Electrical cables in the substation

  • Electrical cables in the substation

    As regards the cables, in substations we can have the following situations:

    – accessible sub-panel compartment, or a walkable basement under the supporting plane of the switchgear, for which a net height of not less than 1.7 m is recommended. This solution is suitable for cases where the cables entering and leaving the substation are many and buried  

    • floating floor with removable tiles and inspectable space for cables for which we a height of not less than 0.6 m is

    recommended. This solution is suitable for cases where the number of incoming and outgoing cables is limited while the number of interconnection cables between the switchgear of the same substation is high

    • conduits, or a fixed floor and cables contained in conduits prefabricated or built on site
    • walkways or equivalent systems (see CEI EN 61936-1); the connections between the various components are mainly implemented by using ceiling cable suspension systems and wall mounted systems made of insulating or metal material
    • The Distributor normally indicates how the holes for passing of cables are to be created, between the part of the Distributor's part of the substation structure and the customer's receiver In any case, all openings to the outside must be made in accordance with the Standards CEI 11-17 and CEI EN 61936-1 to prevent the entry of animals and water and to prevent the spread of any fires
    • we also emphasize compliance with Standard CEI EN 61936-1 and the product standard for the cables for the sizing and positioning of the conduits and protective tubes embedded in the masonry that must ensure compliance with the curvature radii of the cables and allow free expansion
    • the armor or metal screen of the cables can be considered as metallic segregation. Therefore the MV cables that are laid on walkways, conduits, piping, etc., can coexist with low voltage cables


    Earthing systems

    Particular attention should be paid to the design and construction of the substation earthing system. The earthing system is not addressed directly by CEI 99-4 that refers for such details to CEI EN 50522 and for MV systems and to CEI 64-8 only for the relevant LV systems. 

    The figure below, taken from CEI 99-4, illustrates - merely by way of example - some characteristics of the earthing system in order to draw attention to the elements that make up the earthing system.




    a - Connection between earthing collector and earthing system of structures other than the substation  b - Connection between transformer 1 star centre and earthing collector

    c -  Connection between transformer 2 star centre and earthing collector d - Connection to the public distributor in the manner requested

    e -  Connection between earthing collector and electro-welded mesh under the foundation  f - Connection between the peripheral earthing ring and collector

    x - Reinforcement bars of the perimeter earthing ring for connections to the conductive parts


    We will look at the main requirements of the earthing system for the three types of substations.

    Isolated substation (separated from the building)

    • If necessary, insert a perimeter ring earth plate in the foundation excavation, consisting of a conductor buried directly in virgin
    • All the reinforcement steel elements of the foundation can be used as an earthing
    • Given that the lateral structural elements of the building are connected together for reasons of staticity, if necessary they should be connected to the underfloor electrically welded mesh to obtain better
    • All the elements that contribute to the formation of the earth plate must be connected (individually or in groups) to the collector of the


    Substation included within the volume of the building

    • The earth plate will be part of the general earth plate of the building (considered as meshed or a ring or equivalent) and will consist of current elements, using the iron bars of the foundations, possibly supplemented by an intentional earthing
    • All the elements that contribute to the formation of the earth plate are connected to the main earth collector of the building, which is not necessarily located in the substation
    • The collector of the substation is in any case connected to the main
    • In particular, the substation and in its immediate vicinity, measures must be taken to achieve equalization of the In addition, the measures to limit the potentials due to earth faults must be evaluated.


    Substation in traditional masonry or prefabricated on the roof of a building

    • The earth plate will be part of the main earth plate of the building, and must consist of current elements; in particular, for equalization of potential it is recommended that the reinforcing bars of the pillars and the floor slab of the roof should be
    • The collector of the substation will be connected to the structure of the building directly on the floor where the substation will be


    Elements of the earth plate

    • The earth plate can be implemented as a ring with copper wire or a steel bar according to Standard CEI EN 50522 and CEI Guide 11-37.
    • If necessary, the ring may be supplemented with outreaches or with a second outer ring at a greater depth, possibly supplemented, for example, with copper-plated steel pegs

    1.50 m in length complete with a collar for fastening the copper rope.

    • Due the type and use of current elements, please refer to the Guide CEI 64-12 and CEI 11-37.
    • All the necessary measures must be taken to limit the effects of corrosion, with particular attention to

    combinations of different metals (see UNI standards or the Guides CEI 11-37 and CEI 64-12 within the relevant scope).

    • The filling soil around the earth plate must be of vegetation type and must not contain any waste


    Earthing terminal

    • The earth collector (in the form of not necessarily just one drawbar, or a ring) must constitute the point of connection between the elements of the substation earth plate, the earth plate of the building where the substation is inserted (if required), the MV earth conductors, the LV protective conductors LV and the equipotential conductors. The horizontal ducts for implementing these connections are preferably arranged in the floor of the
    • The studs of reinforcing rods provided as connection points must be left free from the pouring of the
    • To facilitate the maintenance and checking operations the individual conductors arriving at the collector should be signposted.
    • The protective, equipotential and earthing conductors, if not exposed, must have a yellow/green
    • The sizing for the sections of the collector and of all the protective conductors is performed by the designer, according to the MV and LV earth fault
    • The equipotential conductors for the MV installation, if made of copper, must have a minimum cross-section of 16 mm2, (see CEI EN 50522). For LV installations, the main equipotential conductors, if made of copper, must have a minimum cross section of 6 mm2 (see Standard CEI 64-8 for the relevant installations).

     Reference: ABB

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