Before our college switched to fiber optics for inter-building computer
communications, I was the primary person trying to figure out how to
protect the communication lines during lightning storms. I had some
successes and a lot of failures. Here's of few of the things I think I
learned. At the risk of being chastised for personifying lightning,
I'm going to write in that manner of speaking. I apologize that this
will also be long. But I'm giving a physics exam right now (I'm in my
office because we give exams on the honor system... no proctor) but I
need to stay nearby. So I have two hours to kill.
(1) Asking where lightning is going to go is a bit like asking where a
500-pound canary sleeps. Answer: anywhere it wants. I get a kick out
of people who think cars are safe in lightning because cars are
insulated and lighting won't strike them (as opposed to the real reason
that it forms a Faraday cage around the occupants. ) The common
opinion is the rubber tires of a car are what insulates a car and keeps
lightning from hitting it. Good grief, the lightning just "jumped
through" tens or even hundreds of feet of air... someone thinks it
can't jump from the frame of the car to the ground!
Likewise, if it was sufficient charge to breakdown hundreds of feet of
air, and it's now surging through your home's plumbing or wiring, are
we to believe it can't jump to you or split to include you in its path.
I find it useful to think of the charge "trying to get to ground"
anyway it can. It "explores" various paths, sometimes zig-zagging
around, sometimes splitting into multiple paths. But by the time the
strike has developed, there's no stopping it... it will go where it
wants.
(2) Nonetheless, it's not dumb... i.e. it doesn't disobey the laws of
nature. As it is searching out its path, the one(s) it takes are the
easiest one(s) at each moment... at each zig or zag. Huge potential
differences build up across the air and across objects. If the object
is a conductor, current naturally flows because of the potential
difference. If the object is an insulator, the potential might be
great enough to cause it to break down (dielectric failure), or the
electric field might break down the air and cause sparking around the
object, or both, or it might take some other easier path.
(3) If electronic equipment is in the wrong position at the wrong time
such that a significant potential difference develops across it... it's
toast. The same can be true for people. I don't necessarily think of
this as being "in the path of the lightning," because the path is not
predefined. Rather, I think of it as being near enough to the
charge/current to experience significant electric fields and hence
significant potential differences across the object/person. These
electric fields can be both electrostatically caused (i.e. from being
near large amounts of charge) or from induced emf's (i.e. from being
near large currents).
So, if lightning strikes anywhere near me, I am one of the options for
the path. I am definitely going to have some electric fields through
me and hence potential differences developing across me. The question
is, how big will that potential difference be, and as a result, what
current will go through me? And another question: is there anything I
can do to minimize the electric fields and potential differences I
experience? I think the general answer (short of surrounding myself in
a Faraday cage) is to try to stay away from where the bulk of the
charge might be expected to go. Thus, even though I expect a huge
amount of that charge to go through the metal plumbing, or the
building's wires, I should stay away from the plumbing and wires as
much as possible. When those wires or pipes have a large instantaneous
current flowing through them, the electric field around them is high,
and that puts large potential differences across objects around them,
and that can cause currents (or lightning splitting or zig-zagging) in
the objects surrounding the "primary path." Hence, if you want to
reduce your risk (from small to smaller) go to the part of your house
with the least amount of plumbing and wiring. Of course I don't do
this. I go out on my front porch and watch the show. In that position
I worry the most about the lightning hitting the big tree out in the
yard, and I will get hit by shrapnel from the tree.
I think people-phone-lightning problems are not so much because the
house got hit, but because the telephone pole or line got hit and/or
the phone lines were near a lightning hit. Suppose lightning hits a
telephone pole in my neighborhood. The phone wires in that vicinity
rise in potential either from direct deposit of charge, or from induced
emf's. If I'm talking on the hard-wired phone, then the wiring in the
handset I'm holding next to my head rises in potential just like its
doing on the phone lines all through my neighborhood. And the question
is... how high? High enough to cause a breakdown of the air between
the handset and me? Maybe. If so, it's going to spark right into my
ear or mouth... not too exciting. This is clearly what takes out
modems on computers. The computer is grounded via the power wires, and
the phone line rises in potential, so the electronics break down
because a "spark" goes across the junctions in the transistors in the
modem from the phone lines into the grounded computer. The computer
didn't get hit. Rather, lightning hit near enough that the phone lines
rose in potential (from direct-deposit of charge for from induced
emf's). Interestingly, the modem can also get fried if the wires that
rise in potential are the 120-volt power wires. If the power line
rises in potential, but the phone line doesn't, the spark goes "the
other way" through the modem (from the higher potential power wires to
the lower potential phone wires).
Surge absorbers "protecting" the modem line and power lines have
varistors not only connected from the wires to ground, but also from
one wire to another. If the power lines in my house rise in potential
because of a hit or near hit, but the phone line stays at a lower
potential, the varistor connecting the power lines and phone lines can
allow a current path from the power lines to the phone lines without
going through my modem. Perhaps the varistor will survive... perhaps
it will fry... but a fried surge absorber is less expensive than a
modem.
Well. That's probably enough. I'm no expert... just a person who
tried (for several years) to route wires and install varistors and/or
spark gaps in a manner to try to make lightning and induced currents go
where I wanted them to go instead of where they wanted to go. Like I
said, I sometimes won... but mostly I failed. Fiber-optic
communication sure is nice... that's how I ultimately won.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817