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THE UNIVERSITY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunications Session 1 2011 ELEC9713 – Industrial and Commercial Power Systems Quiz #2 SolutionQ1. The possible fault types that may occur in a three-phase system are: Three phase (symmetrical) faults (the most severe in terms of current) Phase to phase fault Single phase to earth fault Three phase to earth fault Phase to phase to earth fault Each of these fault types will have different fault current when they occur at the same location and the electrical protection system will need to take this into account when operating time is determined. A symmetrical 3-phase fault often gives maximum fault current level at any location and represents worst case situations. Typical value at point of supply for 230/400V NSW distribution systems Suburban residential areas: 10 kA Commercial and industrial areas: 25 kA
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