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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:
Types of distribution switchboards
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:
Fig. E27: Examples of main LV switchboard and motor control centre
Fig. E28: A sub-distribution switchboard (Prisma G)
Fig. E29: Final distribution switchboards
Distribution switchboards for specific applications (e.g. heating, lifts, industrial processes) can be located:
Sub-distribution and final distribution switchboards are generally distributed throughout the site.
Two technologies of distribution switchboards
Universal 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:
The Prisma G and P ranges of functional distribution switchboards from Schneider Electric cover needs up to 3200 A and offer:
Figures FigureE27a, E28 and E29 show examples of functional distribution switchboards ranging for all power ratings and FigureE27b shows a high-power industrial functional distribution switchboard.
Main types of functional units
Three basic technologies are used in functional distribution switchboards.
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 G)
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
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 IEC 61439
The IEC standard series 61439 ('Low-voltage switchgear and controlgear assemblies') have been developed in order to provide to the End-Users of switchboards a high level of confidence in terms of safety and power availability.
Safety aspects include:
Power availability is a major issue in many activity sectors, with high possible economical impact in case of long interruption consecutive to a switchboard failure.
The standards give the design and verification requirements so that no failure should be expected in case of fault, disturbance, or operation in severe environment conditions.
Compliance to the standards shall ensure that the switchboard will operate correctly not only in normal conditions, but also in difficult conditions.
Standard structure
The IEC 61439 standard series consist in one base standard (IEC 61439-1) giving the general rules, and several related standards detailing which of these general rules apply (or not, or should be adapted) for specific types of assemblies:
The first edition (IEC 61439-1 and 2) of these documents has been published in 2009, with a revision in 2011.
Major improvements with IEC61439 standard
Compared to the previous series IEC60439, several major improvements have been introduced, for the benefit of the End-User.
Requirements based on End-User expectationsSub A Board Assy Diagram
The different requirements included in the standards have been introduced in order to fulfil the End-User expectations:
Clear definition of responsibilities
The role of the different actors has been clearly defined, and can be summarized by the following FigureE33.
Fig. E33: Main actors and responsibilities, as defined by the IEC 61439-1&2 standard
Switchboards are qualified as Assembly, including switching devices, control, measuring, protective, regulating equipment, with all the internal electrical and mechanical interconnections and structural parts. Assembly systems include mechanical and electrical components (enclosures, busbars, functional units, etc.).
The original manufacturer is the organization that has carried out the original design and the associated verification of an assembly in accordance with the relevant standard. He is responsible for the Design verifications listed by IEC 61439-2 including many electrical tests.
The verification may be supervised by a Certification body, providing certificates to the Original Manufacturer. These certificates can be conveyed to the Specifier or End-User at their request.
The assembly manufacturer, generally a Panel Builder, is the organization taking responsibility for the completed assembly. The assembly must be completed according to the original manufacturer's instructions. If the assembly manufacturer derivates from the instructions of the original manufacturer he has to carry out again new design verifications.
Such deviations should also be submitted to the original manufacturer for validation.
At the end of assembly, routine verifications must be carried out by the assembly manufacturer (Panel-builder).
The result is a fully tested assembly, for which design verifications have been carried out by the original manufacturer, and routine verifications carried out by the assembly manufacturer.
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This procedure gives a better visibility to the end-user, compared to the 'Partially Type Tested' and 'Totally Type Tested' approach proposed by the previous IEC60439 series.
Clarifications of design verification, new or updated design requirements and routine verifications
The IEC61439 standards also include:
The following paragraphs provide details on these evolutions.
Design requirements
For an Assembly System or switchboard to be compliant with the standards, different requirements are applicable. These requirements are of 2 types:
See Fig.E34 for the detailed list of requirements.
The design of the assembly system must follow these requirements, under the responsibility of the original manufacturer.
Design verification
Design verification, under the responsibility of the original manufacturer, is intended to verify compliance of the design of an assembly or assembly system with the requirements of this series of standards.
Design verification can be carried out by:
The IEC61439 standard have clarified a lot the definition of the different verification methods, and specifies very clearly which of these 3 methods can be used for each type of design verification, as shown in Fig.E34.
