# IEC 60909-3:2009

(Main)## Short-circuit currents in three-phase AC systems - Part 3: Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth

## Short-circuit currents in three-phase AC systems - Part 3: Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth

IEC 60909-3:2009 is also available as IEC Standards+ 60909-3:2009 which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.

IEC 60909-3:2009 specifies procedures for calculation of the prospective short-circuit currents with an unbalanced short circuit in high-voltage three-phase a.c. systems operating at nominal frequency 50 Hz or 60 Hz, i. e.:

- currents during two separate simultaneous line-to-earth short circuits in isolated neutral or resonant earthed neutral systems;

- partial short-circuit currents flowing through earth in case of single line-to-earth short circuit in solidly earthed or low-impedance earthed neutral systems.

The currents calculated by these procedures are used when determining induced voltages or touch or step voltages and rise of earth potential at a station (power station or substation) and the towers of overhead lines. Procedures are given for the calculation of reduction factors of overhead lines with one or two earth wires. This edition constitutes a technical revision. The main changes with respect to the previous edition are:

- New procedures are introduced for the calculation of reduction factors of the sheaths or shields and in addition the current distribution through earth and the sheaths or shields of three-core cables or of three single-core cables with metallic non-magnetic sheaths or shields earthed at both ends;

- The information for the calculation of the reduction factor of overhead lines with earth wires are corrected and given in the new Clause 7;

- A new Clause 8 is introduced for the calculation of current distribution and reduction factor of three-core cables with metallic sheath or shield earthed at both ends;

- The new Annexes C and D provide examples for the calculation of reduction factors and current distribution in case of cables with metallic sheath and shield earthed at both ends.

This publication is to be read in conjunction with IEC 60909-0:2001.

## Courants de court-circuit dans les réseaux triphasés à courant alternatif - Partie 3: Courants durant deux courts-circuits monophasés simultanés séparés à la terre et courants de court-circuit partiels s'écoulant à travers la terre

IEC 60909-3:2009 est disponible sous forme de IEC Standards+ 60909-3:2009 qui contient la Norme internationale et sa version Redline, illustrant les modifications du contenu technique depuis l'édition précédente.

IEC 60909-3:2009 spécifie les procédures applicables au calcul des valeurs présumées des courants de court-circuit lors d'un court-circuit déséquilibré dans les réseaux triphasés à haute tension à courant alternatif fonctionnant à une fréquence nominale de 50 Hz ou 60 Hz, c'est-à-dire:

- les courants durant deux courts-circuits monophasés simultanés séparés à la terre dans les réseaux à neutre isolé ou mis à la terre par une bobine d'extinction;

- les courants de court-circuit partiels s'écoulant à travers la terre, dans le cas d'un seul court-circuit monophasé à la terre dans les réseaux à neutre mis à la terre directement ou par une faible impédance.

Les courants calculés suivant ces procédures sont utilisés pour la détermination des tensions induites ou des tensions de contact ou de pas, et de la montée du potentiel de terre d'un poste (groupe de production, poste), ainsi que les pylônes des lignes aériennes. Des procédures de calcul des facteurs de réduction des lignes aériennes avec un ou deux câbles de garde sont fournies. Cette édition constitue une révision technique. Les modifications principales par rapport à l'édition précédente sont les suivantes:

- De nouvelles procédures sont fournies pour le calcul des facteurs de réduction des gaines ou des écrans, ainsi que pour la répartition du courant par la terre et les gaines ou les écrans des câbles à trois conducteurs ou de trois câbles monoconducteurs avec gaines ou écrans non magnétiques métalliques mis à la terre à chaque extrémité;

- Les informations pour le calcul du facteur de réduction pour lignes aériennes avec câbles de garde sont corrigées et fournies en un nouvel Article 7;

- Un nouvel Article 8 est introduit pour le calcul de la répartition du courant et du facteur de réduction des câbles avec gaine ou écran métallique, mis à la terre à chaque extrémité;

- Les nouvelles Annexes C et D donnent des exemples pour le calcul des facteurs de réduction et de la répartition du courant dans le cas de câbles avec gaine ou écran métallique, mis à la terre à chaque extrémité.

