Download Radical restructuring and revision of the IEC 6043

The development of a new international series of standards for low-voltage switchgear and
controlgear assemblies
by T V McGhie, Consultant, 12, Duchess Drive, Seaford, East Sussex, BN25 2XL, UK; e-mail: [email protected]
Abstract
The current series of international standards for low-voltage switchgear and controlgear assemblies, IEC 60439 [1], has
evolved over more than two decades, but it does not cater for the manufacture of customised assemblies, where it is
impracticable to perform type tests. There is also continuing confusion over the distinction between type-tested and
partially type-tested assemblies, the identification of these categories and the tests that are required to demonstrate
conformity without damaging the equipment to be placed in service. The IEC is in the process of undertaking a radical
restructuring and revision of the standard, which will involve the development of a general rules together with several
individual product standards. This has led to the proposal to dispense with the terms type-tested and partially type-tested
and to introduce the new concept of design verification, which will offer a number of alternative and equally acceptable
methods consisting of verification testing (the old type-testing), measurement, calculation or the application of strict
design rules. Manufacturers of unique customised assemblies would thus have a recognised route to conformity.
Introduction
The current series of international standards for low-voltage switchgear and controlgear assemblies (ASSEMBLIES),
IEC 60439 [1], comprises the following product standards:
Part 1 – Type-tested and partially type-tested assemblies [2];
Part 2 – Particular requirements for busbar trunking systems (busways)[3];
Part 3 – Particular requirements for low-voltage switchgear and controlgear assemblies intended to be installed in
places where unskilled persons have access for their use - Distribution boards [4];
Part 4 – Particular requirements for assemblies for construction sites (ACS) [5];
Part 5 – Particular requirements for assemblies intended to be installed outdoors in public places - Cable distribution
cabinets (CDCs) for power distribution in networks [6].
There is, however, no part for ASSEMBLIES which are designed to meet the requirement of a specific contract, where
the product is unique and will probably never be manufactured again. In these circumstances, it is impracticable and
uneconomic to perform type tests, as they are potentially destructive and may render the tested ASSEMBLY unfit to be
placed in service. Nevertheless, vast numbers of ASSEMBLIES which are neither type-tested nor partially type-tested
are sold throughout the world, leaving both the manufacturer and the purchaser without the security of conformity to a
recognised standard.
Confusion also exists regarding the interpretation of what constitutes a type-tested and a partially type-tested
ASSEMBLY and the distinction between the two categories. It is clear that standardised ASSEMBLIES may be
classified as type-tested, but large distribution switchboards and composite switchgear and controlgear ASSEMBLIES
may contain parts which are unique to the application and are not type-tested. It is unlikely that the latter
ASSEMBLIES can be regarded as type-tested and it is possible that they may not even satisfy the definition of partially
type-tested.
The need to broaden the scope of IEC 60439 [1], to make it accessible to the large number of small manufacturers, was
recognised formally by the International Electrotechnical Commission (IEC) in 1998 and a special Project Team was
established to assess the feasibility of undertaking a radical restructuring and revision of the standard. The Project Team
concluded that a new series of standards [7] should be developed to replace IEC 60439 [1] comprising a general rules
document together with parts for specific product standards, employing a similar relationship to that adopted by the IEC
60947 series [8] for low-voltage switchgear and controlgear devices. The parts, to be issued in conjunction with the
general rules, will probably include:

Power switchgear and controlgear assemblies (new part 2);

Busbar trunking systems (replacement for Part 2);

Distribution boards (replacement for Part 3);

Assemblies for construction sites (replacement for Part 4);

Assemblies for power distribution (replacement for Part 5);

Small assemblies;

Empty enclosures (not under the new series);

Guidance to specifiers.
The Project Team decided that it should develop initially and simultaneously the general rules together with the
standard for power switchgear assemblies, so that the principle of the new structure could be established. The paper
addresses the new concepts introduced into those draft standards and examines the potential benefits that they will
provide to the manufacturers and users of customised ASSEMBLIES.
Interface characteristics
When designing an ASSEMBLY, it is necessary to determine how it will interface with the supply systems, the loads to
which it will be connected and the service environment. Furthermore, the switching devices and components
incorporated within an ASSEMBLY must be selected to be compatible with one another and with the characteristics of
the complete ASSEMBLY. A new term, interface characteristics, has thus been proposed to describe these properties
and to differentiate between those which will apply to the complete ASSEMBLY and those which will be restricted to a
particular circuit of an ASSEMBLY.
The external interface characteristics of the complete ASSEMBLY will be required to be specified to ensure that the
ASSEMBLY is suitable for the intended application. The purchaser, therefore, will have to provide the manufacturer
with sufficient information to enable the characteristics to be selected.
The interface characteristics which will have to be assigned to the complete ASSEMBLY will be:

