Usually the power supply systems are named with single phase, three-phase three wire or three-phase four-wire. The German VDE and the IEC60364-1 have unified the supply systems based on grounding.
We have a TN system, divided in TN-C, TN-S and TN-C-S, a TT system and an IT system.
What is the difference?
Let’s take a closer look.
The TN-C System
French abbreviation from: Terre-Neutre-Combiné
In a TN-C system the neutral conductor (N) and the protective conductor (PE) sharing the same wire. It’s called PEN.
This system is in Germany outdated.
The TN-S System
French abbreviation from: Terre-Neutre-Separé
In this system the neutral conductor and the protective conductor are completely separated.
The TN-C-S System
French abbreviation from: Terre-Neutre-Combiné–Separé
The protective conductor (PE) is combined with the neutral (N) on the way to the local distribution system (PEN). The PE is separated from the N typically at the switchboard close to the energy meter. Here the PE is grounded individually for each building.
This system is commonly used for domestic installations in Germany.
When the lines (L1, L2, L3) are heavy unbalanced, the PE is affected by the N potential. This system can reduce the potential difference of e.g. a motor housing to the ground, but it can not eliminate the voltage completely.
The voltage magnitude depends on the load unbalance and wiring length. A repeated grounding of the PE is mandatory.
The TT System
French abbreviation from: Terre Terre
In a TT system the neutral-point of the transformer and the consumer housings are directly grounded. The neutral wire (N) does not have a connection to PE. Therefore, the N has no protection function.
The IT System
French abbreviation from: Isolé Terré
The transformer is not grounded, the consumer devices are grounded. In case of a fault (line to housing) there will be no closed circuit and hazardous currents are prevented.
The protection is done by isolation monitor devices.
This was brief information about the different power supply systems defined in IEC. In real world the grounding is sometimes strange. I heard stories about potential differences of >100 V between neutral and PE.
For safety, it’s a big problem.
Energy theft with this method is detected by the partial earthing tamper feature of CLOU energy meters. The billing is still accurate and the event is logged.
Thank you for reading.
Editor's note: This article was originally published in September 2020 and has been updated for comprehensiveness.