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



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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