On Jul 13, 2:18*am, Owen Duffy wrote:
Tom Horne wrote in news:e802f6fa-b0e1-471b-bf31-
:

Can anyone make a recommendation, based on actual training and
experience, as to what width and thickness of copper strap would be ...

In this part of the world, we have an Australian / New Zealand Standard
(our version if you like of ANSI, BS etc) which explains the rationale
behind lightning protection, a method of estimating the downcurrent for
protection design purposes and a process for designing down conductors.

Broadly, the scheme is that downconductors are designed to withstand a
few donwstrokes in quick succession without melting the down conductor.
If you work from a peak current of 20kA, it would lead to a down
conductor in copper of at least 25mm^2 which is about #2 to you folk.

I regularly see hams recommend much thinner down conductors, and can only
assume that there is not regulatory guidance or requirement, and I wonder
at the effectiveness of using #6 as often recommended, especially
aluminium as is often the case. Note that reducing conductor size is a
double whammy, you increase the resistance (so the power), and decrease
the mass that has to be heated to melting point, and so the energy
required.

But, firstly, you should determine if there are regulatory requirements,
such as NEC etc.

The question of equipotential bonding conductors ought be dealt with in
the same way, though that is not to imply that they will be the same
size.

Owen

Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke I was hoping to elicit best
practice advise on what size the conductors should actually be as well
as advise on how to accomplish the bonding of the interior single
point grounding buss bar to the exterior grounding conductors and
Grounding Electrode System.
--
Tom Horne, W3TDH

On Jul 15, 9:18*pm, Tom Horne wrote:


Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. *Since
those sizes are at best a bad joke I was hoping to elicit best
practice advise on what size the conductors should actually be as well
as advise on how to accomplish the bonding of the interior single
point grounding buss bar to the exterior grounding conductors and
Grounding Electrode System.
--
Tom Horne, W3TDH

It's not that bad a joke.. If the ground connection is good, #10
is plenty thick enough. In fact, it would barely get warm if it
took a strike. Of course, if the connection to ground is bad,
it will be toast. But so would a lot of heavier gauges also..
The connection to ground is the critical factor in such a case.
But I would still follow what the local code says.
The main reason I'm making this post is only to clarify that
under proper conditions, #10 is plenty thick enough to safely
route the strike to ground with no damage to the wire.

Tom Horne wrote:


Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke

Perhaps you could explain why you think it's a bad joke? Do you think a
13 square mm conductor couldn't carry the strike current? (it can)

Or, perhaps, you're thinking that there are some other design criteria
that might push one towards a larger conductor (mechanical strength in
the face of icing and storms might be one).

Jim Higgins wrote:
On Mon, 19 Jul 2010 09:23:53 -0700, Jim Lux
wrote:

Tom Horne wrote:

Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke
Perhaps you could explain why you think it's a bad joke? Do you think a
13 square mm conductor couldn't carry the strike current? (it can)

Or, perhaps, you're thinking that there are some other design criteria
that might push one towards a larger conductor (mechanical strength in
the face of icing and storms might be one).

Maybe E=IR has something to do with wanting a larger conductor. The
voltage between the strike point and true ground is going to be the 20
- 100 kA of the strike times the resistance of the down conductor from
the strike point to true ground. With a smaller conductor,
fewer/shorter ground rods, or other conditions that raise the
resistance of the path to ground that voltage will be higher and if
high enough the strike will seek additional paths to ground by arcing
to nearby objects closer to ground potential.


Resistance isn't actually a big deal here. It's all about inductance on
that microsecond rise time pulse. And there's not much difference in
inductance between a AWG 6 and 2/0 (it's very weakly dependent on
diameter and strongly dependent on length.. 1 microhenry/meter is a good
estimate, pretty much independent of diameter)

The other problem is that for fast transients, skin effect means that
the AC resistance goes more as the diameter than as the cross sectional
area (hollow tubes work just as well as solid conductors).

