Date: 9-May-85 17:35:39-PDT
Subject: Re: lightning protection
In the antenna world, where lightning strikes are a routine problem,
several layers of defenses are needed to protect equipment. The first is
typically a spark gap followed by about a ten-turn coil of half-inch square
copper busbar. The lightning spike is sharp enough that the inductance of
the coil forces most of the energy across the spark gap, which must connect,
via 000 copper or better, to a good solid ground such as a small
farm of 6' ground rods. Heathkit used to sell such a unit, which looked
line an orange-juice can with coax connectors on both ends and a big
ground terminal on the side; this unit was rated as able to damp a direct
lightning strike down to 90 volts or less.
With one of these out front, a MOV-type surge suppressor can probably
do the rest of the job, and the wire should melt down before your equipment
From: John De Armond
Subject: Re: Lightning Strikes
Date: Thu, 16 Dec 1999 00:03:24 EST
"Arnold B. Christensen" wrote:
> I have a friend that has had much of the electrical system in his MH
> fried. Dealership says it came from an outside source. He was plugged
> into the park electricity and a line of severe thunderstorms did come
> thru the area prior to his noticing smoke coming out of several
> recepticles. His MH has/had a 30 amp service.
> Anyone else had this experience?
> Is there such a thing as a surge protector for a motorhome?
Camping World and others sell a rather expensive RV-oriented surge
suppresser. One is designed to be wired in and the other is in the
middle of a "dogbone". I have no idea how they work. Intertec
makes an OK unit, available from Home Depot and Lowe's, etc, that
will work about as well for much less money (about $35.)
Nothing is going to work very well in an RV because there is no good
low impedance ground available. All surge suppressers requires a
ground that remains at ground potential during the transient in
order to dispose of the excess energy.
I have a lot of experience making commercial radio equipment live on
mountaintops where lightning strikes can be a daily occurrence. In
more than once instance, I've had the antenna literally blown off
the tower with no damage to the electronics on the ground below.
Here are some things we do.
Lightning is the big old gorilla stomping through the china shop so
stopping it is out of the question. The approach that works is to
divert the energy wherever possible while providing more conductive
paths to ground and by dividing the energy into parts which are more
easily disposed of.
First off, the gold standard in lightning protection is Lightning
Protection Corp. in Santa Barbara, CA. They have a web site. In
addition to THE BEST surge suppressers, they supply engineered
systems-oriented solutions to lightning protection. I use their
model LPC-10262 120/240 volt 3 wire secondary arrester. This unit
costs about $150 (or did the last time I bought a batch.)
Notice I said "secondary arrester". This is part of a partitioning
scheme. The first partition is the lightning arresting apparatus on
the utility pole. In areas with high lightning frequency, it is not
uncommon to find the pole arrester to not be working. Typically
they fail by exploding. So, though you can't do much about an RV
park's utility system, you CAN look up at the pole for the
arrester. It's a long, typically slightly mushroom-shaped object
hooked in parallel with the transformer. Maybe it's just my utility
background but I find my eyes seeking out the utility connection
right up front when I enter a campground.
One of the simplest and yet most effective means of diverting
lightning energy is to put a loop or two in all leads entering or
leaving the equipment building. These loops form chokes that
present a higher impedance to the lightning pulse than simply arcing
to surrounding grounds. This is good. It's not uncommon to see
burn marks all over the ground near where we bring the coax cable
off the tower with a couple of loops. In RV terms, looping the
power cable a couple of loops near the outlet box will do wonders.
Back in the late 70s, Motorola sent out a service bulletin to their
2-way radio shops outlining a new and completely counter-intuitive
technique for lightening protection. Their recommendation was to
UNGROUND all the radio equipment! That is remove all grounds and
insulate the equipment from ground. Code-required safety grounds
are brought to the equipment through air chokes. The theory is, it
does not matter how high the lightning voltage is, as long as all
parts of the equipment is at the SAME potential, there can be no
damage. When we adopted this technique, lightning-related damage
effectively stopped. We put our equipment cabinets up on wooden
pallets. All wiring was insulated from ground and the building.
All grounds were brought back to one insulated point and this point
went to safety ground though a low inductance choke. The choke
would pass 60hz fault current with little impedance while blocking
the fast rise time lightning surge. Key to making this work is the
partitioning of the surge energy so that the voltage on the
equipment can't rise high enough to flash over. In RV terms, this
is fairly easy to do if one does not hook to water or sewer. A
coiled up shore power cord will serve as the choke to ground.
Tires, though somewhat conductive, insulate enough for this
purpose. Leveling jacks need to be on wood.
All that said, the easiest way to protect an RV is to simply unplug
it when a storm approaches! I have installed an LPC arrester on the
breaker panel of my RV and I coil up the cord near the power
pedestal. This is "just in case" protection. But I unplug if a
storm is near! That's a no-brainer. I can do without power for a
little while or if it is really hot, I'll fire off the genset.
In the instance you cite, I suspect that something else was at work
by there being smoke coming from several locations. Lightning by
itself typically does its damage in one area. Diffuse damage like
that is likely caused by high voltage line transients caused by the
lightning. In one situation, the lightning causes a
primary-secondary fault in the pole transformer. Some fraction of
the primary voltage (typically 7200 or 14.4kv in this area) appears
on the house power before the upstream breakers trip. In another
situation, a dual voltage transformer has one winding shorted to
ground by the lightning. This causes twice the normal voltage to
appear on the house side. This is indicated when multiple devices
are damaged from overheating, as opposed to arc damage. Many
utilities use these transformers because they have distribution
feeders at both 7200 and 14.4KV and the dual voltage transformers
allow the same transformer to be used on both lines by simply
flipping an internal switch.
