IEC 60364-4-43:2008

IEC 60364-4-43
Edition 3.0 2008-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Low-voltage electrical installations –
Part 4-43: Protection for safety – Protection against overcurrent

Installations électriques à basse tension –
Partie 4-43: Protection pour assurer la sécurité – Protection contre les
surintensités
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IEC 60364-4-43
Edition 3.0 2008-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Low-voltage electrical installations –
Part 4-43: Protection for safety – Protection against overcurrent

Installations électriques à basse tension –
Partie 4-43: Protection pour assurer la sécurité – Protection contre les
surintensités
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
V
CODE PRIX
ICS 29.120.50; 91.140.50 ISBN 2-8318- 8318-9922-2
– 2 – 60364-4-43 © IEC:2008
CONTENTS
FOREWORD.4

43 Protection against overcurrent.6
430.1 Scope.6
430.2 Normative references.6
430.3 General requirements.7
431 Requirements according to the nature of the circuits.7
431.1 Protection of line conductors .7
431.2 Protection of the neutral conductor .7
431.3 Disconnection and reconnection of the neutral conductor in multi-phase
systems .8
432 Nature of protective devices .8
432.1 Devices providing protection against both overload current and short-circuit
current.8
432.2 Devices ensuring protection against overload current only.9
432.3 Devices ensuring protection against short-circuit current only.9
432.4 Characteristics of protective devices.9
433 Protection against overload current .9
433.1 Coordination between conductors and overload protective devices.9
433.2 Position of devices for overload protection.10
433.3 Omission of devices for protection against overload .10
433.4 Overload protection of conductors in parallel .11
434 Protection against short-circuit currents .12
434.1 Determination of prospective short-circuit currents .12
434.2 Position of devices for short-circuit protection.12
434.3 Omission of devices for protection against short-circuit .12
434.4 Short-circuit protection of conductors in parallel .13
434.5 Characteristics of short-circuit protective devices .13
435 Coordination of overload and short-circuit protection .15
435.1 Protection afforded by one device.15
435.2 Protection afforded by separate devices .15
436 Limitation of overcurrent by characteristics of supply .15

Annex A (informative) Protection of conductors in parallel against overcurrent.16
Annex B (informative) Conditions 1 and 2 of 433.1.21
Annex C (informative) Position or omission of devices for overload protection .22
Annex D (informative) Position or omission of devices for short-circuit protection .25
Annex E (informative) List of notes concerning certain countries.28

Bibliography.30

Figure A.1 – Circuit in which an overload protective device is provided for each of the
m conductors in parallel.18
Figure A.2 – Circuit in which a single overload protective device is provided for the m
conductors in parallel.18

60364-4-43 © IEC:2008 – 3 –
Figure A.3 – Current flow at the beginning of the fault.19
Figure A.4 – Current flow after operation of the protective device cs.19
Figure A.5 – Illustration of linked protective device .20
Figure B.1 – Illustration of conditions 1 and 2 of 433.1.21
Figure C.1 – Overload protective device (P ) NOT at the origin of branch circuit (B)
(refer to 433.2.2a)).22
Figure C.2 – Overload protective device (P ) installed within 3 m of the origin of the
branch circuit (B) (refer to 433.2.2b)) .23
Figure C.3 – Illustration of cases where overload protection may be omitted (refer to
433.3.1a), b) and d)).23
Figure C.4 – Illustration of cases where overload protection may be omitted in an IT
system .24
Figure D.1 – Limited change of position of short-circuit protective device (P ) on a
branch circuit (refer to 434.2.1).25
Figure D.2 – Short-circuit protective device P installed at a point on the supply side of
the origin of a branch circuit (refer to 434.2.2) .26
Figure D.3 – Situation where the short-circuit protective device may be omitted for
some applications (refer to 434.3).27

Table 43A – Values of k for conductors.14

– 4 – 60364-4-43 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE ELECTRICAL INSTALLATIONS –

