How do power transformers work in houses


Electricity purchase and conversion
From the large power plants, which usually generate three-phase alternating current with voltages ≤ 10,000 V, the electrical energy is transferred to the European network with high voltages of 110 to 380 kV. This network connects cities, districts and larger industrial plants with one another. The high voltage is transformed in switchgears and substations into medium voltage of usually 10 kV. Larger residential complexes with more than 20 residential units and other larger consumers such as factories, hospitals, department stores and administrative buildings are connected to the medium-voltage network. Small consumers and private households are supplied via the public low-voltage network. Here, the medium voltage must be converted to 400/230 V in network stations. These substations - also called transformer stations, network stations or transformer stations for short - are housed in transformer houses or on the ground floor or in the basement of buildings.

The responsible supply network operators are generally responsible for the planning, execution and operation of the substations. The architect should only plan in cooperation with the electrical planner and the network operator the corresponding space requirements and the corresponding requirements of entry and exit openings, entrances and transport routes.

House connection
The house connection forms the interface between the households and the supply network; it connects it to the customer's consumer system and consists of:

  • the mains connection, e.g. house connection branch joint
  • the house connection line
  • the house entry (wall duct)
  • the building entry line with protective tube or cover
  • the house connection box with fuses
A basic distinction is made between underground cable connections and overhead line connections. Whereby the house connection to the public distribution network is nowadays mainly made by underground cable.

House connection room, main power supply system and meter
The house connection line ends in the house connection room in which the house connection box is arranged. The house connection fuses are housed here. These overcurrent protection devices ensure the protection of the house connection and entry line as well as the protection of the main power supply system in the event of overload and short circuit. The main power supply system consists of main lines and safety devices and, for reasons of security of supply, is constructed as a radiation network with centralized or decentralized meters. The meters are housed in meter cabinets.

Circuit distributor
The electricity already recorded by the meter is distributed to the individual circuits via the circuit distributor. The following facilities can be found here:

  • Overcurrent protection devices, i.e. line circuit breakers or fuses with a maximum rated current of 63 A.
  • Residual current circuit breaker (FI protection)
  • Terminals
  • Remote switch
  • Contactors, time switches, etc.
Every customer system, i.e. every household and every office unit, etc., requires its own circuit distributor.


The number of circuits required depends on the size and equipment of the apartment. Separate circuits must be set up for large appliances such as stoves, washing machines, water heaters and the like, as well as for each room in an apartment for light and sockets.

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