"Rick (W-A-one-R-K-T)" wrote in
news
On Sun, 15 Jul 2007 17:23:13 +0000, Walter Maxwell wrote:

This topic has aroused my curiosity. As a grounding device, why would a
solid rod be better than a hollow pipe, except for the current carrying
capability?

Good afternoon, Walt.

The thing is, the current carrying capability for transient events like
lightning strikes should be about the same for the same diameter pipe or
rod, since most of the current is carried in skin effect anyway.

I think that is flawed thinking.

A lighning down conductor needs to carry something like 20kA for 100ms, so
it needs to be substantial enough that it doesn't melt and remains in place
to protect against the next strike.

Owen

On Jul 15, 1:47 pm, Owen Duffy wrote:


I think that is flawed thinking.

Dunno.. Not if the ground connection itself is good enough..

A lighning down conductor needs to carry something like 20kA for 100ms, so
it needs to be substantial enough that it doesn't melt and remains in place
to protect against the next strike.

A #10 wire can handle that job if the ground connection is up to par.
Course, most real down conductor will be flat copper strap.
The rod or tube in the ground is not the down conductor per say,
although I guess you could consider it the end of it..
As far as DC contact with earth, it's possible the tubing could be
slightly better, as it's got surface both on the outside, and the
inside..
But I imagine RF flowing along tubing would mostly flow on the
outside,
rather than inside. But maybe a bit of both.. Not sure..
Copper tubing itself can easily handle any lightning strike. Way
overkill actually. If you had a well grounded copper tube lightning
rod, and it was struck, all you would see on the tip would be a tiny
arc spot, which might even be unnoticable unless you were looking
for it. Even a lowly #10 wire will barely raise temp when struck as
long as the connection to ground is real good. Of course, if it isn't,
it may well vaporize into blobs of metal..
MK

Owen Duffy wrote:
"Rick (W-A-one-R-K-T)" wrote in
news

On Sun, 15 Jul 2007 17:23:13 +0000, Walter Maxwell wrote:


This topic has aroused my curiosity. As a grounding device, why would a
solid rod be better than a hollow pipe, except for the current carrying
capability?

Good afternoon, Walt.

The thing is, the current carrying capability for transient events like
lightning strikes should be about the same for the same diameter pipe or
rod, since most of the current is carried in skin effect anyway.


I think that is flawed thinking.

A lighning down conductor needs to carry something like 20kA for 100ms, so
it needs to be substantial enough that it doesn't melt and remains in place
to protect against the next strike.

Owen

The fusing/melting current for 1/2" copper pipe is probably well above
20kA, even for 100ms pulses. A more interesting potential failure mode
might be from the mechanical forces due to the magnetic field. (see,
e.g., quarter shrinking or can-crushing)

Jim Lux wrote in
:

Owen Duffy wrote:
"Rick (W-A-one-R-K-T)" wrote in
news

On Sun, 15 Jul 2007 17:23:13 +0000, Walter Maxwell wrote:


This topic has aroused my curiosity. As a grounding device, why
would a solid rod be better than a hollow pipe, except for the
current carrying capability?

Good afternoon, Walt.

The thing is, the current carrying capability for transient events
like lightning strikes should be about the same for the same diameter
pipe or rod, since most of the current is carried in skin effect
anyway.


I think that is flawed thinking.

A lighning down conductor needs to carry something like 20kA for
100ms, so it needs to be substantial enough that it doesn't melt and
remains in place to protect against the next strike.

Owen

The fusing/melting current for 1/2" copper pipe is probably well above
20kA, even for 100ms pulses. A more interesting potential failure mode
might be from the mechanical forces due to the magnetic field. (see,
e.g., quarter shrinking or can-crushing)


Hi Jim,

I note the "probably" in your comment, and the "dunno" in N5MK's
response.

The uncertainty in my statement is over the exact lightning scenario,
they vary, and the circuit response (ie current waveshape, amplitude,
duration, ringing etc) depend on the specific excitation and circuit
elements (parameters of the down conductor, nature of the earth system,
ground, environment etc).

