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 Figure 4-3 shows an asymmetrical short-circuit wave with the maximum possible asymmetry.
The magnitude of current offset for a typical fault will be between the two extremes of complete
symmetry and complete asymmetry, because the odds are against the fault occurring at either
peak or zero voltage. The offset of the asymmetrical current wave from a symmetrical wave,
having equal peak-to-peak displacement, is a positive value of current that may be considered as
a direct current.
The asymmetrical current, therefore, may be thought of as the sum of an alternating current
component b and a direct current component a. The direct current component decreases
eventually to zero, as the stored energy it represents is expended in the form of I 2 R losses in
the resistance of the system. The initial rate of decay of the direct current component is
inversely proportional to the X/R ratio of the system from the source to the fault. The decay
becomes more rapid as the X/R ratio is decreased. This decay is called the direct current
decrement. The total short-circuit current is thus affected by both an alternating current and a
direct current decrement before reaching its steady-state value.
4-8
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