ASYMMETRICAL AND SYMMETRICAL SHORT CIRCUIT CURRENTS DEFINITION AND BASICS TUTORIAL

Symmetrical and Asymmetrical are terms used to describe the symmetry of the short-circuit current waveform around the zero axis. If a short-circuit occurs in an inductive reactive circuit at the peak of the voltage waveform, the resulting short-circuit current will be totally symmetrical.

If a short-circuit, in the same circuit, occurs at the zero of the voltage waveform, the resulting short-circuit current will be totally asymmetrical. If a short-circuit, in the same circuit, occurs at some time between the zero and peak of the voltage waveform, the resulting short-circuit current will be partially asymmetrical.



The amount of offset or asymmetry depends on the point when the fault occurs. In circuits containing both resistance and inductive reactance, the amount of asymmetry will vary between the same limits as before.

However, the X/R ratio (ratio of inductive reactance to resistance looking upstream from the fault point) will determine the rate of decay of the DC component.

As X/R increases, the rate of decay decreases. Interrupting current ratings may have to be derated for high X/R values. Practically speaking, most all short-circuit currents are partially asymmetrical during the first few cycles after a short-circuit occurs.

Modern personal computers can now be used to easily calculate symmetrical and asymmetrical current values at various times after a fault. Low-voltage protective switchgear has a momentary and an interrupting rating.

The momentary rating is the short-circuit duty during the first cycle after a fault, and defines the equipment's ability to close and latch against worst-case mechanical stresses. The interrupting rating is the short-circuit duty as the equipment contacts part, and is expressed in symmetrical amperes or MVA.

Medium-voltage fuses have interrupting ratings expressed in symmetrical amperes.

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