ARCAD online short circuit calculator outputs the hierarchical system equipment tree with short circuit MVA calculated at each node. Short circuits contributed by upstream and downstream equipment are listed across each node. For a three phase fault, divide total calculated SC MVA value by 1.73 * kVLL to get short circuit current values in kA.
Three phase faults are unique in that they are balanced (symmetrical) in the three phases, and can be calculated from the single one line diagram. The resulting short circuit current values in kA are suitable for arc flash calculations, or any other study requiring symmetrical three phase bolted fault current analysis.
The protection specialist is interested in a wider variety of faults than just three-phase fault. The most common fault is a single phase to earth fault, which in low voltage systems can produce a higher fault current than a three-phase fault. Experience has shown that between 70 to 80 percent of transmission line faults are single line to ground faults, which arise from the flash-over of only one line to the tower and ground. Similarly, because protection is expected to operate correctly for all types of fault, it may be necessary to consider the fault currents due to many different types of fault. Since the three-phase fault is unique in being a balanced fault, a method of analysis that is applicable to unbalanced faults is required. It can be shown that, by applying the principle of superposition, any general three-phase system of vectors may be replaced by three sets of balanced (symmetrical) vectors; two sets are three-phase but having opposite phase rotation and one set is co-phasal. These vector sets are described as the positive, negative and zero sequence sets respectively.
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