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This paper describes a series of long spark investigations that were carried out to study the leader initiation process, strike probabilities and the protection area of lightning rods. The tests involved the breakdown of different configurations of air gaps, 5-26 m in length, using lightning rods of differing geometries. The objectives of these tests were to: (i) obtain an estimate of the "critical radius" using an inverted rod-plane configuration and compare this with previous values; (ii) investigate the effectiveness of "sharp" vs "blunt" lightning rods as strike receptors; (iii) obtain estimates of some of the streamer-leader parameters that are important in studies of lightning attachment; and (iv) develop new ways of testing lightning rods with long sparks. Estimates of the critical radius were found to he a little lower than previous values, a recommendation is made with respect to the minimum air gap for "competition tests" between lightning rods, a minimum current condition and critical streamer length is specified for leader inception from lightning rods, and a novel experimental configuration is proposed, tested and discussed.