As far as supposition as to the fusing current for conductors, that is
determinable for a given scenario. I have at hand the Protective Earthing
Code of Practice published by the Electricity Authority of NSW June 1975
and it shows that a 35mm^2 copper conductor has a fault current withstand
of 20kA for 100ms. (I have considered implementing the underlying
formulas in an online calculator.)

N5MK stated "A #10 wire can handle that job". If he is talking copper, I
understand that #10 means 2.5mm diameter, or ~5mm^2, or less than 15% of
the recommended conductor csa for the stated scenario. I am not familiar
with your water pipe sizes. If it were, say, a half inch diameter #19, it
has a CSA of around 35mm^2, so the #10 wire should melt before the pipe
electrode, thus protecting the pipe electrode from failure. Yes,
mechanical forces are also relevant to lightning conductors, but my
comment was about the fusing current.

In this part of the world there is an Australian Standard (AS1768)
relating to lightning protection, there may be a similar standard or
"code" in other jurisdictions, and they would not be a bad place to start
in understanding lightning protection and designing a protection scheme.

Another source of information is to walk around a mobile phone base
station and look at the earthing system from the outside. It is even more
enlightning (no pun) to look inside. These things withstand lightning
events quite well. Are they over engineered? Probably not, they do suffer
damage from time to time.

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

Owen

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 (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

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.

Owen

The fusing/melting current for 1/2" copper pipe is probably well above
20kA, even for 100ms pulses. A more interesting potential failure mode
might be from the mechanical forces due to the magnetic field. (see,
e.g., quarter shrinking or can-crushing)



Hi Jim,

I note the "probably" in your comment, and the "dunno" in N5MK's
response.
That's because I was lazy and didn't want to actually compute it. I've
put multi tens of kA pulses through 1/4" copper pipe, but they're not
100ms long.



The uncertainty in my statement is over the exact lightning scenario,
they vary, and the circuit response (ie current waveshape, amplitude,
duration, ringing etc) depend on the specific excitation and circuit
elements (parameters of the down conductor, nature of the earth system,
ground, environment etc).

One could certainly use the standard double exponential approximations..
either a 2/50 waveform for a strike or the longer surge impulse (I can't
remember the exact rise/fall times for the surge..)



As far as supposition as to the fusing current for conductors, that is
determinable for a given scenario. I have at hand the Protective Earthing
Code of Practice published by the Electricity Authority of NSW June 1975
and it shows that a 35mm^2 copper conductor has a fault current withstand
of 20kA for 100ms. (I have considered implementing the underlying
formulas in an online calculator.)

Preece or Onderdonk?
(http://home.earthlink.net/~jimlux/hv/fuses.htm

N5MK stated "A #10 wire can handle that job". If he is talking copper, I
understand that #10 means 2.5mm diameter, or ~5mm^2, or less than 15% of
the recommended conductor csa for the stated scenario.


Preece equation gives fusing current for AWG10 (2.5mm diameter, as you
say) as 316 amps, but that's sort of for a steady state.

Onderdonk's equation, plugging in 100 ms for the melt time, gives 4.7kA,
which I can believe. I've blown up a lot of AWG10 wire with those sorts
of currents in a quarter shrinker. Partly melting, partly mechanical
stresses in that application.

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.

Lightning protection is usually things like 2/0 (0.364 inch diameter,
9.25 mm), which has a fusing current (viz Onderdonk) of 65kA.

I am not familiar
with your water pipe sizes. If it were, say, a half inch diameter #19, it
has a CSA of around 35mm^2, so the #10 wire should melt before the pipe
electrode, thus protecting the pipe electrode from failure. Yes,
mechanical forces are also relevant to lightning conductors, but my
comment was about the fusing current.

35 mm^2 would have a fusing current of around 30-35 kA.

1/2" Copper pipe is 0.625" od and 0.545" id (very close to 1mm wall)
so, has about 47 mm^2 area.


In this part of the world there is an Australian Standard (AS1768)
relating to lightning protection, there may be a similar standard or
"code" in other jurisdictions, and they would not be a bad place to start
in understanding lightning protection and designing a protection scheme.

Another source of information is to walk around a mobile phone base
station and look at the earthing system from the outside. It is even more
enlightning (no pun) to look inside. These things withstand lightning
events quite well. Are they over engineered? Probably not, they do suffer
damage from time to time.

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

I would agree..



Owen

....

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

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