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1/4Schneider Electric - Electrical installation guide 2005

E5

E - Distribution in low-voltage installations 1 LV distribution schemes

c Location of the substation and the main LV switchboard

The starting point for the design of an electrical installation, and the physical locationof sub-distribution and final distribution boards, is a drawing of the building(s) concerned

with the location of the loads together with their power requirements. The MV/LVsubstation, replacement sources and the main LV distribution board, should, for bothtechnical and economic reasons, be placed as near to the electrical centre of the loadarea as possible. On a large industrial site, a number of MV/LV substations and MLVSscan be located in the same manner, i.e. based on the electrical centre of the load area.

However, many other factors must be considered, and in particular, the agreementof the utility concerning the location of the MV/LV substation, and its related civilengineering works.

Busways, also referred to as busbar trunking systems, can be used to ensure a highdegree of flexibility for future extension or modifications in the electrical distributionsystem. To make sure that the enhanced flexibility for future modifications is notdetrimental to ease of operation, it may be necessary to install protective devices asclose to the loads as possible.

Examples of distribution schemes

c Radial branched distribution

This scheme of distribution is the most widely used and generally followsarrangements similar to those illustrated below.

v Advantages

- One circuit only is de-energised (by fuses or MCCBs) in the event of a fault

- Faults are easily located

- Maintenance or extensions to the circuit can be carried out with the rest of theinstallation in service. Conductor sizes can be tapered to suit the decreasing currentlevels towards the final sub-circuits

v Disadvantages

- A fault occurring on one of the conductors from the main LV switchboard will cut offsupply to all circuits of related downstream sub-distribution and final distribution boards.

v Conventional wiring (see Fig. E6 )

Conventional wiring is suitable for buildings intended for specific use, where theelectrical distribution system is relatively stable, such as homes, hotels, agricultural

activities, schools, etc.Specific advantages: Virtually unrestricted passage for ducts, cable trays, conduits,trunking, etc.

Fig. E6: Radial branched distribution by conventional wiring at 3 levels

MLVS(main LV switchboard)

Sub-distribution board(workshop A)

Process

M M

Powerfinal distribution

board

Lighting & heatingfinal distribution

board

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E - Distribution in low-voltage installations

Fig. E7: Radial branched distribution using busways at sub-distribution level

v Busways for sub-distribution (see Fig. E7 )

Busways are an excellent solution for industrial and commercial sector installationsthat will be subject to future changes.

Specific advantages: Flexible and easy installation in large non-partitioned areas

Fig. E8: Radial branched distribution using busways for final distribution to lighting and socket-outlets


To lighting & heating

final distribution board

M M M

Busway

Second busway

Process

v

Busways for final distribution (see Fig. E8 )For offices, laboratories and all modular premises subject to frequent rearrangements.

Specific advantages: A flexible, attractive and easily installed solution for final distributionin locations where partitioning may change according to consumers requirements.


Sub-distributionboard (office C)

BA C

To heating finaldistribution board

Lighting

buswaySocket-outletbusway

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E - Distribution in low-voltage installations 1 LV distribution schemes

c Simple (unbranched) radial distribution

This scheme (see Fig. E9 ) is used for the centralised control, management,maintenance and monitoring of an installation or process dedicated to a particular

application.v Advantages

- A fault (other than at busbar level) will interrupt one circuit only

v Disadvantages

- Surplus of copper due to the number and length of circuits

- High ratings of protective devices (proximity of source, dependent on short-circuitcurrent at the point considered)

Fig. E9: Simple radial distribution


M M M

Process

M

c Mixed distribution from MLVSs and high-power busways

PrincipleA high-power busway connected to the MLVS can be used to supply feeders at otherlocations throughout the site. These feeders supply sub-distribution boards and/orsub-distribution busways. For high power requirements, the transformers andMLVSs can also be located throughout the site. In this case, busways are used tointerconnect the different MLVSs.

Here are a few examples (see Fig. E10 below and Fig. E11 next page).

v Single MLVS

Fig. E10: Example with a single MLVS

Feederssupplied

by MLVS

Feederssupplied

by busway

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E - Distribution in low-voltage installations

v Advantages- Greater design flexibility, independence of main LV switchboard level design andinstallation with respect to the sub-distribution level, higher energy availability for thesite- Parallel sources ensure the availability of electrical power in the event of failure ofone of them. They also make it possible to take into account non-uniformity in the

distribution of load power requirements over the site.

Changing neutral systems

In large LV installations, two voltage levels are normally used:

c 380 V, 400 V or 415 V (or exceptionally 480 V), mainly for motors (processapplications)

c220 V, 230 V or 240 V (or exceptionally 277 V) for lighting and socket-outlet circuits

When the neutral is not distributed, LV/LV transformers will be installed wherever aneutral is required. These transformers provide galvanic isolation of the circuits,making it possible to change the neutral system and improve the main isolationcharacteristics (see Fig. E12 ).

Fig. E11 : Example with 2 substations

Feederssuppliedby MLVS

Feederssuppliedby MLVS

Feederssuppliedby busway

v MV/LV transformers and MLVSs located throughout the site

Fig. E12: Use of a single-phase or three-phase transformer to change from an IT to a TT system

IT system

400 V / 230 V

transformerTT system forlighting circuits

Residual current

device

PE protective

earthing conductor

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