Cleaning of overhead transmission line is not required because any dust and dirt that falls out of the sky onto them will be limited thickness and will not affect the electricity flow in any way nor will it decrease clearance (which is the insulation) in any meaningful way. Thus it is not necessary to clean overhead conductors but it is necessary to clean the insulators that support the conductors when they become contaminated with salt spray from the ocean or magnesium chloride used to melt ice on roadways in cold regions as well as from other contamination products.
One method for cleaning insulators is to use water through a pressure washer similar to what is used at a car wash. This cleaning can take place with the transmission line de-energized or energized depending on the utility’s operational practices. If the line is energized, the water used needs to be distilled and free of minerals and salts to reduce the risk of a flash-over back to the nozzle or phase-to-tower.