Arc length vs. electrode gap for underground cable arc flash hazard analysis

The test lab, in association with a major US utility conducted laboratory arc flash testing on various types of cables and cable joints. The objective was to provide the utility with an accurate tool to predict the incident energy levels released by arc faulted underground cables, and thus assist in PPE selection for workers in underground vaults. From the testing results and subsequent analyses, it was deduced that arc lengths from cables can be considerably longer than the distance between the conductor and the shield of the cable; arc lengths should be used in the calculations instead of bus gap distances. The actual arc length at a cable fault in the testing was as long as 18". Further testing is recommended to improve the accuracy for a variety of cable types, arc currents and durations, and to allow the results to be more confidently extrapolated. This paper presents laboratory arc flash testing results for several types of underground cables and cable joints widely used on the utility's distribution system. It demonstrates the differences between the electrode or bus gap distance and arc length, as well as the importance of using the arc length as one of the parameters when conducting incident energy calculations.