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This paper presents a simplified method for evaluation of the technical aspects of arrester application on modern underground distribution systems. The methods presented here can be used to calculate protective levels (and margins) for all equipment locations on such a system for any one of the three protection schemes identified in the paper. The calculations are presented by way of a worked example of protection design for Ontario Hydro's 27.6/16 kV URD system. For this system, the industry requirement ot 20% minimum protective margin can be met by the installation of only one additional arrester at the open point of the cable loop, provided arrester selections are restricted to those with the lowest protective levels. Without any restrictions on arrester selections, a second additional arrester is required at the first transformer upstream from the open point, but this practice results in protective margins much superior to the minimum requirements.
Date of Publication: Oct. 1987