Part 4-43: Protection for safety –
Protection against overcurrent

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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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.
International Standard IEC 60364-4-43 has been prepared by IEC technical committee 64:
Electrical installations and protection against electric shock.
This third edition cancels and replaces the second edition, published in 2001, and constitutes
a technical revision.
The main changes with respect to the previous edition are listed below:
– Annex B “IEC 60364 – Parts 1 to 6: Restructuring” deleted.
– Introduction of new informative Annexes B, C and D.
– Information concerning flexible cables added to Scope.
– The word “phase” changed to “line” throughout the standard.
– Requirement not to distribute the neutral in IT systems changed to a NOTE.
– Requirements added for overload detection for the neutral conductor for harmonic
currents.
60364-4-43 © IEC:2008 – 5 –
– Requirement that devices for protection against short-circuit current be capable of making
as well as breaking short-circuit current added.
– Information added to clarify protection against overload current.
– Requirements where devices for protection against overload need not be provided
expanded.
– More examples given where omission of devices for protection against overload is
permitted.
– Requirements where devices for protection against short-circuit need not be provided
expanded.
– Requirements for short-circuit current ratings of busbar trunking systems added.
The text of this standard is based on the following documents:
FDIS Report on voting
64/1641/FDIS 64/1656/RVD
Full information on the voting for the approval of this standard 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.
The reader's attention is drawn to the fact that Annex E lists all of the "in-some-country"
clauses on differing practices of a less permanent nature relating to the subject of this
standard.
A list of all parts in the IEC 60364 series, under the general title Low-voltage electrical
installations, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
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.
The contents of the corrigendum of October 2008 have been included in this copy.

– 6 – 60364-4-43 © IEC:2008
LOW-VOLTAGE ELECTRICAL INSTALLATIONS –

Part 4-43: Protection for safety –
Protection against overcurrent

43 Protection against overcurrent
430.1 Scope
This part of IEC 60364 provides requirements for the protection of live conductors from the
effects of overcurrents.
This standard describes how live conductors are protected by one or more devices for the
automatic disconnection of the supply in the event of overload (Clause 433) and short-circuit
(Clause 434) except in cases where the overcurrent is limited in accordance with Clause 436
or where the conditions described in 433.3 (omission of devices for protection against
overload) or 434.3 (omission of devices for protection against short-circuit) are met.
Coordination of overload protection and short-circuit protection is also covered (Clause 435).
NOTE 1 Live conductors protected against overload in accordance with Clause 433 are considered to be
protected also against faults likely to cause overcurrents of a magnitude similar to overload currents.
NOTE 2 The requirements of this standard do not take account of external influences.
NOTE 3 Protection of conductors according to this standard does not necessarily protect the equipment
connected to the conductors.
NOTE 4 Flexible cables connecting equipment by plugs and socket-outlet to fixed installations are not part of the
scope of this standard and for this reason are not necessarily protected against overcurrent.
NOTE 5 Disconnection does not mean isolation in this standard.
430.2 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 60269-2, Low-voltage fuses – Part 2: Supplementary requirements for fuses for use by
authorized persons (fuses mainly for industrial application) – Examples of standardized
systems of fuses A to I
IEC 60269-3, Low-voltage fuses – Part 3: Supplementary requirements for fuses for use by
unskilled persons (fuses mainly for household and similar applications) – Examples of
standardized systems of fuses A to F
IEC 60269-4, Low-voltage fuses – Part 4: Supplementary requirements for fuse-links for the
protection of semiconductor devices
IEC 60364-4-41, Low-voltage electrical installations – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60364-5-52:2001, Electrical installations of buildings – Part 5-52: Selection and erection
of electrical equipment – Wiring systems
IEC 60439-2, Low-voltage switchgear and controlgear assemblies – Part 2: Particular
requirements for busbar trunking systems (busways)

