IEC TR 61912-1:2007
(Main)Low-voltage switchgear and controlgear - Overcurrent protective devices - Part 1: Application of short-circuit ratings
Low-voltage switchgear and controlgear - Overcurrent protective devices - Part 1: Application of short-circuit ratings
This technical report, which serves as an application guide for the short-circuit ratings given in IEC standards for low-voltage switchgear and controlgear and assemblies, summarises the definitions of the ratings and provides examples of their application. This edition cancels and replaces the first edition of IEC 61912, published in 2006, and corrigendum 1 (May 2006). The changes are only editorial.
Appareillage à basse tension - Dispositifs de protection contre les surintensités - Partie 1: Application des caractéristiques de court-circuit
Le présent rapport technique, qui sert de guide d'application pour les caractéristiques de court-circuit données dans les normes CEI de l'appareillage à basse tension et des ensembles d'appareillage, résume les définitions des caractéristiques et donne des exemples de leur application. Cette édition annule et remplace la première édition de la CEI 61912, publiée en 2006, et le corrigendum 1 (mai 2006).Les modifications ne sont que rédactionnelles.
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Standards Content (Sample)
IEC/TR 61912-1
Edition 1.0 2007-08
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Low-voltage switchgear and controlgear – Overcurrent protective devices –
Part 1: Application of short-circuit ratings
Appareillage à basse tension – Dispositifs de protection contre les surintensités –
Partie 1: Application des caractéristiques de court-circuit
IEC/TR 61912-1:2007
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IEC/TR 61912-1
Edition 1.0 2007-08
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Low-voltage switchgear and controlgear – Overcurrent protective devices –
Part 1: Application of short-circuit ratings
Appareillage à basse tension – Dispositifs de protection contre les surintensités –
Partie 1: Application des caractéristiques de court-circuit
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
S
CODE PRIX
ICS 29.130 ISBN 2-8318-9273-2
– 2 – TR 61912-1 © IEC:2007
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Alphabetical list of definitions and characteristics .8
4 Principle of application – Installation .9
5 Characteristics – Low-voltage assemblies (switchboard, distribution board, etc.).9
6 Characteristics – Switching devices.10
6.1 General .10
6.2 Switching devices – Self-protection against short-circuit.10
6.3 Switching devices – Application as SCPD.11
6.3.1 Fuse-combination units and fuses as SCPD .11
6.3.2 Circuit-breakers to IEC 60947-2 as SCPD .11
6.3.3 Control and protective switching devices (CPS) according to
IEC 60947-6-2 as SCPD .12
6.3.4 Circuit-breakers according to IEC 60898-1 (MCBs) and residual
current operated circuit-breaker with integral overcurrent protection
(RCBOs) according to IEC 61009-1 as SCPD .12
7 Examples of the practical application of the product characteristics.13
7.1 General .13
7.2 Protection of cables.13
7.3 Short-circuit protection for LV assemblies.13
7.3.1 Switchgear and controlgear assemblies (switchboard/motor-control
centre (MCC)).13
7.3.2 Busbar trunking systems (BTS) .13
7.4 Short-circuit protection for contactors and starters .14
7.4.1 General .14
7.4.2 Protected switching device and protected starter .14
7.4.3 Control and protective switching device (CPS) according to
IEC 60947-6-2 .
7.5 Short-circuit protection using circuit-breakers for household and similar
installations according to IEC 60898-1 (usually known as MCBs) and residual
current operated circuit-breakers with integral overcurrent protection
(RCBOs) according to IEC 61009-1 .15
Annex A (informative) Interpolation of the suitability of an alternative SCPD for the
protection of contactors and starters (substitution).21
Bibliography.22
Figure 1 – Example of the I t characteristic of a fuse.17
Figure 2 – Example of the I t characteristic of a circuit-breaker .17
Figure 3 – Example of SCPDs in combination .18
Figure 4 – Example of the derivation of a conditional rating from type-test parameters .19
Figure 5a – Co-ordination of a motor-starter with a fuse.20
TR 61912-1 © IEC:2007 – 3 –
Figure 5b – Co-ordination of a motor-starter with circuit-breaker.20
Figure 5 – Illustration of co-ordination between motor-starter and SCPD.20
Table 1 – Ratio k between service short-circuit capacity (I ) and rated short-circuit
cs
capacity (I ) for an MCB according to IEC 60898-1 .16
cn
Table 2 – Value of the prospective test current according to the rated operational
current .16
– 4 – TR 61912-1 © IEC:2007
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
OVERCURRENT PROTECTIVE DEVICES –
Part 1: Application of short-circuit ratings
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 61912-1, which is a technical report, has been prepared by subcommittee 17B: Low-
voltage switchgear and controlgear, of IEC technical committee 17: Switchgear and
controlgear.
