IEC 60724:2000+AMD1:2008 CSV

IEC 60724
Edition 3.1 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Short-circuit temperature limits of electric cables with rated voltages
of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV)
m m
Limites de température de court-circuit des câbles électriques de tensions
assignées de 1 kV (U = 1,2 kV) et 3 kV (U = 3,6 kV)
m m
IEC 60724:2000+A1:2008
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IEC 60724
Edition 3.1 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Short-circuit temperature limits of electric cables with rated voltages
of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV)
m m
Limites de température de court-circuit des câbles électriques de tensions
assignées de 1 kV (U = 1,2 kV) et 3 kV (U = 3,6 kV)
m m
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CC
CODE PRIX
ICS 29.060.20 ISBN 978-2-88910-259-4
– 2 – 60724 © IEC:2000+A1:2008
CONTENTS
FOREWORD.3
INTRODUCTION.5

1 Scope.6
2 Normative references.6
3 Factors governing the application of the temperature limits .6
3.1 General .6
3.2 Cables.7
3.3 Accessories.7
3.4 Installation conditions .8
4 Maximum permissible short-circuit temperatures for cables with rated voltages
of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV) .8

m m
4.1 Insulation materials.8
4.2 Oversheath and bedding materials where there are no electrical
or other requirements .9
4.3 Conductor/metallic sheath/screen/armour materials and methods of connection .9

Table 1 – Temperature limits for insulation materials .9
Table 2 – Temperature limits for oversheath materials.9
Table 3 – Temperature limits for current-carrying components .10

60724 © IEC:2000+A1:2008 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
SHORT-CIRCUIT TEMPERATURE LIMITS
OF ELECTRIC CABLES WITH RATED VOLTAGES
OF 1 kV (U = 1,2 kV) AND 3 kV (U = 3,6 kV)

m m
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
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60724 has been prepared by IEC technical committee 20: Electric
cables.
This consolidated version of IEC 60724 consists of the third edition (2000) [documents
20/399/FDIS and 20/418/RVD] and its amendment 1 (2008) [documents 20/951/FDIS and
20/974/RVD].
The technical content is therefore identical to the base edition and its amendment and has
been prepared for user convenience.
It bears the edition number 3.1.
A vertical line in the margin shows where the base publication has been modified by
amendment 1.
– 4 – 60724 © IEC:2000+A1:2008
The committee has decided that the contents of the base publication and its amendments 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.
60724 © IEC:2000+A1:2008 – 5 –
INTRODUCTION
Editorially, this third edition of IEC 60724 is brought into line with IEC 60986, second edition,
and IEC 61443, first edition.
The following four aspects may be applicable when selecting the short-circuit rating of a cable
system:
a) the permissible maximum temperature limits for cable components (e.g. conductor,
insulation, screen or metallic sheath, bedding, armour and oversheath). For practical
purposes, the energy producing the temperature rise is usually expressed by an equivalent
(I t) value so that the permitted maximum duration for a given short-circuit current can be
calculated;
b) the maximum value of current which will not cause mechanical failure (such as bursting)
due to electromagnetic forces. Irrespective of any temperature limitations, this determines a
maximum current which should not be exceeded;
c) the thermal performance of joints and terminations at the limits of current and duration
specified for the associated cable. Accessories should also withstand the thermo-
mechanical and electromagnetic forces produced by the short-circuit current in the cable;
d) the influence of installation conditions on the above three aspects.
Aspect a) is dealt with in detail, and the limits given are based on a consideration of the cable
only. A single short-circuit application is not expected to produce any significant damage to the
cable, but repeated short-circuits may cause cumulative damage. Guidance is given, where
appropriate, on aspects c) and d) mainly as they concern thermo-mechanical forces in the
conductors and metallic sheath. Aspect b) is not covered in this standard.
The limits recommended in this International Standard should be used for guidance only.
It is not possible to provide complete limits for joints and terminations because their
construction is not standardized and performance varies. Where the full short-circuit capability
of the cable is needed, the accessories should be designed appropriately, but this is not always
economically justified and the short-circuit capability of a cable system may be determined by
the performance of its joints and terminations. Where possible, guidance has been included on
the performance of accessories when they are installed on cables subject to the short-circuit
limits given in this standard.

