[geeks] Whee! Lightning strikes, AGAIN!

[Shannon Hendrix]

On Jul 29, 2009, at 03:42 , gsm at mendelson.com wrote:

> On Wed, Jul 29, 2009 at 02:28:41AM -0400, Shannon Hendrix wrote:
>
>> 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.
>
>
> If you are referring to me, I said something completely different. I  
> said
> that they were grounded. I pointed out that the one I used was a ham  
> radio
> antenna of which the entire thing was DC grounded.

I was responding to wa2egp at att.net.

Looking at the list, my reply shows up in response to him, not you.

He was promoting dissipators, which do are not lightning rods and they  
don't work.

>> The lightning research community generally rejects the idea and  
>> further says that it increases strike probability rather than  
>> reduce it.
>
> I'd love to see that. Everything you say about ungrounded dissipater  
> is
> correct, but where do you see that for grounded ones? Maybe you did  
> not
> and lost in all the noise was the point I was referring to grounded  
> ones.

Like I said, I was responding to bob's post in support of dissipators,  
not you.

All lightning research indicates that while not perfect, lightning  
rods work, and dissipators do not.

Dissipators which appear to work are always really lightning rods in  
disguise, except they cost a lot more for no good reason.

> Since Franklin's time, it has been shown that except for rare  
> occasions
> (mostly existing to make the WikiPedia seem fair), they cause  
> lightening
> strikes.

No, they don't cause them.  That's a myth.

They intercept local strikes, usually those which would have hit the  
structure or very close.

They don't reach out a half mile and pull in a falling leader.

Well, I guess a really large one might...

> While it's a great idea to channel them safely, in Franklin's
> day the EMP problems caused by them were of no consequence.

EMP is rarely a problem today.  Surge is the primary issue for most  
people, either direct or induced.

Franlin's type of rods are also generally not suitable for today's  
structures, and modern lightning rods are no longer pointed, they are  
round, blunt, or are long wires instead.

>> 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.
>
> There you go mixing intercepts (downward) and creating upward  
> streamers.

Eh, no.

ESE lightning rods are marketed specifically as better interceptors  
because they induce upward streamers on purpose.  The theory is that  
by creating the channel ahead of the leader it is more effective.   
Testing shows a slight benefit, but perhaps not near enough to justify  
the increase in cost.

The terms are certainly not mutually exclusive.

Even a conventional system with an induced field above it can  
sometimes have an upward streamer happen.

So can natural strikes.

> Guide or attract would be a much better, IMHO word.

Some people say divert.

However, everything I have read says that lightning researchers reject  
those terms, and prefer "intercept".

I think it is because they see the ground system as the part that  
directs the current, and the "rod" as the part that does an intercept  
of the strike.

But you'd really have to ask them... :)


-- 
Shannon Hendrix
shannon at widomaker.com