Behaviour of red fir poles for 11-kV lines under electrical stress due to insulator failure

When a potential was applied between the ends of poles 10 ft long the potential difference between points on the lower portions of the poles and earth (the bases of the poles were earthed) was substantially reduced by using earthing collars or spikes. The former were most effective under damp conditions, whereas the latter were equally effective under damp and dry conditions. Shocks would in any event be obtained only when the surfaces of the poles are wet, and these should not be dangerous. These results were confirmed on a number of 30-ft poles used on 11-kV lines. The risk of shock is eliminated by the use of earthing collars. There is little or no danger of fire to poles having moisture contents up to 23%, which corresponds to an atmosphere of 95% relative humidity. There are, however, indications that the moisture content of the heart wood of poles in service for a considerable time in very wet positions may be abnormally high, so that fire readily occurs should the pole be subjected to a high electric stress (0.5 kV per foot of pole length). The field tests described in Section 9 show that abnormal moisture contents of the heart wood are extremely rare. It should also be borne in mind that poles are subjected to high electric stress only when the conductor comes into contact with the pole, which could normally happen only when disintegration of an insulator allows the conductor to rest on the cross-arm. The rarity of this, coupled with the normal dryness of most poles, probably accounts for the scarcity of recorded cases of ignition of poles, which were intentionally arranged to operate with the insulator steelwork unearthed.