– 6 – 60724 © IEC:2000+A1:2008
SHORT-CIRCUIT TEMPERATURE LIMITS
OF ELECTRIC CABLES WITH RATED VOLTAGES
OF 1 kV (U = 1,2 kV) AND 3 kV (U = 3,6 kV)

m m
1 Scope
This International Standard gives guidance on the short-circuit maximum temperature limits of
electric cables having rated voltages of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV), with regard
m m
to the following:
– insulating materials;
– oversheath and bedding materials;
– conductor and metallic sheath materials and methods of connection.
The design of accessories and the influence of the installation conditions on the temperature
limits are taken into consideration.
The calculation of the permissible short-circuit current in the current-carrying components of
the cable should be carried out in accordance with IEC 60949.
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 60055 (all parts), Paper-insulated metal-sheathed cables for rated voltages up to 18/30 kV
(with copper or aluminium conductors and excluding gas-pressure and oil-filled cables)
IEC 60502-1:1998, Power cables with extruded insulation and their accessories for rated
voltages from 1 kV (U = 1,2 kV) up to 30 kV (U = 36 kV) – Part 1: Cables for rated voltages
m m
of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV)
m m
IEC 60949:1988, Calculation of thermally permissible short-circuit current, taking into account
the non-adiabatic heating effects
3 Factors governing the application of the temperature limits
3.1 General
The short-circuit temperatures given in clause 4 are the actual temperatures of the current-
carrying component as limited by the adjacent material in the cable and are valid for short-
circuit durations of up to 5 s. When calculating the allowable short-circuit current, these
temperatures will be obtained if heat loss into the insulation during the short-circuit is taken
into account (non-adiabatic heating). If heat loss during the short-circuit is neglected (adiabatic
heating), the calculations give short-circuit currents that are on the safe side.
NOTE The temperature limits given in clause 4 should also not be exceeded with repeated short-circuits occurring
in a short time.
60724 © IEC:2000+A1:2008 – 7 –
The 5 s time period mentioned is the limit for the temperatures quoted to be valid and not for
the application of the adiabatic calculation method. The time limit for the use of the adiabatic
method has a different definition, being a function of both the short-circuit duration and the
cross-sectional area of the current-carrying component. This is dealt with in IEC 60949.
Caution may be needed when using the conductor temperatures specified when the cables are
sheathed with a lower temperature material, especially for cables with conductor cross-
sectional areas of 1 000 mm and above. This is because the high thermal time constant of
these cables will cause the oversheath to attain high temperatures for longer times. In addition,
the high mechanical forces could result in insulation deformation. Nevertheless, it should be
stressed that for conductor cross-sectional areas above 1 000 mm the permissible short-
circuit current is so high that it is not normally attained in common systems.
Where other temperature limits are known with certainty to be more appropriate for the
materials or the cable design, then these may be used.
3.2 Cables
3.2.1 Paper insulated cables (mass-impregnated cables according to IEC 60055)
The temperature limits for paper insulated cables impregnated with oil/resin or non-draining
compound are imposed by the tendency to compound migration and void formations. All paper
insulated cables are also limited by thermal degradation of the cable components and by
possible tearing of paper tapes due to movement of the cores.
3.2.2 Polymeric insulated cables (according to IEC 60502-1)
For thermoplastic insulating materials, the temperature limits should be applied with caution
when the cables are either directly buried or securely clamped when in air. Local pressure due
to clamping or the use of an installation radius less than that specified for the cable, especially
for cables that are rigidly restrained, can lead to high deforming forces under short-circuit
conditions. Where these conditions cannot be avoided it is suggested that the limit be reduced
by 10 °C.
3.3 Accessories
Attention should be given to the design and installation of joints and terminations if the short-
circuit limits set out in this standard are to be safely used. The following aspects are not
exclusive and are provided for guidance only. It is desirable that the performance of an
accessory be considered in the context of the particular installation.
a) Longitudinal thrust in cable conductors can be considerable, depending on the degree of
lateral restraint imposed on the cable. Conductor stresses as high as 50 N/mm can easily
occur. These forces may cause buckling of conductors and other damage in a joint or
termination.
b) Longitudinal tension in cable conductors is also to be expected after a short-circuit. This
tension may exist for a very long period, particularly if the cable is only partly loaded after
the short-circuit. A minimum conductor stress of 40 N/mm should be used for design
purposes.
c) With impregnated paper cables, compound expansion can give rise to considerable fluid
pressure. If compound leaks out at joints and terminations, it could cause softening of the
bitumen filling. Moisture may also be drawn back into the accessory and cable in a
sufficient quantity to affect the performance of the insulation.
d) The use of a temperature limit only implies that any combination of current and time which
produces temperatures not exceeding that limit is permissible. For short-circuit currents this
is not sufficient. An additional limit should be set for the peak value of the current in order
to avoid excessive electromagnetic forces. These forces are of particular importance at
terminations and proper support is necessary to avoid undesirable movement and damage.

