The problem of outages on overhead transmission lines due to streamer-type flashovers is caused by typical bird behavior after perching on a tower cross-arm. Before taking-off, they usually empty their bowels and release up to 60 cm3 of excrement mixed with conductive urine at one time and coming out at quite a pressure. This creates a conductive path that bridges the air gap between tower structure and conductors resulting in a flashover either in parallel to or a little apart from the insulator string.
Detailed investigation of these streamers has shown that the continuous part of the streamer can attain lengths up to 2 m and move at speeds of 2 to 5 m/s. The angle of the streamer to the cross-arm can even be 60-70° yet still lead to flashover. The resistivity of the dried excrement has been found to vary from 800-2000 Ω∙cm. In its natural state, this figure will vary slightly by species. It should be taken into account, of course, that the average body temperature of a bird is 41.5°C and that actual resistivity will be less at lower ambient temperatures.
Experiments confirm that, given the above parameters, it is indeed possible to flash over insulator strings up to 500 kV AC and ± 400 kV DC. In Russia, for example, past research found that flashover occurs at a resistivity of about 600 Ω∙cm and a dielectric strength of 70 kV/m, which is less than the maximum operating voltage for the 110 kV insulators tested. Similarly, in the U.S., a resistivity of about 120 Ω∙cm, was enough to flash over a 500 kV suspension cap & pin string at 320 kV or about 1.1 times nominal operating voltage. It is worth noting that, during these experiments, the streamer could be as far as 70 cm from the string yet still affect it.
It is certainly clear that, from an engineering point of view, the problem of bird streamer flashover on transmission lines has been clarified and confirmed. The main issue now is how best to protect a line from birds perching in the vicinity of insulator strings and periodically flashing them over.