In the TN-C system, a common PEN conductor is used for the protective and neutral conductors. This allows a low voltage to be applied to appliance housings during normal operation. However, if the PEN conductor is interrupted, there is a risk that the full mains voltage (up to 230 V) will be applied to parts that can be touched - a significant safety risk. Despite the lower cabling costs, the TN-C system has only been permitted for large conductor cross-sections (from 10 mm² Cu) since 1973.

A TN-S system is a power supply system in which the neutral conductor (N) and protective earth conductor (PE) are routed separately from the transformer to the load. This achieves a higher level of safety, as problems such as an interrupted PEN conductor do not occur in the TN-C or TN-C-S system.
TN-S systems are mainly used in larger commercial installations with their own transformer station or in older British residential buildings. From the transition from TN-C to TN-S, the blue neutral conductor is added separately and carries the operating current alone.

A TN-C-S system combines a TN-C system in the upstream network (usually from the energy supplier) with a TN-S system in the building. The PEN conductor (combined protective earth and neutral conductor) is split into protective earth (PE) and neutral conductor (N) at a defined point - usually at the building entrance. From this point, PE and N must be permanently separated.
The TN-C-S system is the standard for building distribution systems in Germany, Austria and Switzerland. The early separation of the PEN conductor improves electromagnetic compatibility (EMC) and is prescribed in particular for buildings with IT technology (according to DIN VDE 0100-444).
In accordance with DIN VDE 0100-410, measures such as protective earthing, potential equalization and automatic disconnection in the event of a fault are also mandatory to ensure fault protection.

A TT system (French terre-terre) is a type of network in which both the transformer neutral point and the load systems are earthed separately. The protective earth conductor (PE) of the system is not connected to the earthing system of the energy supplier, but uses its own system earthing (RA).
The advantage: There are no equalizing currents between the earth of the supplier and the consumer, which reduces corrosion on conductive external parts (e.g. water pipes). One disadvantage, however, is that high residual currents are difficult to switch off with fuses. A residual current circuit breaker (RCD) is therefore usually prescribed.
The TT system is often used where there are high requirements for EMC or earthing safety - for example in rural areas or for railroad separations.

An IT system (French: Isolé Terre) is a form of low-voltage power supply in which no active conductor is directly earthed - even the neutral conductor is insulated or only earthed via high impedance.
The main feature: a first earth fault does not lead to disconnection, which offers a high level of supply reliability. IT systems are therefore ideal for critical applications with high reliability, e.g. in hospitals, industry or emergency power systems.
However, the network is limited to a small extent, as fault currents across the capacitance of the lines increase with increasing line length and troubleshooting becomes more difficult. IT systems also have good EMC properties and are suitable for both alternating and direct current. In medical IT systems, the power is limited to a maximum of 8 kVA and special sockets are required.