For solidly grounded systems, the arc currents are, in percent of bolted three-phase faulted: three-phase, 89% line-line, 74% and line-ground, 38%. The arcing fault is a discontinuous current that alternately strikes, is extinguished and restrikes again. In addition to these types, one must also define the arcing fault: an intermittent failure between phases or phase to ground. Most of these are man-made in other words, they are accidents caused by improper operating procedures. Three-phase faults are less than 0.5% of all faults. Phase-to-phase faults are less than 1.5% of the total, and are usually the result of line-to-ground faults that aren’t cleared.
Line-to-ground accounts for 98% of these failures. There are generally three types of faults in industrial power systems: The basic definition of power system grounding, says Jack Woodham, P.E., senior electrical engineer with Cincinnati-based Jedson Engineering, Inc., “is the grounding of the power system transformer winding connection.” Woodham delivered the keynote presentation of the symposium: a tutorial on low-voltage-480 volts to 4,160 volts-applications for the three types of grounding mentioned above. Discussion centered on three types of grounding systems: ungrounded, solidly grounded and high-resistance grounding (HRG). On October 9, a one-day symposium, hosted by Post Glover, Inc., Erlanger, Ky., offered a number of expert opinions on grounding systems both for industrial and commercial facilities. After all, grounding problems are often the result of very unpredictable events, and so, many power quality issues are rooted in inferior grounding practices. It isn’t easy designing an effective grounding system.
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