Le contenu du corrigendum de septembre 2013 a été pris en considération dans cet exemplaire.

Cette publication doit être lue conjointement avec la IEC 60909-0:2001.

### General Information

### Standards Content (sample)

IEC 60909-3

Edition 3.0 2009-03

INTERNATIONAL

STANDARD

NORME

INTERNATIONALE

Short-circuit currents in three-phase AC systems –

Part 3: Currents during two separate simultaneous line-to-earth short circuits

and partial short-circuit currents flowing through earth

Courants de court-circuit dans les réseaux triphasés à courant alternatif –

Partie 3: Courants durant deux courts-circuits monophasés simultanés séparés

à la terre et courants de court-circuit partiels s'écoulant à travers la terre

IEC 60909-3:2009

---------------------- Page: 1 ----------------------

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---------------------- Page: 2 ----------------------

IEC 60909-3

Edition 3.0 2009-02

INTERNATIONAL

STANDARD

NORME

INTERNATIONALE

Short-circuit currents in three-phase AC systems –

Part 3: Currents during two separate simultaneous line-to-earth short circuits

and partial short-circuit currents flowing through earth

Courants de court-circuit dans les réseaux triphasés à courant alternatif –

Partie 3: Courants durant deux courts-circuits monophasés simultanés séparés

à la terre et courants de court-circuit partiels s'écoulant à travers la terre

INTERNATIONAL

ELECTROTECHNICAL

COMMISSION

COMMISSION

ELECTROTECHNIQUE

PRICE CODE

INTERNATIONALE

CODE PRIX

ICS 17.220.01; 29.240.20 ISBN 978-2-88910-328-7

® Registered trademark of the International Electrotechnical Commission

Marque déposée de la Commission Electrotechnique Internationale

---------------------- Page: 3 ----------------------

– 2 – 60909-3 © IEC:2009

CONTENTS

FOREWORD...........................................................................................................................5

1 Scope and object..............................................................................................................7

2 Normative references .......................................................................................................8

3 Terms and definitions .......................................................................................................8

4 Symbols .........................................................................................................................10

5 Calculation of currents during two separate simultaneous line-to-earth shortcircuits ...........................................................................................................................12

5.1 Initial symmetrical short-circuit current ..................................................................12

5.1.1 Determination of M and M ...............................................................12

(1) (2)5.1.2 Simple cases of two separate simultaneous line-to-earth short

circuits.......................................................................................................13

5.2 Peak short-circuit current, symmetrical short circuit breaking current andsteady-state short-circuit current ...........................................................................13

5.3 Distribution of the currents during two separate simultaneous line-to-earthshort circuits..........................................................................................................14

6 Calculation of partial short-circuit currents flowing through earth in case of an

unbalanced short circuit..................................................................................................14

6.1 General .................................................................................................................14

6.2 Line-to-earth short circuit inside a station ..............................................................15

6.3 Line-to-earth short circuit outside a station............................................................16

6.4 Line-to-earth short circuit in the vicinity of a station...............................................18

6.4.1 Earth potential U at the tower n outside station B ..................................19

ETn6.4.2 Earth potential of station B during a line-to earth short circuit at the

tower n ......................................................................................................19

7 Reduction factor for overhead lines with earth wires .......................................................20

8 Calculation of current distribution and reduction factor in case of cables with

metallic sheath or shield earthed at both ends................................................................21

8.1 Overview ...............................................................................................................21

8.2 Three-core cable ...................................................................................................22

8.2.1 Line-to-earth short circuit in station B ........................................................22

8.2.2 Line-to-earth short circuit on the cable between station A andstation B ....................................................................................................23

8.3 Three single-core cables .......................................................................................26

8.3.1 Line-to-earth short circuit in station B ........................................................26

8.3.2 Line-to-earth short circuit on the cable between station A andstation B ....................................................................................................26

Annex A (informative) Example for the calculation of two separate simultaneous line-

to-earth short-circuit currents................................................................................................30

