Electric power distribution

Low-voltage networks are a part of the electricity network for distribution of electrical power to most of the electrical consumers, consisting of low-voltage devices. To avoid voltage loss, low-voltage networks are limited to a few kilometers in the spatial extent of an area of some 100 m. They are therefore fed regionally transformer stations from a parent medium voltage grid. Low-voltage networks are not established, in contrast to other voltage levels as a three, rather than four-wire systems, in order to allow the connection of single-phase loads. You are in Europe usually with a mains voltage between 230 V / 400 V ( single-phase / three-phase) up to 1000 V,. The rated performance of individual distribution transformers are at 250, 400 or 630 kVA.


The types of network design used in Europe for three-phase alternating current are set by the International Electrotechnical Commission (IEC) and are divided into:

  • TN system (French Terre Neutre ): It is in Central Europe, the most common type, has a solidly grounded neutral point, and in different sub- types of protective conductor together the neutral and protective conductors as a so-called PEN conductor of the transformer station to the individual sub-distributions out. This required five or four parallel conductors. The TN system is divided depending on the concrete implementation of the protective conductor and neutral conductor and their combinations in the TN -C system, TN -S system and the most common, the TN- CS system.
  • TT system (French Terre Terre ): The star point at the transformer as well as the consumer is electrically connected to the grounding system. It exists between the consumer and transformer station no common protective conductor connection, the connection of the protective conductor is exclusively via the ground. This system is less expensive than the TN system, since this additional conductor is not necessary for the protective conductor at the main distribution. However, it requires little grounding resistors, which can not always be guaranteed.
  • IT system (French Isolé Terre ): So-called isolated system. Use in most small-scale industrial networks and in hospitals. As a special feature in this network is not grounded star point. IT systems have the advantage that a simple earth fault does not immediately result in a failure. The error is indicated by an insulation monitoring device and can then to be resolved without interruption. At the fault location flows a small idle current. The magnitude of the current depends on the capacitance of the interconnected in the network area network cable. The use of residual current devices is possible in this network only for two- or multiple errors for immediate shutdown.

Outside of Europe also other low-voltage systems are common. In North America and partly in Asia, the single-phase three-wire system and builds on it Red -Leg Delta system is widespread.


Low-voltage networks are starting usually divided by the area of ​​the main distribution in several harnesses, supply in the local area which individual houses or groups of houses. The strands are usually implemented in a star shape, which take place in the area of ​​service connection via a so-called loop box, the branches to the sub-distribution. For in rural areas still usual overhead lines Offtake on roof rack is done. In special cases, a low-voltage network be constructed as a ring and be fed by several points.

In the area of ​​distribution is a star-like power of the individual consumer and sockets. In England and some former British colonies also ring-shaped distributions in the context of BS 1363 in homes occur. The ring topology has the advantage that the same power smaller conductor cross-sections can be used, but the installation cost is higher.


Low-voltage systems are fed from the medium-voltage network through local transformer stations. Sometimes a little less expensive transformer with vector group Yy0 ( YY 0) is used, the secondary side of the windings are connected together at one end to the star point. Better than the star circuit the switching group Yz5 (star - zigzag 5), in which the distribution of the asymmetrical outer conductor current, the asymmetry is largely balanced on each leg two halves of a transformer. Does not happen this compensation, the individual phase voltages will be equal to the consumer have high voltage and unbalanced loads are the result.

On the low voltage side of the neutral point of the local transformer is solidly grounded. Here, a ground resistance of RE is usually required less than 2 Ω. If this value is higher due to aging, damage, soil drought or other circumstances that may occur in case of failure to impermissibly high touch voltages or step voltages.

Phase identifiers

  • Phase conductors (L1, L2, L3 ): Identification of the high potential leading end of the transformer winding. Three phase-shifted by 120 ° offset outer conductors in three-phase system. Colloquially, the outer conductor is also referred to as a phase. An outer conductor to the neutral conductor is colloquially referred to as the luminous flux and the three-phase system as a force power.
  • Neutral ( N): This describes the balancing neutral conductor in a three phase system. In a symmetrically loaded three-phase system, the currents cancel in the outer conductors. In the entrained neutral ( neutral), no electric current flows. Only if an unbalanced load is not generated by a 230 - V loads, the current balances in the neutral conductor of the asymmetry.
  • Protective conductor ( PE): This describes the conductor which is used to protect against dangerous contact voltages only to conductive surfaces. He is always / in green yellow and connected to the conductive metallic enclosures of electrical equipment.
  • PEN conductor (PEN ): A neutral conductor from the TN -C system, the protective conductor functions with perceives. This design is problematic because in case of failure the outer conductor voltage may be present on conductive surfaces. The return line in unbalanced three-phase system would be missing in case of failure and the voltage applied on the housings.
  • Grounding: When we talk about grounded or grounding, it is always a conductive connection to the earth meant. It is produced by an earth electrode. Are usual foundation grounding in the house foundation, ground rod up to 20 m depth or Bandeisenerder that are dug 15 m long 1 m deep. The earthing of the transformer stations should have no more than 2 Ω resistor. All other grounding device are usually much worse and much of the soil and the weather conditions.
  • Equipotential bonding: Man thus complements conducting surfaces on the earth electrode, which normally have no access to electricity. This concerns, for example, shower trays, baths, heating pipes, water pipes and gas pipes.

The path must include among their catenaries all metallic parts in the potential equalization, also railing, watches and panels. In the plan area of the catenary lines even have 25 kA / 1 s thermal withstand short circuit currents. The electrical voltage drop through catenary shorts would otherwise produce excessive stresses.


To distinguish between the phase conductor and the neutral conductor uniform colors are specified in cable systems for low voltage depending on the region. In the EU, the coloring by the standard IEC 60446 ( EN 60446 ) is fixed. In other countries also deviating color schemes are used. Some common color schemes in three-phase systems are:

Notes to the color table:

Special shapes

Low-voltage systems with nominal voltages of 690 V are used inter alia in industrial plants or power stations for supply of larger electric motors for driving pumps, conveyor belts and the like with powers of several 100 kW to several megawatts.

Another specific use of higher low voltages are extensive low-voltage networks in rural areas in Europe to keep up with 960 V as an intermediate voltage by the voltage drop between the transformer station and the end customer connection with 400 V within the permissible limits. It is in close proximity to the end customer connection, typically the remote farms or individual secluded chalets, an additional transformer from 980 V to 400 V are provided in the power range of some 10 kVA. The transformer station, which is operated at medium voltage, can be located several kilometers away. The advantage of the intermediate voltage is in addition to a reduction according to the voltage variation of the long low voltage supply is that no time-consuming and costly isolation from the medium voltage power line to the remote buildings is necessary. It can be approved up to 1 kV Low voltage wiring and electrical installation devices are used.

Other applications

Low-voltage networks can be used not only for supplying electrical power, but also for communication. In particular, take place on low voltage networks, the transmission of control signals of the ripple control technology and in some countries even higher-frequency data signals by means of carrier frequency modems (English powerline). These were also attempts or applications with wire radio.