[time-nuts] Thunderbolt cabling questions
engineering at mt.net
Wed Jun 13 15:17:54 UTC 2012
Sometimes the "cone of protection" simply does not exist. A couple of weeks ago I had to respons to one of my sites which contains an 800 foot tower. While there a large storm cell developed and a severe thunderstorm ensued. Shortly before leaving lighting hit the tower which was unnerving enough. But as I was walking out the door to get in the truck, another strike hit the ground about 200 yards in front of me. So much for "protection."
The house I live in is a 50 year-old two-story structure with metal siding. After moving in ten years ago and getting to know the neighbors, I was hearing rumors about the old lady who previously owned it having an issue of smelling smoke in the house shortly after a lightning storm occurred and having noticed a nearby strike. The fire department responded and investigated but could not find anything.
After moving in I decided to remodel the second floor. As I was demo'ing the walls, I came upon the older style asphalt covered cloth AC wiring in one outside wall with its insulation completely burned away over a couple of vertical feet. Fortunately the rock wool insulation in the wall refused to continue ignition and the burning ceased. Additional investigation revealed that the main lightning strike had hit the tree next to the house and a secondary streamer traveled over to the siding. But this is where it gets interesting...
The streamer hit the metal siding and under that location was a nail that apparently was fastening the old lap siding (under the later added metal overlay). The lightning then arced over to this nail which extended through the under sheath and then to the AC wire starting the fire. Looking at the point of the nail tip I found copper that was deposited from the wire over to the nail. The migration indicated that the lightning strike had been of negative polarity given the propensity for metals to migrate from the positive to the negative electrodes in an arcing situation.
> Hi gang
> Just to upset the apple cart a bit, high trees do not necessarily
> protect a large area from lightning.
> In my past life as a range officer at a large shooting facility, we were
> hit by lightning directly in front of the firing line during a storm.
> The tree line was about 20 to 30 ft behind us and at least 50+ feet higher.
> There was even evidence of arcing between 2 pieces of conduit stuck in
> the ground about 5 ft apart. first and last time I saw a fire ball.
> For induced charges from nearby strikes nothing beats a good ground and
> lightning protector system.
> If a direct hit, I hope your insurance is paid up.
> My gps antennas are about 25 above ground just above my roof line. The
> mast is grounded to 2 ground rods and the coax fed through gas tube
> lightning protects. No problems even with hits several hundred yards away.
> Ewing (Rix) Seacord
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