SIST EN IEC 60909-0:2026
(Main)Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents
- Abstract
This part of IEC 60909 is applicable to the calculation of short-circuit currents
– in low-voltage three-phase AC systems,
– in high-voltage three-phase AC systems, and
– operating at a nominal frequency of 50 Hz or 60 Hz.
Systems at highest voltages of 550 kV and above with long transmission lines are outside the scope of this document.
This document establishes a general, practicable and concise procedure leading to results which are generally of acceptable accuracy. For this calculation method, an equivalent voltage source at the short-circuit location is introduced. This does not exclude the use of special methods, for example the superposition method, adjusted to particular circumstances, if they give at least the same precision. The superposition method gives the short-circuit current related to the one load flow presupposed. This method, therefore, does not necessarily lead to the maximum respective minimum short-circuit current.
This document deals with the calculation of short-circuit currents in the case of balanced or unbalanced short circuits.
A single line-to-earth fault occurring in networks with isolated or resonant earthing of the neutral point is beyond the scope of this document.
For currents during two separate simultaneous single-phase line-to-earth short circuits in an isolated neutral system or a resonance earthed neutral system, see IEC 60909-3.
Short-circuit currents and short-circuit impedances can also be determined by system tests, by measurement on a network analyser, or numerical simulations with electromagnetic programs.
In existing low-voltage systems, it is possible to determine the short-circuit impedance on the basis of measurements at the location of the prospective short circuit considered.
The calculation of the short-circuit impedance is in general based on the rated data of the electrical equipment and the topological arrangement of the system and has the advantage of being possible both for existing systems and for systems at the planning stage.
In general, two types of short-circuit currents, which differ in their magnitude, are considered:
– maximum short-circuit current which determines the capacity or rating of electrical equipment;
– minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors.
NOTE The current in a three-phase short circuit is assumed to be made simultaneously in all poles. Investigations of non-simultaneous short circuits, which can lead to higher aperiodic components of short-circuit currents, are beyond the scope of this document.
This document does not cover short-circuit currents deliberately created under controlled conditions (short-circuit testing stations).
This document does not deal with the calculation of short-circuit currents in installations on board ships and aeroplanes.
This document does not distinguish between grid-forming and grid-following converters for the calculation of short-circuit currents.
- Status
- Published
- Public Enquiry End Date
- 30-Jun-2025
- Publication Date
- 27-Oct-2026
- Technical Committee
- NTF - Electricity energy supply systems
- Current Stage
- 6060 - National Implementation/Publication (Adopted Project)
- Start Date
- 12-Oct-2026
- Due Date
- 17-Dec-2026
- Completion Date
- 28-Oct-2026
SIST EN IEC 60909-0:2026 is published by SIST and covers the calculation of short-circuit currents in three-phase a.c. systems. It gives a practical method for finding maximum and minimum short-circuit currents, including initial, peak, symmetrical breaking, steady-state, and thermal equivalent values, for engineers who design, operate, protect, or assess electrical networks and equipment.
What does SIST EN IEC 60909-0:2026 specify?
SIST EN IEC 60909-0:2026 specifies a general procedure for calculating short-circuit currents in low-voltage and high-voltage three-phase a.c. systems operating at a nominal frequency of 50 Hz or 60 Hz. It is intended to give results of acceptable accuracy for planning and equipment rating.
The scope is based on an equivalent voltage source at the short-circuit location, while allowing special methods such as superposition if they give at least the same precision. The standard covers balanced and unbalanced short circuits.
It excludes single line-to-earth faults in networks with isolated or resonant earthing of the neutral point. It also excludes deliberately created short-circuit tests, installations on board ships and aeroplanes, and special consideration is needed for systems at highest voltages of 550 kV and above with long transmission lines.
The document is organized into:
- Clause 1 Scope
- Clause 2 Normative references
- Clause 3 Terms and definitions
- Clause 4 Symbols, subscripts and superscripts
- Clause 5 Characteristics of short-circuit currents: calculating method
- Clause 6 Modelling of electrical equipment for short-circuit calculations
- Clauses 7 to 14 for the calculation of specific current values and special cases
- Annex A and Annex B
- Bibliography
| Annex | What it covers |
|---|---|
| Annex A | Short-circuit current inside a power station unit with synchronous generator |
| Annex B | Formulas for calculating the factors m and n |
What are the key requirements of SIST EN IEC 60909-0:2026?
SIST EN IEC 60909-0:2026 requires the user to calculate short-circuit currents from a simplified network representation and then refine the result for the fault type and source type involved. The key point is that the method is not just one formula - it is a sequence of modelling choices that affect the final current used for equipment rating and protection settings.
Calculation method and assumptions
Clause 5 sets out the main calculation approach. The standard introduces an equivalent voltage source at the fault point, which means the internal voltages of generators, motors, and feeders are replaced by one source and the network is reduced to an equivalent short-circuit impedance.