Annex B (informative) Examples for the calculation of partial short-circuit currents

through earth ........................................................................................................................33

Annex C (informative) Example for the calculation of the reduction factor r and the

current distribution through earth in case of a three-core cable .............................................43

Annex D (informative) Example for the calculation of the reduction factor r and the

current distribution through earth in case of three single-core cables ....................................48

---------------------- Page: 4 ----------------------60909-3 © IEC:2009 – 3 –

Figure 1 – Driving point impedance Z of an infinite chain, composed of the earth wire

impedance Z =Zd and the footing resistance R of the towers, with equalQQ T

distances d between the towers.............................................................................................9

Figure 2 – Driving point impedance Z of a finite chain with n towers, composed of the

earth wire impedance Z =Z d , the footing resistance R of the towers, with equal

Q Q Tdistances d between the towers and the earthing impedance Z of station B from

T EB

Equation (29) ........................................................................................................................10

Figure 3 – Characterisation of two separate simultaneous line-to earth short circuits

and the currents I ...........................................................................................................12

kEEFigure 4 – Partial short-circuit currents in case of a line-to-earth short circuit inside

station B ...............................................................................................................................15

Figure 5 – Partial short-circuit currents in case of a line-to-earth short circuit at a

tower T of an overhead line ..................................................................................................16

Figure 6 – Distribution of the total current to earth I ......................................................17

ETtotFigure 7 – Partial short–circuit currents in the case of a line-to-earth short circuit at a

tower n of an overhead line in the vicinity of station B ...........................................................18

Figure 8 – Reduction factor r for overhead lines with non-magnetic earth wiresdepending on soil resistivity ρ ...............................................................................................21

Figure 9 – Reduction factor of three-core power cables ........................................................23

Figure 10 – Reduction factors for three single-core power cables .........................................27

Figure A.1 – Two separate simultaneous line-to-earth short circuits on a single fed

overhead line (see Table 1) ..................................................................................................30

Figure B.1 – Line-to-earth short circuit inside station B – System diagram for stations

A, B and C ............................................................................................................................34

Figure B.2 – Line-to-earth short circuit inside station B – Positive-, negative- and zero-

sequence systems with connections at the short-circuit location F within station B...............34

Figure B.3 – Line-to-earth short circuit outside stations B and C at the tower T of an

overhead line – System diagram for stations A, B and C .......................................................36

Figure B.4 – Line-to-earth short circuit outside stations B and C at the tower T of an

overhead line – Positive-, negative- and zero-sequence systems with connections at

the short-circuit location F.....................................................................................................37

Figure B.5 – Earth potentials u = U /U with U = 1,912 kV and u = U /UETn Etn ET ET EBn Ebn EB

with U = 0,972 kV, if the line-to-earth short circuit occurs at the towers n = 1, 2, 3, ...

in the vicinity of station B ......................................................................................................42

Figure C.1 – Example for the calculation of the cable reduction factor and the current

distribution through earth in a 10-kV-network, U = 10 kV; c = 1,1; f = 50 Hz ........................44

Figure C.2 – Short-circuit currents and partial short-circuit currents through earth for

the example in Figure C.1.....................................................................................................45

Figure C.3 – Example for the calculation of current distribution in a 10-kV-network with

a short circuit on the cable between A and B (data given in C.2.1 and Figure C.1)...............46

Figure C.4 – Line-to-earth short-circuit currents, partial currents in the shield and

partial currents through earth................................................................................................47

Figure D.1 – Example for the calculation of the reduction factor and the currentdistribution in case of three single-core cables and a line-to-earth short circuit in

station B ...............................................................................................................................49

Figure D.2 – Positive-, negative- and zero-sequence system of the network in Figure

D.1 with connections at the short-circuit location (station B) .................................................50

Figure D.3 – Current distribution for the network in Figure D.1, depending on the

length, ℓ, of the single-core cables between the stations A and B..........................................51

---------------------- Page: 5 ----------------------– 4 – 60909-3 © IEC:2009

r and the current

Figure D.4 – Example for the calculation of the reduction factors

distribution in case of three single-core cables and a line-to-earth short circuit

between the stations A and B ...............................................................................................52

