Electrical Distribution System Explained

Electrical distribution system: low-voltage electrical system consisting of a distribution network and an electrical installation [defined in the IEC TS 62257-5-2015].

The structure of the distribution system and its examples.

Note 1 to entry: The distribution system usually includes an electrical installation of building which is connected to the low-voltage distribution network consisting of a step-down transformer substation and an overhead line or an underground cable (see Figure B.1).

Note 2 to entry: The smallest distribution system includes a power source and one item of a current-using equipment (see Figure B.2).

Power sources can also be: a local power plant, a separate small power generator driven by an internal combustion engine, and even an isolation transformer, on the basis of which the IT system is implemented in a part of the building’s electrical installation. However, the listed power sources are exceptions to the general rule. In the vast majority of cases in the low-voltage distribution networks to which the building installation is connected, the power sources are transformers installed in step-down transformer substations.

Figure B.1 below illustrates the connection of a building’s electrical installation to the low-voltage electrical distribution network.

Distribution system
Figure B.1 [1] – General outline of the distribution system
Distribution system of the smal lest type
Figure B.2 [1] – Distribution system of the smallest type

The electrical installation of the building is conventionally shown in Figure B.1 as a three-phase class I current-using equipment, the exposed conductive parts of which are subject to protective earthing in accordance with the features of the type of system earthing in question. The input terminals of the switchgear used in the building installation are connected to the corresponding conductors of the power line. The PEN conductor is separated at the point of entry into the building installation. Therefore, neutral and protective conductors are used throughout the building installation.

The electrical installation of building, including an individual residential building, is usually connected to a low-voltage electrical distribution network consisting of a 10/0.4 kV transformer substation and an overhead transmission line. Electrical installations of apartment buildings in cities are usually connected to transformer substations by cable power lines.

In a transformer substation, the transmission line conductors are connected respectively to the three-phase busbars (L1, L2, L3) and to the PEN bus of its 0.4 kV switchgear, and in a building installation, to the input clamps of the same name of the input and distribution switchgear or the input device installed in the building. The power line conductors may also be connected to the terminals connecting the branch conductors from the overhead line to the feeder to the input cable(s) of the building electrical installation. The power source in the electrical distribution network in question is a transformer installed at the substation.


  1. IEC TS 62257-5-2015