To address this situation, Eskom has designed and implemented measures that do not require further intervention once an existing overhead line network has been reengineered in the affected areas.
Pole-top fires have been recorded in diverse locations, including the coastal regions of South Africa and Australia, the Middle Eastern desert and the industrial areas of North America. While these areas differ in many ways, they all have certain climatic conditions that encourage insulator pollution and the potential for insulators to get lightly wet at different times of the year.
- The surface of one or more of the overhead line phase conductor’s insulators has surface pollution.
- The polluted insulator gets lightly wet from mist, fog, drizzle, light rain or condensation. Conversely, heavy rain can help to reduce the risk of pole-top fires because it cleans the insulators, reducing the buildup of surface pollution.
- Wet and polluted insulators offer a conductive surface, resulting in leakage current.
- The leakage current finds its way into the interior and/or onto the surface of the wood pole or cross arm of the distribution structure.
- Arcing results, leading to tracking on the surface of the wood, which can be ignited.
The increased impedance of the wood-pole path that includes the air gap also improves bird safety, as the risk of birds being electrocuted is reduced. However, there is an increased risk the pole may burn at the air gap because of leakage current flow to earth. Field experience has shown the risk of burning in this position is significantly less than that of a wood cross arm burning.
Other Wood-Pole Fires
Using Eskom’s engineering mitigation measures has the advantages of not requiring frequent labor-intensive and costly interventions — such as insulator washing and leakage current detection — once the measures have been implemented. However, it is not practical or cost-effective to implement these design measures on existing overhead line networks in a short span of time, so long-term programming of these remedial changes is required to eliminate or limit the risk of pole-top fires to an acceptable level.
Eskom has included these principles in its engineering philosophy for a while now on its medium-voltage overhead line network. To date, no unexplained pole-top fires have been reported on structures built following this philosophy. This result is as expected because of the extensive investigation conducted to verify the philosophy’s soundness, including controlled testing in a high-pollution environment and determining the circumstances of field failures. This process has proven worthwhile because of the confidence Eskom gained from the recommendations.