Wood Pole Transmission Lines

Abstract

IN order to examine the incidence of risks from shocks and fire due to the use of wood-pole transmission lines, tests were undertaken of primary insulation at 19, 11 and 6-35 kv. The work is described by,G. T. Garwood in the Electrical Reviewof April 12. The structure on which the tests were carried out comprised a plain-sawn English oak cross-arm, 5 in. by 3 in., bolted to a red fir-pole, 32 ft. by 11 in. in diameter at 5 ft. from the butt. The pole was sound in every way and had been creosoted. The instruments used for the tests were two electrostatic voltmeters. The resistance between the cross-arm bolt and earth was measured on frequent occasions in the period during which the test was being taken and also under varying conditions of dryness. The megger readings ranged from 200,000 ohms with the pole dry to 50,000 ohms with it wet. With the 11 kv. connexions a current of 140 micro-amperes could be obtained from below a plain earthing collar. As 15 micro-amperes may give a fatal shock to a normal man, if his heart lie in the circuit, the experiments showed that a plain earthing collar round the pole is insufficient protection. In the destruction tests, about ten minutes after the wetting of the pole smoke appeared from the cross-arm near the insulator pin. Longitudinal 'shakes' opened up in the cross-arm and showed signs of carbonizing. After about twenty minutes, smoke appeared from these shakes, the source of the smoke moving steadily towards the pole. After thirty minutes, smoke appeared from the pole top and ‘tracking’ with small intermittent flames at the earthing collar. Heavy smoke and flames appeared at the pole-top after seventy-five minutes and after eighty-five minutes the test was stopped, it being clear that the complete destruction of the structure was inevitable.