IEC 60909-3:2009

FOREWORD
1 Scope and object
2 Normative references
3 Terms and definitions
4 Symbols
5 Calculation of currents during two separate simultaneous
  line-to-earth short circuits
  5.1 Initial symmetrical short-circuit current
      5.1.1 Determination of M[(1)] and M[(2)]
      5.1.2 Simple cases of two separate simultaneous
            line-to-earth short circuits
  5.2 Peak short-circuit current, symmetrical short circuit
      breaking current and steady-state short-circuit
      current
  5.3 Distribution of the currents during two separate
      simultaneous line-to-earth short circuits
6 Calculation of partial short-circuit currents flowing
  through earth in case of an unbalanced short circuit
  6.1 General
  6.2 Line-to-earth short circuit inside a station
  6.3 Line-to-earth short circuit outside a station
  6.4 Line-to-earth short circuit in the vicinity of a station
      6.4.1 Earth potential U[Etn] at the tower n outside
            station B
      6.4.2 Earth potential of station B during a line-to
            earth short circuit at the tower n
7 Reduction factor for overhead lines with earth wires
8 Calculation of current distribution and reduction factor
  in case of cables with metallic sheath or shield
  earthed at both ends
  8.1 Overview
  8.2 Three-core cable
      8.2.1 Line-to-earth short circuit in station B
      8.2.2 Line-to-earth short circuit on the cable between
            station A and station B
  8.3 Three single-core cables
      8.3.1 Line-to-earth short circuit in station B
      8.3.2 Line-to-earth short circuit on the cable between
            station A and station B
Annex A (informative) Example for the calculation of two
        separate simultaneous line-to-earth short-circuit
        currents
Annex B (informative) Examples for the calculation of
        partial short-circuit currents through earth
Annex C (informative) Example for the calculation of the
        reduction factor r[1] and the current distribution
        through earth in case of a three-core cable
Annex D (informative) Example for the calculation of the
        reduction factor r[3] and the current distribution
        through earth in case of three single-core cables

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.