So, the net effect is that you need to design for several things:
1) the wire not melting..
2) The wire not breaking from mechanical impact (ladders hitting it,
lawnmowers, etc.
3) The wire not breaking under electromagnetic forces (this is why you
don't want loops and why NFPA 780 says 8" bend radius.. while a 1
microsecond pulse at 10kA won't melt a AWG 10 wire, if it's in a loop,
it will destroy it from EM forces)

You'll see heavier conductors where they have to be able to move (say on
a gate or actuated device), not only for mechanical life, but also
because the flexible wire is more subject to destruction by EM forces.

Side flash is a consideration, but usually accommodated by making sure
your downleads are far from potential victim circuits.

Jim Lux wrote in
:
....
The other problem is that for fast transients, skin effect means that
the AC resistance goes more as the diameter than as the cross
sectional area (hollow tubes work just as well as solid conductors).

The problem is that while we might characterise the raw excitation caused
by lightning, and use assumptions about the shape, rise and fall times
and peak field strength, the response of circuits (such as those that
include the down conductor) is quite different, and it is unsafe to
assume in the general case that skin effect is fully effective for all or
even most of the energy spectrum.

Perhaps that is why some of these standards tend to treat the conductor
as having a resistance equal to that implied by just the conductivity (or
resistivity) and CSA. It might be conservative, but then standards tend
to be so.

Having seen the results of fairly detailed EM modelling of EMP and
lightning excitation of major infrastructure, and the effects of some
small changes to the model, I wonder a bit about the effectiveness of
some measures... but over engineering probably saves the day in a lot of
cases.

The real danger with lightning protection is that a half baked approach
my give the implementor some comfort, but actually increase the risk of
adverse outcome.

The most thorough and consistent practice I have seen is that employed
here in mobile phone base stations. Sure, they are occasionally damaged
by lightning, but the vast majority of lightning incidents do not cause
permanent damage.

Owen

On 7/19/2010 6:54 PM, Owen Duffy wrote:
The real danger with lightning protection is that a half baked approach
my give the implementor some comfort, but actually increase the risk of
adverse outcome.

The most thorough and consistent practice I have seen is that employed
here in mobile phone base stations. Sure, they are occasionally damaged
by lightning, but the vast majority of lightning incidents do not cause
permanent damage.

Owen

The biggest problem with lightning protection in my area is that the
local power company leaves the ground lines coiled up at the bottom of
the poles. On about half the poles I've checked.

When they put the new transformer in across the street from my house the
crew said they would be back to put in the ground rod. Nope. So I
called a friend that si a troubleshooter for the power company about 2
months later and told him about it. "Yup, I'll get somebody right
over." No joy 4 years later.

I lost $1500 dollars worth of gear last year because the only decent
ground was connected to my radio. And every bit of current that came
into the house on the power line exited that direction.

The power companies are likely the worst culprits from my perspective.
I have installed more grounds at my house than the rest of the street
has. Probably not all to spec, but safer than what wasn't here before.

I also discovered the dictionary definition of replacement cost is not
the same one the insurance industry uses. Big surprise.

tom
K0TAR

On 7/19/2010 8:06 PM, tom wrote:
On 7/19/2010 6:54 PM, Owen Duffy wrote:
The real danger with lightning protection is that a half baked approach
my give the implementor some comfort, but actually increase the risk of
adverse outcome.

The most thorough and consistent practice I have seen is that employed
here in mobile phone base stations. Sure, they are occasionally damaged
by lightning, but the vast majority of lightning incidents do not cause
permanent damage.

Owen

The biggest problem with lightning protection in my area is that the
local power company leaves the ground lines coiled up at the bottom of
the poles. On about half the poles I've checked.

When they put the new transformer in across the street from my house the
crew said they would be back to put in the ground rod. Nope. So I called
a friend that si a troubleshooter for the power company about 2 months
later and told him about it. "Yup, I'll get somebody right over." No joy
4 years later.

I lost $1500 dollars worth of gear last year because the only decent
ground was connected to my radio. And every bit of current that came
into the house on the power line exited that direction.

The power companies are likely the worst culprits from my perspective. I
have installed more grounds at my house than the rest of the street has.
Probably not all to spec, but safer than what wasn't here before.