Clause 5.2.4 lists the calculation assumptions for maximum and minimum values. In practice, this means bolted faults are assumed, the fault type is kept unchanged during the fault, the network topology is treated as fixed, and transformer impedances are referred to the tap-changers in their main positions. Shunt admittances of non-rotating loads, lines, and transformers are neglected in the positive- and negative-sequence systems, with specific zero-sequence treatment where needed.
Symmetrical components
Clause 5.2.5 uses symmetrical components to handle balanced and unbalanced faults. The positive-sequence, negative-sequence, and zero-sequence systems are combined to obtain the currents in the line conductors.
This matters because it is the basis for calculating unbalanced faults without modelling every conductor directly. It is also why the standard treats positive-sequence and negative-sequence impedances as equal for many passive network parts, while rotating machines can differ.
Modelling of electrical equipment
Clause 6 gives the modelling rules for overhead lines, cables, transformers, reactors, capacitors, network feeders, synchronous machines, asynchronous machines, power electronic converters, power station units, power plants, converter-fed motors, and FACTS devices and HVDC transmission systems.
This clause matters because the current depends strongly on the source type and the equipment impedance. For example, transformers may need impedance correction factors, reactors are modelled from their rated short-circuit voltage, and power electronic converters may contribute as current sources rather than as voltage sources.
Current values to be calculated
Clauses 7 to 11 define the current quantities used in practice:
- initial short-circuit current
- peak short-circuit current
- symmetrical breaking current
- DC component and unbalanced asymmetrical breaking current
- steady-state short-circuit current
These quantities are not interchangeable. The initial current is often used for equipment rating and protection studies, the peak current for mechanical stress, the breaking current for switching devices, and the steady-state current for thermal and control assessments.
Maximum and minimum cases
Clauses 7.1.1 and 7.1.2 distinguish maximum and minimum initial short-circuit currents. For maximum values, the standard uses the system configuration that gives the highest current and includes asynchronous motors and power station units unless their contribution can be neglected under the stated condition. For minimum values, it uses the configuration and source contribution that give the lowest current, neglects motor and wind or photovoltaic contributions, and uses higher line resistance at end-of-fault temperature.
This distinction matters in protection coordination. Maximum current is used for interrupting duty and equipment withstand, while minimum current is used to check whether protective devices will still operate reliably.
Special source types and network cases
Clauses 8 and 9 distinguish between single-fed and multiple-fed faults. Power station units with full size converter are treated as current sources, and their manufacturer-provided current values are needed for the calculation. Clauses 9.1 to 9.3 give separate rules for synchronous machines, asynchronous machines, doubly fed asynchronous generators, full size converters, and network feeders.
Clause 12 handles low-voltage side faults of transformers when one high-voltage conductor is interrupted. Clause 13 covers terminal short circuits of asynchronous motors. Clause 14 gives the thermal equivalent short-circuit current and Joule integral, which are used when thermal stress over time is the issue.
What terms does SIST EN IEC 60909-0:2026 define?
SIST EN IEC 60909-0:2026 defines the terms used to classify the fault and the current value being calculated.
- Short circuit - an accidental or intentional conductive path between two or more conductive parts that forces the potential difference to zero or near zero.
- Prospective short-circuit current - the current that would flow if the fault were replaced by an ideal connection of negligible impedance.
- Initial symmetrical short-circuit current - the rms value of the AC symmetrical component at the instant of the fault.
- Peak short-circuit current - the maximum possible instantaneous value of the prospective short-circuit current.
- Symmetrical short-circuit breaking current - the rms value at the instant of contact separation of the first pole to open.
- Steady-state short-circuit current - the rms value remaining after transient effects have decayed.
- Voltage factor c - the factor that links the equivalent voltage source to the nominal system voltage and is used in the calculation formulas.
- Equivalent voltage source - the ideal source applied at the short-circuit location for the calculation method.
Who uses SIST EN IEC 60909-0:2026?
SIST EN IEC 60909-0:2026 is used by power system engineers, utility planners, protection engineers, and equipment specifiers who need calculated short-circuit currents for design and verification. It is also relevant to network operators, industrial plant engineers, and manufacturers of transformers, switchgear, circuit-breakers, machines, converters, and power plant units.
In practice, users apply it to:
- size and rate electrical equipment
- set and coordinate protective devices
- check breaking duty and peak mechanical stress
- assess minimum fault current for fuse and relay operation
- estimate thermal stress in conductors and equipment
- model generator, motor, feeder, and converter contributions in network studies
What changed in SIST EN IEC 60909-0:2026 from the previous edition?
SIST EN IEC 60909-0:2026 is the third edition and cancels and replaces the second edition published in 2016. It is identified as a technical revision.
The listed changes are:
- restructuring and complementing of the chapters, especially Clause 6
- restructuring of the subscripts
Which standards are used with SIST EN IEC 60909-0:2026?
SIST EN IEC 60909-0:2026 cites the following normative references and companion documents:
- IEC 60038:2009 - provides the standard voltages used for nominal system voltage values.
- IEC 60050-131 - provides the vocabulary terms for circuit theory used in the definitions.
- IEC TR 60909-1:2002 - gives factors used for calculations according to IEC 60909-0.
- IEC TR 60909-2:2008 - gives data for electrical equipment used in short-circuit calculations.
