SIST EN 61660-2:1998

Short-circuit currents in d.c. auxiliary installations in power plants and substations - Part 2: Calculation of effects

Describes a method for calculation of the mechanical and thermal effects on rigid conductors caused by short-circuit currents in d.c. auxilliary installations in power plants and substations, as described in EN 61660-1. Provides a method which has wide application and which gives results of sufficient accuracy. The calculation method is based on substitute functions, which cause approximately the same maximum stresses in the conductors and the same forces on the supports as the actual electromagnetic force.

Kurzschlußströme in Gleichstrom-Eigenbedarfsanlagen in Kraftwerken und Schaltanlagen - Teil 2: Berechnung der Wirkungen

Courants de court-circuit dans les installations auxiliaires alimentées en courant continu dans les centrales et les postes - Partie 2: Calcul des effets

Décrit une méthode de calcul des effets mécaniques et thermiques sur les conducteurs rigides dus aux courants de court-circuit dans les installations auxiliaires alimentées en courant continu dans les centrales et les postes, tels que décrits dans la EN 61660-1. Donne une méthode généralement applicable et fournit des résultats d'une précision suffisante. La méthode de calcul est basée sur des fonctions de substitution produisant par approximation les mêmes contraintes maximales dans les conducteurs et les mêmes forces sur les supports que la force électromagnétique réelle.

Kratkostični toki v pomožnih enosmernih napeljavah elektrarn in transformatorskih postaj - 2. del: Računanje učinkov (IEC 61660-2:1997)

General Information

• Status: Published
• Publication Date: 30-Sep-1998
• ICS: 17.220.01 - Electricity. Magnetism. General aspects
• ICS: 29.240.01 - Power transmission and distribution networks in general
• Technical Committee: NTF - Electricity energy supply systems
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Oct-1998
• Due Date: 01-Oct-1998
• Completion Date: 01-Oct-1998
• Ref Project: EN 61660-2:1997 - Short-circuit currents in d.c. auxiliary installations in power plants and substations - Part 2: Calculation of effects

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 61660-2:1998
01-oktober-1998
.UDWNRVWLþQLWRNLYSRPRåQLKHQRVPHUQLKQDSHOMDYDKHOHNWUDUQLQ
WUDQVIRUPDWRUVNLKSRVWDMGHO5DþXQDQMHXþLQNRY,(&
Short-circuit currents in d.c. auxiliary installations in power plants and substations - Part
2: Calculation of effects
Kurzschlußströme in Gleichstrom-Eigenbedarfsanlagen in Kraftwerken und
Schaltanlagen - Teil 2: Berechnung der Wirkungen
Courants de court-circuit dans les installations auxiliaires alimentées en courant continu
dans les centrales et les postes - Partie 2: Calcul des effets
Ta slovenski standard je istoveten z: EN 61660-2:1997
ICS:
17.220.01 Elektrika. Magnetizem. Electricity. Magnetism.
Splošni vidiki General aspects
29.240.01 2PUHåMD]DSUHQRVLQ Power transmission and
GLVWULEXFLMRHOHNWULþQHHQHUJLMH distribution networks in
QDVSORãQR general
SIST EN 61660-2:1998 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 61660-2:1998

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SIST EN 61660-2:1998

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SIST EN 61660-2:1998

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SIST EN 61660-2:1998

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SIST EN 61660-2:1998

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SIST EN 61660-2:1998
NORME
CEI
INTERNATIONALE
IEC
61660-2
INTERNATIONAL
Première édition
STANDARD
First edition
1997-06
Courants de court-circuit dans les installations
auxiliaires alimentées en courant continu
dans les centrales et les postes –
Partie 2:
Calcul des effets
Short-circuit currents in d.c. auxiliary installations
in power plants and substations –
Part 2:
Calculation of effects
 IEC 1997 Droits de reproduction réservés  Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun any form or by any means, electronic or mechanical,
procédé, électronique ou mécanique, y compris la photo- including photocopying and microfilm, without permission in
copie et les microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: [phone] e-mail: [e-mail] IEC web site http: //www.iec.ch
CODE PRIX
Commission Electrotechnique Internationale
W
PRICE CODE
International Electrotechnical Commission
Pour prix, voir catalogue en vigueur
For price, see current catalogue