This edition cancels and replaces the first edition of IEC 61912, published in 2006, and
corrigendum 1 (May 2006). The changes are only editorial.
TR 61912-1 © IEC:2007 – 5 –
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
17B/1547/DTR 17B/1564/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts of the IEC 61912 series, under the general title Low-voltage switchgear
and controlgear − Overcurrent protective devices, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – TR 61912-1 © IEC:2007
INTRODUCTION
Low-voltage equipment standards IEC 60947 and IEC 60439 currently include short-circuit
ratings for products and assemblies respectively, defined in terms of the ability of the
equipment to operate at a level of peak current, an r.m.s. current for a specified time and/or a
level of current conditional upon a short-circuit protective device in series. In practice the
correct application of the various short-circuit ratings needs to be fully understood by the
circuit designer to avoid leaving a circuit or equipment with inadequate short-circuit
protection. It is also useful to take full advantage of the capability of devices and systems to
avoid over-engineering, with the consequent unnecessary additional cost.
TR 61912-1 © IEC:2007 – 7 –
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
OVERCURRENT PROTECTIVE DEVICES –
Part 1: Application of short-circuit ratings
1 Scope
This technical report, which serves as an application guide for the short-circuit ratings given in
IEC standards for low-voltage switchgear and controlgear and assemblies, summarises the
definitions of the ratings and provides examples of their application.
NOTE This document does not concern itself with household (domestic) installations.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60255 (all parts), Electrical relays
IEC 60269-1, Low-voltage fuses – Part 1: General requirements
IEC 60364 (all parts), Low-voltage electrical installations
IEC 60439-1, Low-voltage switchgear and controlgear assemblies – Part 1: Type-tested and
partially type-tested assemblies
IEC 60439-2, Low-voltage switchgear and controlgear assemblies – Part 2: Particular
requirements for busbar trunking systems (busways)
IEC 60898-1, Electrical accessories – Circuit-breakers for overcurrent protection for
household and similar installations – Part 1: Circuit-breakers for a.c. operation
IEC 60947-1, Low-voltage switchgear and controlgear − Part 1: General rules
IEC 60947-2, Low-voltage switchgear and controlgear − Part 2: Circuit-breakers
IEC 60947-3, Low-voltage switchgear and controlgear − Part 3: Switches, disconnectors,
switch-disconnectors and fuse-combination units
IEC 60947-4-1, Low-voltage switchgear and controlgear − Part 4-1: Contactors and motor-
starters − Electromechanical contactors and motor-starters
IEC 60947-6-2, Low-voltage switchgear and controlgear − Part 6-2: Multiple function
equipment – Control and protective switching devices (or equipment) (CPS)
IEC 61009-1, Residual current operated circuit-breakers with integral overcurrent protection
for household and similar uses (RCBOs) – Part 1: General rules
– 8 – TR 61912-1 © IEC:2007
3 Alphabetical list of definitions and characteristics
Clause
B
Breaking capacity of a fuse-link . 6.3.1
C
Circuit-breaker according to IEC 60898-1 . 6.2 b) ii)
Circuit-breaker according to IEC 60947-2 . 6.2 b) ii)
Conditional short-circuit rating (back-up protection). 4
Control and protective switching device (CPS) according to IEC 60947-6-2 . 6.2 b) iv)
Cut-off current of a circuit-breaker according to IEC 60947-2. 6.3.2
Cut-off current of a fuse-link . 6.3.1
F
Fuse-combination unit to IEC 60947-3 . 6.2 b) i)
O
Operating I t (Joule integral) of a circuit-breaker according to IEC 60947-2 . 6.3.2
Operating I t (Joule integral) of a fuse-link. 6.3.1
P
Peak short-circuit current I. 4
p
Prospective (available) short-circuit current I. 4
cp
Protected starter according to IEC 60947-4-1 . 6.2 b) iii)
Protected switching device according to IEC 60947-4-1. 6.2 b) iii)
R
Rated conditional short-circuit current I of a contactor or motor starter . 7.4.2
q
Rated conditional short-circuit current I of a circuit of an assembly . 5
cc
Rated peak withstand current I of a circuit of an assembly . 5
pk
Rated service short-circuit breaking capacity I of a circuit-breaker according to IEC 60947-2 6.3.2
cs
Rated short-circuit breaking capacity I of a CPS . 6.3.3
cs
Rated short-circuit capacity I of a circuit-breaker according to IEC 60898-1 and of a residual
cn
...........................................................................................................................
