An electrical distribution system is a low-voltage electrical system consisting of a distribution network and an electrical installation [this term is defined in the IEC TS 62257-5-2015].
The current standard BS 7671 defines the term “system” as:
An electrical system consisting of a single source or multiple sources running in parallel of electrical energy and an instal lation.BS 7671:2018+A2:2022
This definition reflects the requirements of IEC 60364-1 for low-voltage electrical systems with multiple power supplies.
Structure of the Electrical Distribution System
The distribution system usually includes an electrical installation of a 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 1).
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 1 below illustrates the connection of the electrical installation of a building to the low-voltage electrical distribution network.
The electrical installation of a building is conventionally shown in Figure 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 in the entire electrical installation of a building.
The electrical installation of a 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 line. Electrical installations of apartment buildings in cities are usually connected to transformer substations by cable power lines.
In a transformer substation, the conductors of an overhead line or an underground cable are connected respectively to the three-phase busbars (L1, L2, L3) and to the PEN bus of its 0.4 kV switchgear, and in the electrical installation of a building to the corresponding input terminals of the main distribution board (MDB) installed in the building.
The distribution line conductors may also be connected to the terminals connecting the branch conductors from the overhead line to the input with the cable(wires) origin of the electrical installation of a building. The power source in the electrical distribution network in question is a transformer installed at the substation.
The boundary that separates the low-voltage electrical distribution network and the electrical installation of a building connected to it usually runs:
- On the MDB input terminals, if the electrical installation of a building is connected to the underground cable of the electrical distribution network;
- On MDB input terminals, if the electrical installation of a building is connected to the overhead line of the electric distribution network, and the branch from the overhead line to the input and the origin to the electrical installation of a building is performed by cable, insulated wires or self-supporting insulated wires;
- on the terminals connecting the wires of the overhead line branch to the input with the cable (wires) of the origin of the electrical installation of a building, if the electrical installation of a building is connected to an overhead line of the electric distribution network, and the branch from overhead line to the input is performed with uninsulated wires.
The structure of the real electrical distribution system may be different. If several electrical installations of buildings are connected to the electrical distribution network, a different type of system earthing can be set for each set, which includes the general electrical distribution network and a specific electrical installation of a building. In this case, there are as many electrical distribution systems as there are electrical installations of buildings connected to the common electrical distribution network.
If the transformer substation is placed in a large building, one of the elements of the electrical distribution network – the low-voltage distribution line – is usually missing. Its function is performed by the wiring of the electrical distribution circuits connecting the low-voltage switchgear of the transformer substation to the low-voltage switchgears which are part of the electrical installation of a building.
Moreover, the electrical distribution system may include only a part of the electrical installation of a building, which is performed with a different type of system earthing than the rest of its parts.
Electrical Distribution System of the Smallest Type
The electrical distribution system of the smallest type includes a power source, such as a power generator, and one current-using equipment (see Figure 2 below). It can be performed with one of five types of system earthing (TN-C, TN-S, TN-C-S, TT, IT).
- IEC TS 62257-5-2015
- IEC 60364-1
- BS 7671:2018+A2:2022