Figure D.5 – Positive-, negative- and zero-sequence system of the network in Figure D.4

with connections at the short-circuit location (anywhere between the stations A and B) ........52

Figure D.6 – Current distribution for the cable in Figure D.4 depending on ℓ , R →∞ .........54

A EFFigure D.7 – Current distribution for the cable in Figure D.4 depending on ℓ , R = 5 Ω ......56

A EFTable 1 – Calculation of initial line-to-earth short-circuit currents in simple cases ................13

Table 2 – Resistivity of the soil and equivalent earth penetration depth ................................20

Table C.1 – Results for the example in Figure C.1 ................................................................45

Table C.2 – Results for the example in Figure C.3, l = 5 km .................................................47

Table C.3 – Results for the example in Figure C.3, l = 10 km ...............................................47

---------------------- Page: 6 ----------------------60909-3 © IEC:2009 – 5 –

INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________

SHORT-CIRCUIT CURRENTS IN THREE-PHASE AC SYSTEMS –

Part 3: Currents during two separate simultaneous

line-to-earth short circuits and partial short-circuit

currents flowing through earth

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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equipment declared to be in conformity with an IEC Publication.6) All users should ensure that they have the latest edition of this publication.

7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

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other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and

expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC

Publications.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.

International Standard IEC 60909-3 has been prepared by IEC technical committee 73: Short-

circuit currents.This International Standard is to be read in conjunction with IEC 60909-0.

This third edition cancels and replaces the second edition published in 2003. This edition

constitutes a technical revision.The main changes with respect to the previous edition are listed below:

– New procedures are introduced for the calculation of reduction factors of the sheaths

or shields and in addition the current distribution through earth and the sheaths or

shields of three-core cables or of three single-core cables with metallic non-magnetic

sheaths or shields earthed at both ends;– The information for the calculation of the reduction factor of overhead lines with earth

wires are corrected and given in the new Clause 7;---------------------- Page: 7 ----------------------

– 6 – 60909-3 IEC:2009

– A new Clause 8 is introduced for the calculation of current distribution and reduction

factor of three-core cables with metallic sheath or shield earthed at both ends;– The new Annexes C and D provide examples for the calculation of reduction factors

and current distribution in case of cables with metallic sheath and shield earthed at

both ends.The text of this standard is based on the following documents:

FDIS Report on voting

73/148/FDIS 73/149/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.

A list of all parts of the IEC 60909 series, published under the general title Short-circuit

currents in three-phase a.c. systems, 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.

The contents of the corrigendum of September 2013 have been included in this copy.

---------------------- Page: 8 ----------------------60909-3 © IEC:2009 – 7 –

SHORT-CIRCUIT CURRENTS IN THREE-PHASE AC SYSTEMS –

Part 3: Currents during two separate simultaneous

line-to-earth short circuits and partial short-circuit

currents flowing through earth

1 Scope and object

This part of IEC 60909 specifies procedures for calculation of the prospective short-circuit

currents with an unbalanced short circuit in high-voltage three-phase a.c. systems operating

at nominal frequency 50 Hz or 60 Hz, i. e.:a) currents during two separate simultaneous line-to-earth short circuits in isolated neutral or

resonant earthed neutral systems;b) partial short-circuit currents flowing through earth in case of single line-to-earth short

circuit in solidly earthed or low-impedance earthed neutral systems.The currents calculated by these procedures are used when determining induced voltages or

touch or step voltages and rise of earth potential at a station (power station or substation) and

the towers of overhead lines.Procedures are given for the calculation of reduction factors of overhead lines with one or two

earth wires.The standard does not cover:

a) short-circuit currents deliberately created under controlled conditions as in short circuit

testing stations, orb) short-circuit currents in the electrical installations on board ships or aeroplanes, or

c) single line-to-earth fault currents in isolated or resonant earthed systems.The object of this standard is to establish practical and concise procedures for the calculation

of line-to-earth short-circuit currents during two separate simultaneous line-to-earth short