current operated circuit-breaker with integral overcurrent protection (RCBO) .
according to IEC 61009-1. 6.3.4
Rated short-circuit making capacity I of a circuit-breaker according to IEC 60947-2 6.3.2
cm
Rated short-time withstand current I of a circuit of an assembly . 5
cw
Rated short-time withstand current I of a circuit-breaker according to IEC 60947-2 6.3.2
cw
Rated ultimate short-circuit breaking capacity I of a circuit-breaker according to IEC 60947-
cu
2. 6.3.2
Residual current operated circuit-breaker with integral overcurrent protection (RCBO)
according to IEC 61009-1. 6.2 b) ii)
S
Service short-circuit capacity I of a circuit-breaker according to IEC 60898-1 and of a
cs
residual .
current operated circuit-breaker with integral overcurrent protection (RCBO) .
according to IEC 61009-1. 7.5
Short-circuit protective device (SCPD) . 4
........................................................................... 4
Steady-state short-circuit current I
k
Symmetrical short-circuit breaking current I. 4
b
TR 61912-1 © IEC:2007 – 9 –
4 Principle of application – Installation
In order to ensure the capability of equipment under short-circuit conditions, the circuit
designer shall firstly have available the value of the prospective fault level at the point of
installation of each item of equipment. This is produced by a system protection study. Short-
circuit parameters are defined in terms that include the following:
• prospective (available) short-circuit current I
cp
current that would flow if the short-circuit were replaced by an ideal connection of
negligible impedance without any change of the supply
• peak short-circuit current I
p
maximum possible instantaneous value of the prospective (available) short-circuit current
• symmetrical short-circuit breaking current I
b
r.m.s. value of an integral cycle of the symmetrical a.c. component of the prospective
(available) short-circuit current at the instant of contact separation of the first pole of a
switching device
• steady-state short-circuit current I
k
r.m.s. value of the prospective short-circuit current which remains, after the decay of the
transient phenomena:
− unlimited
− limited by an SCPD (short-circuit protective device)
Additional useful definitions:
• short-circuit protective device (SCPD)
device intended to protect a circuit or part of a circuit against short-circuit currents by
interrupting them
• conditional short-circuit rating (back-up protection)
short-circuit rating, of a device or an assembly, dependent on an SCPD connected in
series with the device or assembly
5 Characteristics – Low-voltage assemblies (switchboard, distribution
board, etc.)
An assembly will have a short-circuit rating assigned by the manufacturer specified as the
maximum permissible prospective short-circuit current, defined in terms of current and time,
at the point of connection to the incoming terminals. The short-circuit rating of the assembly
should be equal to or exceed the maximum prospective short-circuit current at the point of
connection to the system. The assembly manufacturer is responsible for ensuring the
capability of the equipment between the incoming and outgoing terminals of the assembly
(incoming and outgoing devices, busbars, connections, etc.). The short-circuit rating will have
been determined by the manufacturer, in accordance with the applicable part of the
IEC 60439 series.
The terminology to define the short-circuit rating of an assembly is given in the standard as
follows:
• rated short-time withstand current I (of a circuit of an assembly)
cw
Summarised as: r.m.s value of short-time current that a circuit of an assembly can carry
without damage under specified test conditions, defined in terms of a current and time,
e.g. 20 kA, 0,2 s.
