The short-circuit duty is determined by
the maximum short-circuit that the rotating machinery connected to
the system at the time of short circuit can pass through the breaker
to a point just beyond the breaker, at the instant the breaker
contacts open.
The short-circuit current is determined
by the characteristics of synchronous and induction machines
connected to the system at the time of the short circuit, the
impedance between them and the point of short circuit, and the
elapsed time between the starting of the short circuit and the
parting of the breaker contacts.
In calculating short-circuit currents
of high-voltage ac circuit, it is ordinarily sufficiently accurate to
take into account only the reactance of the machines and circuits,
whereas in low-voltage circuit resistance as well as reactance may
enter into the calculation. In dc circuit, resistance only is
ordinarily sufficient.
For first approximations, the reactance
and typical time-decrement curves of the synchronous machines may be
used. For close calculations, the actual reactances and time
characteristics of the equipment should be used, and calculation made
for single- as well as 3-phase faults.
The “per unit” impedance system and
the “internal voltage” method, using “symmetrical components,”
are often used in more exact calculations. Programs are available for
digital computer studies of system shortcircuit currents, both
balanced 3-phase and phase-to-ground.
The interrupting capacity, in kilovolt
amperes, is the product of the phase-to-ground voltage, in kilovolts,
of the circuit and the interrupting ability, in amperes, at stated
intervals and for a specific number of operations. The current taken
is the rms value existing during the first half-cycle of arc between
contacts during the opening stroke.
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