TN system

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A TN system ( French terre neutre ) is a certain type of implementation of a low-voltage network in the electrical energy supply . The most important feature is the type of earth connection of this power supply system at the power source and the electrical equipment within the building installation. Other network systems are the TT system and the IT system .


In contrast to an IT system , in a TN system as in a TT system, the star point on the low-voltage side of the feeding transformer station is earthed . In contrast to a TT-system, a is performed in a TN system zeroing of the circuit with the consumer installation. In the TN system, there is a connection between the system earth and the system earth.

Depending on the design of the protective conductor , TN systems are divided into TN-C systems, TN-CS systems and TN-S systems.

Ground faults in TN networks lead to ground fault currents with sufficient low resistance , which cause the upstream fuse to respond. In the case of a high-resistance earth fault, on the other hand, the earth fault current is often too low for the fuse to respond. These earth currents, also known as fault currents, are particularly dangerous as they can lead to electrical accidents or system fires. To reduce this risk, residual current circuit breakers are used to detect high-resistance earth faults .

TN-C system

A TN-C system

In a TN-C system ( French: terre neutre combiné ), a PEN conductor is used that is both a protective conductor (PE) and a neutral conductor (N).

As a result of the dual function of the PEN conductor, a (low) voltage to earth is already applied to the housings of earthed devices during normal operation, since the current flowing through the PEN conductor causes a voltage drop according to Ohm's law . In multi-phase installations, uneven loading of the outer conductors also leads to zero point shifts, and in unfavorable cases almost the full voltage between the outer conductors (up to 400 V) can be applied to the device, which in most cases leads to electrical destruction of the devices concerned. If a PEN conductor is even interrupted in an installation, then the conductive housings of the devices connected upstream of the interruption point - due to the connection from the outer conductor to the PEN conductor in the device - the full outer conductor voltage to earth, i.e. usually 230  V. . Therefore, a TN-C system in the household represents a significant potential source of danger .

Nevertheless, the TN-C system was used for a long time as "classic zeroing " in the entire house installation - the main advantage is the lower cabling effort (two-wire lines are sufficient for single-phase circuits). Since 1973 the TN-C system has only been permitted for conductors with a cross-section of at least 10 mm² copper or 16 mm² aluminum . This is to keep the risk of an interrupted PEN conductor with the consequences described above low. Old installations with a smaller cross-section are subject to grandfathering (for country-specific details see zeroing # grandfathering ).

In the TN-C network, residual current circuit breakers in the low-voltage distribution can only be used provided that the system is properly installed (the PEN conductor may only be earthed once at the feed point and not additionally outside the low-voltage distribution) - and only with significant restrictions. In contrast to the Austrian Electrical Engineering Ordinance, however, the use of residual current protective devices (RCD) in TN-C systems is expressly prohibited for Germany in DIN VDE 0100-410: 2007-06 under point 411.4.5 .

There is only additional protection when external conductors come into contact with earth potential, but not against PEN (e.g. earthed housing). So-called RCD sockets, on the other hand, offer unrestricted protection in the TN-C network.

The Austrian Electrical Engineering Ordinance ETV prescribes (for the first time with the 2002 / A2 edition) in the new § 7a the retrofitting also in old apartments, if these are rented again. The arrangement in § 7a reads accordingly: When re-letting an apartment that does not have additional protection, the “installation of at least one residual current circuit breaker with a nominal residual current of not more than 30 mA, immediately before the in line protection devices located in the apartment. "

TN-CS system

A TN-CS system

A TN-CS system ( French: terre neutre combiné séparé ) is made up of a TN-C system, preferably for the utility's distribution network, and a TN-S system in the customer's facility.

The PEN conductor is divided into a protective conductor “PE” and a neutral conductor “N”, if possible in the main power supply system (in Germany according to TAB 2007 point 6.1 (10)). This point marks the transition from the TN-C system to the TN-CS system. From the transition to the TN-CS system, the protective conductor (PE) and neutral conductor (N) are kept strictly separate in the further course of the line; it is not permissible to connect the neutral conductor in the further course of the line to any other earthed part of the system or to the protective conductor again merged (Germany: according to DIN VDE 0100-540: 2007-06 (point 543.4.3)).

This system is widely used in building supplies in Germany, Austria and Switzerland and is standard for new installations (see also lecture: “Introduction of the TN system at RWE”).

In the previous VDE standards, there was no explicit requirement for a PEN allocation as early as possible. However, PEN conductors cause considerable stray currents and fields and are extremely unfavorable for EMC . DIN VDE 0100-444 "Protection against electromagnetic interference (EMI) in systems in buildings" requires separation in section 444.3.12 "in buildings that have a significant amount of information technology equipment or from which this is expected for the future" of the PEN conductor in PE and N from entering the building.