rated voltage (Un);

rated operational voltage (Ue);

rated impulse withstand voltage (Uimp);

rated current (In);

rated diversity factor (RDF);

rated peak withstand current (Ipk);

rated short-time withstand current (Icw);

rated conditional current (Icc);

rated frequency;

EMC classification.
The internal interface characteristics, relating to the circuits within the ASSEMBLY, will be the concern of the
manufacturer, whose responsibility it will be to ensure that the characteristics of the switching devices and components
have been selected to be compatible with one another and with the interface characteristics of the complete
ASSEMBLY.
Design verification
In order to avoid the controversy associated with the type-tested and partially type-tested ASSEMBLIES, the Project
Team proposed that the terms should not be employed in the new series of standards and that the concept of design
verification should be introduced to demonstrate that the ASSEMBLY meets the requirements of the standard.
The intention of design verification is to offer the manufacturer a number of alternative and equally acceptable means of
conformity consisting of:

verification testing (potentially destructive);

non-destructive measurement;

calculation;

application of design rules.
It is intended that none of the alternatives should be considered superior to the others, but it is probable that moving
from the first, through the options, to the last will result in increasingly more conservative design approach which will
be required to compensate for the omission of testing.
The multi option approach will enable the manufacturer to choose the most appropriate method to be employed for the
verification of a particular requirement. If he has invested in a comprehensive testing laboratory, he may decide to
perform verification tests, previously classified as type tests, but if he does not possess the necessary test equipment, he
may choose to follow strict design rules which could incur higher manufacturing costs. The opportunity will thus exist
for diverse manufacturers with different resources and circumstances to conform to a common standard at
approximately equivalent overall costs.
The requirements to be verified and the alternative verification methods are listed in Table 1.
Requirement to be verified
Verification method
Test
Strength of materials
√
Degree of protection of enclosures
√
Measurement
Clearances and creepage distances
√
Earth effectiveness for internal faults
√
√
Earth effectiveness for external faults
Calculation
Design rules
√
√
Incorporation of devices and components
√
Internal electrical circuits and connections
√
Terminals for external conductors
√
Power-frequency withstand voltage
√
Impulse withstand voltage
√
√
Temperature-rise limits
√
√
Short-circuit withstand strength
√
Electromagnetic compatibility
√
Mechanical operation.
√
√
√
√
√
√
√
√ – Verification allowed by this method
Table 1: Requirements to be verified
Not all requirements will lend themselves to the same number of options, as the avoidance of testing may be
inappropriate in some instances. For example, it may be inadvisable to design an assembly with 6300 A busbars and a
short-time withstand rating of 120 kA for 1 s without performing temperature-rise and short-circuit tests.
A review of the requirements to be verified, as well as the alternative methods to be employed, was undertaken by the
Project Team and the most significant changes are considered in detail.
Strength of materials and parts
Many of the requirements of IEC 62208 – Empty enclosures for low-voltage switchgear and controlgear assemblies [9]
have been incorporated into the draft standard and applied to the verification of the strength of the enclosure and parts
of the ASSEMBLY. If the enclosure of an ASSEMBLY conforms to that standard and has not been modified such that
its performance has been impaired, it will not be necessary to perform further tests on that individual enclosure.
New requirements to be verified will be:

lifting of transport sections;

resistance to corrosion of ferrous parts of enclosures, hinges, locks and fastenings;

thermal stability and resistance of insulating materials to heat and fire;

