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 List of external influences ).

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 - (see Figure E27a)
• Motor control centres - MCC - (see Figure E27b)

Fig. E27 – Examples of main LV switchboard and motor control centre

• 

  [a] Main LV switchboard - MLVS - (Prisma P) with incoming circuits in the form of busways

• 

  [b] MLVS + motor control centre - MCC - (Okken)

• Sub-distribution switchboards (see Figure E28)

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

• Final distribution switchboards (see Figure E29)

Fig. E29 – Final distribution switchboards

• 

  [a] Resi9

• 

  [b] Kaedra

• 

  [c] Prisma Pack 160

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:

• Universal 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.

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:

• 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 Prisma 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 Figure 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 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 IEC 61439

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

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:

• Safety of people (risk of electrocution),
• Risk of fire,
• Risk of explosion.

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.

Three elements of standards IEC 61439-1 & 61439-2 contribute significantly to dependability:

• Clear definition of functional units
• Forms of separation between adjacent functional units in accordance with user requirements
• Clearly defined verification tests and routine verification

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:

• IEC/TR 61439-0: Guidance to specifying assemblies
• IEC 61439-1: General rules
• IEC 61439-2: Power switchgear and controlgear assemblies
• IEC 61439-3: Distribution boards intended to be operated by ordinary persons (DBO)
• IEC 61439-4: Particular requirements for assemblies for construction sites (ACS)
• IEC 61439-5: Assemblies for power distribution in public networks
• IEC 61439-6: Busbar trunking systems (busways)
• IEC/TS 61439-7: Assemblies for specific applications such as marinas, camping sites, market squares, electric vehicles charging stations.

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 expectations

The different requirements included in the standards have been introduced in order to fulfil the End-User expectations:

• Capability to operate the electrical installation,
• Voltage stress withstand capability,
• Current carrying capability,
• Short-circuit withstand capability,
• Electro-Magnetic Compatibility,
• Protection against electric shock,
• Maintenance and modifying capabilities,
• Ability to be installed on site,
• Protection against risk of fire,
• Protection against environmental conditions.

Clear definition of responsibilities

The role of the different actors has been clearly defined, and can be summarized by the following Figure E33.

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.

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:

• updated or new design requirements (example: new lifting test)
• highly clarified design verifications to be made, and the acceptable methods which can be used (or not) to do these verifications, for each type of requirement.
• a more detailed list of routine verifications, and more severe requirements for clearances.

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:

• Constructional requirements
• Performance requirements.

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:

• Testing, which should be done on the most onerous variant (worst-case)
• Calculation, including use of appropriate safety margins
• Comparison with a tested reference design.

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.

No.	Characteristic to be verified	Clauses or subclauses	Verification options available
			Testing	Comparison with a reference design	Assessment
1	Strength of material and parts:	10.2
	Resistance to corrosion	10.2.2	YES	NO	NO
	Properties of insulatingmaterials:	10.2.3
	Thermal stability	10.2.3.1	YES	NO	NO
	Resistance to abnormal heat and fire due to internal electric effects	10.2.3.2	YES	NO	YES
	Resistance to ultra-violet (UV) radiation	10.2.4	YES	NO	YES
	Lifting	10.2.5	YES	NO	NO
	Mechanical impact	10.2.6	YES	NO	NO
	Marking	10.2.7	YES	NO	NO
2	Degree of protection of enclosures	10.3	YES	NO	YES
3	Clearances	10.4	YES	NO	NO
4	Creepage distances	10.4	YES	NO	NO
5	Protection against electric shock and integrity of protective circuits:	10.5
	Effective continuity between the exposed conductive parts of the ASSEMBLY and the protective circuit	10.5.2	YES	NO	NO
	Short-circuit withstand strength of the protective circuit	10.5.3	YES	YES	NO
6	Incorporation of switching devices and components	10.6	NO	NO	YES
7	Internal electrical circuits and connections	10.7	NO	NO	YES
8	Terminals for external conductors	10.8	NO	NO	YES
9	Dielectric properties:	10.9
	Power-frequency withstand voltage	10.9.2	YES	NO	NO
	Impulse withstand voltage	10.9.3	YES	NO	YES
10	Temperature-rise limits	10.10	YES	YES	YES
11	Short-circuit withstand strength	10.11	YES	YES	NO
12	Electromagnetic compatibility (EMC)	10.12	YES	NO	YES
13	Mechanical operation	10.13	YES	NO	NO

[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:

Routine verification	Visual inspection	Tests
Degree of protection of enclosures	Yes
Clearances	Yes	if D < minimum clearance: verification by an impulse voltage withstand test if not evident by visual inspection to be larger than the minimum clearance (e.g. if D < 1.5 times minimum clearance), verification shall be by physical measurement or by an impulse voltage withstand test
Creepage distances	Yes	or measurement if visual inspection not applicable
Protection against electric shock and integrity of protective circuits	Yes	random verification of tightness of the connections of protective circuit
Incorporation of built-in components	Yes
Internal electrical circuits and connections	Yes	or random verification of tightness
Terminals for external conductors	-	number, type and identification of terminals
Mechanical operation	Yes	effectiveness of mechanical actuating elements locks and interlocks, including those associated with removable parts
Dielectric properties	-	power-frequency dielectric test. For assemblies with incoming protection rated up to 250A, verification of insulation resistance by measurement is accepted.
Wiring, operational performance and function	Yes	verification of completeness of information & markings, inspection of wiring and function test where relevant

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:

• 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 (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:

• 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 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 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:

• conductive materials accidentally left in the assemblies during manufacture, installation or maintenance
• entry of small animals, e.g. mouse, snake, …
• material default or inadequate personnel qualification
• lack of maintenance
• abnormal operating conditions that cause overheating, and eventually an internal arc-fault;

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 (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:

• assemblies for applications requiring a high-level continuity of service
• assemblies for buildings considered as critical
• assemblies installed in areas accessible to unskilled persons, and for shortcircuit current equal or greater than 16 kA with non-instantaneous tripping.

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):

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:

Classification item	Classifications	Comments
Assembly which is tested according to IEC/TR 61641	Arcing Class A personnel protection.(Criteria 1 to 5)
	Arcing Class B personnel protection plus arcing restricted to a defined area within the assembly.(Criteria 1 to 6)	Where there is an agreement between the user and the manufacturer, less or different criteria may apply
	Arcing Class C personnel protection plus arcing restricted to a defined area within the assembly. Limited operation after the fault is possible. (Criteria 1 to 7)
	Arcing Class I Assembly providing protection be means of arc ignition protected zones.
Access	Restricted (default)	Authorized persons only have access to the assembly.
	Unrestricted	The assembly can be placed in a location accessible to everyone including ordinary persons

Fig. E37 – Classification of assemblies according to internal arc tests (Table A.1 of IEC/TR 60641:2014)

Class I: Arc ignition protected zones

Class I is a totally different approach compared to other classes.

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 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:

• some relays can detect an arc-fault in an assembly, usually by sensing the light of the arc-fault, possibly in combination with current measurement. Such relays can even detect the fault in a few milliseconds
• When an arc fault is detected, this relay can trigger the "instantaneous" trip of an upstream circuit-breaker. This permit to limit dramatically the energy released by the arc-fault. See Fig. E40 below as an example.
• In addition, the operation of an internal arc quenching device can be activated, achieving ultimate performance in reducing arc-fault duration (less than 5ms).

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)