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Source: Oracle 2003
Author: John S Innes
Has It Struck You
that the static drain coil is right in the line of fire for lightning? When there is no other DC path from the mast to ground, the coil is used to prevent DC voltage build-up on the mast due to wind and dust friction, and highly charged atmosphere in the presence of storm clouds. The static drain can reduce the probability of a direct lightning strike by keeping the mast at ground potential, but once a direct strike takes place it plays no further part, as the connection to ground is then provided by the horn gap or ball gap across the mast base.
In the first microseconds of a lightning strike, either direct or nearby, the static drain coil is very highly stressed. Because the lightning voltage appears as a pulse with a very steep wave front, most of the voltage appears across the top turn or two of the coil until the voltage distribution across the winding has time to even out. Consequently, coil failures in the top few turns are not uncommon.
The power industry has the same problem with distribution transformers, particularly pole mounted units, and they incorporate voltage grading rings to cause the lightning pulse to divide evenly over the whole transformer winding. A well designed static drain coil will have the top few turns wound with wide spacing, to withstand the voltage pulse. In critical situations, the design may also include a grading ring, sometimes described as a corona ring, round the top of the coil. Well designed coils do not fail, and they help protect the whole antenna system.
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