– 10 – TR 61912-1 © IEC:2007
• rated peak withstand current I (of a circuit of an assembly)
pk
Summarised as: value of peak current that a circuit can withstand satisfactorily under
specified test conditions.
• rated conditional short-circuit current I (of a circuit of an assembly)
cc
Summarised as: r.m.s. value of prospective short-circuit current that a circuit, protected by
a specified short-circuit protective device (SCPD), can withstand satisfactorily for the
operating time of that device, under specified test conditions.
NOTE The short-circuit protective device may form an integral part of the assembly or may be a separate
unit.
An assembly may be assigned a value of I alone.
cc
An assembly may be assigned values of I and I (but cannot be assigned a value of I or
cw pk cw
I alone).
pk
An assembly may be assigned values of I , I and I .
cw pk cc
An assembly may be assigned different values of I for different circuit protective devices
cc
and/or system voltages.
An assembly may be assigned different values of I for different short-time periods e.g.
cw
0,2 s, 0,5 s, 1 s.
6 Characteristics – Switching devices
6.1 General
In terms of short-circuit capability, switching devices shall be considered in respect of their
function in the particular application. A switching device is considered in two respects, self-
protection and use as a short-circuit protective device (SCPD) where applicable.
6.2 Switching devices – Self-protection against short-circuit
The following cases are considered:
a) Load and overload switching alone, without any short-circuit switching capability.
In this case the switching device will be short-circuit rated on a similar basis to a circuit of
an assembly (see Clause 5), with a rating of I and/or a conditional short-circuit rating,
cw
but will in addition have a rated short-circuit making capacity I .
cm
b) Load, overload and short-circuit switching capability:
i) Fuse-combination units according to IEC 60947-3 – a fuse-combination unit is normally
self-protecting up to the breaking capacity of the fuse. In this case the short-circuit
breaking function is provided by the integral fuses and the switching device will have a
conditional short-circuit rating.
ii) Circuit-breakers according to IEC 60947-2, circuit-breakers according to IEC 60898-1
and residual current operated circuit-breakers with integral overcurrent protection
(RCBOs) according to IEC 61009-1 – the device will be self-protecting up to its
breaking capacity rating (see 6.3.2). At fault levels above the breaking capacity rating,
a circuit-breaker may be capable of operating with “back-up” protection by an SCPD
(this is in effect a conditional rating, but the term is not generally used in this context).
iii) Protected switching devices and protected starters according to IEC 60947-4-1 – a
contactor, semiconductor controller or a motor-starter, including overload protection, a
manual switching device and an SCPD rated as a unit. These devices have a rated
conditional short-circuit current I and are self-protecting up to this level.
q
TR 61912-1 © IEC:2007 – 11 –
iv) Control and protective switching devices (CPS) according to IEC 60947-6-2 – a
switching device (or equipment) capable of operation other than by hand, but with or
without local manual operating means. A CPS is capable of making, carrying and
breaking currents under normal conditions, including specified operating overload
conditions and of making, carrying for a specified time and breaking currents under
specified abnormal conditions such as those of short-circuits. A CPS has a rated
service short-circuit breaking capacity and is self-protecting up to this level.
6.3 Switching devices – Application as SCPD
6.3.1 Fuse-combination units and fuses as SCPD
Since the short-circuit breaking function in fuse-combination units is provided by the fuses, it
is the fuse characteristics that are considered. These are given in IEC 60269-1 as follows:
• breaking capacity of a fuse-link
Summarised as: value (for a.c. the r.m.s. value of the a.c. component) of prospective
current that a fuse-link is capable of breaking at a stated voltage under prescribed
conditions of use.
• cut-off current of a fuse-link
Summarised as: maximum instantaneous value reached by the current during the breaking
operation of a fuse-link when it operates to prevent the current reaching the prospective
peak.
t (Joule integral) of a fuse-link
• operating I
Summarised as: integral of the square of the current over the operating time of the fuse-
link under short-circuit conditions.
