TN-C-S system - Wiring

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Updated - October 7, 2025

The TN-C-S system is one of the possible mains forms for three-phase systems in accordance with IEC60364-1 / VDE0100-100 and represents the usual mains form for public supply. Special attention must be paid to the wiring when using RCDs.

TN, from terre neutral (French), describes as the most important feature the earthing at the power source and the consumers in building installations.

Depending on the type of protective conductor design TN-C (combinéFrench), TN-S (séparéFrench) and TN-C-S (combiné séparéFrench) systems. Here the TN-C-S system in more detail.

Feature of the TN-C-S system

On the input side, i.e. as seen from the supplier, four conductors (L1, L2, L3 and PE/N) are made available. In addition to the three phases (L1, L2 and L3), protective (P.E) and neutral conductor (N) are combined in ONE conductor (PE/N) are separate in the consumer unit, although they are connected to each other by a bridge.

Special feature for FI installation

This feature is particularly important when installing RCDs, because: The separation of PE/N into the single ladder P.E and N must be carried out BEFORE the RCD(s) in order to maintain the protective function.

Function of an RCD

The RCD (made up of Fehler and the physical symbol for electricity I), in English RCD (Residual Current Device), compares the data collected by the Lx and N flowing current. Both currents are usually identical.

If there is an insulation leakage or another leakage current, e.g. caused by a person, this current component is missing. If the difference rises above the response threshold of the RCD, usually nominally 30 mA - in practice corresponding to an actual difference of 18 ... 24 mA - the RCD switches off all poles.

The switch-off time must be less than 200 ms to avoid cardiac fibrillation in humans.

Wiring of an RCD

Installations are generally constructed from the bottom (coming from the supplier) to the top (towards the consumers). Accordingly, phases L1 ... L3 are fed in via a phase bar attached to the lower screw contact of the fuses.

Although there is no defined input or output for the RCD, the term input side (coming from the supplier; the lower contacts) and output side (towards the circuit breakers; the upper contacts) is used.
In fact, however, there is no such distinction.

Practical example 3-phase

The following Wiring diagram illustrates this. The color coding of the conductors and terminals corresponds to the current standard. Deviating codes from older installations must be adapted accordingly.

The PE rail assembled

• the incoming supply PE/N-Manager
• on the incoming N-Connection of the RCD leading conductors
• to the loads as their protective conductor (P.E)

The N-rail unites in itself

• The respective N-output of each RCD as a neutral conductor (N)
• The neutral conductors leading to the loads (N) of this RCD

In a 3-phase installation with 3 RCDs, 3 N busbars (N1 .. N3) must therefore be provided. If a residual current occurs at a load of one of the three RCCBs, the resulting difference in current between the phase and neutral conductor causes the associated RCCB to switch off all poles immediately if the threshold value is exceeded.

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All information without guarantee - installations on electrical systems may only be carried out by authorized specialist personnel!