[a]Verification of temperature-rise limits by assessment (e.g. calculation) has been restricted and clarified with IEC61439 (2011) standard. As a synthesis:
- for rated current ≤ 630 A and single compartment switchboards: calculation is permitted, based on a comparison between the total power losses of all the components inside the enclosure, and the power loss capability of the enclosure (measured by a test with heating resistors), and a mandatory 20% de-rating of the rated current of the circuits - for rated current ≤ 1600 A and switchboard with one or several compartments with maximum 3 horizontal partitions for every section: calculation is permitted based on IEC/TR 60890, but with a mandatory 20% de-rating of the rated current of the circuits. - for rated current > 1600 A, no calculation, only tests permitted [b]Verification of short-circuit withstand strength by comparison with a reference design has been clarified with IEC61439 standard. In practice, in most cases it is mandatory to do this verification by testing (type-testing), and in any case the comparison with a reference design is only possible for short-circuit protection devices of the same manufacturer, and provided that all other elements of a very strict comparison checklist are verified (Table 13 – 'Short-circuit verification by comparison with a reference design: check list' of IEC61439-1).
Fig. E34: List of design verifications to be performed, and verification options available (table D.1 of Annex D of IEC61439-1)
Routine verification
Routine verification is intended to detect faults in materials and workmanship and to ascertain proper functioning of the manufactured assemblies. It is under the responsibility of the Assembly Manufacturer or Panel Builder. Routine verification is performed on each manufactured assembly or assembly system.
Check to be carried out:
Fig. E35: List of routine verifications to be performed
A precise approach
The IEC 61439 series introduces a precise approach, intended to give to switchboards the right level of quality and performance expected by End-Users.
Detailed design requirements are given, and a clear verification process is proposed, which differentiates design verification and routine verification.
Responsibilities are clearly defined between the original manufacturer, responsible for the design, and assembly manufacturer, responsible for assembly and delivery to the End-User.
Functional units
The same standard defines functional units:
What is more, distribution switchboard technologies use functional units that may be fixed, disconnectable or withdrawable (see Service Index & Fig.E30, E31 and E32).
Forms
(see Fig.E36)
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:
The decision on which form to implement results from an agreement between the manufacturer and the user.The Prima functional range offers solutions for forms 1, 2b, 3b, 4a, 4b.
Fig. E36: Representation of different forms of LV functional distribution switchboards
Beyond the standard
Despite the improvements brought by the IEC 61439 series compared to the previous IEC 60439, there are still some limitations. In particular, for an Assembly manufacturer or Panel Builder combining equipment and devices from different sources (manufacturers), the design verification cannot be complete. All the different combinations of equipment from different sources cannot be tested at the design stage. With this approach, the compliance with the standard cannot be obtained in all particular configurations. Compliance is limited to a reduced number of configurations.
In this situation, End-users are encouraged to ask for test certificates corresponding to their particular configuration, and not only valid for generic configurations.
On the other hand, IEC 61439 sets strict limitation to the device substitution by a device from another series, for temperature rise and short-circuit withstand verification in particular. Only substitution of devices of the same make and series, i.e. same manufacturer and with the same or better limitation characteristics (I2t, Ipk), can guarantee that the level of performance is maintained. As a consequence, substitution by another device not of same manufacturer can only be verified by testing (e.g. 'type-testing') to comply to IEC61439 standard and guarantee the safety of the Assembly.
By contrast, in addition to the requirements given by the IEC 61439 series, a full system approach as proposed by a manufacturer like Schneider Electric provides a maximum level of confidence. All the different parts of the assembly are provided by the Original Manufacturer. Not only generic combinations are tested, but all the possible combinations permitted by the Assembly design are tested and verified.
The high level of performance is obtained through Protection Coordination, where the combined operation of protective and switching devices with internal electrical and mechanical interconnections and structural parts is guaranteed. All these devices have been consistently designed with this objective in mind. All the relevant device combinations are tested. There is less risk left compared with assessment through calculations or based only on catalogued data. (Protection coordination is further explained in chapter LV switchgear: functions and selection).
Only the full system approach can provide the necessary peace of mind to the End-user, whatever the possible disturbance in his electrical installation.