Sometimes referred to as “let-through energy”. When expressed in A s gives the energy
dissipated per ohm and thus represents the thermal effect on the circuit.
See Figure 1: Example of the I t characteristic of a fuse.
6.3.2 Circuit-breakers according to IEC 60947-2 as SCPD
The short-circuit breaking function is provided by the circuit-breaker itself and the following
characteristics should be considered.
Moulded-case circuit-breakers (MCCBs) and air circuit-breakers (ACBs) are rated according
to IEC 60947-2 as follows:
• rated short-circuit making capacity I
cm
Summarised as: maximum peak prospective current that the circuit-breaker can make
satisfactorily.
• rated ultimate short-circuit breaking capacity I
cu
Summarised as: r.m.s prospective current that the circuit-breaker is capable of breaking at
a specified voltage, under defined test conditions which include one break operation and
one make/break operation.
The I rating of a circuit-breaker should be equal to or exceed the prospective (available)
cu
short-circuit current at the point of installation. The exception being where the circuit-
breaker is itself protected by another SCPD, the combination being rated for a higher
short-circuit current.
See Figure 3: Example of SCPDs in combination.
• rated service short-circuit breaking capacity I
cs
Summarised as: r.m.s prospective current that the circuit-breaker is capable of breaking at
a specified voltage, under defined test conditions which include one break operation and
two make/break operations.
– 12 – TR 61912-1 © IEC:2007
The standard specifies fixed relationships to I /I of 25 %, 50 %, 75 % or 100 %.
cs cu
The I rating of a circuit-breaker is applied where assurance of continuity of service is
cs
required after a short-circuit fault.
• rated short-time withstand current I
cw
Summarised as: r.m.s value of short-time current assigned by the manufacturer, based on
specified test conditions.
Minimum values are given in the standard.
A circuit-breaker can only be assigned rated short-time withstand currents I if it is equipped
cw
with a time-delay short-circuit release.
A circuit-breaker may be assigned different values of I for different short-time periods e.g.
cw
0,2 s, 0,5 s, 1 s.
All circuit-breakers according to IEC 60947-2 have values of I and I .
cu cs
Circuit-breaker characteristics not specified in IEC 60947-2 but having application to short-
circuit protection:
• cut-off current of a circuit-breaker
Summarised as: maximum instantaneous value reached by the current during the breaking
operation of a circuit-breaker when it operates to prevent the current reaching the
prospective peak.
NOTE A current limiting circuit-breaker exhibits cut-off under short-circuit conditions. A non-current limiting
circuit-breaker does not exhibit cut-off.
• operating I t (Joule integral) of a circuit-breaker
Summarised as: integral of the square of the current over the operating time of the circuit-
breaker under short-circuit conditions.
Sometimes referred to as “let-through energy”. When expressed in A s gives the energy
dissipated per ohm and thus represents the thermal effect on the circuit.
See Figure 2: Example of the I t characteristic of a circuit-breaker.
Non-automatic circuit-breakers (i.e. without overcurrent sensing) are also used as SCPD in
combination with external overcurrent protective relays according to IEC 60255.
6.3.3 Control and protective switching devices (CPS) according to IEC 60947-6-2 as
SCPD
A CPS has a rated short-circuit breaking capacity I and the application of the CPS as an
cs
SCPD is the same as that for a circuit-breaker (see 6.3.2).
6.3.4 Circuit-breakers according to IEC 60898-1 (MCBs) and residual current
operated circuit-breaker with integral overcurrent protection (RCBOs)
according to IEC 61009-1 as SCPD
The short-circuit breaking function is provided by the circuit-breaker/RCBO itself and the
following characteristic should be considered:
• rated short-circuit capacity I
cn
Summarised as: the ultimate short-circuit breaking capacity of the circuit-breaker.
An MCB/RCBO is also tested for a service short-circuit capacity I , which has a fixed
cs
relationship to I (see Table 1).
cn
TR 61912-1 © IEC:2007 – 13 –
7 Examples of the practical application of the product characteristics
7.1 General
In simple studies only the r.m.s value of steady-state prospective short-circuit current I is
k
quoted. The peak current is assumed to be in a standard relationship to the r.m.s current,
determined by the overall power factor, and taken into account by the respective IEC
standards, e.g. IEC 60947-1, Table 16.
