Lightning Protection System
Lightning Protection System
(OP)
I have culled NFPA 780 for any information on sizing a down conductor for a Class II installation, but to no avail. I am specifically looking for a value for current resulting from a lightning strike.





RE: Lightning Protection System
Stand-by for others' input.
RE: Lightning Protection System
You might want to look at some of the info on this website, in particular there is a link to a NASA webpage on lightning.
Because lightning has a lot of high frequency components and steep wavefronts, it probably makes more sense to think in terms of watts and total energy rather than current.
RE: Lightning Protection System
I realize that this is not the specific information you asked for, but offer it as a reality check for your design. If you find that you are off by an order of magnitude, you may have missed something.
Bill
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"Why not the best?"
Jimmy Carter
RE: Lightning Protection System
RE: Lightning Protection System
However, before I declare my question answered, I would like to make sure that sizing the cable has to do with the wieght per 1000' and individual strand diameter.
RE: Lightning Protection System
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RE: Lightning Protection System
RE: Lightning Protection System
Article by Martin A. Uman (University of Florida)
" A fully developed stepped leader has about 5 coulombs of negative charge on it, it has travelled to ground in about 20ms with an average velocity of 1.5 X 106 m/s and an average current of about 100 A, and has an electrical potential with respect to ground of about 10[su]-8[/sup] V. The intermittent leader steps have a pulse current of about 1 kA.
When the stepped leader is near the ground, its electric feild causes upward moving discharges to be launched from the ground. When one of these discharges contacts the leader some tens of meters above the ground, the leader bottom is connected to ground potential.
.........the return stroke propigates continuosly up the leader channel at a velocity of typically one third the speed of light, the trip taking less than 100us. The return stroke channel carries a peak current of typically 20 kA with a time to peak of a few mircoseconds. currents measured at the channel base fall to half of peak value in about 50us, and currents on the order of hundreds of amperes may flow for milliseconds or longer"
RE: Lightning Protection System
On the other hand in my case I also have to try to protect a TV transmitter. To this end the sheild of the coax cable is bonded to the grounded tower and at the transmitter a small split iron ring around the coax (over the plastic sheath) introduces a small inductance that will not interfere with the transmission characteristics of the coax BUT does delay current of the high freq components and as such allows charge to 'find' it easier to take the path to ground at the sheild bond to the tower.
... at least in theory!
RE: Lightning Protection System
Dan - Owner

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