Distribution switchboards

Distribution switchboards, including the main LV switchboard (MLVS), are critical to the dependability of an electrical installation. They must comply with well-defined standards governing the design and construction of LV switchgear assemblies

A distribution switchboard is the point at which an incoming-power supply divides into separate circuits, each of which is controlled and protected by the fuses or switchgear of the switchboard. A distribution switchboard is divided into a number of functional units, each comprising all the electrical and mechanical elements that contribute to the fulfilment of a given function. It represents a key link in the dependability chain.
Consequently, the type of distribution switchboard must be perfectly adapted to its application. Its design and construction must comply with applicable standards and working practises.
The distribution switchboard enclosure provides dual protection:

• Protection of switchgear, indicating instruments, relays, fusegear, etc. against mechanical impacts, vibrations and other external influences likely to interfere with operational integrity (EMI, dust, moisture, vermin, etc.)
• The protection of human life against the possibility of direct and indirect electric shock (see degree of protection IP and the IK index in section 3.3 of Chapter E).
  

Types of distribution switchboards

The load requirements dictate the type of distribution switchboard to be installed

Distribution switchboards may differ according to the kind of application and the design principle adopted (notably in the arrangement of the busbars).
Distribution switchboards according to specific applications
The principal types of distribution switchboards are:

• The main LV switchboard - MLVS -

Fig. E27a:A main LV switchboard - MLVS - (Prisma Plus P) with incoming circuits in the form of busways -

• Motor control centres - MCC -

Fig. E27b:A LV motor control centre - MCC - (Okken)

• Sub-distribution switchboards

Fig. E28:A sub-distribution switchboard (Prisma Plus G)

• Final distribution switchboards

Fig. E29: Final distribution switchboards [left] Prisma Plus G Pack; [center] Kaedra; [right] mini-Pragma

Distribution switchboards for specific applications (e.g. heating, lifts, industrial processes) can be located:

• Adjacent to the main LV switchboard, or
• Near the application concerned

Sub-distribution and final distribution switchboards are generally distributed throughout the site.

Two technologies of distribution switchboards

A distinction is made between: Traditional distribution switchboards in which switchgear and fusegear, etc. are fixed to a chassis at the rear of an enclosure Functional distribution switchboards for specific applications, based on modular and standardised design.

Traditional distribution switchboards
Switchgear and fusegear, etc. are normally located on a chassis at the rear of the enclosure. Indications and control devices (meters, lamps, pushbuttons, etc.) are mounted on the front face of the switchboard.
The placement of the components within the enclosure requires very careful study, taking into account the dimensions of each item, the connections to be made to it, and the clearances necessary to ensure safe and trouble-free operation.
Functional distribution switchboards
Generally dedicated to specific applications, these distribution switchboards are made up of functional modules that include switchgear devices together with standardised accessories for mounting and connections, ensuring a high level of reliability and a great capacity for last-minute and future changes.

• Many advantages

The use of functional distribution switchboards has spread to all levels of LV electrical distribution, from the main LV switchboard (MLVS) to final distribution switchboards, due to their many advantages:
  - System modularity that makes it possible to integrate numerous functions in a single distribution switchboard, including protection, 
    distribution switchboard maintenance, operation and upgrades
  - Distribution switchboard design is fast because it simply involves adding functional modules
  - Prefabricated components can be mounted faster
  - Finally, these distribution switchboards are subjected to type tests that ensure a high degree of dependability.
The new Prisma Plus G and P ranges of functional distribution switchboards from Schneider Electric cover needs up to 3200 A and offer:
  - Flexibility and ease in building distribution switchboards
  - Certification of a distribution switchboard complying with standard IEC 61439 and the assurance of servicing under safe conditions
  - Time savings at all stages, from design to installation, operation and modifications or upgrades
  - Easy adaptation, for example to meet the specific work habits and standards in different countries.
Figures E27a, E28 and E29 show examples of functional distribution switchboards ranging for all power ratings and figure E27b shows a high-power industrial functional distribution switchboard.

• Main types of functional units

Three basic technologies are used in functional distribution switchboards.
  - Fixed functional units (see Fig. E30)
These units cannot be isolated from the supply so that any intervention for maintenance, modifications and so on, requires the shutdown of the entire distribution switchboard. Plug-in or withdrawable devices can however be used to minimise shutdown times and improve the availability of the rest of the installation.