7.2 Protection of cables
The application of short-circuit protective devices to cable protection is detailed in the
installation rules of IEC 60364 and is given by:
2 2 2
(I t) ≤ (k S )
SCPD cable
where
k is a factor depending upon the materials of the cable (conductivity and insulation), and
S is the nominal cross-sectional area of the conductor.
It is accepted that the selection of the protective device on the basis of overload protection
and compliance with the above formula at the breaking capacity of the SCPD, provides
complete short-circuit protection, in the case of non-time delayed devices.
In the case of fuses according to IEC 60269-1 and MCBs according to IEC 60898-1 it is
accepted that selection of the protective device on the basis of overload protection of a cable
allows determination of the cable details for short-circuit protection, since the operating
characteristics are defined in the respective standards. The details are generally presented in
the form of a table in installations rules.
7.3 Short-circuit protection for LV assemblies
7.3.1 Switchgear and controlgear assemblies (switchboard/motor-control centre
(MCC))
The prospective short-circuit current, given as an r.m.s. value, at the input to the switchboard
is obtained from a system protection study.
a) If the switchboard/MCC has an I higher than the prospective current level, then the only
cw
requirement is to limit the time for which a short-circuit could persist to within the
corresponding short-time value. This is achieved by the time-delay setting of short-circuit
releases upstream or at the incomer to the switchboard/MCC.
b) If the switchboard/MCC has an I higher than the prospective current level, then the only
cc
requirement is to include the specified SCPD in the circuit. This may be added in the
circuit upstream or may already be integral to the switchboard/MCC.
NOTE It is important that the SCPDs specified by the manufacturer are used, e.g. fuses not replaced by fuses of
a higher rating or links.
7.3.2 Busbar trunking systems (BTS)
The prospective short-circuit current, given as an r.m.s. value, at the input to the BTS, is
obtained from a system protection study.
a) If the BTS has an I higher than the prospective current level, then the only requirement
cw
is to limit the time for which a short-circuit could persist to within the short-time value. This
is achieved by the time-delay setting of short-circuit releases upstream.
b) If the BTS has an I lower than the prospective current level I but has an I rating
cw k cc
higher than I , then the only requirement is to include the specified SCPD in the circuit
k
– 14 – TR 61912-1 © IEC:2007
upstream or in the busbar trunking feeder unit. The suitability of any given SCPD may be
derived from the cut-off current and Joule-integral characteristics by comparison with type
test parameters.
See Figure 4: Example of the derivation of a conditional rating from type-test parameters.
7.4 Short-circuit protection for contactors and starters
7.4.1 General
Motor-starters and contactors are not generally self-protecting against the effects of short-
circuit and therefore need to be associated with an SCPD. In this particular case, test
procedures according to IEC 60947-4-1 recognise the difficulty of protecting sensitive devices
from damage under heavy short-circuit conditions. Thus a special case of conditional rating is
obtained which allows two types of co-ordination with an SCPD:
– Type “1” co-ordination requires that, under short-circuit conditions, the contactor or starter
shall cause no damage to persons or installation and may not be suitable for further
service without repair or replacement of parts.
– Type “2” co-ordination requires that, under short-circuit conditions, the contactor or starter
shall cause no damage to persons or installation and shall be suitable for further use. The
risk of contact welding is recognised, in which case the manufacturer shall indicate the
measures to be taken as regards the maintenance of the equipment.
These ratings can only be obtained by type-testing and thus the data for the selection of the
SCPD shall be obtained from the manufacturer of the contactor/starter, taking into account the
rated operational current, rated operational voltage and the corresponding utilisation
category.
The rated conditional short-circuit current of contactors and starters backed up by short-circuit
protective device(s) (SCPD(s)), combination starters and protected starters is verified by
short-circuit tests at two levels of prospective current:
a) at the rated conditional short-circuit current I ; and
q
b) an additional test is made at a current “r” as shown in Table 2. The test current “r” is
considered a critical current for a contactor and the test ensures the performance of the
contactor at this level.
NOTE Further information about co-ordination between fuses and contactors/motor-starters is given in
IEC/TR 61459.
