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.:
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a) currents during two separate simultaneous line-to-earth short circuits in isolated neutral or resonant earthed neutral systems;
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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:
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a) short-circuit currents deliberately created under controlled conditions as in short circuit testing stations, or
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b) short-circuit currents in the electrical installations on board ships or aeroplanes, or
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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.