Tom Horne wrote:
On Jul 13, 12:52 am, Richard Clark wrote:
On Mon, 12 Jul 2010 21:21:24 -0700 (PDT), Tom Horne

wrote:
Can anyone make a recommendation, based on actual training and
experience, as to what width and thickness of copper strap would be
needed as the down conductor from the antenna mounts at the peak of my
house roof, some twenty five feet above local terrain.
Hi Tom,

Consult the NEC code for your area. I have never seen them specify
strap in any situation, and don't try your own interpretations of what
"continuous"or "direct" means.

73's
Richard Clark, KB7QHC

Richard
I will certainly make the installation code compliant. What I was
looking for help on was how to make it effective in avoiding damage
from lightning. I have read all of the applicable material from the
Polyphaser, NIST, and several other sites but what I was hoping to
elicit was specific guidance on what size strap to use for the down
conductors and what size to use for the ground ring so that they might
actually work rather than just comply with the code. I can certainly
add any regular wire conductor that would keep the electrical
inspectors happy since I already have 2/0 bare copper for the ground
ring; were only #2 is required and, the ridiculously undersized,
number ten that the code requires for down conductors in hand.


Hmmm you say "ridicuously undersized"..

Why?

have you studied the fusing current of AWG 10?
Have you compared the inductance of various sizes?

Do you know *why* the code only requires AWG 10?

The code requirements are based on actual science, engineering, and test
data, so if the code requires AWG 10, it's probably for a fairly good
reason.

Note that they allow bronze and copper clad steel as well as solid
copper, so clearly, DC resistance isn't what they're worried about.

(hint.. think of mechanical reasons)

Note, especially, that the NEC (NFPA 70) grounding/bonding requirements
are NOT for lightning protection. (that's in NFPA 780, not NFPA 70)

Tom Horne wrote:

Owen
Believe it or not the NEC only calls for number ten American Wire Gage
(AWG) or 5.261 (mm)2 for protective grounding conductors. Bonding
conductors between electrodes are only required to be number six AWG
or 13.30 (mm)2. So leaving aside the bad joke that is the NEC
requirements I'm trying to get some idea of what best practice might
be.


Tom is a bit confused here about the purpose of NEC vs NFPA 780..

The bonding requirements in the NEC are designed to keep the building
from burning down in the event of an accidental fault to an energized
conductor. The basic requirement is that it carry enough fault current
for long enough to trip the overcurrent protection device on the
energized conductor.

It's not for lightning protection, per se. (although NEC bonding will,
incidentally, provide some degree of protection against induced transients)

I'd also note that AWG 10 wire is more than sufficient to carry a 50kA
pulse for the 20 microseconds or so that a lightning stroke lasts
without melting.
Using the Onderdonk equation, you can calculate that a AWG16 copper wire
will carry about 90kA for a 20 microsecond pulse. AWG10 should be able
to carry 4 times that much. AWG6, 10 times, because it scales with cross
sectional area.

Having only really paid attention to this recently, I noticed that in
Rome (a place with a fair number of thunderstorms), they use fairly
small down conductors (AWG 10 or 6, just by eye), and similar for 7
story tall wooden pagodas in Nara, Japan (another place with lots of
thunderstorms).

I'm not quite sure where the fashion for 2/0 grounding conductors comes
from (maybe Phelps-Dodge has a representative on the NFPA 780 review
committee?grin)

Tom Horne wrote:
On Jul 13, 1:52 pm, Owen Duffy wrote:
Owen Duffy wrote :

...

But, firstly, you should determine if there are regulatory
requirements, such as NEC etc.
Is "NFPA 780: Standard for the Installation of Lightning Protection
Systems" a relevant standard in your jurisdiction?

Owen

Owen
It is relevant as a consensus standard but it is not adopted as local
or State law. Do you have a link to a copy that can be read online?


NFPA 780, like NFPA 70, is a copyrighted document *sold* by NFPA.
However, there *are* online copies of various provenance and age around.
http://www.atmo.arizona.edu/students...A_780_2004.pdf

Unfortunately, the bare code doesn't tell you much about the "why" for
various code provisions, so if you're thinking of going "off code" for
one reason or another, you don't have a lot of information to tell you
whether it's a good idea.

There's also some interesting seeming inconsistencies.. NFPA 780
requires a minimum length of a ground rod of 8 feet (4.13.2.1) but also
requires that they extend vertically not less than 10 feet into the
earth (4.13.2.3(A))) The figure makes it clear.. the top of an 8 foot
rod is 2 feet below the surface of the soil.

NFPA 780 says 29 square millimeters for main conductors (6 mm in
diameter or a strip that is 1.3mm thick x 22.3 mm wide).. That's AWG 6
roughly.


There's also a great site by Carl Malamud: publicresource.org that has
all the California Building Codes (including an older rev of the NEC)
although it doesn't have NFPA 780 on it, as far as I know.

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).

