Burning of Wood Structures by Leakage Currents

Wood pole or structure fires of leakage-current origin have caused serious wood-member damage and service interruption. These fires often occur at the end of prolonged dry periods. Operating experience indicates surface tree carbonization on crossarms and pocket burninig at the crossarm-pole junction to be the two general types of burn damage. Pocket burning on some properties has been responsible for the larger part of the cases that required wood-member replacement. Laboratory tests conducted on approximately 200 specimens indicate that a coincident occurrence of dry wood and selective wetting of pole and crossarm surfaces by a light rain or fog, wind directed, which leaves a dry wood area in series with the leakage path, can result in a fire of a pocket type. This dry area, being high in resistance in comparision with adjacent series wetted-wood surfaces, results in a voltage concentration across the dry zone. If the dry zone is sufficiently short for a given circuit voltage, electrical breakdown occurs across the dry zone. This electrical breakdown being located favorably in dry wood and encouraged by a breeze may result in a pocket burn. Shunting devices which by-pass leakage currents around dry high resistance areas have proven successful in laboratory tests in eliminating pocket fires within the by passed zone.