Relative value of different types of overcurrent protection for distribution circuits

LINE sectionalizing in the main feeder and individual protection at branch-line junctions offers a practical solution to the problem of service interruption caused by overcurrent faults on distribution lines. Actual operating data have shown that as high as 96 per cent of all consumer minutes outage (minutes of outage per consumer per year) are caused by faults out beyond the substation, with 85 to 90 per cent of the outage time resulting from faults occurring on the primary lines. These faults are divided between those which persist so as to require servicing by a line crew (permanent faults) and those which are temporary, so that they cause a negligible momentary opening of the circuit or a momentary collapse of the voltage.¹ Operating experience shows that the percentage of temporary faults varies from 15 to 85 per cent of the total,² depending on local conditions and the amount spent on such things as tree trimming and pulling up slack.