[geeks] Whee! Lightning strikes, AGAIN!
shannon at widomaker.com
Wed Jul 29 01:28:41 CDT 2009
On Jul 28, 2009, at 19:59 , wa2egp at att.net wrote:
> NO, I did not.
You original post didn't make it clear what you were saying.
It doesn't really matter, because you are still wrong.
In fact, the OP is a little bit wrong too, but not near as much.
For one thing, you guys are talking about two different things:
1) lightning rods
2) lightning dissipators
You are talking about a lightning dissipator. They are ungrounded,
pointed rods in most cases, but some designs are spiky or even
blunted. The theory is that they will bleed off charge and reduce the
potential difference between ground and cloud.
The lightning research community generally rejects the idea and
further says that it increases strike probability rather than reduce it.
There *IS* current when you use one, but it isn't enough to complete
the ionization path, and it results in a larger electrical field and
heavily ionized (conductive) air rather than a reduction.
Most of what is sold out there is pseudo-scientific crap.
A lightning rod is, by contrast, designed to intercept a strike and
direct it to a controlled ground. The theory here is to channel it
through a safe path to prevent it from channeling where it will do
harm. All lightning rods are grounded to earth or water, sometimes
via a grounding network or a building frame designed for that purpose.
Some earth grounds used are salted underground cable networks.
The lightning rod creates a field above the rod which intercepts
nearby strikes by acting as an upward streamer channel which connects
with cloud->ground stepped leader and connects that to a controlled
ground. This helps prevent the strike from going through structure,
people, electronics, etc.
Regarding sharp points. New research indicates that round or blunt
points produce better intercept performance, and in the US all new
lightning rods are round or blunt.
Ben Franlkin did originally claim rods were dissipators, but he later
retracted after he realized that they needed to be fully grounded, and
that they attracted, or intercepted strikes. However, he never did
fully understand the mechanics of it, ionization, leaders, etc. He
ended by saying "it is somewhat of a mystery".
> Oh yes it is designed to prevent strikes.
No, they aren't. They are strike interceptors.
Lightning dissipators are designed to prevent or reduce strikes, but
don't appear to work.
> And if it supposed to be struck by lightning then why to they put
> them on buildings with metal frames and no ground wires to the
> lightning rod.
They don't. If there is no rod->ground connection, then it is not a
lightning rod. See above.
> If the frame takes the lightning strike, then why use lightning
> rods at all?
To intercept the strike and bring it to a controlled ground.
> A good place to see where they are positioned is in Florida. They
> have them all over the place at Sea World.
I'm sure if you ask someone who knows, you'll find they are grounded,
they are lightning rods.
"Where some they sell their dreams for small desires."
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