....

Jim, a lot of interesting stuff with which I generally agree.

The approach that my reference took to rating the conductor for a
lightning discharge includes a safety factor (as you might expect), and
so will rate the conductor at lower I^2*t than finding the conditions to
melt the wire. In real life, you would want the conductor to withstand a
second strike or fault soon after, and you would want to allow some
tolerance for other variables, hence the safety factor. The approach is
to find the I^2*t that raises the conductor one third of the way from
ambient (323K) to melting point. The calculator you used might assume
resistivity is at 0°C , ambient is 0°C, and the material is raised to
melting point with no heat loss, and that would give a fusing current
close to double of the approach that I used.

BTW, we have half inch copper water pipe over here (we still do but it
has a nominal metric size) and it is half in od... whereas half inch
galvanised steel pipe is half inch nominal bore... actually about 5/8"
id. Don't you like consistency in the same field!

Some years ago I did extensive modelling of a double exponential
excitation of structures and facilities (not lightning, faster than
lightning) and it was interesting how much the circuit configuration
affected the transformation of the excitation waveform to structure
current, including ringing. The same software could run a lightning
scenario, but that wasn't the main goal of the analysis so my experience
with the lightning scenario is more limited. So, as I said, the nature of
the current waveform is the big uncertainty and so measures are usually
quite conservative to cover that uncertainty.


Owen

"Rick (W-A-one-R-K-T)" wrote in
news
On Mon, 16 Jul 2007 21:26:16 +0000, Owen Duffy wrote:

It is my view that there is a significant risk that an inadequate
lightning protection scheme may be much worse than doing nothing.

Owen, certainly optimal is better than sub-optimal, but I don't
understand why sub-optimal can be worse than nothing at all. So far
you have been exceedingly helpful and I have learned a lot. Can you
explain why something isn't necessarily better, and in fact can be
much worse, than nothing?



Rick,

I guess to some extent it goes to the meaning of do nothing.

If you did not install a lightning protection system, but only connected
antennas at a time of low risk, then you might be much better off than
trusing an inadequate protection scheme.

It does reach a point where the disconnect strategy is not convenient /
practical / effective, so you are faced with performing a risk assessment
and designing a solution to mitigate the high risk factor risks. (Risk
factor considers the likelihood of an outcome and the severity of an
outcome.)

Owen

Jim Lux wrote in
:

The purpose of the National Electrical Code (National, here, referring
chauvinistically to the U.S.) required AWG 6 (diam 0.15 inches, 3.8
mm) bonding wire for grounds is NOT to carry the lightning current
(which it wouldn't, in most cases) but to carry fault currents from
things like shorts from line to grounding conductor, which are usually
in the hundreds of amps range. Say an energized power line falls down
and hits the antenna. You want the antenna's grounding conductor to
carry the likely fault current and not go open, and carry enough
current to trip any overcurrent protective devices.

I meant to comment:

I think that it is common in electricity distribution level networks,
that they are designed to hold fault current to about 20 times the
maximum working current.

In this part of the world, a single phase 240 home probably has a 80A
rated service, and fault current would usually be not worse than than
about 1600A, so the specified 6mm^2 earthing conductor and 4mm^2 bonding
conductor should withstand that current for 100ms until the protective
device operates. (4mm^2 withstands 3200A for 0.1s with a safety factor of
3.)

You probably know the numbers for your own distribution network practice,
they may be of interest to readers.

So you raise a good point, that if your tower falls onto power lines, it
would be good if your earth system could withstand the likely fault
current to take out the protection on the power lines and leave the tower
un-energised. In this part of the world with LV distribution, the
protection is probably a 500A HRC fuse with a fault current level of 10
+kA. It may be much lower for you if power lines are HV where the fault
current level should be a good deal lower.

Owen

Owen Duffy wrote in
:
protective device operates. (4mm^2 withstands 3200A for 0.1s with a
safety factor of 3.)

That should read:


protective device operates. (4mm^2 withstands 2400A for 0.1s with a
safety factor of 3.)

On Tue, 17 Jul 2007 03:29:55 +0000, Owen Duffy wrote:

If you did not install a lightning protection system, but only connected
antennas at a time of low risk, then you might be much better off than
trusting an inadequate protection scheme.

Ah, I see your point.

My main objective in all this is keeping the house from burning down. I
have a very low level of confidence that ANYTHING I do will prevent the
radio from receiving damage if the tower gets a direct lightning hit
while the antenna is connected to the radio. So, I'll continue to
disconnect antennas, ground feedlines, etc. when a storm is near or we're
going to be away for a while.

Maybe I'll get lucky and end up with a ground system that will protect
everything so I can continue to merrily yak or tap away during the worst
thunderstorm, but I'll be satisfied if I can just be confident that any
hit on the tower will go to ground and not to the house. If I have to
replace lengths of coax after a hit, I can live with that.

We aren't exactly in a high-occurrence area, here. In 22 years of living
here we have only had one lightning hit on our property, which hit (and
utterly destroyed) a tree in our backyard, damaged my Internet router,
tripped the main house circuit breaker, and blew out the timer and display
on the microwave oven. The tower was 10 feet shorter then and wasn't hit.
Now the tower is 10 feet taller AND will have a 25-foot-high VHF antenna
and mast on it, so it's time to do something to improve protection.