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 3 −
CONTENTS
Page
FOREWORD . 5
Clause
1 General . 7
1.1 Scope. 7
1.2 Normative references. 9
1.3 Symbols and units . 9
1.4 Definitions. 15
2 Electromagnetic effect on rigid conductors . 17
2.1 General. 17
2.2 Calculation of electromagnetic forces. 19
2.3 Calculation of stresses in rigid conductors and forces on supports . 21
2.4 Design load for post insulators, their supports and connectors. 33
3 Thermal effect on bare conductors and electrical equipment . 33
3.1 General. 33
3.2 Calculation of temperature rise . 35
Tables . 39
Figures .47
Annexes
A Equations for calculations of diagrams . 67
B Bibliography. 74

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 5 −
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SHORT-CIRCUIT CURRENTS IN DC AUXILIARY INSTALLATIONS
IN POWER PLANTS AND SUBSTATIONS –
Part 2: Calculation of effects
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization
comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to
promote international co-operation on all questions concerning standardization in the electrical and electronic
fields. To this end and in addition to other activities, the IEC publishes International Standards. Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt
with may participate in this preparatory work. International, governmental and non-governmental organizations
liaising with the IEC also participate in this preparation. The IEC collaborates closely with the International
Organization for Standardization (ISO) in accordance with conditions determined by agreement between the
two organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the
form of standards, technical reports or guides and they are accepted by the National Committees in that
sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the
subject of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61660-2 has been prepared by IEC technical committee 73: Short-
circuit currents.
The text of this standard is based on the following documents:
FDIS Report on voting
73/85/FDIS 73/98/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.
Annexes A and B are for information only.
IEC 61660 consists of the following parts, under the general title: Short-circuit currents in d.c.
auxiliary installations in power plants and substations:
– Part 1: 1997: Calculation of short-circuit currents
– Part 2: 1997: Calculation of effects
– Part 3: 199X: Examples of calculations (in preparation)

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 7 −
SHORT-CIRCUIT CURRENTS IN DC AUXILIARY INSTALLATIONS
IN POWER PLANTS AND SUBSTATIONS –
Part 2: Calculation of effects
1 General
1.1 Scope
This part of IEC 61660 describes a method for calculation of the mechanical and thermal
effects on rigid conductors caused by short-circuit currents in d.c. auxiliary installations in
power plants and substations. Such systems may contain the following items of equipment
which act as sources, as well as contributing to the short-circuit currents:
– rectifiers in three-phase a.c. bridge connection for 50 Hz;
– stationary lead-acid batteries;
– smoothing capacitors;
– d.c. motors with independent excitation.
This standard provides a method which has wide application, and which gives results of
sufficient accuracy. The calculation method is based on substitute functions, which cause
approximately the same maximum stresses in the conductors and the same forces on the
supports as the actual electromagnetic force.
The standardized calculation procedures of clauses 2 and 3 are applicable for the
electromagnetic effect on rigid conductors and the thermal effect on bare conductors and
electrical equipment, respectively.
For cables and insulated conductors, however, reference is made to IEC 60949 and
IEC 60986, for example.
Only d.c. auxiliary installations in power plants and substations are dealt with in this standard.
In particular, the following points should be noted:
– The calculation of short-circuit currents should be based on IEC 61660-1.
– Short-circuit duration used in this standard depends on the protection concept, and
should be considered in that sense.
– These standardized procedures are adjusted to practical requirements, and contain
simplifications with safety margins. Testing or more detailed methods of calculation or both
may be used.
– In clause 2 of this standard, only the stresses caused by short-circuit currents are
calculated. Furthermore, other stresses can exist, such as those caused by dead-load,
operating forces, or earthquakes. The combination of these loads with the short-circuit
loading should be part of an agreement and/or given by standards, for example erection
codes.