- IEC 60909-3:2009 - covers two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth.
- IEC TR 60909-4:2021 - provides examples and additional information for short-circuit calculations.
What does the SIST EN IEC 60909-0:2026 document contain?
SIST EN IEC 60909-0:2026 contains calculation procedures, equipment models, and worked relationships for the main short-circuit current quantities used in system studies. The body of the document includes figures for fault types and network diagrams, plus tables for voltage factors and specific calculation factors.
It also includes formulas for equivalent impedances, correction factors, peak current factors, breaking current factors, and thermal equivalent current calculations. Annex A gives a worked treatment of short-circuit current inside a power station unit with a synchronous generator, and Annex B gives the formulas for the factors m and n used in thermal calculations.
The document is structured for design, planning, protection studies, and equipment rating rather than for test methods or certification procedures.
Relations
- Effective Date
- 01-Dec-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
- Effective Date
- 22-Sep-2026
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Frequently Asked Questions
SIST EN IEC 60909-0:2026 is a standard published by the Slovenian Institute for Standardization (SIST). Its full title is "Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents". This standard covers: This part of IEC 60909 is applicable to the calculation of short-circuit currents – in low-voltage three-phase AC systems, – in high-voltage three-phase AC systems, and – operating at a nominal frequency of 50 Hz or 60 Hz. Systems at highest voltages of 550 kV and above with long transmission lines are outside the scope of this document. This document establishes a general, practicable and concise procedure leading to results which are generally of acceptable accuracy. For this calculation method, an equivalent voltage source at the short-circuit location is introduced. This does not exclude the use of special methods, for example the superposition method, adjusted to particular circumstances, if they give at least the same precision. The superposition method gives the short-circuit current related to the one load flow presupposed. This method, therefore, does not necessarily lead to the maximum respective minimum short-circuit current. This document deals with the calculation of short-circuit currents in the case of balanced or unbalanced short circuits. A single line-to-earth fault occurring in networks with isolated or resonant earthing of the neutral point is beyond the scope of this document. For currents during two separate simultaneous single-phase line-to-earth short circuits in an isolated neutral system or a resonance earthed neutral system, see IEC 60909-3. Short-circuit currents and short-circuit impedances can also be determined by system tests, by measurement on a network analyser, or numerical simulations with electromagnetic programs. In existing low-voltage systems, it is possible to determine the short-circuit impedance on the basis of measurements at the location of the prospective short circuit considered. The calculation of the short-circuit impedance is in general based on the rated data of the electrical equipment and the topological arrangement of the system and has the advantage of being possible both for existing systems and for systems at the planning stage. In general, two types of short-circuit currents, which differ in their magnitude, are considered: – maximum short-circuit current which determines the capacity or rating of electrical equipment; – minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors. NOTE The current in a three-phase short circuit is assumed to be made simultaneously in all poles. Investigations of non-simultaneous short circuits, which can lead to higher aperiodic components of short-circuit currents, are beyond the scope of this document. This document does not cover short-circuit currents deliberately created under controlled conditions (short-circuit testing stations). This document does not deal with the calculation of short-circuit currents in installations on board ships and aeroplanes. This document does not distinguish between grid-forming and grid-following converters for the calculation of short-circuit currents.
This part of IEC 60909 is applicable to the calculation of short-circuit currents – in low-voltage three-phase AC systems, – in high-voltage three-phase AC systems, and – operating at a nominal frequency of 50 Hz or 60 Hz. Systems at highest voltages of 550 kV and above with long transmission lines are outside the scope of this document. This document establishes a general, practicable and concise procedure leading to results which are generally of acceptable accuracy. For this calculation method, an equivalent voltage source at the short-circuit location is introduced. This does not exclude the use of special methods, for example the superposition method, adjusted to particular circumstances, if they give at least the same precision. The superposition method gives the short-circuit current related to the one load flow presupposed. This method, therefore, does not necessarily lead to the maximum respective minimum short-circuit current. This document deals with the calculation of short-circuit currents in the case of balanced or unbalanced short circuits. A single line-to-earth fault occurring in networks with isolated or resonant earthing of the neutral point is beyond the scope of this document. For currents during two separate simultaneous single-phase line-to-earth short circuits in an isolated neutral system or a resonance earthed neutral system, see IEC 60909-3. Short-circuit currents and short-circuit impedances can also be determined by system tests, by measurement on a network analyser, or numerical simulations with electromagnetic programs. In existing low-voltage systems, it is possible to determine the short-circuit impedance on the basis of measurements at the location of the prospective short circuit considered. The calculation of the short-circuit impedance is in general based on the rated data of the electrical equipment and the topological arrangement of the system and has the advantage of being possible both for existing systems and for systems at the planning stage. In general, two types of short-circuit currents, which differ in their magnitude, are considered: – maximum short-circuit current which determines the capacity or rating of electrical equipment; – minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors. NOTE The current in a three-phase short circuit is assumed to be made simultaneously in all poles. Investigations of non-simultaneous short circuits, which can lead to higher aperiodic components of short-circuit currents, are beyond the scope of this document. This document does not cover short-circuit currents deliberately created under controlled conditions (short-circuit testing stations). This document does not deal with the calculation of short-circuit currents in installations on board ships and aeroplanes. This document does not distinguish between grid-forming and grid-following converters for the calculation of short-circuit currents.