A good heavy duty surge arrester will catch most of these
transients. The LPC one certainly will. Lightweight arresters will
overheat and fail before the utility's protective relaying can
de-energize the feeder.
From: John De Armond
Subject: Re: Lightning Strikes
Date: Fri, 17 Dec 1999 02:23:03 EST
G Sanders wrote:
> Neon John wrote:
> > I have a lot of experience making commercial radio equipment live on
> > mountaintops where lightning strikes can be a daily occurrence. In
> > more than once instance, I've had the antenna literally blown off
> > the tower with no damage to the electronics on the ground below.
> GOOD POST, John!
> Lot's of good information there, that I can put to immediate
> I've had a GMRS repeater on a "skyscraper" (a skyscraper in central
> Florida is only 12 stories!), and I've been expecting it to get
> hit for over 2 years now. I'm living on borrowed time with it.
> Several of the other ham and commercial systems have been
> clobbered already up there.
> We used to call the damaged antennas the Roman Candle effect.
yeah, don't you just love it when you climb up to find that all
that's left of your Super Stationmaster is the base and a couple of
feet of fiberglass that looks like the cartoon exploding cigar?
There is another approach that you might use in your situation. If
your "skyscraper" :-) has a steel core, it is probably a good enough
conductor that if your equipment is properly bonded to the steel,
lightning probably won't bother it. We used to have an amateur
radio voice repeater and a KA9Q high speed packet router up at the
1000 foot level of Channel 11's tower in Atlanta. (that's all gone
now, compliments of HDTV :-( Our repeater was in a large NEMA-1 box
and the antenna was mounted on some brackets attached to the box.
Conduit carried the coax to the antenna and was bonded to the
antenna base. That tower was hit at least once a week - rain or
shine. The tower is heavy enough and the square mile of buried
copper screen is conductive enough that the lightning is harmlessly
conducted away to ground. We've even been on the tower when it has
been hit - no idea of the event until someone called up from the
ground crew to tell us.
Two key features. One, the NEMA-1 box provided total Faraday
shielding of everything inside the box. Two, the conduit with the
coax inside acted as a waveguide-beyond-cutoff choke that kept
lightning transients from riding the feedline into the box.
This waveguide-beyond-cutoff choke is another trick I forgot to
mention yesterday. If a conductor is placed inside a piece of
conduit, the assembly forms a waveguide whose cutoff frequency is up
in the microwave region. Anything below the cutoff is greatly
attenuated. Brick wall effect, actually. Routing the feedline,
power leads and such through a few feet of conduit with a spark gap
from the feedline to ground upstream of the conduit essentially
stops lightning energy. The conduit should be grounded, but only to
keep capacitive coupling from putting enough potential on it to arc
back onto the feedline on the other end. On our mountain top
repeater, we just attach the conduit to the outside of the repeater
shack using conduit clips and then bond them all together and to a
ground stake (NOT to the tower!!!) The ground doesn't need to be
heavy enough to handle a big hit.
I certainly don't envy you guys trying to keep electronics alive in
Florida. It's bad enough around here; can't imagine having to
survive several times that many strikes.
PS: Interesting story about the Ch 11 tower, something that's
become a legend in the region. When we decided to put a data router
on the tower, we faced the problem of how to rack out the equipment
in a conditioned environment. The router was a PC running KA9Q's
TCP/IP code plus some 56k RF modems (not bad in the late 80s). I
got the brilliant idea that a refrigerator would make an excellent
environmental chamber if it was equipped with a thermostat that
would hold it at about room temperature. Apparatus heat would warm
it in the winter and the refrigeration system would cool it in the
summer. Sooo, I found an old refrigerator, we racked out the
equipment in it and got it ready to go on the tower. This tower is
one of the few in the US that actually had (pre-HDTV) an elevator
running up the inside. The elevator was too small to take the
'fridge so we lashed it under the cab with ropes. When we got it a
thousand feet in the air, we had to slide planks across the shaft,
slide the 'fridge across them and onto the equipment platform. I'd
gotten the idea to screw a couple of paint cans and a length of
black PVC pipe to the 'fridge so that from a distance it looked like
a giant walkie-talkie :-) This thing worked like a champ until the
station manager got wind of it. The member of our group who worked
for Ch11 had kinda forgotten to get permission for us to put the
'fridge up there. Given till dark to get it down, he went up with a
demolition saw, cut the 'fridge up into pieces and heaved them over
the side. You outta see the crater a compressor leaves after a
free-fall of 1000 feet!
There are some photos of this escapade on my little web site. Click
on the url below and then go to /files/misc and look at tower1
through tower3.jpg. The top part of tower2.jpg shows how we had to
operate the elevator. The radio link (old tube-type CB) remote
control from the elevator cab had long since quit working so we
operated the elevator by having the passenger radio down to the
ground with motion requests. Motion required standing on the brake
arm to release it while pushing the motor contactor with a wooden
stick! That's what my friend Randy is doing there.
Funny thing about that tower. It's the only one made by that
manufacturer that has not fallen...