Re: [TCML] comments on designing a lightning protection system

["Peter Terren" <pterren@xxxxxxxxxxxx>]

Jon, Mike and others

When I fire my cap bank with measured currents over 100kA I develop a 
voltage of something like 600V over 12 inches of 2 inch x 1/4inch busbar 
which has a bend in it.  This explains why I was getting a shower of sparks 
between the earthed cases of two adjacent pulse caps. It had me puzzled 
until I did the math.

The inductance of any narrow (ie inches wide) lightning conductor means that 
the roof/aerial potential is still going to be (?hundreds) of  thousands of 
volts above ground potential.  This will of course also appear on the aerial 
wires to the TV etc.  Grounding to a broad structure of lower impedance like 
the steel building structure of the whole building will do better but you 
can't avoid a huge voltage appearing on the highest point.

As you say it is not simple and nothing I can think of will save the TV 
except unplugging it which we try to remember to do in a storm. It's all the 
other appliances like computers, phones and controllers that will have to 
put up with a multi KV spike on ground potential even in the best 
circumstances.

Here is a 100kA spark channel seeded by an exploding wire. It's a daytime 
shot unlike most of my other spark shots.
http://tesladownunder.com/PulseCapExplodingWire35G2kJNoInductor.jpg

Peter

----- Original Message ----- 
From: "Jon Busack" <boo1062@xxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, June 06, 2008 4:12 AM
Subject: [TCML] comments on designing a lightning protection system


> Please have a professional do your lightning protection system design and
> installation. It's more complicated than it seems.
>
> That cabling is designed especially for lighting, and for good reason.
>
> Lightning amperage is so high it actually "water hammers" your conductors.
> In plumbing if your pipe has a 90-degree bend, the pressure-surge of water
> hammers the bend as it turns the corner. In the same way, when lightning
> current hits a corner in the conductor it can blow out from sheer force of
> the current and the EM fields.
>
> No sharp bends in the cable. Point your T-splices "downstream" so the 
> cable
> bends in the way the current wants to go. There are rules about what kind 
> of
> cable mounting to use and how far apart.
>
> Ground rods- some rules here also- at least two, no less than 6 feet 
> apart,
> depends on soil conductivity. Use an underground "ground ring" that you
> splice your downfeeders to. In the ground ring use copper 400kcmil 
> minimum,
> and best to use thermal connections like cadweld. Consider a ground rod at
> each downfeeder.
>
> Ground the lightning protection system to your service entrance. Make sure
> your building has a single service ground that all your grounding 
> conductors
> feed back to. There are risks involved with multiple building grounds
> although intuitively it seems right. Especially with lightning strikes, if
> you have multiple grounds you can have a ground surge pass under the
> building and one side of your building might be several thousand volts
> higher than the other side as the surge rolls through.
>
> There are also design rules like the "rolling sphere" which tell you how 
> far
> apart to put the rods, and how high, depending on the topology of your
> towers and buildings and nearby trees.
>
> Don't do this one yourself.
>
> Some  suppliers will actually design a system for you if you use their
> products.
>
> Here's a supplier with a good reputation-- http://www.harger.com/
> Also check out NFPA 780 as Bert recommended. It has complete design
> recommendations, drawings, diagrams.
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