According to DIN VDE 0100-410, low-voltage systems (including customary customer systems) must meet the requirements for fault protection. According to point 411.3, the measures "protective earthing and equipotential bonding", "protective equipotential bonding via the main earthing rail" and "automatic shutdown in the event of a fault" are meant.

Since the June 2007 edition (with a transition period until the end of January 2009), DIN-VDE 0100-410 has required one as "additional protection for final circuits for outside areas and for sockets" for all socket-outlet circuits that are used by laypersons in electrical engineering Residual current circuit breaker (RCD) with a rated residual current of a maximum of 30 mA (in indoor areas, circuits up to 25 A, outdoors for all final circuits up to 32 A). For rooms with showers or bathtubs in new buildings, DIN VDE 0100-701 (see lecture: DIN VDE 0100-701) has required an RCD as described above for all circuits (except permanently connected water heaters) since 1984.

TN-S system

TN-S system

In a TN-S system ( French: terre neutre séparé ) separate neutral conductors and protective conductors are routed from the transformer to the consumables.

A TN-S system is safer than the TN-C or TN-CS system. The problems that can result from an interrupted PEN conductor do not occur here, the protection is guaranteed much better. However, it is not used very often and is mainly used in larger commercial systems that are usually supplied with medium voltage and equipped with their own transformers (then corresponds to a TN-CS system). Older city and suburban houses in Great Britain are also often supplied via the TN-S system.

The transition from a TN-C to a TN-S system is signaled with a blue line.

Grounding in TN systems

In the event of a possible earth fault of an external conductor, other conductors such as PEN and PE conductors can assume a voltage to earth that exceeds the permissible contact voltage of 50 V. In order to prevent this voltage increase , the total earth resistance in the low-voltage network is reduced by using several earth electrodes , i.e. operational earth electrodes (RB) on the network transformer and plant earth electrodes (RA) in the consumer systems .


  • DIN VDE 0100-100: 2009-06 Installation of low-voltage systems - Part 1: General principles, provisions of general characteristics, terms
  • DIN VDE 0100-410: 2007-06 Installation of low-voltage systems - Part 4-41: Protective measures - Protection against electric shock
  • DIN VDE 0100-540: 2012-06 Erection of low-voltage systems - Part 5-54: Selection and erection of electrical equipment - Earthing systems, protective conductors and protective equipotential bonding conductors
  • DIN VDE 0100-444: 2010-10 Setting up low-voltage systems - Part 4-444: Protective measures - Protection against interference voltages and electromagnetic interference
  • DIN VDE 0100-701: 2008-10 Erection of low-voltage systems - Part 7-701: Requirements for production facilities, rooms and special types of systems - rooms with bathtub or shower


Gerhard Kiefer: VDE 0100 and practice . 12th edition. VDE-Verlag GmbH, Berlin / Offenbach 2009, ISBN 978-3-8007-3130-5 .

Günter Springer: Expertise in electrical engineering . 18th edition. VDE-Europa-Lehrmittel Verlag, Wuppertal 1989, ISBN 3-8085-3018-9 .

Werner Hörmann, Bernd Schröder :: VDE series 140 - Protection against electric shock in low-voltage systems - Commentary on DIN VDE 0100-410 (VDE 0100-410): 2007-06 . 18th edition. VDE-Europa-Lehrmittel Verlag, Berlin 2010, ISBN 978-3-8007-3190-9 .

Hans Schultke: ABC of electrical installation . 14th edition. EW Medien und Kongress GmbH, Frankfurt 2009, ISBN 978-3-8022-0969-7 , p. 131 ff .


  1. Schrack, Safety Requirements for Consumer Systems ( Memento from March 9, 2016 in the Internet Archive ), September 2007 (PDF; 3.4 MB)
  2. BMWFJ , Safety of Electrical Systems - Electrical Engineering Ordinance ( Memento of December 28, 2010 in the Internet Archive )
  3. Introduction of the TN system at RWE ( Memento from July 5, 2016 in the Internet Archive ), February 26, 2010 (PDF; 297 kB)
  4. Electrical installation and EMC in a building ( Memento from July 21, 2016 in the Internet Archive ) (PDF; 623 kB)
  5. EMC-compliant network for machines and devices ( Memento from January 13, 2018 in the Internet Archive ), January 2007 (PDF; 412 kB)
  6. Moeller, Explanations on DIN VDE 0100-410 ( Memento of January 13, 2018 in the Internet Archive ), 2008 (PDF; 246 kB)
  7. New construction regulations for rooms with bathtub or shower ( Memento of February 21, 2017 in the Internet Archive ) (PDF; 5.2 MB), February 1, 2002