60364-4-43 © IEC:2008 – 7 –
IEC 60724, Short-circuit temperature limits of electric cables with rated voltages of 1 kV (U =
m
1,2 kV) and 3 kV (U = 3,6 kV)
m
IEC 60898 (all parts), Electrical accessories – Circuit-breakers for overcurrent protection for
household and similar installations
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 60947-6-2, Low-voltage switchgear and controlgear – Part 6-2: Multiple function
equipment – Control and protective switching devices (or equipment) (CPS)
IEC 61009 (all parts), Residual current operated circuit-breakers with integral overcurrent
protection for household and similar uses (RCBOs)
IEC 61534 (all parts), Powertrack systems
430.3 General requirements
Protective devices shall be provided to disconnect any overcurrent in the circuit conductors
before such a current could cause danger due to thermal or mechanical effects detrimental to
insulation, joints, terminations or material surrounding the conductors.
431 Requirements according to the nature of the circuits
431.1 Protection of line conductors
431.1.1 Detection of overcurrent shall be provided for all line conductors, except where
431.1.2 applies. It shall cause the disconnection of the conductor in which the overcurrent is
detected but not necessarily the disconnection of the other live conductors.
If disconnection of a single phase may cause danger, for example in the case of a three-
phase motor, appropriate precautions shall be taken.
431.1.2 In a TT or TN system, for a circuit supplied between line conductors and in which
the neutral conductor is not distributed, overcurrent detection need not be provided for one of
the line conductors, provided that the following conditions are simultaneously fulfilled:
a) there exists, in the same circuit or on the supply side, protection intended to detect
unbalanced loads and intended to cause disconnection of all the line conductors;
b) the neutral conductor is not distributed from an artificial neutral point of the circuits
situated on the load side of the protective device mentioned in a).
431.2 Protection of the neutral conductor
431.2.1 TT or TN systems
Where the cross-sectional area of the neutral conductor is at least equivalent to that of the
line conductors, and the current in the neutral is expected not to exceed the value in the line
conductors, it is not necessary to provide overcurrent detection for the neutral conductor or a
disconnecting device for that conductor.

– 8 – 60364-4-43 © IEC:2008
Where the cross-sectional area of the neutral conductor is less than that of the line
conductors, it is necessary to provide overcurrent detection for the neutral conductor,
appropriate to the cross-sectional area of that conductor; this detection shall cause the
disconnection of the line conductors, but not necessarily of the neutral conductor.
In both cases the neutral conductor shall be protected against short-circuit current.
NOTE This protection may be achieved by the overcurrent protective devices in the line conductors. In that case it
is not necessary to provide overcurrent protection for the neutral conductor or a disconnecting device for that
conductor.
Where the current in the neutral conductor is expected to exceed the value in the line
conductors, refer to 431.2.3.
Except for disconnection the requirements for a neutral conductor apply to a PEN conductor.
431.2.2 IT systems
Where the neutral conductor is distributed, it is necessary to provide overcurrent detection for
the neutral conductor of every circuit. The overcurrent detection shall cause the disconnection
of all the live conductors of the corresponding circuit, including the neutral conductor. This
measure is not necessary if
– the particular neutral conductor is effectively protected against overcurrent by a
protective device placed on the supply side, for example at the origin of the installation,
or if
– the particular circuit is protected by a residual current operated protective device with a
rated residual current not exceeding 0,20 times the current-carrying capacity of the
corresponding neutral conductor. This device shall disconnect all the live conductors of
the corresponding circuit, including the neutral conductor. The device shall have sufficient
breaking capacity for all poles.
NOTE In IT systems, it is strongly recommended that the neutral conductor should not be distributed.
431.2.3 Harmonic currents
Overload detection shall be provided for the neutral conductor in a multi-phase circuit where
the harmonic content of the line currents is such that the current in the neutral conductor is
expected to exceed the current-carrying capacity of that conductor. The overload detection
shall be compatible with the nature of the current through the neutral and shall cause the
disconnection of the line conductors but not necessarily the neutral conductor. Where the
neutral is disconnected, the requirements of 431.3 apply.
NOTE Further requirements regarding protection of neutral conductors are given in IEC 60364-5-52.
431.3 Disconnection and reconnection of the neutral conductor in multi-phase systems
Where disconnection of the neutral conductor is required, disconnection and reconnection
shall be such that the neutral conductor shall not be disconnected before the line conductors
and shall be reconnected at the same time as or before the line conductors.
432 Nature of protective devices
The protective devices shall be of the appropriate types indicated by 432.1 to 432.3.
432.1 Devices providing protection against both overload current and short-circuit
current
Except as stated in 434.5.1, a device providing protection against both overload and short-
circuit current shall be capable of breaking and, for a circuit-breaker, making any overcurrent

