AS 3851:1991 (R2015)

Specifies standard methods for the calculation of short-circuit currents in three-phase a.c. systems, both radial and meshed with and without contribution from generators and motors, which are suitable for either computer or manual calculation.

Scope

This Standard specifies a standardized procedure for the calculation of prospective short-circuit currents in the fault for-
(a) low voltage a.c. systems; and
(b) high voltage three-phase a.c. systems, operating at a nominal frequency of 50 Hz.
The procedure is intended to be, as far as possible, in a form to facilitate its use by non-specialist engineers.
NOTE: The procedure is applicable generally to other frequencies close to 50 Hz; however, care should be exercised in the use of frequency-dependent factors.
Except in Appendix A, the basis of the calculation procedure in this Standard is ohmic. Appendix A converts this procedure to per-unit calculation.
NOTE: Worked examples are provided in Appendices D and E on ohmic and per unit basis respectively.
The current, calculated in this Standard forms the basis for the determination of the currents in the paths feeding the fault.
This Standard specifies a standardized method for the calculation of the joule integral (I2t) of a short-circuit current.
This Standard does not cover-
(i) short-circuits deliberately created under controlled conditions as in short-circuit testing stations;
(ii) short-circuits in the electrical installations of ships and aircraft;
(iii) non-simultaneous short-circuits, which can lead to higher aperiodic components of short-circuit current; or
(iv) methods for analysis of short-circuit currents in non-linear systems.