IEC 60781:1989

FOREWORD
PREFACE
Clause
1 Scope
2 Object
3 General calculation methods and calculation assumptions
4 Definitions
   4.1 Short circuit
   4.2 Far-from-generator short circuit
   4.3 Short-circuit current
   4.4 Prospective (available) short-circuit current
   4.5 Initial symmetrical short-circuit current
   4.6 Peak short-circuit current
   4.7 Symmetrical short-circuit breaking current
   4.8 Steady-state short-circuit current
   4.9 Nominal system voltage
   4.10 Short-circuit impedances of electrical equipment
   4.11 Short-circuit impedances at the short-circuit location
   4.12 Voltage source
   4.13 Equivalent voltage source
   4.14 Voltage factor c
5 Symbols, subscripts and superscripts
   5.1 Symbols
   5.2 Subscripts
   5.3 Superscripts
6 Short-circuit types
7 Calculation methods and assumptions
   7.1 Symmetrical components
   7.2 Equivalent voltage source at short-circuit location
   7.3 Conditions for disregarding the influence of motors
8 Short-circuit impedances of electrical equipment
   8.1 High-voltage network feeders
   8.2 Transformers
   8.3 Overhead lines and cables
   8.4 Other impedance elements
   8.5 Conversion of impedances
9 Calculation of short-circuit currents
   9.1 Balanced three-phase short-circuit current
   9.2 Line-to-line short-circuit current without earth
        connection
   9.3 Line-to-earth short-circuit current
10 Example of short-circuit current calculations using forms
   10.1 General procedure
   10.2 Explanations for using the calculation forms
11 Example for a housing installation
   11.1 Network diagram
   11.2 Data of electrical equipment
   11.3 Special remarks
FORMS

This application guide presents a practical method to be used when calculating short-circuit currents in low-voltage networks. The method corresponds strictly with IEC 60909 and leads to conservative results with sufficient accuracy.Two short-circuit currents which differ in magnitude are to be calculated:-the maximum short-circuit current which causes the maximum thermal and electromagnetic effects on electrical equipment and determines the required capacity or rating;-the minimum short-circuit current which may be a basis for the adjustment of protective devices.