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 9 −
1.2 Normative references
The following normative documents contain provisions which, through reference in this text
constitute provisions of this part of IEC 61660. At the time of publication, the editions indicated
were valid. All normative documents are subject to revision, and parties to agreements based
on this part of IEC 61660 are encouraged to investigate the possibility of applying the most
recent editions of the normative documents indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 60865-1: 1993, Short-circuit currents — Calculation of effects – Part 1: Definitions and
calculation methods
IEC 60865-2: 1994, Short-circuit currents — Calculation of effects – Part 2: Examples of
calculation
IEC 60949: 1988, Calculation of thermally permissible short-circuit currents, taking into account
non-adiabatic heating effects
IEC 60986: 1989, Guide to the short-circuit temperature limits of electric cables with a rated
voltage from 1,8/3 (3,6) kV to 18/30 (36) kV
IEC 61660-1: 1997, Short-circuit currents in d.c. auxiliary installations in power plants and
substations – Part 1: Calculation of short-circuit currents
1.3 Symbols and units
All equations used in this standard are quantity equations in which quantity symbols represent
physical quantities possessing both numerical values and dimensions.
The symbols used in this standard and their exemplary SI units are given in the following lists.
1.3.1 Symbols for clause 2: electromagnetic effects
2
A Impulse for determining the parameters of the substitute rectangular A s
i
function
2
A Cross-section of one subconductor m
s
a Centre line distance between conductors m
a Effective distance between neighbouring main conductors m
m
a Effective distance between subconductors m
s
a Centre line distance between subconductor 1 and subconductor n m
1n
a Centre line distance between subconductors m
1s
b Dimension of a subconductor perpendicular to the direction of the force m
b Dimension of a main conductor perpendicular to the direction of m
m
the force
c Factor for the influence of connecting pieces 1
D Outer diameter of tubular conductor m
d Dimension of a subconductor in the direction of the force m

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 11 −
d Dimension of a main conductor in the direction of the force m
m
2
E Young's modulus N/m
F Force acting between two, parallel, long conductors during a short circuit N
F Force between main conductors caused by the substitute rectangular N
R
function
F Force between subconductors caused by the substitute rectangular N
Rs
function
F Force on support (peak value) N
d
F Force between main conductors during a short circuit (peak value) N
m
F Force between subconductors during a short circuit N
s
f Relevant natural frequency of a main conductor Hz
c
f Relevant natural frequency of a subconductor Hz
cs
2
g Value of acceleration of gravity m/s
n
2 3
I Value for determining of the parameters of the substitute rectangular A s
g
function
I Current of the substitute rectangular function for the calculation of the A
R
force between main conductors
I Current of the substitute rectangular function for the calculation of the A
Rs
force between subconductors
I Quasi steady-state short-circuit current A
k
i Peak short-circuit current A
p
i , i Instantaneous values of current in conductors in the sections of the A
1 2
standard approximation function
i , i Instantaneous values of currents in the conductors L1 and L2 A
L1 L2
4
J Second moment of main conductor area m
4
J Second moment of subconductor area m
s
k Number of sets of spacers or stiffening elements 1
k Factor for effective conductor distance between subconductor 1 and 1
1n
subconductor n
k Factor for effective conductor distance 1
1s
l Centre line distance between supports m
l Centre line distance between connecting pieces m
s
Mass per unit length of main conductor kg/m
m′
Mass per unit length of subconductor kg/m
m′
s
m Total mass of one set of connecting pieces kg
z
m ,m , Factors for determining the parameters of the substitute rectangular 1
g1 g2
function
m ,m ,
Ig1 Ig2
m ,m
θ1 θ2
n Number of subconductors of a main conductor 1
p Ratio I /i 1
k p
q Factor of plasticity 1
2
R Stress corresponding to the yield point N/m
p 0,2
s Wall thickness of tubes m