60364-4-43 © IEC:2008 – 9 –
up to and including the prospective short-circuit current at the point where the device is
installed. Such devices may be:
– circuit-breakers incorporating overload and short-circuit release;
– circuit-breakers in conjunction with fuses;
– fuses having fuse links with gG characteristics.
NOTE 1 The fuse comprises all the parts that form the complete protective device.
NOTE 2 This subclause does not exclude the use of other protective devices if the requirements in 433.1 and
434.5 are fulfilled.
432.2 Devices ensuring protection against overload current only
These protective devices shall satisfy the requirements of Clause 433 and may have an
interrupting capacity below the value of the prospective short-circuit current at the point where
the devices are installed.
NOTE 1 These devices are generally inverse time lag protective devices.
NOTE 2 Fuses type aM do not protect against overload.
432.3 Devices ensuring protection against short-circuit current only
A device providing protection against short-circuit current only shall be installed where
overload protection is achieved by other means or where Clause 433 permits overload
protection to be dispensed with. Such a device shall be capable of breaking, and for a circuit-
breaker making, the short-circuit current up to and including the prospective short-circuit
current. Such a device shall satisfy the requirements of Clause 434.
Such devices may be
− circuit-breakers with short-circuit release only,
− fuses with gM, aM type fuse links.
432.4 Characteristics of protective devices
The operating characteristics of overcurrent protective devices shall comply with those
specified in, for example, IEC 60898, IEC 60947-2, IEC 60947-6-2, IEC 61009, IEC 60269-2,
IEC 60269-3, IEC 60269-4 or IEC 60947-3.
NOTE The use of other devices is not excluded provided that their time/current characteristics provide an
equivalent level of protection to that specified in this clause.
433 Protection against overload current
433.1 Coordination between conductors and overload protective devices
The operating characteristics of a device protecting a cable against overload shall satisfy the
two following conditions:
I ≤ I ≤ I (1)
B n Z
≤ 1,45 × I (2)
I
2 Z
where
is the design current for that circuit;
I
B
I is the continuous current-carrying capacity of the cable (see Clause 523);

Z
is the rated current of the protective device;
I
n
NOTE 1 For adjustable protective devices, the rated current I is the current setting selected.
n
– 10 – 60364-4-43 © IEC:2008
I is the current ensuring effective operation in the conventional time of the protective
device.
The current I ensuring effective operation of the protective device shall be provided by the
manufacturer or as given in the product standard.
Protection in accordance with this clause may not ensure protection in certain cases, for
example where sustained overcurrents less than I occur. In such cases, consideration should

be given to selecting a cable with a larger cross-sectional area.
NOTE 2 I is the design current through the line or the permanent current through neutral in case of a high level
B
of the third harmonic
NOTE 3 The current ensuring effective operation in the conventional time of protective devices may also be
named I or I according to the product standards. Both I and I are multiples of I and attention should be given to

t f
t f n
the correct representation of values and indexes.
NOTE 4 See Annex B for an illustration of conditions (1) and (2) of 433.1.
NOTE 5 Design current I can be considered as an actual current I after applying correction factors. See Clause
B a
311.
433.2 Position of devices for overload protection
433.2.1 A device ensuring protection against overload shall be placed at the point where a
change, such as a change in cross-sectional area, nature, method of installation or in
constitution, causes a reduction in the value of current-carrying capacity of the conductors,
except where 433.2.2 and 433.3 apply.
433.2.2 The device protecting the conductor against overload may be placed along the run
of that conductor if the part of the run between the point where a change occurs (in cross-
sectional area, nature, method of installation or constitution) and the position of the protective
device has neither branch circuits nor socket-outlets and fulfils at least one of the following
two conditions:
a) it is protected against short-circuit current in accordance with the requirements stated in
Clause 434;
b) its length does not exceed 3 m, it is carried out in such a manner as to reduce the risk of
short-circuit to a minimum, and it is installed in such a manner as to reduce to a minimum
the risk of fire or danger to persons (see also 434.2.1).
NOTE For installation according to a) see Figure C.1. For installation according to b) see Figure C.2.
433.3 Omission of devices for protection against overload
The various cases stated in this subclause shall not be applied to installations situated in
locations presenting a fire risk or risk of explosion or where the requirements for special
installations and locations specify different conditions.
433.3.1 General
Devices for protection against overload need not be provided:
a) for a conductor situated on the load side of a change in cross-sectional area, nature,
method of installation or in constitution, that is effectively protected against overload by a
protective device placed on the supply side;
b) for a conductor that is not likely to carry overload current, provided that this conductor is
protected against short-circuit in accordance with the requirements of Clause 434 and that
it has neither branch circuits nor socket-outlets;
c) at the origin of an installation where the distributor provides an overload device and
agrees that it affords protection to the part of the installation between the origin and the
main distribution point of the installation where further overload protection is provided.