I use 4 foot to 8 foot long, half inch dia., copper pipe put into the
ground with the use of a water hose clamped to the top of the pipe and
a pump - this hydraulically drills the pipe into the soil... They are
laid out in a grid around the towers and along the back side of the
shack with 16 feet separation... They are all interconnected with bare
copper #2 gauge, stranded ground wire (mostly) which is slit into the
ground an inch or so to keep it away from the mower blades... All of
this creates a large area of metal in contact with the soil..

All of this agonizing over contact of hydraulically driven pipe versus
pounded stakes, etc., etc., amounts to counting the number of angels
dancing on the head of a pin... Just get in as many ground pipes/
stakes as you can on 16 foot centers, interconnect them in a grid with
bare wire slit into the soil, run heavy gauge wire from the stake just
outside of wall of the house/shack to the buss bar behind the radio,
and be happy...

I also competely isolate the radios and amps when they are not in
use...

denny / k8do

Owen Duffy wrote:
...

Jim, a lot of interesting stuff with which I generally agree.

The approach that my reference took to rating the conductor for a
lightning discharge includes a safety factor (as you might expect), and
so will rate the conductor at lower I^2*t than finding the conditions to
melt the wire. In real life, you would want the conductor to withstand a
second strike or fault soon after, and you would want to allow some
tolerance for other variables, hence the safety factor. The approach is
to find the I^2*t that raises the conductor one third of the way from
ambient (323K) to melting point. The calculator you used might assume
resistivity is at 0°C , ambient is 0°C, and the material is raised to
melting point with no heat loss, and that would give a fusing current
close to double of the approach that I used.

BTW, we have half inch copper water pipe over here (we still do but it
has a nominal metric size) and it is half in od... whereas half inch
galvanised steel pipe is half inch nominal bore... actually about 5/8"
id. Don't you like consistency in the same field!
But they're not the same field.. the stuff made of copper is actually
"tubing" and the stuff made of steel is "pipe", and historically,
they've been measured differently.

Tubing is usually soldered/sweated/brazed into fittings with a
receptacle, so the OD is important, because even with different wall
thicknesses, the fittings are all the same.

Pipe is based on something else (King John's toe diameter or something)

Some years ago I did extensive modelling of a double exponential
excitation of structures and facilities (not lightning, faster than
lightning) and it was interesting how much the circuit configuration
affected the transformation of the excitation waveform to structure
current, including ringing. The same software could run a lightning
scenario, but that wasn't the main goal of the analysis so my experience
with the lightning scenario is more limited. So, as I said, the nature of
the current waveform is the big uncertainty and so measures are usually
quite conservative to cover that uncertainty.

There's some fascinating papers out there that use NEC to model response
to a nearby lightning stroke (a much more common occurance than a direct
hit). It's actually quite involved, since they model the traveling
impulse of the stroke.


Owen

On Tue, 17 Jul 2007 04:48:48 -0700, Denny wrote:

I use 4 foot to 8 foot long, half inch dia., copper pipe put into the
ground with the use of a water hose clamped to the top of the pipe and
a pump - this hydraulically drills the pipe into the soil... They are
laid out in a grid around the towers and along the back side of the
shack with 16 feet separation... They are all interconnected with bare
copper #2 gauge, stranded ground wire (mostly) which is slit into the
ground an inch or so to keep it away from the mower blades... All of
this creates a large area of metal in contact with the soil..

All of this agonizing over contact of hydraulically driven pipe versus
pounded stakes, etc., etc., amounts to counting the number of angels
dancing on the head of a pin... Just get in as many ground pipes/
stakes as you can on 16 foot centers, interconnect them in a grid with
bare wire slit into the soil, run heavy gauge wire from the stake just
outside of wall of the house/shack to the buss bar behind the radio,
and be happy...

Like Denny I have a lot of ground rods (32 or 33) in a network
CadWelded (TM) to over 600 feet of bare #2 copper wire that ties into
the station forming a single point ground or as near to it as I can
get. The coax shields are grounded at the top and bottom of the tower
as well as to a ground plate where they enter the house.

I also competely isolate the radios and amps when they are not in
use...

Unlike him I rarely disconnect anything for several reasons. The
first is the stuff is just too difficult to get to. The second is, I'm
lazy and although I could disconcert the station when not in use it
takes a good 10 to 15 minutes to get things hooked back up. Typically
I want to turn it on, listen, and then operate if the bands are open
or shut down if not. In addition, nature threw me a curve a bit over 3
months ago making it very difficult for me to get into places like
behind the rigs to disconnect things although I hope to be able to get
back in there and up the tower again before long.
One other thing and it may be, and probably is due to luck, but with
an average of three verified direct hits on the tower per year I've
had no damage to any rig since finishing up the ground system.

Roger (K8RI)

denny / k8do

Denny wrote:

All of this agonizing over contact of hydraulically driven pipe versus
pounded stakes, etc., etc., amounts to counting the number of angels
dancing on the head of a pin

Amen Brother!



-73 de Mike KB3EIA -

We had a near by lightning strike last night... All my radios and
antennas survived... My son's Dish Network receiver did not... He's
quite bummed out...

denny