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 13 −
T Short-circuit duration s
k
T Vibration period of the main conductor s
me
T Vibration period of the subconductor s
mes
t Time to peak s
p
t Time of substitute rectangular function for the calculation of the force s
R
between main conductors
t Time of the substitute rectangular function for the calculation of the force s
Rs
between subconductors
V Ratio of dynamic and static force on supports 1
F
V Ratio of dynamic and static main conductor stress 1
σ
V Ratio of dynamic and static subconductor stress 1
σs
3
Z Section modulus of main conductor m
3
Z Section modulus of subconductor m
s
Factor for force on support 1
α
Factor for main conductor stress 1
β
Factor for relevant natural frequency estimation 1
γ
Magnetic constant, permeability of vacuum H/m
μ
0
2
Bending stress caused by the forces between main conductors N/m
σ
m
2
Bending stress caused by the forces between subconductors N/m
σ
s
2
Resulting conductor stress N/m
σ
tot
Rise-time constant s
τ
1
Decay-time constant s
τ
2
1.3.2 Symbols for clause 3: Thermal effects
2
A Main conductor cross-section m
2
A Impulse for determining of the parameters of the substitute rectangular A s
i
function
I Thermal equivalent short-time current (r.m.s.) A
th
I Rated short-time withstand current (r.m.s.) A
thr
0,5 2
K Factor for calculating S As /m
thr
2
S Thermal equivalent short-time current density (r.m.s.) A/m
th
2
S Rated short-time withstand current density (r.m.s.) A/m
thr
T Short-circuit duration s
k
T Rated short-time s
kr
t Time to peak s
p
Conductor temperature at the beginning of the short circuit
θ °C
b
Conductor temperature at the end of the short circuit
θ °C
e

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 15 −
1.4 Definitions
For the purpose of this part of IEC 61660, the following definitions apply.
1.4.1 Definitions for clause 2: electromagnetic effects
1.4.1.1 main conductor: A conductor, or an arrangement composed of a number of
conductors, which carries the total current.
1.4.1.2 subconductor: A single conductor which carries a certain part of the total current and
is a part of the main conductor.
1.4.1.3 fixed support: A support of a conductor which does not permit the conductor to move
angularly at the point of the support.
1.4.1.4 simple support: A support of a conductor which permits angular movement at the
point of support.
1.4.1.5 connecting piece: Any additional mass within a span which does not belong to the
uniform conductor material. This includes among others: spacers, stiffening elements, bar
overlappings, branchings, etc.
1.4.1.5.1 spacer: A mechanical element between subconductors which, at the point of
installation, maintains the clearance between subconductors.
1.4.1.5.2 stiffening element: A special spacer intended to reduce the mechanical stress.
1.4.1.6 short-circuit duration, T : The time interval between the initiation of the short circuit
k
and the breaking of the current.
1.4.1.7 standard approximation function: A curve, calculated according to IEC 61660-1,
describing the momentary value of the short-circuit current, including the value of rise-time
constant τ , decay-time constant τ , duration of the short-circuit current flow T and the time to
1 2 k
peak t .
p
NOTE — For further information, see IEC 61660-1.
1.4.1.8 squared standard approximation function: A curve representing the momentary
value of the square of the standard approximation function. It represents the form of the
momentary value of the electromagnetic force caused by the short-circuit current as well as the
envelope for the Joule integral.
μ
0
2
*
NOTE — For further information, see [1] , in which figure 8 also represents i (t) multiplied by the factor
2πa
giving F'(t).
1.4.1.9 substitute rectangular function: A function with rectangular form representing a
current that causes the same mechanical stresses and forces as the squared standard
approximation function.
μ
0
2
NOTE — For further information, see [1] in which figure 8 also represents I multiplied by the factor
R
2πa
giving F′ .
R
_________
*
 Figures in square brackets refer to the bibliography given in annex B.

