Vegetation clearance distances to prevent wildland fire caused damage to telecommunication and power transmission infrastructure (2)

Abstract

Towers and poles supporting power transmission and telecommunication lines have collapsed due to heating from wildland fires. Such occurrences have led to interruptions in power or communication in large municipal areas with associated social and political implications as well as increased immediate danger to humans. Vegetation clearance standards for overhead conductors in the US have been specified to prevent ignition due to arcing from the power line to ground or to minimize potential fire ignition by hot sparks from a line fault that fall into vegetation on the ground. The California Public Resources Code (PRC) section 4292 suggests "clearance of flammable fuels for a 10-foot horizontal radius from the outer circumference of power line poles and towers." Unfortunately, no studies were found addressing the question of what is the appropriate clearance needed to prevent damage to the conductors and support towers by wildland fires. Recognizing the lack of quantitative information, The Fire Sciences Laboratory (USFS) analyzed this question. The results suggest that steel towers provide the greatest resistance to fire damage; however when failure occurs it is catastrophic, wood poles and towers do not fail catastrophically and thus may provide longer term resistance to failure. Minimum clearance for steel towers in surface and crown fires is 1 to 5 m. The minimum clearances for wood poles exposed to surface fires of low to moderate intensity are on the order of 1 to 5 m. For crown fires in tall brush and tree canopies, wood poles and towers require clearances of 20 to 30 m. The study indicates that aluminum towers are most similar to steel in terms of clearance distances for fires in all vegetation types. The susceptibility of wood poles to ignition and sustained burning is dependent on the age and condition of the wood surface: aged poles that present fissures for ember accumulation have the greatest risk. There is evidence from the USFS study simulations that the possibility exists for overhead conductors to fail although this occurrence has not be observed. Transformer and junction enclosures were not susceptible to damage primarily due to the steel exterior material. Clearance around telecommunication towers is dependent on the exposure of cables, guy wires, and other materials near the ground. Analysis and conclusions from this study should be characterized as preliminary.

Parent Publication

• Proceedings of the large wildland fires conference; May 19-23, 2014; Missoula, MT

Citation

Butler, B. W.; Wallace, T.; Hogge, J. 2015. Vegetation clearance distances to prevent wildland fire caused damage to telecommunication and power transmission infrastructure (2). In: Keane, Robert E.; Jolly, Matt; Parsons, Russell; Riley, Karin. Proceedings of the large wildland fires conference; May 19-23, 2014; Missoula, MT. Proc. RMRS-P-73. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. p. 257-261.