resistance of outdoor enclosures to ultra-violet radiation.
Insulation
Clearances
Clearances must be sufficient to enable the declared impulse withstand voltage to be achieved and they will be specified
for standard values of rated impulse withstand voltage. Case B, for homogeneous fields, will be removed as such ideal
conductor configurations are considered not to exist in practical ASSEMBLIES.
The previous view that clearances could be confirmed by the application of an impulse voltage test is no longer
supported, as clearances are a physical requirement and must be verified by measurement.
Creepage distances
Creepage distances are determined by the rated insulation voltage of a circuit together with the material group and
pollution degree. Pollution degree 4 will be removed as an environmental condition for ASSEMBLIES, as insulation
co-ordination cannot be achieved if pollution is persistently conductive.
Creepage distances may only be verified by measurement.
Dielectric properties
The ability of the ASSEMBLY to withstand temporary and transient overvoltages on the system will be verified
respectively by the application of power-frequency withstand voltage and impulse withstand voltage tests. At the
prescribed test voltages, disruptive discharges may occur and hence the tests may be potentially destructive. The
duration of the power-frequency test has been reduced in a recent amendment from 1 minute to 5 s and this value will
be used for design verification in order to minimise the damage that might be sustained by ASSEMBLIES which are
intended to be placed in service.
Impulse withstand voltage test
As the impulse withstand voltage determines the clearances required and defines the ability of the ASSEMBLY to
interface with the supply system, it will be necessary for it to be a mandatory rated characteristic.
The multi option approach will offer the following alternative means for the verification of the rated impulse withstand
voltage:

impulse voltage test at the specified peak value corresponding to an altitude of 2000 m;

power-frequency test at the same peak value but with a duration of 15 ms to 100 ms;

d.c. voltage test at the same peak value but with a duration of 15 ms to 100 ms;

avoidance of testing by the adoption of clearances of 1,5 times the standard values.
Temperature-rise
General
The present standard requires that, for type-tested ASSEMBLIES, representative combinations of circuits should be
tested for temperature-rise by either the passage of current or, in the case of comparatively low rated currents, the use of
heating resistors. Extrapolation from reference data is permitted for partially type-tested ASSEMBLIES as described in
IEC 60890 – A method of temperature-rise assessment by extrapolation for partially type-tested assemblies of lowvoltage switchgear and controlgear [10].
In practice, the number of permutations and combinations of circuits that may be required to provide customised
distribution switchboards and motor control centres is so great that it is commercially impractical to test all possible
arrangements.
It was decided, therefore, that a modular approach should be adopted whereby each functional unit could be tested
separately so that it could be employed within permissible defined arrangements.
The alternative methods of verification will be extended also to include:

testing with current;

testing of enclosure power loss;

measurement of dimensions;

calculation of power loss;

design rules.
Testing with current
General
ASSEMBLIES will be considered to be either modular systems or specific arrangements.
Modular system
The concept of the modular system will be introduced to enable complex ASSEMBLIES, such as motor control centres,
to be constructed by arranging individually verified functional units within the ASSEMBLY in an approved manner.
The functional units to be verified will be:

main busbars tested at their rated current with no incoming or outgoing conductors connected;

distribution busbars tested at their rated current with no outgoing conductors connected. The main busbars will
be connected and they will carry their normal current;

incoming and outgoing functional units tested singly at their rated current and placed in the most onerous
location within the ASSEMBLY. The main or distribution busbars will be connected and they will carry their
normal current;

integrated ASSEMBLIES, such as moulded case circuit-breaker distribution boards, tested at the rated current
of the incoming device with each outgoing circuit carrying its rated current multiplied by the rated diversity
factor.
Functional units, with the exception of busbars, will be considered to have similar thermal behaviour and constitute a
range if they are the same in the following respects:

function and wiring diagram;

device frame size and manufacturer’s series;

compartment size;

device mounting and arrangement within the compartment;

type and arrangement of conductors;

functional unit location.
The particular critical functional units selected for test will be those with the highest rated current or power dissipation
and they will be assigned a de-rating factor as appropriate to take account of their enclosed location. This data will be
used to establish the rated current of other functional units in the range.
Specific arrangements
It will be permissible to verify a specific arrangement by testing the combination of functional units within the
ASSEMBLY or as individual functional units.
As a combined ASSEMBLY, the functional units will be tested at their rated currents. If the sum of the rated currents of
the outgoing circuits exceeds the rated current of the incoming unit or distribution busbar, the outgoing units will be
arranged in groups which correspond to the rating of the incomer. The functional units within each group will be
arranged to produce the highest temperature-rise and each functional unit will be required to be included within at least
one group.
If it is decided to consider the ASSEMBLY as a collection of individual functional units, each functional unit will be
verified in the manner required for a modular system. The ASSEMBLY comprising the verified functional units will
then be tested with the incoming circuit carrying its rated current and with each outgoing circuit carrying its rated
current multiplied by the rated diversity factor.
The rated diversity factor will be redefined, however, as the “per unit value of the rated current, declared by the
manufacturer, to which outgoing circuits of an ASSEMBLY can be continuously and simultaneously loaded”, as it is
considered previously to have been misunderstood and misapplied.
The extent to which the construction of an ASSEMBLY may be allowed to deviate from the verified design without
invalidating the verification will be limited to the following:

reduction in the rating of the busbars by reducing the height, thickness or number of the laminations, provided
that there is no change to the busbar arrangement, distance between phases, enclosure or compartment;

increase in overall dimensions of the ASSEMBLY;

increased ventilation of the ASSEMBLY;

reduction in the power loss in a section.
Testing of enclosure power loss
The power loss capability of an enclosure for use within an ASSEMBLY will be permitted to be verified by placing
heating resistors at suitably distributed locations throughout the enclosure. The temperature-rise of the air at the top of
the enclosure will be limited to 20 K, where the power loss of the load carrying components is proportional to the
square of the current, and to 15 K, if the component losses are independent of current. The temperature-rise of the
enclosure will also be required to be within the limits of the standard.
Measurement of dimensions
Where it is intended to use the same components in a larger enclosure than that of an ASSEMBLY which has been
verified, the height, width and depth of the proposed enclosure will have to be measured to confirm that none of the
dimensions is less than the reference design. The examination will also establish that the ASSEMBLY is of the same
construction and that the positioning of components, internal partitions and barriers together with ventilation is the
same.
Calculation of power loss
Assessment of power loss for an ASSEMBLY will be permitted to be undertaken in accordance with IEC 60890 [11], if
the supply current does not exceed 1600 A and the following conditions are met:

the power loss capability of the switching devices and components is known;

the ratings of the switching devices and components in the main circuit will not exceed the lower of either 80
% of the enclosed thermal current or that current which would, by calculation, raise the air temperature
adjacent to the component to the maximum value permitted by the component manufacturer;

control circuit components are able to operate at the calculated local air temperature;

conductors have a minimum cross section as specified by this standard.
Verification will be achieved if the calculated air temperature in the vicinity of any component is within the limit
declared by the component manufacturer.
Design rules
The observance of design rules will be limited to single compartment ASSEMBLIES where the supply current does not
exceed 630 A and the following conditions are met:

the power loss capability of the enclosure, switching devices and components is known;

the ratings of the switching devices and components in the main circuit will not exceed the lower of either 80
% of the enclosed thermal current or that current permitted at an ambient temperature of 55 °C;

control circuit components are able to operate at an ambient temperature of 55 °C;

there is an even distribution of power loss throughout the enclosure;

the flow of air inside the enclosure is not impeded;

precautions are taken to minimise eddy current and hysteresis losses at currents exceeding 200 A;

conductors have a minimum cross section as specified by this standard.
Verification will be achieved if the total power loss of all components and conductors within the ASSEMBLY does not
exceed the power loss capability of the enclosure.
Short-circuit strength
General
ASSEMBLIES will continue to be exempt from the verification of short-circuit withstand strength in the following
cases:

the rated short-time withstand current of the ASSEMBLY does not exceed 10 kA;

ASSEMBLIES protected by a current limiting device with a cut-off current not exceeding 17 kA;

auxiliary circuits supplied by a transformer with a rating of 10 kVA at not less than 110 V or 1,6 kVA at less
than 110 V.
Busbar systems, switching devices and components which have already been tested for the conditions of the
ASSEMBLY will not be exempt from verification.
The alternative methods of verification will be by testing, the application of design rules or by derivation from a
verified and tested reference design.
In order to verify an ASSEMBLY by any of the alternative methods, a tested reference design will have to exist.
Testing
All reference ASSEMBLY designs will be required to be tested.
Busbars will be tested at their rated short-time withstand current and peak withstand current.
In addition, if the ASSEMBLY incorporates a short-circuit protective device (SCPD), the ASSEMBLY will be required
to be tested at the incoming prospective short-circuit current for a sufficient time for the SCPD to operate. Where the
ASSEMBLY is intended to be protected by an upstream SCPD, the test will be performed at the incoming let-through
peak current and energy as specified by the SCPD manufacturer.
Each outgoing circuit will be required to be tested with a short-circuit applied to its outgoing terminals. If there is a
switching device in that circuit, it will be closed before commencement of the test. Where the device is an SCPD, it will
be allowed to clear the fault, but if it has no fault interrupting capability, the duration of the test will correspond to the
short-time current rating of the busbars.
Design rules
The rules will require that the ASSEMBLY satisfies the following criteria when compared to a verified reference
design:

short-circuit withstand of each circuit is not greater;

busbar supports are of the same type, shape and material;

cross sectional dimensions of the busbars and connections are not less;

spacing of the busbars and connections is not less;

axial distance between busbar supports is not greater;

material and material properties of the conductors are the same;

short-circuit protective devices are of the same manufacturer, make and type;

short-circuit protective devices have the same or better current limiting characteristics;

non-protected live conductor lengths are not greater;

ASSEMBLY enclosure of the same design and type;

ASSEMBLY enclosure dimensions are not less;

ASSEMBLY compartments of the same mechanical design;

ASSEMBLY compartment dimensions are not less.
In addition, if the ASSEMBLY includes an enclosure, the reference design will have to have been verified by test
together with its enclosure. So called certified busbar systems which have not been tested in the enclosure with which
they are to be used will not be permitted.
Derivation from a reference design
The verification of the short-time withstand strength of an ASSEMBLY by extrapolation from a tested arrangement was
established by the introduction of IEC 61117 – A method of assessing the short-circuit withstand strength of partially
type-tested assemblies [11]. It will be permitted to use this standard where the busbar arrangement may be derived from
a reference design which has been verified by test and where the acceptance criteria for the application of design rules
have been met.
Completion of the new standard
The new series of standards has been assigned the designation IEC 61439 [7] . Part 1 – General rules and Part 2 –
Power switchgear and controlgear assemblies exist as committee drafts for voting (CDVs) and will be circulated to
national committees towards the end of 2004. As earlier drafts received a significant number of national committee
comments, it is anticipated that the documents will be supported without major changes. The final draft international
standards (FDISs) should be issued in 2005 and may be published in that year.
Work has not commenced on the remaining product standards, but this will probably start shortly after the CDVs have
received positive support and, thereafter, they will be issued progressively.
References
[1] IEC 60439 series: Low-voltage switchgear and controlgear assemblies, International Electrotechnical Commission,
Geneva, Switzerland.
[2] IEC 60439-1: Low-voltage switchgear and controlgear assemblies – Part 1: Type-tested and partially type-tested
assemblies, International Electrotechnical Commission, Geneva, Switzerland, September 1999.
[3] IEC 60439-2: Low-voltage switchgear and controlgear assemblies – Part 2: Particular requirements for busbar
trunking systems (busways), International Electrotechnical Commission, Geneva, Switzerland, March 2000.
[4] IEC 60439-3: Low-voltage switchgear and controlgear assemblies – Part 3: Particular requirements for lowvoltage switchgear and controlgear assemblies intended to be installed in places where unskilled persons have access
for their use – Distribution boards, International Electrotechnical Commission, Geneva, Switzerland, May 2001.
[5] IEC 60439-4: Low-voltage switchgear and controlgear assemblies – Part 4: Particular requirements for assemblies
for construction sites (ACS), International Electrotechnical Commission, Geneva, Switzerland, June 2004.
[6] IEC 60439-5: Low-voltage switchgear and controlgear assemblies – Part 5: Particular requirements for assemblies
intended to be installed outdoors in public places – Cable distribution cabinets (CDSs) for power distribution in
networks, International Electrotechnical Commission, Geneva, Switzerland, October 1998.
[7] Draft CDV: IEC 61439 series – Low-voltage switchgear and controlgear assemblies, International Electrotechnical
Commission, Geneva, Switzerland.
[8] IEC 60947 series: Low-voltage switchgear and controlgear, International Electrotechnical Commission, Geneva,
Switzerland.
[9] IEC 62208: Empty enclosures for low-voltage switchgear and controlgear assemblies – General requirements,
International Electrotechnical Commission, Geneva, Switzerland, November 2002.
[10] IEC 60890: A method of temperature-rise assessment by extrapolation for partially type-tested assemblies (PTTA)
of low-voltage switchgear and controlgear, International Electrotechnical Commission, Geneva, Switzerland, July
1987.
[11] IEC 61117: A method of assessing the short-circuit withstand strength of partially type-tested assemblies (PTTA),
International Electrotechnical Commission, Geneva, Switzerland, June 1992.