60364-4-43 © IEC:2008 – 11 –
d) for circuits for telecommunications, control, signalling and the like.
NOTE For installations according to a), b) and d), see Figure C.3.
433.3.2 Position or omission of devices for protection against overload in IT systems
433.3.2.1 The provisions in 433.2.2 and 433.3.1 for an alternative position or omission of
devices for protection against overload are not applicable to IT systems unless each circuit
not protected against overload is protected by one of the following means:
a) use of the protective measures described in Clause 412 of IEC 60364-4-41;
b) protection of each circuit by a residual current protective device that will operate
immediately on a second fault;
c) for permanently supervised systems only use of insulation monitoring which either:
− causes the disconnection of the circuit when the first fault occurs, or
− gives a signal indicating the presence of a fault. The fault shall be rectified according
to the operational requirements and recognizing the risk from a second fault.
NOTE It is recommended to install an insulation fault location system according to IEC 61557-9. With the
application of such a system it is possible to detect and locate the insulation fault without interruption of the supply.
433.3.2.2 In IT systems without a neutral conductor, the overload protective device may be
omitted in one of the phase conductors if a residual current protective device is installed in
each circuit.
433.3.3 Cases where omission of devices for overload protection shall be considered
for safety reasons
The omission of devices for protection against overload is permitted for circuits supplying
current-using equipment where unexpected disconnection of the circuit could cause danger or
damage. Examples of such cases include:
• exciter circuits of rotating machines;
• supply circuits of lifting magnets;
• secondary circuits of current transformers;
• circuits which supply fire extinguishing devices;
• circuits supplying safety services (burglar alarm, gas alarms, etc.).
NOTE In such cases, consideration should be given to the provision of an overload alarm.
433.4 Overload protection of conductors in parallel
Where a single protective device protects several conductors in parallel, there shall be no
branch circuits or devices for isolation or switching in the parallel conductors.
This subclause does not preclude the use of ring final circuits.
433.4.1 Equal current sharing between parallel conductors
Where a single device protects conductors in parallel sharing currents equally, the value of I
z
to be used in 433.1 is the sum of the current-carrying capacities of the various conductors.
It is deemed that current sharing is equal if the requirements of the first indent of 523.7 a) of
IEC 60364-5-52:2001 are satisfied.

– 12 – 60364-4-43 © IEC:2008
433.4.2 Unequal current sharing between parallel conductors
Where the use of a single conductor, per phase, is impractical and the currents in the parallel
conductors are unequal, the design current and requirements for overload protection for each
conductor shall be considered individually.
NOTE Currents in parallel conductors are considered to be unequal if the difference between any currents is more
than 10 % of the design current for each conductor. Guidance is given in Clause A.2.
434 Protection against short-circuit currents
This standard only considers the case of short-circuit between conductors belonging to the
same circuit.
434.1 Determination of prospective short-circuit currents
The prospective short-circuit current at every relevant point of the installation shall be
determined. This may be carried out either by calculation or by measurement.
NOTE The prospective short-circuit current at the supply point may be obtained from the supply utility.
434.2 Position of devices for short-circuit protection
A device ensuring protection against short-circuit shall be placed at the point where a
reduction in the cross-sectional area of the conductors or another change causes a change to
the current-carrying capacity of the conductors, except where 434.2.1 434.2.2 or 434.3
applies.
434.2.1 The various cases stated in the following subclause shall not be applied to
installations situated in locations presenting a fire risk or risk of explosion and where special
rules for certain locations specify different conditions. The device for protection against short-
circuit may be placed other than as specified in 434.2, under the following conditions.
In the part of the conductor between the point of reduction of cross-sectional area or other
change and the position of the protective device there shall be no branch circuits nor socket-
outlets and that part of the conductor shall
a) not exceed 3 m in length, and
b) be installed in such a manner as to reduce the risk of a short-circuit to a minimum, and
NOTE 1 This condition may be obtained for example by reinforcing the protection of the wiring against
external influences.
NOTE 2 See Figure D.1.
c) not be placed close to combustible material.
434.2.2 A protective device may be placed on the supply side of the reduced cross-sectional
area or another change made, provided that it possesses an operating characteristic such
that it protects the wiring situated on the load side against short-circuit, in accordance with
434.5.2.
NOTE The requirements of 434.2.2 may be met by the method given in Annex D.
434.3 Omission of devices for protection against short-circuit
Provided that both of the following conditions are simultaneously fulfilled:
• the wiring is installed in such a way as to reduce the risk of a short-circuit to a
minimum (see item b) of 434.2.1), and