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 17 −
1.4.2 Definitions for clause 3: thermal effects
1.4.2.1 thermal equivalent short-time current, I : The r.m.s. value of current having the
th
same thermal effect and the same duration as the actual short-circuit current.
1.4.2.2 rated short-time withstand current, I : The r.m.s. value of current that the
thr
electrical equipment can carry during a rated short time under prescribed conditions of use and
behaviour.
NOTES
1 It is possible to state several pairs of values of rated short-time withstand current and rated short time; for
thermal effect 1 s is used in most IEC specifications.
2 The rated short-time withstand current, as well as the corresponding rated short time, are stated by the
manufacturer of the equipment.
1.4.2.3 thermal equivalent short-time current density, S : The ratio of the thermal
th
equivalent short-time current and the cross-section area of the conductor.
1.4.2.4 rated short-time withstand current density, S for conductors: The r.m.s. value
thr
of the current density which a conductor is able to withstand for the rated short time.
NOTE — The rated short-time withstand current density is determined according to 3.2.
1.4.2.5 short-circuit duration, T : The time interval between the initiation of the short circuit
k
and the breaking of the current.
1.4.2.6 rated short time, T : The time duration for which:
kr
– an electrical equipment can withstand a current equal to its rated short-time withstand
current;
– a conductor can withstand a current density equal to its rated short-time withstand
current density.
2 Electromagnetic effect on rigid conductors
2.1 General
The time patterns of short-circuit currents in d.c. auxiliary installations are manifold. A standard
approximation function is defined in IEC 61660-1 by six parameters: i , I , t , τ , τ , T (see
p k p 1 2 k
figure 4a). A variation of any parameter will result in a variation of stress. For practical use, a
substitute rectangular function is introduced for the square of the current, and therefore for the
electromagnetic force, which leads to the same mechanical stresses and forces as the real
short-circuit current. Figure 4b shows the square of the standard approximation function of the
short-circuit current and its substitute rectangular function which is defined by two parameters:
2
I and t .
R R
With the calculation method presented in this clause
– stresses in rigid conductors, and
– forces on insulators and substructures, which result in bending, tension and/or
compression,
can be estimated.

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 19 −
Electromagnetic forces are induced in conductors by the currents flowing through them. Where
such electromagnetic forces interact on parallel conductors, they cause stresses which have to
be taken into account at the auxiliary installations. For this reason:
– the forces between parallel conductors are set forth in the following subclauses,
– the electromagnetic force components set up in conductors with bends and/or crossovers
may normally be disregarded.
When parallel conductors L1 and L2 are long compared to the distance between them, the
forces will be evenly distributed along the conductors and are given by the equation:
μ l
0
Fi= i (1)
L1 L2
2π a
where
i and i are the instantaneous values of the currents in the conductors L1 and L2;
L1 L2
l is the centre line distance between the supports;
a is the centre line distance between the conductors.
When calculating the maximum possible short-circuit current, additional details from other
IEC standards may be considered if these result in stress reduction.
2.2 Calculation of electromagnetic forces
2.2.1 Calculation of peak value of forces between the main conductors
The maximum force is given by:
μ l
0
2
Fi= (2)
m p
2πa
m
where
i is the peak short-circuit current;
p
l is the maximum centre line distance between supports;
a is the effective distance between main conductors according to 2.2.3.
m
2.2.2 Calculation of peak value of forces between coplanar subconductors
The maximum force due to the currents in the subconductors acts on the outer subconductors.
This maximum between two adjacent connecting pieces is given by:
2
 
i
μ l
p
0 s
F= (3)
 
s
2π na
 
s
where
n is the number of subconductors;
l is the maximum existing centre line distance between two adjacent connecting pieces;
s
a is the effective distance between subconductors according to 2.2.3.
s

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 21 −
2.2.3 Effective distances between main conductors and between subconductors
The forces between conductors carrying short-circuit currents depend on the geometrical
configuration and the profile of the conductors. For this reason the effective distance a
m
between main conductors has been introduced in 2.2.1, and the effective distance a between
s
subconductors in 2.2.2. They shall be taken as follows.
Effective distance a between coplanar main conductors with the centre line distance a:
m
– main conductors consisting of single circular cross-sections:
aa= (4)
m
– main conductors consisting of single rectangular cross-sections and main conductors
composed of subconductors with rectangular cross-sections:
a
a = (5)
m
k
12
k shall be taken from figure 1, with a = a, b = b and d = d according to figure 2.
12 1s m m
Effective distance a between the n coplanar subconductors of a main conductor:
s
– subconductors with circular cross-sections:
11 1 1 1 1
=+ + +LL+ + + (6)
aa a a a a
s12 1314 1s 1n
– subconductors with rectangular cross-sections:
Some values for a are given in table 1. For other distances and subconductor dimensions
s
the equation
k k k k k
1
12 13 14 1s 1n
=+ + +LL+ + + (7)
a a a a a a
s 12 13 14 1s 1n
can be used. The values for …,
k k shall be taken from figure 1.
12 1n
2.3 Calculation of stresses in rigid conductors and forces on supports
2.3.1 General
The conductors may be supported in different ways, either by fixed or simple supports, or by a
combination of both. Depending on the type of support and the number of supports, the
stresses in the conductors, and the forces on the supports will be different for the same short-
circuit current. The equations given also include the elasticity of the supports.
The stresses in the conductors and the forces on the supports also depend on the relevant
natural frequency of the mechanical system and the short-circuit duration.
2.3.2 Calculation of stresses in rigid conductors
The assumption that the conductor is rigid means that the axial forces are disregarded. Under
this assumption, the acting forces are bending forces, and the general equation for the bending
stress caused by the forces between main conductors is given by:
Fl
m
σβ=
V (8)
m σ
8Z

