You do not need to fly near high voltage lines to get electricity to the cargo hook. I've seen soldiers knocked completely off the back of 2.5T trucks when trying to hook them up to Chinooks. I've done many hours of slingloads in BH206 models, and they all create static electricity. The ground guys are taught how to dissipate the current before actually connecting the sling line to the hook.
Lots of old wives' tales on this thread, and apparently a lot of old wives, too.