Lightning Protection System

[MSRhodes]

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.

 

[VEBill]

I've read 200,000 amps for the largest monster strokes in Florida, to more like 10,000 amps for the run-of-the-mill strokes.

Stand-by for others' input.

 

[dpc]

http://www.lightningsafety.com/index.html

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.

 

[waross]

In some areas, a braided aluminum conductor is used to avoid the inductance of twisted strands. Installation is with no sharp bends and always in a downward direction. As I remember the cable was about #4 AWG braided aluminum to ground the lightning rods for the inflatable roof of a major stadium.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[MSRhodes]

Thanks for the information. The existing system utilizes two, #4 condcutors and the whole appears to be meet code-for a thirty year old installation. We are in the preliminary stages and will have the system inspected/tested, but we wanted to see if it is adequate before we begin.

 

[MSRhodes]

Excuse me while I eat some humble pie. I am a new EE and I allowed myself to soley rely on the NEC's methods of conductor sizing and focus on the required typical data for doing so. I found my answer in NFPA 780-the same source I culled.

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.

 

[7anoter4]

I think the down conductors for the class II are more specific stated on UL 96[or UL 96A] para. 19 or 24 see:

http://ulstandardsinfonet.ul.com/tocs/tocs.asp?fn=0096.toc

http://ulstandardsinfonet.ul.com/tocs/tocs.asp?fn=0096A.toc

 

[alehman]

780 specifies the conductor size in circular mils. I think class 2 copper works out to be somewhere between #2 and #1 AWG.

 

[TVtech]

Encyclopedia of Physics 2nd Edition copyright 1991
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"

 

[TVtech]

Because of the high freq components of a lightning strike the path to ground should contain as little inductance as possible, meaning as straight a line as possible.

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!

 

[MacGyverS2000]

Speaking of Dr. Uman... he has some really cool fulgerites on a shelf in his office. During my time at UF, I always wished I could spend at least one summer doing field research with his team, launching rockets and making lightning strike on command.

Dan - Owner

http://www.Hi-TecDesigns.com