circuits and partial short-circuit currents through earth, earth wires of overhead lines and

sheaths or shields of cables leading to conservative results with sufficient accuracy. For this

purpose, the short-circuit currents are determined by considering an equivalent voltage

source at the short-circuit location with all other voltage sources set to zero. Resistances of

earth grids in stations or footing resistances of overhead line towers are neglected, when

calculating the short-circuit currents at the short-circuit location.This standard is an addition to IEC 60909-0. General definitions, symbols and calculation

assumptions refer to that publication. Special items only are defined or specified in this

standard.The calculation of the short-circuit currents based on the rated data of the electrical

equipment and the topological arrangement of the system has the advantage of being

possible both for existing systems and for systems at the planning stage. The procedure is

suitable for determination by manual methods or digital computation. This does not exclude

the use of special methods, for example the super-position method, adjusted to particular

circumstances, if they give at least the same precision.As stated in IEC 60909-0, short-circuit currents and their parameters may also be determined

by system tests.---------------------- Page: 9 ----------------------

– 8 – 60909-3 © IEC:2009

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 60909-0:2001, Short-circuit currents in three-phase a.c. systems – Part 0: Calculation of

currentsIEC/TR 60909-2:2008, Short-circuit currents in three-phase a.c. systems – Part 2: Data of

electrical equipment for short-circuit current calculations3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

3.1

two separate simultaneous line-to earth short circuits

line-to-earth short circuits at different locations at the same time on different conductors of a

three-phase a.c. network having a resonant earthed or an isolated neutral3.2

initial short-circuit currents during two separate simultaneous line-to-earth

short circuits I

kEE

r.m.s value of the initial short-circuit currents flowing at both short-circuit locations with the

same magnitude3.3

partial short-circuit current through earth I

r.m.s. value of the current flowing through earth in a fictive line in the equivalent earth

penetration depth δNOTE In case of overhead lines remote from the short-circuit location and the earthing system of a station, where

the distribution of the current between earthed conductors and earth is nearly constant, the current through earth

depends on the reduction factor of the overhead line (Figures 4 and 5). In case of cables with metallic sheaths or

shields, earthed at both ends in the stations A and B, current through earth between the stations A and B (Figures

9a) and 10a)), respectively between the short-circuit location and the stations A or B (Figures 9b) and 10b)).

3.4total current to earth I at the short-circuit location on the tower T of an overhead

ETtotline

r. m. s. value of the current flowing to earth through the footing resistance of an overhead line

tower far away from a station connected with the driving point impedances of the overhead

line at both sides, see Figure 53.5

total current to earth I at the short-circuit location in the station B

EBtot

r.m.s. value of the current flowing to earth through the earthing system of a station B (power

station or substation) with connected earthed conductors (earth wires of overhead lines or

sheaths or shields or armouring of cables or other earthed conductors as for instance metallic

water pipes), see Figure 43.6

current to earth I

ETn

r.m.s. value of the current flowing to earth causing the potential rise at an overhead line tower

n in the vicinity of a station---------------------- Page: 10 ----------------------

60909-3 © IEC:2009 – 9 –

3.7

current to earth I

EBn

r.m.s. value of the current flowing to earth causing the potential rise U of a station B, in

EBncase of a line-to-earth short circuit at an overhead line tower n in the vicinity of the station B

3.8reduction factor r

for overhead lines, which determines the part of the line-to-earth short-circuit current flowing

through the earth remote from the short-circuit location and the earthing systems of the

stations3.9

reduction factor r

for three-core cables with metallic sheath or shield earthed at both ends

3.10

reduction factor r

for three single-core cables with metallic sheaths or shields earthed at both ends

3.11driving point impedance Z of an infinite chain

composed of the earth-wire impedance Z between two towers with earth return and the

footing resistance R of the overhead line towers (Figure 1):Z = 0,5Z +()0,5Z +R Z (1)

P Q Q T Q

IEC 160/09

Figure 1 – Driving point impedance Z of an infinite chain, composed of the earth wire

impedance Z =Zd**...**

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