– 8 – 60724 © IEC:2000+A1:2008
e) Soldered joints should not be used if conductor temperatures greater than 160 °C are
contemplated.
f) Attention is drawn to the need to examine the design for short-circuit stability of the
electrical contact of all connectors used for jointing conductors and connecting armour and
metallic sheath bonds.
g) Screen and/or armour wires, when gathered together at a joint or termination, may have a
lower short-circuit performance than when in the cable. At such connections the expected
temperature rise should not be excessive for the materials involved and adequate
mechanical support should be provided.
h) Account should be taken of the risk of longitudinal shrinkage of polymeric components at
the cut ends of cables at short-circuit temperatures.
3.4 Installation conditions
When it is intended to make full use of the short-circuit limit of a cable, consideration should be
given to the influence of the installation conditions. An important aspect concerns the extent
and nature of the mechanical restraint imposed on the cable. Longitudinal expansion of
the cable during a short-circuit can be significant and when this expansion is restrained, the
resultant forces are considerable.
For cables in air, it is advisable to install them so that expansion is absorbed uniformly along
the length by snaking rather than permitting it to be relieved by excessive movement at a few
points only. Fixings should be spaced sufficiently far apart to permit lateral movement of
multicore cables or groups of single-core cables.
Where cables are installed directly in the ground, or require restraining by frequent fixing, then
provision should be made to accommodate the resulting longitudinal forces on accessories.
Sharp bends should be avoided because the longitudinal forces are translated into radial
pressures at bends in the cable route and these may damage thermoplastic components of the
cable such as insulation and sheaths. Attention is drawn to the minimum radius of installed
bend recommended by the appropriate installation regulations. For cables in air, it is also
desirable to avoid fixings at a bend which may cause local pressure on the cable.
4 Maximum permissible short-circuit temperatures for cables with
rated voltages of 1 kV (U = 1,2 kV) and 3 kV (U = 3,6 kV)

m m
The following tables should be read in conjunction with the comments in clause 3. Values given
are actual temperatures of the current-carrying components. Limits are for short-circuits of up
to 5 s duration.
The following subclauses 4.1 to 4.3 should be considered together when selecting a
temperature limit for a particular cable construction.
4.1 Insulation materials
The temperature limits for all types of conductors when in contact with the insulation materials
specified are given in table 1.

60724 © IEC:2000+A1:2008 – 9 –
Table 1 – Temperature limits for insulation materials
Temperature
1)
Material
°C
Paper 250
Polyvinyl chloride (PVC/A)
– conductor cross-section ≤300 mm
– conductor cross-section >300 mm 140
Cross-linked polyethylene (XLPE)
Ethylene propylene rubber (EPR and HEPR)
1)
Materials and designations according to IEC 60055 and IEC 60502-1.

4.2 Oversheath and bedding materials where there are no electrical or
other requirements
The screen/metallic sheath/armour temperature limits, when in contact with, or embedded in
the oversheath materials, but thermally separated from the insulation by layers of suitable
material and sufficient thickness, are given in Table 2. If thermal separation is not provided, the
temperature limit of the insulation should be used if it is lower than that of the oversheath.
Table 2 – Temperature limits for oversheath materials
2),
Temperature
1)
Material
°C
Polyvinyl chloride (ST and ST) 200
1 2
Polyethylene (ST ) 150
(ST ) 180
Polychloroprene, chlorosulphonated polyethylene 200
or similar polymers (SE )
1) Materials and designations according to IEC 60502-1.
2) Higher temperature
...