SIST EN IEC 60909-0:2026 is classified under the following ICS (International Classification for Standards) categories: 17.220.01 - Electricity. Magnetism. General aspects; 29.240.20 - Power transmission and distribution lines. The ICS classification helps identify the subject area and facilitates finding related standards.
SIST EN IEC 60909-0:2026 has the following relationships with other standards: It is inter standard links to SIST EN 60909-0:2016, SIST EN 60027-7:2010, SIST EN 60865-1:2012, SIST EN 50368:2004, SIST EN 60909-3:2010, SIST HD 60364-5-54:2011, SIST-TS CLC/TS 50654-1:2020, SIST EN 60519-1:2015, SIST EN 50628:2016, SIST-TS CLC/TS 50654-1:2018. Understanding these relationships helps ensure you are using the most current and applicable version of the standard.
SIST EN IEC 60909-0:2026 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2026
Nadomešča:
SIST EN 60909-0:2016
Kratkostični toki v izmeničnih trifaznih sistemih - 0. del: Računanje tokov
Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents
Kurzschlussströme in Drehstromnetzen - Teil 0: Berechnung der Ströme
Courants de court-circuit dans les réseaux triphasés à courant alternatif - Partie 0: Calcul
des courants
Ta slovenski standard je istoveten z: EN IEC 60909-0:2026
ICS:
17.220.01 Elektrika. Magnetizem. Electricity. Magnetism.
Splošni vidiki General aspects
29.240.20 Daljnovodi Power transmission and
distribution lines
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EUROPEAN STANDARD EN IEC 60909-0
NORME EUROPÉENNE
EUROPÄISCHE NORM September 2026
ICS 17.220.01; 29.240.20 Supersedes EN 60909-0:2016
English Version
Short-circuit currents in three-phase AC systems - Part 0:
Calculation of currents
(IEC 60909-0:2026)
Courants de court-circuit dans les réseaux triphasés à Kurzschlussströme in Drehstromnetzen - Teil 0:
courant alternatif - Partie 0: Calcul des courants Berechnung der Ströme
(IEC 60909-0:2026) (IEC 60909-0:2026)
This European Standard was approved by CENELEC on 2026-08-27. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60909-0:2026 E
European foreword
The text of document 73/220/CDV, future edition 3 of IEC 60909-0, prepared by TC 73 "Short-circuit
currents" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-09-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-09-30
document have to be withdrawn
This document supersedes EN 60909-0:2016 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60909-0:2026 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60038:2009 NOTE Approved as EN 60038:2011
IEC 60865-1 NOTE Approved as EN 60865-1
IEC 60909-3 NOTE Approved as EN 60909-3
IEC 62428 NOTE Approved as EN 62428
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60050-131 - International Electrotechnical Vocabulary - - -
Part 131: Circuit theory
IEC 60909-0 ®
Edition 3.0 2026-07
INTERNATIONAL
STANDARD
Short-circuit currents in three-phase AC systems -
Part 0: Calculation of currents
ICS 17.220.01; 29.240.20 ISBN 978-2-8327-1382-2
IEC 60909-0:2026-07(en)
IEC 60909-0:2026 © IEC 2026
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 9
3 Terms and definitions . 9
4 Symbols, subscripts and superscripts . 14
4.1 General . 14
4.2 Symbols . 14
4.3 Subscripts . 16
4.4 Superscripts . 17
5 Characteristics of short-circuit currents: calculating method . 17
5.1 General . 17
5.2 Method of calculation . 20
5.2.1 General. 20
5.2.2 Equivalent voltage source at the short-circuit location . 22
5.2.3 Installations modelled as current sources . 23
5.2.4 Calculation assumptions . 24
5.2.5 Symmetrical components . 24
6 Modelling of electrical equipment for short-circuit calculations . 25
6.1 General . 25
6.2 Overhead lines and cables . 25
6.3 Transformers. 25
6.3.1 Two-winding transformers . 25
6.3.2 Three-winding transformers . 26
6.3.3 Impedance correction factors for two- and three-winding network
transformers . 28
6.4 Reactors . 29
6.4.1 General. 29
6.4.2 Shunt reactors . 29
6.4.3 Series reactors . 29
6.5 Capacitors . 29
6.5.1 Shunt capacitors . 29
6.5.2 Series capacitors . 29
6.6 Non-rotating loads . 30
6.7 Network feeders . 30
6.8 Synchronous machines . 31
6.8.1 General. 31
6.8.2 Synchronous generators . 33
6.8.3 Synchronous compensators . 33
6.8.4 Synchronous motors. 33
6.9 Asynchronous machines . 33
6.9.1 General. 33
6.9.2 Asynchronous generators . 34
6.9.3 Asynchronous motors . 35
6.10 Power electronic converters . 35
6.10.1 Power electronic converters with diodes or with thyristors . 35
IEC 60909-0:2026 © IEC 2026
6.10.2 Power electronic converters with semiconductor devices that can be
turned off, for example IGBT . 35
6.11 Power station units . 36
6.11.1 General. 36
6.11.2 Power station units with synchronous generators . 36
6.11.3 Power station units with asynchronous generator . 37
6.11.4 Power station units with doubly fed asynchronous generator . 37
6.11.5 Power station units with full-size converter . 38
6.12 Power plants . 39
6.13 Converter-fed motors . 40
6.13.1 General. 40
6.13.2 Converter-fed motors with grid-side converters using diodes or
thyristors . 41
6.13.3 Converter-fed motors with grid-side converters using semiconductor
devices that can be turned off, for example IGBT . 41
6.14 FACTS devices and HVDC transmission systems . 41
6.14.1 FACTS devices . 41
6.14.2 HVDC transmission systems . 41
7 Calculation of initial short-circuit currents . 42
7.1 Overview . 42
7.1.1 General. 42
7.1.2 Maximum short-circuit currents . 44
