Indoor substation

From Electrical Installation Guide

General arrangement of a LV metering substation

Fig. B47 and 48 shows a typical layout recommended for a LV metering substation.

Remark: The cast-resin dry-type transformer does not need a fire protection oil sump. However, periodic cleaning of the transformer is needed.

Fig. B47 – General arrangement of a LV metering substation
1: HV Switchboard, 2: Transformer, 3: LV switchboard, 4: Electronic devices / Capacitors
Fig. B48 – Examples of general arrangements of LV metering substations, plan view

Connection to the utility and internal MV and LV interconnections

Connection to the MV utility network is made by, and is under the responsibility of the utility.

Connection between the MV switchgear and the transformer may be realized by:

  • Short copper bars when the transformer is housed in a panel part of the MV switchboard
  • By single-core or three cores screened cables with PR or EPR insulation, and possible connection to the transformers by plugin type terminals.

Connection between the LV terminals of the transformer and the LV switchgear may be realized with:

  • Single-core cables
  • LV busway with heat-shrinkable insulation.

It is highly recommended to use busway for the connection of transformers requiring more than five single LV cables in parallel per phase. Above five single core cables per phase the equal share of the current in each cable cannot be ensured and the laying becomes a real difficulty.

Earthing circuits

To ensure the safety of the persons an equipotential system must be created within the substation. It is realized according the following recommendations:

  • Creation of an earthing electrode under the substation by burying copper conductors
  • Inter-connection by means of protective conductors of all the exposed conductive parts of the installation:
    • Enclosures of the electrical equipment
    • Screens of the MV cables
    • Frame of the transformer
    • Metallic doors
    • Etc.
  • Connection of all protective conductors at one single common point
  • Connection of the common point of the protective conductors and the reinforcing rods of the concrete slab supporting the substation, should be connected to the earth electrode.


The supply of the lighting circuits can be taken upstream or downstream from the main incoming LV circuit breaker. Appropriate LV circuit breakers must be provided for the protection of LV lighting circuits.

The lighting must adequately illuminate:

  • The switchgear operating handles
  • The mechanical flags indicating the position of electrical apparatus
  • All the information displayed on the meters and on the protection relays
  • All the instruction plates dedicated to the operations and the safety.

For safety reasons, it is recommended to add emergency lighting boxes including each an individual battery.

Materials for operation and safety

According to local safety rules, the substation shall be equiped with the following safety equipment:

  • Devices for the safe exploitation of the substation:
    • An Insulated stool
    • An insulated mat
    • A pair of insulated gloves stored in a dedicated box
    • A detector of MV voltage presence
  • Fire-extinguishing devices complying with the local regulations
  • Warning and instruction plates dedicated to:
    • Operation of the substation
    • Safety of the persons
    • First-aid care to victims of electrical accidents.