Some Arcing Mechanisms in Lightning Protectors at Atmospheric Pressure

Some aspects of discharges occurring between carbon and carbon-copper electrodes in lightning protector blocks are examined. Various mechanisms that can cause arc transfer from high-to low-resistivity cathodes are considered. High resistivity in the cathode spot is a necessary but not sufficient property to cause arc motion. Diffusion of the arc plasma from an initial carbon-carbon arc to initiate a carbon-copper arc appears to be the most likely means for the transfer mechanism above about 30 A. Below 30 A the probability of arc transfer is considerably reduced. Effects of ac currents on these protectors are examined, and lifetimes have been predicted to within a factor of <2 of the experimental values.