7.1.3 Minimum short-circuit currents. 45
7.2 Three-phase short circuit . 45
7.3 Line-to-line short circuit . 46
7.4 Line-to-line short circuit with earth connection . 47
7.5 Line-to-earth short circuit. 48
8 Calculation of peak short-circuit currents . 48
8.1 Single-fed three-phase short circuit . 48
8.2 Multiple-fed three-phase short circuits . 49
8.3 Unbalanced short circuits . 50
9 Calculation of symmetrical breaking currents . 51
9.1 Single-fed three-phase short circuits . 51
9.1.1 Symmetrical breaking currents fed by synchronous machines . 51
9.1.2 Symmetrical breaking current fed by asynchronous machines . 52
9.1.3 Symmetrical breaking current fed by power station units with doubly fed
asynchronous generator . 53
9.1.4 Symmetrical breaking current fed by power station units with full-size
converter . 54
9.1.5 Symmetrical breaking current fed by network feeders . 54
9.2 Multiple-fed three-phase short circuits . 54
9.3 Single-fed and multiple-fed unbalanced short circuits . 55
10 DC component and unbalanced asymmetrical breaking current . 55
11 Calculation of steady-state short-circuit currents . 56
11.1 General . 56
11.2 Three-phase short circuits . 56
11.2.1 Steady-state short-circuit currents single-fed by one synchronous
machine or one power station unit with synchronous generator . 56
11.2.2 Steady-state short-circuit currents of asynchronous machines . 59
IEC 60909-0:2026 © IEC 2026
11.2.3 Steady-state short-circuit currents of power station units with doubly fed
asynchronous generators . 59
11.2.4 Steady-state short-circuit currents of a power station unit with full-size
converter . 59
11.2.5 Steady-state short-circuit currents of a network feeder . 60
11.2.6 Steady-state short-circuit currents in case of multiple-fed short circuits . 60
11.3 Unbalanced single-fed and multiple-fed short circuits . 60
12 Short circuits at the low-voltage side of transformers, if one line conductor is
interrupted at the high-voltage side . 61
13 Terminal short circuits of asynchronous motors. 62
14 Thermal equivalent short-circuit current and Joule integral . 62
Annex A (informative) Short-circuit current inside a power station unit with
synchronous generator. 66
Annex B (normative) Formulas for the calculation of the factors m and n . 70
Bibliography . 71
Figure 1 – Short-circuit current (schematic diagram) . 19
"
Figure 2 – Illustration for calculating the initial symmetrical short-circuit current I . 21
k
Figure 3 – Characterization of short circuits and their currents . 22
Figure 4 – Three-winding transformer . 27
Figure 5 – System diagram and equivalent circuit diagram for network feeders . 31
Figure 6 – Example of a power plant with short-circuit locations consisting of power
station units according to 6.11.3 to 6.11.5 . 39
Figure 7 – Equivalent circuit diagram (positive-sequence system) of a power plant with
power station units PA, PD, and PC for a short circuit at location F2 . 39
Figure 8 – Examples of single-fed short circuits . 43
Figure 9 – Example of multiple-fed short circuit . 44
Figure 10 – Factor κ for series circuit as a function of ratio R/X or X/R . 49
Figure 11 – Factor μ for calculation of symmetrical three-phase short-circuit breaking
current I fed by synchronous or asynchronous machines . 52
b
Figure 12 – Factor q for the calculation of the symmetrical three-phase short-circuit
breaking current of asynchronous machines . 53
Figure 13 – Factors λ and λ factors for cylindrical rotor synchronous machines . 58
min max
Figure 14 – Factors λ and λ for salient-pole synchronous machines . 58
min max
Figure 15 – Transformer secondary short-circuits, if one line (fuse) is opened on the
high-voltage side of a transformer Dy(n)5. 61
Figure 16 – Factor m for the heat effect of the DC component of the short-circuit
current . 64
Figure 17 – Factor n for the heat effect of the AC component of the short-circuit current . 65
Figure A.1 – Initial short-circuit currents and initial partial short-circuit currents for
three different short-circuit locations inside a power station unit with synchronous
generator . 68
IEC 60909-0:2026 © IEC 2026
Table 1 – Voltage factor c . 23
Table 2 – Factors α and β for the calculation of short-circuit currents with Formula (94),
rated transformation ratio t = U /U . 62
r rTHV rTLV
Table 3 – Calculation of short-circuit currents of asynchronous motors in the case of a
short circuit at the terminals. 63
IEC 60909-0:2026 © IEC 2026
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Short-circuit currents in three-phase AC systems -
Part 0: Calculation of currents
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 this end and
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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. 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 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 IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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
members of its technical committees and IEC National Committees for any personal injury, property damage or
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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). [IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch and/or
www.iso.org/patents. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 60909-0 has been prepared by IEC technical committee 73: Short-circuit currents. It is an
International Standard.
