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, or
b) short-circuit currents in the electrical installations on board ships or aeroplanes,
or
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