Fig. E30: Assembly of a final distribution switchboard with fixed functional units (Prisma Plus G)

- Disconnectable functional units (see Fig. E31)
Each functional unit is mounted on a removable mounting plate and provided with a means of isolation on the upstream side (busbars) and disconnecting facilities on the downstream (outgoing circuit) side. The complete unit can therefore be removed for servicing, without requiring a general shutdown.

Fig. E31: Distribution switchboard with disconnectable functional units

- Drawer-type withdrawable functional units (see Fig. E32)
The switchgear and associated accessories for a complete function are mounted on a drawer-type horizontally withdrawable chassis. The function is generally complex and often concerns motor control.
Isolation is possible on both the upstream and downstream sides by the complete withdrawal of the drawer, allowing fast replacement of a faulty unit without de-energising the rest of the distribution switchboard.

Fig. E32: Distribution switchboard with withdrawable functional units in drawers

Standards

Compliance with applicable standards is essential in order to ensure an adequate degree of dependability

Different standards
Certain types of distribution switchboards (in particular, functional distribution switchboards) must comply with specific standards according to the application or environment involved.
The reference international standard is IEC 61439-1 type-tested and partially type-tested assemblies
Standard IEC 61439-1

Three elements of standard IEC 61439-1 contribute significantly to dependability: Clear definition of functional units Forms of separation between adjacent functional units in accordance with user requirements Clearly defined routine tests and type tests

• Categories of assemblies

Standard IEC 61439-1 distinguishes between two categories of assemblies:
  - Type-tested LV switchgear and controlgear assemblies (TTA), which do not diverge significantly from an established type or systemfor 
    which conformity  is ensured by the type tests provided in the standard
  - Partially type-tested LV switchgear and controlgear assemblies (PTTA), which may contain non-type-tested arrangements provided 
    that the latter are derived from type-tested arrangements
When implemented in compliance with professional work standards and manufacturer instructions by qualified personnel, they offer the same level of safety and quality.

• Functional units

The same standard defines functional units:
  - Part of an assembly comprising all the electrical and mechanical elements that contribute to the fulfilment of the same function
  - The distribution switchboard includes an incoming functional unit and one or more functional units for outgoing circuits, depending on
    the operating requirements of the installation 
What is more, distribution switchboard technologies use functional units that may be fixed, disconnectable or withdrawable

• Forms (see Fig. E33)

Fig. E33: Representation of different forms of LV functional distribution switchboards

Separation of functional units within the assembly is provided by forms that are specified for different types of operation.
The various forms are numbered from 1 to 4 with variations labelled “a” or “b”. Each step up (from 1 to 4) is cumulative, i.e. a form with a higher number includes the characteristics of forms with lower numbers. The standard distinguishes:
  - Form 1: No separation
  - Form 2: Separation of busbars from the functional units
  - Form 3: Separation of busbars from the functional units and separation of all functional units, one from another, except at their output 
    terminals
  - Form 4: As for Form 3, but including separation of the outgoing terminals of all functional units, one from another
The decision on which form to implement results from an agreement between the manufacturer and the user.
The Prima Plus functional range offers solutions for forms 1, 2b, 3b, 4a, 4b.

• Type tests and routine tests

They ensure compliance of each distribution switchboard with the standard. The availability of test documents certified by independent organisations is a guarantee for users.

Remote monitoring and control of the electrical installation

Total accessibility of electrical information and intelligent distribution switchboards are now a reality

Remote monitoring and control are no longer limited to large installations.
These functions are increasingly used and provide considerable cost savings.
The main potential advantages are:

• Reductions in energy bills
• Reductions in structural costs to maintain the installation in running order
• Better use of the investment, notably concerning optimisation of the installation life cycle
• Greater satisfaction for energy users (in a building or in process industries) due to improved power availability and/or quality

The above possibilities are all the more an option given the current deregulation of the electrical-energy sector.
Modbus is increasingly used as the open standard for communication within the distribution switchboard and between the distribution switchboard and customer power monitoring and control applications. Modbus exists in two forms, twisted pair (RS 485) and Ethernet-TCP/IP (IEEE 802.3).
The www.modbus.org site presents all bus specifications and constantly updates the list of products and companies using the open industrial standard.
The use of web technologies has largely contributed to wider use by drastically reducing the cost of accessing these functions through the use of an interface that is now universal (web pages) and a degree of openness and upgradeability that simply did not exist just a few years ago.