This third edition cancels and replaces the second edition published in 2016. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) restructuring and complementing of the clauses, especially restructuring and updating of
Clause 6.
b) restructuring of subscripts.
IEC 60909-0:2026 © IEC 2026
The text of this International Standard is based on the following documents:
Draft Report on voting
73/220/CDV 73/236/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60909 series, published under the general title Short-circuit currents
in three-phase AC systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
IEC 60909-0:2026 © IEC 2026
INTRODUCTION
Main aim of IEC 60909-0 is calculation of maximum and minimum short-circuit currents of bolted
short circuits (see 5.2.4). Calculation results can serve as input for further evaluations such as
calculation of arcing current as described in, for example, IEEE 1584.
IEC 60909-0:2026 © IEC 2026
1 Scope
This part of IEC 60909 is applicable to the calculation of short-circuit currents
– in low-voltage three-phase AC systems,
– in high-voltage three-phase AC systems, and
– operating at a nominal frequency of 50 Hz or 60 Hz.
Systems at highest voltages of 550 kV and above with long transmission lines are outside the
scope of this document.
This document establishes a general, practicable and concise procedure leading to results
which are generally of acceptable accuracy. For this calculation method, an equivalent voltage
source at the short-circuit location is introduced. This does not exclude the use of special
methods, for example the superposition method, adjusted to particular circumstances, if they
give at least the same precision. The superposition method gives the short-circuit current
related to the one load flow presupposed. This method, therefore, does not necessarily lead to
the maximum respective minimum short-circuit current.
This document deals with the calculation of short-circuit currents in the case of balanced or
unbalanced short circuits.
A single line-to-earth fault occurring in networks with isolated or resonant earthing of the neutral
point is beyond the scope of this document.
For currents during two separate simultaneous single-phase line-to-earth short circuits in an
isolated neutral system or a resonance earthed neutral system, see IEC 60909-3.
Short-circuit currents and short-circuit impedances can also be determined by system tests, by
measurement on a network analyser, or numerical simulations with electromagnetic programs.
In existing low-voltage systems, it is possible to determine the short-circuit impedance on the
basis of measurements at the location of the prospective short circuit considered.
The calculation of the short-circuit impedance is in general based on the rated data of the
electrical equipment and the topological arrangement of the system and has the advantage of
being possible both for existing systems and for systems at the planning stage.
In general, two types of short-circuit currents, which differ in their magnitude, are considered:
– maximum short-circuit current which determines the capacity or rating of electrical
equipment;
– minimum short-circuit current which can be a basis, for example, for the selection of fuses,
for the setting of protective devices, and for checking the run-up of motors.
NOTE The current in a three-phase short circuit is assumed to be made simultaneously in all poles. Investigations
of non-simultaneous short circuits, which can lead to higher aperiodic components of short-circuit currents, are
beyond the scope of this document.
This document does not cover short-circuit currents deliberately created under controlled
conditions (short-circuit testing stations).
This document does not deal with the calculation of short-circuit currents in installations on
board ships and aeroplanes.
This document does not distinguish between grid-forming and grid-following converters for the
calculation of short-circuit currents.
IEC 60909-0:2026 © IEC 2026
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 60050-131, International Electrotechnical Vocabulary (IEV) - Part 131: Circuit theory
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-131 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
short circuit
accidental or intentional conductive path between two or more conductive parts (e.g. three-
phase short circuit) forcing the electric potential differences between these conductive parts to
be equal or close to zero
[SOURCE: IEC 60050-151:2001, 151-12-04, modified – The parenthesis "(e.g. three-phase
short circuit)" has been added.]
3.1.1
line-to-line short circuit
two-phase short circuit
accidental or intentional conductive path between two line conductors with or without earth
connection
3.1.2
line-to-earth short circuit
single-phase short circuit
accidental or intentional conductive path in a solidly earthed neutral system or an impedance
earthed neutral system between a line conductor and local earth
3.2
short-circuit current
over-current resulting from a short circuit in an electric system
Note 1 to entry: It is necessary to distinguish between the short-circuit current at the short-circuit location and
partial short-circuit currents in the network branches (see Figure 3) at any point of the network.
