[Boatanchors] Testing a ground connection / lightning

[Barry H]

Some additional input on this....

AC?  Thought it was DC -- potential building up between the clouds etc and the ground and actually electrons going skyward, not downward as the flash makes it appear.

As for turning corners -- the answer is yes and no.  I had a direct lighning strike some years ago on a house I rented.  The lighning took several paths, seemingly simultaneously.

It apparently first struck my TV antenna, went down the mast and through the foam twinlead I used back then.  This was a wood frame house with shingles, but was covered with aluminum skin over the trim and under the soffets.  The lightning ran down (up?) the mast and into the aluminum trim at the peak, down and all around through (I suspect the inside surfaces of) the trim, completely or almost completely "circumnavigating" the trim and soffets -- along with the gutters.  At the corners, it looked as though the aluminum were shot through with BB's.  There was nothing there for it to jump to -- just didn't make the corners too easily.  Some went 'round the bend, a lot shot out.

This made it to the back of the house and through an outside light fixture to a combo wall switch/outlet in the kitchen.  The outlet exploded -- cover was on the floor and the switch/socket was in pieces.  This also took the power out and several fuses were blown down in the basement.

Meanwhile, the lighting that was coming through the downlead followed into the house where the twinlead passed near a baseboard radiator.  Some of it did jump to the radiator passing through the bottom of a synthetic curtain which was singed/melted a bit.  However, it did not completely "choose" the radiator.  It continued to the basement where I had a network of Blonder-Tongue (better quality) splitter/couplers routing the antenna around to the various TV's and FM tuner.  These couplers were made of a very heavy beige plastic top, with two studs heat-formed to hold onto the phenolic board where the balun was mounted.  They are practically impossible to open up without breakage.  The plastic top was missing -- blown off.  I didn't find it until we moved several years later.  The balun was vaporized -- just some smudge marks -- however, not before the strike reached my Dynaco FM-5 Tuner and 400 Amp and possibly also the TV.  The TV was out -- I found a pigtail fuse for the AC supply -- exploded.  When I replaced it, still no luck -- but made it look good so was repaired under warranty.  It's possible the TV received a double hit - through the AC and antenna.

The damage to the FM-5 and amplifier was weird.  The 300-75 ohm balun just inside the tuner was likewise vaporized - the little ferrite core was rolling around inside.  Some part of the trace on the front end board was also gone.  However, when I replaced the balun and bridged the missing copper traces, it was back in business.  Likewise with the amp.  Just a 555 IC and one or two other parts.

Much of the antenna wiring was no good -- the copper had vaporized within it.  The only permanent loss was an air conditioner -- could't figure out what was wrong with it and wasn't worth the bother.

We had, as I recall, $20,000 renter's insurance.  We learned however, that (a) it did not cover lightning, but only fire damage resulting from lightning and (b) they apportion out the coverage in a claim.  Some (unknown) portion is for furniture, some is for clothing, some is for this and some is for that -- but only some small portion was for electronics and electrical appliances.  I was very lucky.  (check the fine print in your insurance policies)

So ... not sure of the exact sequence of events.  I was just getting to sleep at around 1 AM and there was a massive ka-boom and vibration.  Apparently it was not instantaneously over with.  I got up and looked out the window.  Everything was dark and eerily silent in the house (fuses blown) yet there was some sense of bright light looking out across the street.  That's because WE were the light source!  Neighbors later told us that the whole place lit up.  Bear in mind, it took me a few seconds to get up and look out the window.

To wind up a long story -- the "stuff" both turns corners and fails to turn corners and "splashes" out.  From the pattern of damage, it will take several paths at the same time -- not just the one of the least resistence -- though that's dynamic, I guess -- as it may be burning/vaporizing a conductor as it's passing through it.

Among other things, ol' Ben Franklin really had guts.  Not only should you not fly a kite in a storm -- but be sure to keep your keys in your pocket. ;-)

Barry







> Lightning,
> Impedance? Yeah. We're dealing with AC, not DC, so it's impedance, not
> resistance. The tiniest bit of reactance raises impedance. Good thing to
> remember. Also, surges and lightning do not turn corners or generally
> cooperate with your way of doing things. Gently radius any turns.
> Wrapping a wire around an eave will be the point at which lightning
> decides it will not make the bend and will shoot straight out.
> 
 
Lightning doesn't care if it is going straight or turning a corner in a
wire. It travels on the wire just like any other RF current.

What the bend does do is add inductance to the wire at the point of the bend
which raises the impedance at that point as you noted above.
It is the higher impedance point, where the voltage goes up, that may allow
the lightning energy to find an alternative or better path than it is
currently on.
It is not that it is turning a corner which sometimes causes an arc to
something else.

73
Gary  K4FMX