Annex A gives conditions for interpolation of the suitability of an alternative SCPD for the
protection of contactors and starters.
7.4.2 Protected switching device and protected starter
These devices according to IEC 60947-4-1 have a rated conditional short-circuit current I .
q
The I rating should equal or exceed the prospective short-circuit current at the point of
q
installation.
The rated conditional short-circuit current I is derived under test conditions which include the
q
method of mounting of the devices, including any enclosure. Within the test procedure
according to IEC 60947-4-1, it is established that the SCPD takes over the current interruption
at a level of current within the breaking capacity of the contactor or controller or motor-starter,
as applicable.
See Figure 5: Illustration of co-ordination between motor-starter and SCPD.
TR 61912-1 © IEC:2007 – 15 –
7.4.3 Control and protective switching device (CPS) according to IEC 60947-6-2
The ability of a CPS to operate on short-circuit is stated in terms of the rated service short-
circuit capacity I and the CPS is self-protecting up to this level.
cs
Additional tests are made on a CPS at two levels of critical current:
a) conventional current “r”, as for contactors and motor-starters (see 7.4.1);
b) conventional current I , at between 15 − 30 times rated current I according to rating.
cr e
A CPS effectively provides a level of co-ordination which provides continuity of service in the
event of a short-circuit, the test conditions for which do not allow contact welding.
7.5 Short-circuit protection using circuit-breakers for household and similar
installations according to IEC 60898-1 (usually known as MCBs) and residual
current operated circuit-breakers with integral overcurrent protection (RCBOs)
according to IEC 61009-1
NOTE This document does not concern itself with household (domestic) installations.
MCBs/RCBOs have a rated short-circuit capacity I , summarised as: r.m.s prospective
cn
current that the circuit-breaker is capable of breaking at a specified voltage, under defined
test conditions which include one break operation and one make/break operation.
MCBs/RCBOs also have a service short-circuit capacity I , summarised as: r.m.s.
cs
prospective current that the device is capable of breaking at a specified voltage, under
defined test conditions which include two break operations and one make/break operation.
The product standard specifies a fixed relationship between I and I (see Table 1).
cs cn
MCBs and RCBOs are marked with the values of I but not with the I values as these are
cn cs
predefined as stated above.
The I rating of an MCB/RCBO should equal or exceed the prospective (available) short-
cn
circuit current at the point of installation.
When applied in other than domestic (household) situations, the MCB may need to be
“backed-up” by another SCPD. Only testing of the required combination is satisfactory and
thus the data shall be obtained from the manufacturer of the SCPD or the manufacturer of the
MCB.
For application outside the scope of IEC 60898-1, i.e. over 125 A rating and/or 440 V rating,
MCBs can be rated in accordance with IEC 60947-2 and applied accordingly (see 6.3.2).
– 16 – TR 61912-1 © IEC:2007
Table 1 – Ratio k between service short-circuit capacity (I ) and
cs
rated short-circuit capacity (I ) for an MCB according to IEC 60898-1
cn
I k
cn
I ≤ 6 000 A
cn
a
0,75
6 000 A < I ≤ 10 000 A
cn
b
I > 10 000 A 0,5
cn
a
Minimum value of I : 6 000 A I = k⋅I
cs cs cn
b
Minimum value of I : 7 500 A
cs
Table 2 – Value of the prospective test current according to
the rated operational current
Rated operational current Prospective test current “r”
a
I (AC-3)
e
A kA
0 < I ≤ 16
e
16 < I ≤ 63
e
63 < I ≤ 125
e 5
125 < I ≤ 315
e
315 < I ≤ 630
e
630 < I ≤ 1 000
e
1 000 < I ≤ 1 600
e
1 600 < I
e
Subject to agreement between manufacturer and user
a
If the contactor or starter is not specified according to utilization category AC-3, the prospective
current “r” shall correspond to the highest rated operational current for any utilization category
claimed by the manufacturer.
TR 61912-1 © IEC:2007 – 17 –
Let-through energy (A²s)
0,1 kA 1 kA 10 kA 100 kA I
cp
IEC 1922/07
Figur
...
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