Internal arc withstand tests
International standard IEC 61439-2[1] enables the design and manufacturing of reliable assemblies, and ensures high energy availability. However, there is always a risk, however very limited, of an internal arc-fault during the operating life of the assemblies. For example, this can be due to:
The ignition of an arc inside an assembly generates various physical phenomena, causes very high overheating (thermal avalanche) and especially high overpressure inside the enclosure, which endanger people in the close proximity of the assembly (doors sudden opening, projection of hot materials or gases outside the enclosure …).
To evaluate the capability of an assembly to sustain internal overpressures, publication IEC/TR 61641[2] (which is a technical report) has been drafted. It provides a common reference with a standardized method for tests, as well as criteria for the validation of tests results.
IEC/TR 61641 assesses the ability of an assembly to limit the risk of personal injury and damage to the assemblies, as well as the downtime and time needed to return to service after an arc due to an internal fault.
It is important to note that this is a voluntary test done at the discretion of the manufacturer and in agreement with the customer. Internal arc performance could be evaluated, for example, in the following cases:
The 7 evaluation criteria
IEC/TR 61641 defines 7 evaluation criteria for the internal arc test results (refer to IEC/TR 61641:2014 for more details):
Sub A Board Assyrian
1 = Doors and panels remain securely fastened and do not open;
2 = No part of the assembly of a mass exceeding 60 g shall be ejected; 3 = The arc does not cause holes to develop in the external parts of the envelope below 2 m, at the sides declared to be accessible; 4 = The indicators (cotton fabric placed vertically close to the assembly) do not ignite. Indicators ignited as a result of paint or stickers burning are excluded from this assessment; 5 = The protective circuit for accessible part of the enclosure is still effective in accordance with IEC 61439-2; 6 = The assembly is capable of confining the arc to the defined area where it was initiated, and there is no propagation of the arc to other areas within the assembly; 7 = After clearing of the fault or after isolation or disassembly of the affected functional units in the defined area, emergency operation of the remaining assembly is possible. Classification (arcing class)
Based on the results of the tests on the 7 evaluation criteria, the following classification is defined:
Fig. E37: Classification of assemblies according to internal arc tests (Table A.1 of IEC/TR 60641:2014)
Class I: Arc ignition protected zones
In the unlikely case of an arc appearing in an assembly, classes A, B and C focus on the consequences of the effects of the arc, whilst Class I adopts the philosophy of 'prevention is better than cure'.
Class I seeks to reduce dramatically the risk of occurrence of an arc fault by insulating each conductor individually, as much as possible, with solid-insulation.
Class I may be limited to specific zones of an assembly, as declared by the manufacturer, for example a functional unit or the busbar compartment(s). These zones offering protection in accordance with the Class I are called arc ignition protected zones. The insulation shall provide protection against direct contact in accordance with IP 4X as per IEC 60529[3] and withstand a dielectric test of 1.5 times the normal test value for an assembly.
Fig. E38: Example of totally insulated busbar reducing the risk of internal arc-fault ignition (Okken MCC vertical busbar, Schneider Electric)
The internal arc test
The main objective of an internal arc test is to demonstrate, as much as possible, an improved level of safety for personnel in the proximity of an assembly, when an internal arc-fault occurs.
During the test, personnel clothing is simulated by 'indicators' around the assembly. The indicators are composed of different shades of cotton to simulate standard clothing or light work clothing (i.e. to represent assembly installation in non-restricted or restricted access zones).
Fig. E39: Example of assembly prepared for internal arc test, with “indicators” visible on the front and side (Okken, Schneider Electric)
Another rationale to perform internal arc tests on an assembly is to demonstrate the impact of the fault on the assembly itself. In certain cases, and as defined by the Arcing class, it is worth limiting the arc damage to a part of the assembly, so that the rest of the assembly (or part of it) could be re-powered for a limited use after a few maintenance.
Arc-fault detection and mitigation
Another approach to internal arc-fault management exists:
This topic is currently evolving in standardization committees, both for product and equipment standards.
Fig. E40: Example of arc-fault mitigation system (Okken + Vamp system)
Notes
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