TT system

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Basic circuit diagram of a TT network

A TT system (French: terre terre ) 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 at the power source and the electrical equipment within the building installation. Further network systems are the TN system and the IT system .


In the TT-system of the current source of the distribution network with one being operational earth R B connected. As in a TN system, the neutral point of the feeding transformer is usually earthed.

The protective conductor connected to the conductive housing of the electrical equipment in the consumer system is not connected to the earthing of the distribution network, but is connected separately to its own local earth electrode R A (system earth).

The missing link between operational ground of the producer and the earth consumer systems provides the advantage that no compensating currents can occur between the two grounding points, as in the TT system, in contrast to the TN system, not raise the ground potential through the loaded PEN conductor on Consumer side takes place. In the case of a system that is not set up in accordance with the standard (missing protective equipotential bonding conductor between externally accessible parts such as water pipes and the main earthing rail), it is possible that equalizing currents between the system earth and the operating earth of the generator (secondary side local network transformer) via directly earthed plants and systems, such as water pipes and other pipeline networks ( Telecommunications, etc.), causing them to corrode electrochemically over time.

The protective measure protective earthing is problematic because a high electrical current is necessary for the overcurrent protection device to respond quickly . This only occurs with very low earthing resistances , which are difficult to achieve. In most cases, a residual current protection circuit can be used instead . In order to avoid repercussions from the 16⅔ Hz network of the railway on the 50 Hz network, reliable grounding measures are necessary.

If a residual current device (RCD) is to guarantee protection against indirect contact, the following condition for the earthing resistance must be met in addition to compliance with the required disconnection times:

. Where:
  • is the sum of the resistances of the earth electrode and the protective conductor of the body (accessible parts).
  • is the rated residual current of the residual current protective device.

With a typical rated residual current of the FI circuit breaker of 30 or 300 mA, the total earth resistance is less than 1667 or 167 ohms.

If the earthing resistance is higher, a more sensitive residual current protective device must be selected for the TT system .


In Germany today operate some distribution network operators (DSOs) to a greater extent TT systems, such as the Regensburg Netz GmbH and in Thuringia the TEAG and several municipal utilities.

In Italy, the TT system is common in households, while industry and networks with in-house transformer cabins tend to use the TN-CS system or the TN-S system .

In Spain, the TT system is the standard for all installations that are supplied from the public low-voltage network (ITC-BT-08 standard). A different system can only be selected for low-voltage installations if you have your own transformer. The IT system can be used for installations with special demands on security of supply (medicine, industry), taking special requirements into account .

In France, the TT system is also used in all installations that are supplied from the public low-voltage network (NFC 15-100 standard). A residual current circuit breaker (usually 500 mA) is installed by the power supplier together with the main fuse. All circuits are to be protected by additional residual current circuit breakers with 30 mA residual current. Since the earth potential of the neutral conductor N cannot always be ensured in the event of poor earthing , this is regarded as an active conductor and must be separated from the overcurrent protection device together with the active outer conductor.

The TT network is also found frequently in Belgium, and here, too, it is mandatory to switch off both active conductors using a two-pole overcurrent protection device.


  • DIN VDE 0100-100: 2009-06
  • DIN VDE 0100-410: 2007-06
  • IEC 60364-3: 1993-03, section 312.2
  • IEC 60364-3: 1993-03, Amendment 1: 1994-02

See also


  • Gerhard Kiefer: VDE 0100 and practice. 13th edition. VDE-Verlag GmbH, Berlin / Offenbach 2009, ISBN 978-3-8007-3130-5 .
  • Günter Springer: Expertise in electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
  • Werner Hörmann, Bernd Schröder: Protection against electric shock in low-voltage systems - Comment from DIN VDE 0100-410 (VDE 0100-410): 2007-06. VDE publication series Volume 140, VDE-Verlag, Berlin 2010, ISBN 978-3-8007-3190-9 .

Individual evidence

  1. DIN VDE 0100-540: 2012-06 section 544.1
  2. DIN VDE 0100-410: 2007-06 according to table 41.1 for the respective system voltage and not longer than 5 s for distribution circuits
  3. DIN VDE 0100-410: 2007-06 section 411.5.3
  4. [1] Information from Regensburg Netz GmbH on the technical connection conditions
  5. ↑ Electrical practitioner, conversion from the TN to the TT system ( memento from January 13, 2018 in the Internet Archive ), February 2002