3.3
prospective short-circuit current
current that would flow if the short circuit were replaced by an ideal connection of negligible
impedance without any change of the supply
Note 1 to entry: The current in a three-phase short circuit is assumed to be made simultaneously in all poles.
Investigations of non-simultaneous short circuits, which can lead to higher aperiodic components of short-circuit
current, are beyond the scope of this document.
IEC 60909-0:2026 © IEC 2026
3.4
symmetrical short-circuit current
RMS value of the AC symmetrical component of a prospective short-circuit current (see 3.3),
the aperiodic component of current, if any, being neglected
3.4.1
initial symmetrical short-circuit current
"
I
k
RMS value of the total AC symmetrical components of a prospective short-circuit current (see
3.3), applicable at the instant of short circuit if the impedance remains at zero-time value
SEE: Figure 1.
Note 1 to entry: The total initial short-circuit current is the sum of the short-circuit currents, caused by voltage and
current sources (see 3.4.2 and 3.4.3).
3.4.2
initial symmetrical short-circuit current
"
I
kVS
RMS value of the AC symmetrical components of a prospective short-circuit
current (see 3.3), caused by a voltage source, applicable at the instant of short circuit if the
impedance remains at zero-time value
3.4.3
initial symmetrical short-circuit current
"
I
kCS
RMS value of the AC symmetrical components of a prospective short-circuit
current (see 3.3), caused by a current source, applicable at the instant of short circuit if the
impedance remains at zero-time value
3.5
initial symmetrical short-circuit power
"
S
k
"
fictitious value determined as a product of the initial symmetrical short-circuit current I
k
(see 3.4.1), the nominal system voltage U (see 3.11) and the factor 3 :
n
""
S= 3⋅⋅UI
k nk
"
Note 1 to entry: The initial symmetrical short-circuit power S is not used for the calculation procedure in this
k
document.
3.6
decaying aperiodic component of short-circuit current
DC component
i
DC
mean value between the top and bottom envelope of a short-circuit current decaying from an
initial value to zero according to Figure 1
3.7
peak short-circuit current
i
p
maximum possible instantaneous value of the prospective short-circuit current
SEE: Figure 1.
IEC 60909-0:2026 © IEC 2026
Note 1 to entry: Sequential short circuits are not considered.
3.8
symmetrical short-circuit breaking current
I
b
RMS value of an integral cycle of the symmetrical AC component of the prospective short-circuit
current at the instant of contact separation of the first pole to open of a switching device
3.9
steady-state short-circuit current
I
k
RMS value of the short-circuit current which remains after the decay of the transient phenomena
SEE: Figure 1.
3.10
symmetrical locked-rotor current
I
LR
symmetrical RMS current of an asynchronous machine with locked rotor fed with rated voltage
U at rated frequency
rA
3.11
nominal system voltage
U
n
voltage (line-to-line) by which a system is designated, and to which certain operating
characteristics are referred
Note 1 to entry: Values are given in IEC 60038.
3.12
equivalent voltage source
cU /3
n
voltage of an ideal source applied at the short-circuit location for calculating the short-circuit
current according to 5.2.2
Note 1 to entry: The equivalent voltage source is the only active voltage of the network.
3.13
voltage factor
c
ratio between the equivalent voltage source and the nominal system voltage U divided by 3
n
Note 1 to entry: The values are given in Table 1.
Note 2 to entry: The introduction of a voltage factor c is necessary for various reasons: voltage variations depending
on time and place, changing of transformer taps, neglecting loads and capacitances by calculations according to
5.2.4 and the subtransient behaviour of generators and motors.
3.14
far-from-generator short circuit
short circuit during which the magnitude of the symmetrical AC component of the prospective
short-circuit current remains essentially constant
SEE: Figure 1 a).
IEC 60909-0:2026 © IEC 2026
3.15
near-to-generator short circuit
short circuit during which the magnitude of the symmetrical AC component of the prospective
short-circuit current decreases
SEE: Figure 1 b).
Note 1 to entry: A near-to-generator short circuit can be assumed if at least one synchronous machine contributes
a prospective initial symmetrical short-circuit current which is more than twice the machine's rated current, or a short
circuit to which asynchronous motors contribute more than 5 % of the initial symmetrical short-circuit current without
motors.
3.16
short-circuit impedances at the short-circuit location F
3.16.1
positive-sequence short-circuit impedance
Z
(1)
impedance of the positive-sequence system as viewed from the
short-circuit location
Note 1 to entry: See 5.2.5.
3.16.2
short-circuit impedance
Z
k
abbreviated expression for the positive-sequence short-circuit
impedance Z according to 3.16.1 for the calculation of three-phase short-circuit currents
(1)
3.16.3
negative-sequence short-circuit impedance
Z
(2)
impedance of the negative-sequence system as viewed from the
short-circuit location
Note 1 to entry: See 5.2.5.
3.16.4
zero-sequence short-circuit impedance
Z
(0)
impedance of the zero-sequence system as viewed from the short-
circuit location
Note 1 to entry: The zero-sequence short-circuit impedance includes three times the neutral-to-earth impedance
Z .
