Re: Close lightning strike effects
- From: "Abstract Dissonance" <Abstract.Dissonance@xxxxxxxxxxx>
- Date: Fri, 7 Jul 2006 02:22:52 -0500
"Paul E. Schoen" <pstech@xxxxxxxxx> wrote in message
news:44acae7d$0$12728$ecde5a14@xxxxxxxxxxxxxxxxxxx
Monday night, while working at my computer, a strong thunderstorm moved
through this area, which is a very damp valley leading to the Loch Raven
Watershed area in MD. Hearing several thunderclaps seconds after flashes,
I considered shutting down my computer and disconnecting the power cord
and phone line. A few seconds later, I saw a bright flash and an immediate
loud "boom", and all power went out. I smelled something that could have
been ozone or burnt phenolic. I set up emergency lighting, looked for any
signs of fire, and took a brief look outside. Electrical power along the
road up to the junction with a main three phase feeder was out, and both
my phone lines were dead. Luckily my pets and I were not.
BGE arrived about 5 AM the next morning, determined that a fuse had blown,
and power was soon restored. When it was light enough to see, I looked for
other signs of damage outside. I have two houses separated by about 80
feet, and at the corner of the second house (used for storage) stands a
huge Sycamore tree, probably 150 feet high. I found some wet, broken
branches, and some pieces of bark, but no signs of major damage. However,
I noticed that several panes of glass in a window just about two feet from
the tree were broken, and two of them appeared to have blown outward. It
looked like a raccoon may have jumped through the glass (which is
possible, as I have found them in that house). Also near the tree is a 240
VAC twisted utility line, actually touching the tree at one point, and a
drop cable to a meter box which is disconnected. I do have power to the
house through a UF cable for convenience, fed from my residence on two 20
amp breakers. Also just below the power line is my active phone line,
which connects to an interface box and then through a four wire line to my
residence.
I discovered that one of the breakers had tripped, and I reset it with no
problem. There was a second storm in the afternoon that caused loss of
power again, but it was soon restored. The telephone technician found 48
VDC and a dial tone on one of the pairs, but not the other, and replaced a
blown protector device which restored one line, but the other seemed to be
an open circuit to the pole. An access box on the pole was open, possibly
from the lightning strike or perhaps it was being repaired by another
technician.
The damage to my equipment included a blown modem in my computer, damage
to my computer speaker internal amplifier, and a damaged phone answering
machine. I feel lucky.
I don't know the exact path of the lightning, but I think it may have hit
the 15 kV (probably) main line and arced over to the transformer secondary
and over to the tree, where it traveled down the ground paths. I think a
direct hit to the tree would have caused more damage. I am curious about
the pattern of the broken glass in the window, however. At first thought,
I would expect lightning to ionize the air and create steam pressure from
rain, so it would be an explosion with outward force. I think this caused
some of the initial damage to the window, as evidenced by glass shards
inside the house. But I think this may have been followed by a partial
vaccuum that caused the sudden expulsion of higher pressure air in the
house through the cracked glass. I will take pictures of it before I
repair it.
It was almost prophetic that I was just reading some posts in SED
referencing high voltage arcs and lightning, and also now reading the
other post about conductivity of trees.
Now it's raining again, and I think I hear thunder.. AIEEE!
Paul
One thing you can do is to make sure you ground is actually grounded and not
floating. I have grounded my wall outlet's ground to a 3 foot rebar outside
the outlet because of a similar incident. You could probably also
mechanically hook up a varistor between the ground and hot and another
between the ground and neutral just incase. These things don't require much
expertise although you would need to be careful when handling the mains(try
and turn off the power if you don't know what your doing ;). It might not be
the easiest method but it would probably be just as good as as a surge
protector(as most are crap). You could even hook up a fuse and make a little
custom box outside that houses these components incase you need to change
them). Its probably cheaper then buying a decent surge protector and would
work well. Ofcourse you could take it farther and add more protection if you
have expensive equipement. Ofcourse this won't stop ESD from getting at your
equipment but will stop voltage spikes from doing any damage(at the cost of
a very cheap varistor). I'm sure someone else more experienced in this area
and make a better suggestion though. Its really up to you... but if you can
drill a hole, splice wires, drive a rod in the ground, and solder then you
could set up a decent amount of protection for your wall outlet in less than
an hr for just a few bucks. (although you could make it more elaborate too
by adding more sophisticated circuits)
.
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