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Artículo 1.- Aprobación de los derechos de trámite por los procedimientos y servicios administrativos
In a typical LV installation, distribution circuits originate at a main low-voltage switchboard (MLVS) from which conductors supply loads via sub-distribution and/or final distribution boards.
LV distribution levels
For medium to large sites, three distribution levels are generally used to supply LV power to all loads:
c Distribution from the main low-voltage switchboard (MLVS)
At this level, power from one or more MV/LV transformers connected to the MV network of the electrical utility is distributed to:
v Different areas of the site: shops in a factory, homogeneous production areas in industrial premises, floors in office buildings, etc.
v Centralised high-power loads such as air compressors and water cooling units in industrial processes or air conditioners and lifts in office buildings
c Sub-distribution used to distribute electricity within each area c Final distribution, used to supply the various loads
Basic topologies (see Fig. E1)
All distribution schemes are combinations of two basic topologies:
c Star topology: Radial (or centralised) distribution
c Bus topology: Distribution using busways (also referred to as busbar trunking systems)
Fig. E1 : The two basic topologies distribution schemes
Star Bus
Selecting a distribution scheme
The LV distribution scheme is selected according to a number of criteria including:
v Energy availability requirements
v Size of the site (area and total power to be distributed) v Load layout (equipment and power density)
v Installation flexibility requirements c Energy availability requirements
The creation of independent circuits to different parts of an installation makes it possible to:
v Limit the consequences of a fault to the circuit concerned v Simplify fault locating
v Carry out maintenance work or circuit extensions without interrupting the supply of power to the whole installation.
In general, the following circuit groups are required:
v Lighting circuits (the circuits on which the majority of insulation faults occur) v Socket-outlet circuits
v Heating, ventilation and air-conditioning circuits v Power circuits for motor-driven fixed plant
v Power-supply circuits for auxiliary services (indication and control) v Circuits for safety systems (emergency lighting, fire-protection systems and uninterruptible power supplies (UPS) circuits for computer systems, etc.), the installation of which is normally subject to strict regulations and codes of practice.
c Size of the site
v Small sites are supplied directly by the utility’s LV network and the size and power requirements of the electrical installation do not justify a 3-level distribution system (see Fig. E2 opposite page). Electrical distribution in small premises (stores, homes, small offices, etc.) most often involves only one or two levels.
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M M M
Final distribution Supply to loads Repartition within the different areas (floor, factory shop, etc.)
M M M M
Final distribution Supply to loads Repartition within the different areas (floor, factory shop, etc.)
Electrical distribution to the different areas of the building Fig. E2 : Small sites
v Medium-sized sites (e.g. factory, office building) are generally connected to the utility’s MV network (see Fig. E3 ). One or more transformers and their MLVSs supply the entire site.
v Large industrial or infrastructure sites (e.g. airports) are generally connected to the utility’s HV network. An MV distribution system supplies MV/LV substations located at different points on the site as illustrated in Figure E4 next page.
Fig. E3 : Medium-sized sites
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M M M M
Final distribution Supply to loads Repartition within the different areas (floor, factory shop, etc.)
Electrical distribution to the different areas of the building
c Load layout on the site
Two types of loads, depending on their layout on the site, must be taken into account:
v Concentrated loads, generally corresponding to building utilities used for the entire site and requiring high power (e.g. centralised air conditioning units, lifts,
refrigeration units in supermarkets, air compressors in industrial applications) v Distributed loads that can be dealt with in groups corresponding to a homogeneous area (floor, factory shop, production line) and characterised by two parameters: power density (in VA/m2) and equipment density (in number of devices per 10 or 100 m2) (see Fig. E5).
Fig. E4 : Large industrial or infrastructure sites
Low power density High power density
< 100 VA/m2 > 100 VA/ m2
Low equipment density c Machine centres
c Roof-top air conditioners in factories or supermarkets High equipment density c Lighting c Machine shops, presses
c Office computing:
PC, ink-jet printers c Manual work stations, e.g.
in the clothing industry
Fig. E5 : Example of concentrated and distributed loads
c Installation flexibility requirements
Installation flexibility is an increasingly important requirement, in particular for commercial and industrial premises. This need concerns mainly distributed loads and is present at every distribution level:
v Main low-voltage switchboard level: Flexibility in design, allowing electrical power to be distributed to the different areas of the site without detailed knowledge of the needs at sub-distribtuion level.
Typical example: Risers in office buildings are used to distributed electricity to all the floors. They are sized according to the average power requirements of the entire site, making it possible to subsequently meet the very non-uniform power requirements of each floor even though they are not known precisely during the design phase.
v Sub-distribution level: Flexibility for installation and operation v Final distribution level: Flexibility for utilisation
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c Location of the substation and the main LV switchboard
The starting point for the design of an electrical installation, and the physical location of 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/LV substation, replacement sources and the main LV distribution board, should, for both technical and economic reasons, be placed as near to the electrical centre of the load area as possible. On a large industrial site, a number of MV/LV substations and MLVSs can 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 agreement of the utility concerning the location of the MV/LV substation, and its related civil engineering works.
Busways, also referred to as busbar trunking systems, can be used to ensure a high degree of flexibility for future extension or modifications in the electrical distribution system. To make sure that the enhanced flexibility for future modifications is not detrimental to ease of operation, it may be necessary to install protective devices as close to the loads as possible.
Examples of distribution schemes c Radial branched distribution
This scheme of distribution is the most widely used and generally follows arrangements 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 the installation in service. Conductor sizes can be tapered to suit the decreasing current levels towards the final sub-circuits
v Disadvantages
- A fault occurring on one of the conductors from the main LV switchboard will cut off supply 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 the electrical 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
Power final distribution board
Lighting & heating final distribution board
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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 installations that 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
MLVS
(main LV switchboard)
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 distribution in locations where partitioning may change according to consumers requirements.
MLVS
(main LV switchboard)
Sub-distribution board (office C) B
A C
To heating final distribution board
Lighting busway
Socket-outlet busway
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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-circuit current at the point considered)
Fig. E9 : Simple radial distribution
MLVS
(main LV switchboard)
M M M
Process M
c Mixed distribution from MLVSs and high-power busways Principle
A high-power busway connected to the MLVS can be used to supply feeders at other locations throughout the site. These feeders supply sub-distribution boards and/or sub-distribution busways. For high power requirements, the transformers and MLVSs can also be located throughout the site. In this case, busways are used to interconnect the different MLVSs.
Here are a few examples (see Fig. E10 below and Fig. E11next page).
v Single MLVS
Fig. E10 : Example with a single MLVS Feeders supplied
by MLVS
Feeders supplied by busway
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v Advantages
- Greater design flexibility, independence of main LV switchboard level design and installation with respect to the sub-distribution level, higher energy availability for the site
- Parallel sources ensure the availability of electrical power in the event of failure of one 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 (process applications)
c 220 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 a neutral is required. These transformers provide galvanic isolation of the circuits, making it possible to change the neutral system and improve the main isolation characteristics (see Fig. E12).
Fig. E11 : Example with 2 substations
Feeders supplied by MLVS
Feeders supplied by MLVS Feeders supplied
by 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 transformer
TT system for lighting circuits Residual current
device
PE protective earthing conductor
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1.2 Availability of electrical power
High availability of electrical power is achieved by:
c Appropriate division of the installation c Provision of replacement sources
c Sub-division and duplication of important circuits c The type of earthing system (IT for example) c Discriminative protection schemes.