E
3.17
impedances of electrical equipment
3.17.1
positive-sequence impedance
Z
(1)
ratio of the line-to-neutral voltage to the short-circuit current of the
corresponding line conductor of electrical equipment when fed by a symmetrical positive-
sequence system of voltages
Note 1 to entry: See IEC TR 60909-4:2021, Clause 6.
Note 2 to entry: The index of symbol Z can be omitted if there is no possibility of confusion with the negative-
(1)
sequence and the zero-sequence impedances.
IEC 60909-0:2026 © IEC 2026
3.17.2
negative-sequence impedance
Z
(2)
ratio of the line-to-neutral voltage to the short-circuit current of the
corresponding line conductor of electrical equipment when fed by a symmetrical negative-
sequence system of voltages
Note 1 to entry: See IEC TR 60909-4:2021, Clause 6.
3.17.3
zero-sequence impedance
Z
(0)
ratio of the line-to-earth voltage to the short-circuit current of one line
conductor of electrical equipment when fed by an AC voltage source, if the three paralleled line
conductors are used for the outgoing current and either a fourth line or earth as a joint return,
or both
Note 1 to entry: See IEC TR 60909-4:2021, Clause 6.
3.18
subtransient reactance
"
X
d
effective reactance of a synchronous machine at the moment of short circuit
"
Note 1 to entry: For the calculation of short-circuit currents, the saturated value of X is taken.
d
3.19
minimum time delay
t
min
shortest time between the beginning of the short-circuit current and the contact separation of
the first pole to open of the switching device
Note 1 to entry: The time t is the sum of the shortest possible operating time of a protective relay and the shortest
min
opening time of a circuit-breaker. It does not take into account adjustable time delays of tripping devices.
3.20
thermal equivalent short-circuit current
I
th
RMS value of a current having the same thermal effect and the same duration as the actual
short-circuit current, which can contain a DC component and can subside in time
3.21
short-circuit current of full-size converters
current source of full-size converters
3.21.1
source current for three phase short circuits
I
kPC
size converter and current regulation in case of three-phase short circuit at the high-voltage
side of the step-up transformer
3.21.2
source current for line-to-line short circuits
I
(1)k2PC
power station with full-size converter and current regulation in case of a line-to-line short circuit
or a line-to-line short circuit with earth connection at the high-voltage side of the step-up
transformer
IEC 60909-0:2026 © IEC 2026
3.21.3
source current for line-to-earth short circuits
I
(1)k1PC
a power station with full-size converter and current regulation in case of a line-to-earth short-
circuit at the high-voltage side of the step-up transformer
3.22
impedance of the nodal impedances matrix
Z , Z , Z
(1)ii (2ii) (0)ii
diagonal elements of the positive-sequence, or negative-sequence or zero-
sequence nodal impedances matrix for the short-circuit location i
3.23
impedance of the nodal impedances matrix
Z
(1)ij
elements of the positive-sequence nodal impedances matrix, where i is
the node of the short circuit and j the node where the high-voltage side of a power station unit
with full-size converter is connected
4 Symbols, subscripts and superscripts
4.1 General
The formulas given in this document are written without specifying units. The symbols represent
physical quantities possessing both numerical values and dimensions that are independent of
units, provided a consistent unit system is chosen, for example the international system of units
(SI). Symbols of complex quantities are underlined, for example Z = R + jX.
4.2 Symbols
j2π/3
a complex operator a = e
A initial value of the DC component i
DC
c voltage factor
cU /3 equivalent voltage source (RMS)
n
f frequency (50 Hz or 60 Hz)
f equivalent frequency to calculate the peak short-circuit current
c
I symmetrical short-circuit breaking current (RMS)
b
i aperiodic or DC component of short-circuit current
DC
I steady-state short-circuit current (RMS)
k
I steady-state short-circuit current of a generator with compound excitation
kCE
symmetrical locked-rotor current of an asynchronous generator or motor
I
LR
i peak short-circuit current
p
I rated current of electrical equipment
r
I thermal equivalent short-circuit current
th
"
I initial symmetrical short-circuit current (RMS)
k
"
I initial symmetrical short-circuit current (RMS) caused by current sources
kCS
"
I
initial symmetrical short-circuit current (RMS) caused by voltage sources
kVS
IEC 60909-0:2026 © IEC 2026
κ correction factor for impedances
m factor for the heat effect of the DC component
n factor for the heat effect of the AC component
p pair of poles of an asynchronous machine
p range of synchronous generator voltage regulation
S
P total loss in transformer windings at rated current
krT
P rated mechanical power of an asynchronous machine (P = S cos(φ )η )
rA rA rA tA rA
p range of transformer voltage adjustment
T
q factor for the calculation of breaking current of asynchronous machines
q nominal cross-section
n
r relative resistance of a transformer at rated transformation ratio in per unit
rT
R resp. r resistance, absolute respectively relative value
S rated apparent power of electrical equipment
r
T duration of the short-circuit current
k
t minimum time delay
min
t rated transformation ratio (tap-changer in main position); t ≥ 1
r r
u rated short-circuit voltag
...