I also discovered the dictionary definition of replacement cost is not
the same one the insurance industry uses. Big surprise.

tom
K0TAR

Methinks you need a professional engineer and good lawyer?

Marv
W5MTV

On 7/19/2010 9:31 PM, MTV wrote:
On 7/19/2010 8:06 PM, tom wrote:
On 7/19/2010 6:54 PM, Owen Duffy wrote:
The real danger with lightning protection is that a half baked approach
my give the implementor some comfort, but actually increase the risk of
adverse outcome.

The most thorough and consistent practice I have seen is that employed
here in mobile phone base stations. Sure, they are occasionally damaged
by lightning, but the vast majority of lightning incidents do not cause
permanent damage.

Owen

The biggest problem with lightning protection in my area is that the
local power company leaves the ground lines coiled up at the bottom of
the poles. On about half the poles I've checked.

When they put the new transformer in across the street from my house the
crew said they would be back to put in the ground rod. Nope. So I called
a friend that si a troubleshooter for the power company about 2 months
later and told him about it. "Yup, I'll get somebody right over." No joy
4 years later.

I lost $1500 dollars worth of gear last year because the only decent
ground was connected to my radio. And every bit of current that came
into the house on the power line exited that direction.

The power companies are likely the worst culprits from my perspective. I
have installed more grounds at my house than the rest of the street has.
Probably not all to spec, but safer than what wasn't here before.

I also discovered the dictionary definition of replacement cost is not
the same one the insurance industry uses. Big surprise.

tom
K0TAR

Methinks you need a professional engineer and good lawyer?

Marv
W5MTV

To deal with a monopoly? Approved by federal, state and local
government? Of both corrupt colors?

You are a comedian.

tom
K0TAR

On Jul 19, 1:03*pm, Jim Higgins wrote:
On Mon, 19 Jul 2010 09:23:53 -0700, Jim Lux
wrote:

Tom Horne wrote:

Owen
The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. *Since
those sizes are at best a bad joke

Perhaps you could explain why you think it's a bad joke? *Do you think a
13 square mm conductor couldn't carry the strike current? (it can)

Or, perhaps, you're thinking that there are some other design criteria
that might push one towards a larger conductor (mechanical strength in
the face of icing and storms might be one).

Maybe E=IR has something to do with wanting a larger conductor. *The
voltage between the strike point and true ground is going to be the 20
- 100 kA of the strike times the resistance of the down conductor from
the strike point to true ground. *With a smaller conductor,
fewer/shorter ground rods, or other conditions that raise the
resistance of the path to ground that voltage will be higher and if
high enough the strike will seek additional paths to ground by arcing
to nearby objects closer to ground potential.

That's why I tie everything together. In my case, the ground rods
are minimal.. Just a few copper tubes pounded into the ground
around the base of the mast. None are too deep. But I consider
the ground adequate for the purpose, and it seems to be, being
as I've taken strikes on that mast with no damage to anything.
But I tie that ground into the electrical ground, and also the
plumbing, which I clamp to just a few feet away from the base
of the mast. If all grounds are at the same appx potential, and
the connection to ground is up to par as far as resistance, you
shouldn't see flashing over to other objects. I've never had that
problem here so far. In fact, the connection to ground seems
good enough that strikes to that mast are fairly silent and
only make an electrical arc sound which sounds like throwing
a light bulb onto the ground.
On the other hand, a strike to a poorly grounded object with
high resistance is hugely loud.. Say when it strikes the tree
in the front yard.. It's like a 12 gauge going off. And this
sound is separate from the sonic boom of the strike as
it travels through the air. The sonic boom will come from
overhead and is not local like the actual strike noise at the
object being struck.

The NEC only requires 5.261 (mm)2 for the protective down conductor
and 13.30 (mm)2 for the bonding conductor between electrodes. Since
those sizes are at best a bad joke I was hoping to elicit best
practice advise on what size the conductors should actually be as well
as advise on how to accomplish the bonding of the interior single
point grounding buss bar to the exterior grounding conductors and
Grounding Electrode System.
--

In what way is #6 a "bad joke?"

Do you expect it to vaporize and set your roof on fire?