Minimum insulation level for lightning protection of medium-voltage lines

Lightning protection of wood pole lines is a problem of wide interest among operating companies. It is therefore timely to cite the experience of the Philadelphia Electric Company during the six year period, 1935–40, in successfully renovizing its 13 and 33 kv wood pole lines to reduce trouble. In 1935 this company was faced with the problem of determining how to materially reduce lightning trouble on a variety of pole top configurations at a minimum of expense. It had been shown by Andrews and Stroup¹ that wood insulation could be employed effectively. However, no information was available to indicate what reduction in trouble could be obtained for various increases in insulation. The high ridge pin configuration proposed by these authors, while reported to be very effective, could not be applied to many of the existing line structures without making major changes in construction at considerable expense. There was therefore need for an investigation into the degree of protection afforded by lesser amounts of insulation. With this problem in mind, the author set out to make a detailed investigation of the performance of the various types of structures and to correlate their insulation strength with their rate of trouble.