60364-4-43 © IEC:2008 – 13 –
• the wiring is not placed close to combustible material,
devices for protection against short-circuit need not be provided for applications such as:
a) conductors connecting generators, transformers, rectifiers, accumulator batteries to the
associated control panels, the protective devices being placed in these panels;
b) circuits where disconnection could cause danger for the operation of the installations
concerned, such as those cited in 433.3.3;
c) certain measuring circuits;
d) at the origin of an installation where the distributor installs one or more devices providing
protection against short-circuit and agrees that such a device affords protection to the part
of the installation between the origin and the main distribution point of the installation
where further short-circuit protection is provided.
434.4 Short-circuit protection of conductors in parallel
A single protective device may protect conductors in parallel against the effects of short-
circuit provided that the operating characteristics of that device ensures its effective operation
should a fault occur at the most onerous position in one of the parallel conductors. Account
shall be taken of the sharing of the short-circuit currents between the parallel conductors. A
fault can be fed from both ends of a parallel conductor.
If operation of a single protective device is not effective, then one or more of the following
measures shall be taken:
a) The wiring shall be carried out in such a way as to reduce to a minimum the risk of a
short-circuit in any parallel conductor, for example by protection against mechanical
damage, and conductors shall be installed in such a manner as to reduce to a minimum
the risk of fire or danger to persons.
b) For two conductors in parallel, a short-circuit protective device shall be provided at the
supply end of each parallel conductor.
c) For more than two conductors in parallel, short-circuit protective devices shall be provided
at the supply and load ends of each parallel conductor.
Guidance is given in Clause A.3.
434.5 Characteristics of short-circuit protective devices
Each short-circuit protective device shall meet the requirements given in 434.5.1.
434.5.1 The rated breaking capacity shall be not less than the prospective maximum short-
circuit current at the place of its installation, except where the following paragraph applies.
A lower rated breaking capacity is permitted if another protective device having the necessary
breaking capacity is installed on the supply side. In that case, the characteristics of the
devices shall be coordinated so that the energy let through by these two devices does not
exceed that which can be withstood without damage by the device on the load side and the
conductors protected by these devices.
NOTE In certain cases other characteristics may need to be taken into account such as dynamic stresses and
arcing energy for the device on the load side. Details of the characteristics needing coordination should be
obtained from the manufacturers of the devices concerned.
434.5.2 For cables and insulated conductors, all current caused by a short-circuit occurring
at any point of the circuit shall be interrupted in a time not exceeding that which brings the
insulation of the conductors to the permitted limit temperature.
For operating times of protective devices <0,1 s where asymmetry of the current is of
2 2
importance and for current-limiting devices k S shall be greater than the value of the let-
through energy (I t) quoted by the manufacturer of the protective device.

– 14 – 60364-4-43 © IEC:2008
Table 43A – Values of k for conductors
Type of conductor insulation
Property/
PVC EPR Rubber Mineral
PVC
condition
Thermoplastic XLPE 60 °C PVC Bare
Thermoplastic
90°C Thermosetting Thermosetting sheathed  unsheathed
Conductor
cross-sectional ≤ 300 >300 ≤ 300 >300
area mm²
Initial
70 90 90 60 70 105
temperature °C
Final
160 140 160 140 250 200 160 250
temperature °C
Conductor
material:
a
Copper 115 103 100 86 143 141 115 135 -115
Aluminium 76 68 66 57 94 93 – –
Tin-soldered
joints in copper 115 – – – – – – –
conductors
a
This value shall be used for bare cables exposed to touch.
NOTE 1 Other values of k are under consideration for:

– small conductors (particularly for cross-sectional areas less than 10 mm²);
– other types of joints in conductors;
– bare conductors.
NOTE 2 The nominal current of the short-circuit protective device may be greater than the current-carrying
capacity of the cable.
NOTE 3 The above factors are based on IEC 60724.
NOTE 4 See Annex A of IEC 60364-5-54:2002 for the calculation-method of factor k.

For short-circuits of duration up to 5 s, the time t, in which a given short-circuit current will
raise the insulation of the conductors from the highest permissible temperature in normal duty
to the limit temperature can, as an approximation, be calculated from the formula:
t = (k * S / I) (3)
where
t is the duration, in s;
S is the cross-sectional area, in mm ;
I is the effective short-circuit current, in A, expressed as an r.m.s. value;
k is a factor ta
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