On 7/19/2010 10:54 AM, Jim Lux wrote:
Tom Horne wrote:
On Jul 13, 12:52 am, Richard Clark wrote:
On Mon, 12 Jul 2010 21:21:24 -0700 (PDT), Tom Horne

wrote:
Can anyone make a recommendation, based on actual training and
experience, as to what width and thickness of copper strap would be
needed as the down conductor from the antenna mounts at the peak of my
house roof, some twenty five feet above local terrain.
Hi Tom,

Consult the NEC code for your area. I have never seen them specify
strap in any situation, and don't try your own interpretations of what
"continuous"or "direct" means.

73's
Richard Clark, KB7QHC

Richard
I will certainly make the installation code compliant. What I was
looking for help on was how to make it effective in avoiding damage
from lightning. I have read all of the applicable material from the
Polyphaser, NIST, and several other sites but what I was hoping to
elicit was specific guidance on what size strap to use for the down
conductors and what size to use for the ground ring so that they might
actually work rather than just comply with the code. I can certainly
add any regular wire conductor that would keep the electrical
inspectors happy since I already have 2/0 bare copper for the ground
ring; were only #2 is required and, the ridiculously undersized,
number ten that the code requires for down conductors in hand.


Hmmm you say "ridicuously undersized"..

Why?

have you studied the fusing current of AWG 10?
Have you compared the inductance of various sizes?

Do you know *why* the code only requires AWG 10?

The code requirements are based on actual science, engineering, and test
data, so if the code requires AWG 10, it's probably for a fairly good
reason.

Note that they allow bronze and copper clad steel as well as solid
copper, so clearly, DC resistance isn't what they're worried about.

(hint.. think of mechanical reasons)

Note, especially, that the NEC (NFPA 70) grounding/bonding requirements
are NOT for lightning protection. (that's in NFPA 780, not NFPA 70)


IIRC the purpose is to primarily drain off the static charges so the
gnd-cloud potential difference is minimized. A direct strike will
usually just melt whole house wiring, etc. etc.

Marv

I have concluded that if more than 8 foot ground rods were required
for effect or code, the supply houses would be selling them. I cannot
argue with redundancy though. THERE IS A POTENTIAL PROBLEM WITH GROUND
RODS THAT ARE NOT BONDED WITH AT LEAST #6 WIRE! Common sense dictates
that protective system be continuously bled to a common level rather
than allowed to build up a differential charge.


This is actually a big problem.. counterfeit rods with UL markings on
them, and rods that are too small in diameter or too short to meet NEC
requirements.

Electrical Supply places (not the home improvement stores) have had 10
foot rods for a long time.

The NEC has required 8 feet *in the ground* for years and years (at
least 20, I think), so the only way to do that with an 8 foot rod is to
do your exothermic bond to the rod and completely bury it.

The home improvement stores don't like to stock longer rods because most
of the buyers are not "code-aware" and buy on price (hey, an 8 foot rod
is cheaper than a 10 foot rod, I'll use that). Besides, for new
construction in a lot of places, a rod isn't an acceptable primary
grounding means anyway.

And we won't even get into the grounding/bonding practices of satellite
dish and Cable TV installers. There's a great website out there with
pictures of truly lame installations.

IIRC the purpose is to primarily drain off the static charges so the
gnd-cloud potential difference is minimized. A direct strike will
usually just melt whole house wiring, etc. etc.

Not true.

The cloud has SO MUCH charge you don't stand a chance of bleeding it off.

Direct strikes are typically around 20 kA, and can be as high as 100kA.
Both can be adequately carried by the usual AWG6 wire, because the
current pulse only lasts a few microseconds.

On Jul 13, 12:21*am, Tom Horne wrote:
Can anyone make a recommendation, based on actual training and
experience, as to what width and thickness of copper strap would be
needed as the down conductor from the antenna mounts at the peak of my
house roof, some twenty five feet above local terrain. *I have a a
mount for an antenna at one gable end and a mount for a weather
station sensor array at the other. *What thickness and width should I
use in the earth between the two Grounding Electrode Systems. *I will
drive five eighths inch copper rods, each eight feet long as far out
from the foundation as I can get them or eight feet were possible. *On
one end that will be only six feet due to the proximity of the
property line. *At all of the other rod locations I will be able to
keep them at least eight feet from any underground obstructions. *To
compensate for the proximity *to the foundation wall to the first rod
I will use rod couplers and drive it to hard rock or sixteen feet
whichever comes first. *I'm guessing that in keeping the remaining
rods at least eight feet out from the foundation and sixteen feet
apart that I will only have four rods total in a ring around the back
side of the house. *What is the best way to attach the copper strap to
the support masts and eve brackets themselves? *Do you know of
anything that will make a good connection to the one inch galvanized
iron pipes that I'm using for support masts?

Can you recommend a technique for bonding the interior grounding buss
at the operating position to the exterior vertical copper strap. *I
have no idea how that is usually done.
--
Tom Horne, W3TDH

If you want something beyond code typically we use 00 braided copper
at work for lightning protection on towers. NEC and Polyphaser
recommendations usually work pretty good

Jimmie

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.

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.