In the case of locations being energized by multiple feeders, it is necessary to determine the portion of the total arc current passing through each protective device to determine the clearing time for each device. The overcurrent protective device determines the arc duration. The portion of the arcing current flowing through The total arcing current at the point of concern and the portion of that current passing through the upstream protective device(s) must be determined. Therefore, the maximum and minimum short-circuit are both necessary to be considered. Despite the maximum short-circuit is usually considered as worst case, the less arcing current leads to often significantly longer fault clearing times which can results in higher incident energy. Step 4: Calculate the arcing currentĪrcing current is usually less than bolted fault currents however the arcing current is close to bolted fault current for HV. This advancement happens within a few cycles. Arcing faults in devices or air that begin as line-to-ground faults, can also very rapidly step up into 3-phase faults as the air ionizes across phases. However, in most cases three phase short circuit currents typically give the greatest possible short circuit energy and can be considered as the worst-case. The single-phase short-circuit current is used to conduct the single-phase arc flash calculation. The three phase and single-phase short-circuit currents need to be considered. The calculation should take into account the system data and modes of operation. ![]() Step 3: Determine the bolted fault currentsĬalculate the short-circuit current.
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