Currents in power line wood poles : a measuring method

Recently a maintenance worker in Sweden was killed when climbing into a wood pole supporting an 11-kV overhead line. The investigating done afterwards raised the suspicion that a current flowed through the pole was sufficient to kill a person. Due to a broken insulator one of the phases of the 11-kV line had been resting on the metal crossbar. This accident triggered an investigation in order to determine if power line wood poles can conduct current and if so, whether this current can be sufficient to injure a maintenance worker climbing into the pole. The study has been made at the 11 kV level on a pole while a live wire was resting on the metal cross arm as would be the case if the insulator had broken. A proposal for a measuring method has been developed in order to determine under what conditions climbing should be prohibited. In Sweden the use of wood poles as part of 11 kV overhead lines is fairly common. A three phase system is used to distribute 11 kV, one wire for each phase, and the three wires are supported by insulators mounted on a horizontal metal (or wood) cross arm. The poles are treated with some kind of preservative to protect the wood from rot and insects. Three kinds of preservatives are commonly used in Sweden: arsenic, creosote and salt. In this study eight poles with different preservative and age varying from 50 years old to brand new have been examined in a realistic environment. The measurements presented in this paper support the earlier suspicion that a wood pole used for distributing power can conduct current if a fault occurs and a live wire comes in contact with the metal cross arm. It also shows that the current through a parallel, not connected to earth, resistor (i.e. a human climbing the pole) can reach potentially harmful amplitudes. Three main variables that affect the electrical characteristics of the pole have been identified: the preservation of the wood; the environmental temperature; and the humidity of the surroundings. There are however other variables that can have an impact on the conductivity of the pole for example how the pole was dried before preservation.Godkänd; 2010; 20100913 (ronsar