From: stevek@gobi.UUCP (Stephen Kogge)
Newsgroups: sci.electronics
Subject: Re: Thunderstorms
Date: 8 Oct 92 13:20:25 GMT
Organization: Underwater Imaging

In article <1687984FF.FC138001@ysub.ysu.edu> FC138001@ysub.ysu.edu (Phil Munro) writes:
>
>  We've seen lots of theory, but only a few experiences, of lightning
>doing damage to computer equipment.  Would anyone have some details
>about damage done to equipment?  This might give us a better idea of
>how much of a problem lightning actually is, direct hits, EMP, etc.
>- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>  FC138001@ysub.ysu.edu  Phil Munro       Youngstown State University
>  Electrical Engineering Department           Youngstown  Ohio  44555

	My house is in Maryland about 6 miles from the University of
Maryland and 10 miles from the White House. We have lightning hits about
every year or two, sometimes several a year. After the first few we have
learned to take them for granted.

	About 15 years ago a tree next to our house was hit by the primary
bolt, the secondary hits blew holes in the yard, a concrete runoff diverter
under a downspout and the corner of the house near that diverter were
blown up. The tree, one of the smaller oaks in the yard about 30 inch diameter,
was stripped of bark for about 100 ft and about 8 inches wide.
	Windows on that side of the house were blown out by the exploded bark.

	I was up in Canada doing some post-doc work on the spruce budworm
project, my parents were on vacation, so we all had a little suprise
when we got home.

	We found no electrical damage except to those electrical items
which were unplugged.  An unplugged line printer and a computer never
quite worked the same, yet everything else that was plugged in was working.

	My dad commented that when he was a grad in the 30's he worked on a
lightning strike project. They had a truck filled with caps that were charged 
parrallel and discharged in series. Con Ed let them "hit" the power main
and record the transients. Con Ed said it would be impossible to take down
the grid with multiple hits. They computed the time constants for the
grid and proceeded to demonstrate Con Ed was wrong, the phone call came
back and asked them to stop before too many substations dropped off as
the voltage ran up and down.
	
	My dad's theory is that anything plugged in has a clean
path to ground, anything with an unplugged cord was charged up and sparked
to ground.

	About 3 years ago I was on my way to the Red Sea and lightning decided
to hit another of those little 30 inch diameter oaks. This was just over our
property line on park land. The top 50 feet of the oak was turned to
tree bits... This time the TV cable box was history one Sony VCR had
a cute little hole blown in the tuner board and a small TV in another room
has a tuner that no longer works.  The discharge path was a strange one since
other receivers were not touched. The antenna distribution amp was also
history with the bleed resitors connected to the coax charred.

	We stopped counting lightning hits... My dad sort of dryly commented
that he watched one bolt go from the hanging lamp in the kitchen to the
sink anout 8 feet away. He said he paused after the flash and continued
with breakfast. You have to remember this the same fellow that as
a radar expert during WWII was sent to a destroyer to investigate a
problem. He was about 100ft up in the masts where the spark gap
rectifiers were, had taken the "nine hundred ninty ninth bolt" off the
radar compartment hatch, then he woke up in the sick bay. It seems that
the glassohms bleed resistors were broken and the caps were still
charged. He wonders how they got him out of the masts. He also carried
a big screwdriver and a wire to throw into the room first and always
wore rubber overshoes when sent to look at high voltage.

	My advice. Be sure you have a clean path to ground == leave everything
plugged in. When ligthning stikes it will take out what it wants (actually
electronics evolution at work as the poor ground path designs demonstrate
their faulty "genes"). Those one-use 30cent MOV's are of questioanble
use when lightning is measured in a megawatts. I have never met anyone who
actually replaces all the MOVs after every storm, or season. When I have asked
about testing MOVs I get the answer there is no way to test them so you
just replace them often if you expect them to work.

	On a side note one of the UPS vendors sent a warning about MOVs.
It went somthing like this "Be sure to replace the MOVs after every storm,
be sure to replace all of them after every hit or suspected hit. Check
that all 3 (or 5 leads for 3 phase) leads are cross connected with MOVs.
Any design that uses a single MOV to just ground or neutral is dangerous
since the ground (or neutral) lines are not usually designed to survive
a spike - be sure on 110 volt lines that there the required 3 MOVs from
power to neutral, power to ground and ground to neutral are there
... or remove them." Too bad I lost that report.

	Steve Kogge
	stevek@uimage.com
Newsgroups: sci.electronics