable voltage between N and PE under normal load is consistent with a correctly separated TN-S/TN-C-S system after the split point.
A noticeable voltage between N and PE after the split point that varies with the load points to a fault (for example a second, unintended connection between N and PE further into the installation) — this is not an indication of the system type, but a rejection point in its own right.

Note: this N-PE measurement only confirms whether the separation after the split point is intact — it does not replace the actual determination of the system type via the main supply (step 1) and is not a substitute for a loop-impedance or earth-resistance measurement.

Step 4 — Recognising an IT system

An IT system is usually not present by accident — it is deliberately applied in specific environments (operating theatres, industrial continuous processes). Recognition points:

An insulation monitoring device (IMD) in the main distribution board, often with its own alarm indication (see the IT system guide).
No direct, low-resistance connection between one of the live conductors and earth at the origin of the installation.
A building function that makes an IT application likely (operating theatre, laboratory with a continuous process, industrial installation where unexpected disconnection on a first fault is unacceptable).

Practical relevance

During an NEN 3140 inspection or an extension of an existing, (partially) undocumented installation, the earthing system must be established as the very first step — before a loop impedance, an earth-electrode resistance or an RCD trip time is measured and assessed. A measurement that is technically performed correctly but is assessed against the wrong limit values (because the system was assumed incorrectly) leads to a wrong conclusion about the safety of the installation.

Common mistakes

Assuming every Dutch domestic installation is TN-C-S without checking this — TT occurs more often than expected, especially in older, rural or agricultural installations.
Not actually locating the PEN split point and concluding which system applies based purely on an assumption about the number of conductors.
Performing and assessing a loop-impedance measurement as if it were a TN system, while the installation is actually TT — leading to a completely wrong interpretation of the measured value.
Overlooking an IT system because there is no visible earth fault — an IT system is precisely designed not to become immediately visible on a first fault, which makes recognising an IMD all the more important.