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 7/20/2010 11:24 AM, Jim Higgins wrote:
On Mon, 19 Jul 2010 21:40:04 -0500, wrote:

On 7/19/2010 9:31 PM, MTV wrote:
On 7/19/2010 8:06 PM, tom wrote:

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.

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


The value to the consumer of his state Public Utilities Commission
varies by state, but in those states where the PUC is good, the
utilities crap themselves when the PUC contacts them about something
like this. Maybe give it a try.


Minnesota is very similar to Illinois, and seems to be about as corrupt
in the way that only long entrenched Democrat machines can be. They're
just much nicer and friendlier about it. Which is a great disguise in
most cases, and helps the news crews put a positive spin on all the
nonsense that happens here.

I think I'll forego the pleasure of dealing with any state level
bureaucracies, thanks.

But thanks for the advice, I do appreciate it.

tom
K0TAR

"Jim Lux" wrote
...


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.

Each cloud has a charge and the all is flowing to the ground. But only 20%
as the direct stroke.

The rule is simple. A mast with the polished ball on the tip attract the
direct stroke (polished ball do not dissipate).
A mast with many sharp spikes dissipate the static charge and eliminate the
direct strike.

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.

It is the oscillating current which has a canal in the air. It is not
obliged to flow only in the wire.
S*

On Jul 23, 8:48*am, "Szczepan Bialek" wrote:
*"Jim Lux" ...



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.

Each cloud has a charge and the all is flowing to the ground. But only 20%
as the direct stroke.

The rule is simple. A mast with the polished ball on the tip attract the
direct stroke (polished ball do not dissipate).
A mast with many sharp spikes dissipate the static charge and eliminate the
direct strike.

wrong, sharp spikes are designed to start an upward streamer that
connects the downward leader to the lightning rod. that is why they
have a sharp point, to reach the breakdown field gradient before
anything else around them.



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.

It is the oscillating current which has a canal in the air. It is not
obliged to flow only in the wire.
S*

normally they don't oscillate, it is a mono-polarity pulse.

On Jul 23, 3:48*am, "Szczepan Bialek" wrote:


The rule is simple. A mast with the polished ball on the tip attract the
direct stroke (polished ball do not dissipate).
A mast with many sharp spikes dissipate the static charge and eliminate the
direct strike.

A mast with a ball is less likely to attract a strike. That is why
they are
used on flag poles, etc.. They don't stream near as well as a sharp
point. They can still be struck though if nothing more attractive
is around as far as streamers.
A pointed mast streams much easier and will be much more
effective as a lightning rod.
A mast with a lot of spikes is wishful thinking. You can not bleed
off the charge fast enough to eliminate strikes. It's like whizzing
in a whirlwind.
You can not eliminate a direct strike by bleeding off the charge.
You can only offer it a better and easier streaming target than
whatever you do not want struck. And the sharper and more
pointy an object, the better it streams. When is the last time
you saw a lightning rod with a polished ball on top?
They don't sell them, as they would be fairly useless.

wrote:
On Jul 23, 3:48 am, "Szczepan Bialek" wrote:

You can not eliminate a direct strike by bleeding off the charge.
You can only offer it a better and easier streaming target than
whatever you do not want struck. And the sharper and more
pointy an object, the better it streams. When is the last time
you saw a lightning rod with a polished ball on top?

Actually, air terminals with blunt ends and sphere ends have been tested
by Erico, and they work about the same as the traditional "Franklin"
style with the pointy end.

They don't sell them, as they would be fairly useless.

That's not because they work or don't work, it's because fashion
dictates pointy end. You'd have to have a substantial performance
improvement to justify something different (i.e. to be unfashionable, it
had better give you something else).


At the fields in play here, whether you have a 1/10" radius or a 1/2"
radius or 3" radius, it doesn't make much difference.

the breakdown voltage for a curved surface is roughly 3MV/meter or
70kV/inch.

So a 1/2" diameter rod (1/4" radius at largest) will breakdown at about
17-18 kV. A 6" diameter ball breaks down at 200kV or so (assuming it's
perfectly smooth, etc.)

The "free air" electric field without a storm around is on the order of
100 V/meter, rising to 10-20 kv/m under a storm. So, get 10-15 meters
up, and you've got your 200kV needed to start corona on even a pretty
big diameter thing.

If the ball is wet, especially with distinct droplets, then you can get
corona forming much earlier. The electrostatic forces tend to make the
droplets fly off.

On 7/23/2010 4:23 PM, Jim Lux wrote:

If the ball is wet, especially with distinct droplets, then you can get
corona forming much earlier. The electrostatic forces tend to make the
droplets fly off.


Non-rhetorical question(s).

I must be missing something then. Why don't I see corona on the tips of
leaves at the tops of my trees? Trees are pretty conductive when
hundreds of kilovolts are involved.

Or my antenna masts for that matter, 'cause they are grounded, too.

tom
K0TAR