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 23 −
where
F according to equation (2) shall be used.
m
Z is the section modulus of the main conductor, and shall be calculated with respect to
the direction of forces between main conductors.
V is the factor which takes into account the dynamic phenomena; the maximum possible
σ
value shall be taken from table 2.
β is a factor depending on the type and the number of supports and shall be taken from
table 3.
The bending stress caused by the forces between subconductors is given by:
F l
ss
σ =V (9)
ssσ
16 Z
s
where
F according to equation (3) shall be used.
s
Z is the section modulus of the subconductor, and shall be calculated with respect to the
s
direction of forces between subconductors.
V is the factor which takes into account the dynamic phenomena; the maximum possible
σs
value shall be taken from table 2.
NOTE – For the beams in table 3 (except the single span beam with simple supports), the realistic ultimate
loads are calculated with the factors β given in table 3, and q given in table 4.
Non-uniform spans in continuous beams may be treated, with a sufficient degree of accuracy,
by assuming the maximum span is applied throughout. This means that:
– the end supports are not subjected to greater stress than the inner ones;
– span lengths less than 20 % of the adjacent ones shall be avoided. If this does not prove
to be possible, the conductors shall be decoupled using flexible joints at the supports. If
there is a flexible joint within a span, the length of that span should be less than 70 % of the
lengths of the adjacent spans.
If it is not clear that a beam is supported or fixed, the worst case shall be taken into account.
For further consideration, see 2.3.6.
2.3.3 Section modulus and factor q of main conductors composed of subconductors
The bending stress, and consequently the mechanical withstand of the conductor, depend on
the section modulus.
If the stress occurs in accordance with figure 2a, the section modulus Z is independent of the
number of connecting pieces and is equal to the sum of the section moduli Z of the
s
subconductors (Z with respect to the axis x – x). The factor q has then the value 1,5 for
s
rectangular cross-sections, and 1,19 for U and I sections.

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SIST EN 61660-2:1998
61660-2 © IEC:1997 – 25 −
If the stress occurs in accordance with figure 2b, and in case there is only one or no stiffening
element within a supported distance, the section modulus Z is equal to the sum of the section
moduli Z of the subconductors (Z with respect to the axis y – y). The factor q has then the
s s
value 1,5 for rectangular cross-sections, and 1,83 for U and I sections.
When, within a supported distance, there are two or more stiffening elements, higher values of
section moduli may be used:
– for main conductors composed of subconductors of rectangular cross-sections with a
space between the bars equal to the bar thickness, the section moduli are given in table 5;
– for conductor groups having U and I cross-sections, 50 % of the section moduli with
respect to the axis 0 – 0, similar to figure 2b, should be used.
The factor q then has a value of 1,5 for rectangular cross-sections, and 1,83 for U and I
sections.
2.3.4 Permitted conductor stress
A single conductor is assumed to withstand the short-circuit forces when:
σ ≤ q R (10)
m p 0,2
where R is the stress corresponding to the yield point.
p 0,2
The factor q shall be taken from table 4 (see also 2.3.3).
When a main conductor consists of two or more subconductors, the total stress in the
conductor is given by:
σ = σ + σ (11)
tot m s
NOTE — For rectangular cross-sections, σ is the algebraic sum of σ and σ , independent of the loading
tot
m s
directions (see figure 2).
The conductor is assumed to withstand the short-circuit forces when:
σ ≤ q R (12)
tot p 0,2
It is necessary to verify that the short circuit does
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