YOU MUST REMEMBER, lightning is not RF, its not DC, its not even AC... it is a
fast rise time pulse, those can not be handled with normal impedance
equations and lumped models... for instance a big fat wire that runs 300' above
ground from a tower to a very low impedance ground mat... you might expect
that would protect something below it on the tower from the lightning stroke
because it has a low impedance and the current will go out the wire. not so.
when
current comes down this nice fat tower and sees this other possible path all it
knows is the local surge impedance of the wire, surge impedance is what is
calculated taking into account the self inductance and the capacitance to
ground or surrounding objects, for normal size wires this can be hundreds of
ohms,
lets say 300 for now. a surge impedance for a tower section could be a hundred
or so ohms. so current coming to that junction sees 100 ohms going down
and 300 going to the side so it divides and about 3/4 goes down and 1/4 goes
out the wire... a microsecond later that current on the wire gets to the low
impedance mat, now since the mat has a much lower impedance than the surge
impedance of the wire it sends a reflection back on the wire of the opposite
polarity. a microsecond later that reflection gets back to the tower and NOW
about 2 microseconds later the current on the tower see the low impedance
path through the wire and more of it goes off that way. of course in those 2
microseconds anything below the wire on the tower has fried.
oh, and at lightning voltages you have to constantly keep track of the
breakdown voltage of air gaps... while those start as very high impedance and
you would
expect that current on a tower next to house would stay on the tower, it can
easily flash sideways from the tower to the house if there is something there
like a
metal roof flashing or rain gutter or power line.
low resistance doesn't help define current carrying capability or heat
capacity... for example, copperclad steel carrying fast front lightning
currents will have more
of the current in the copper than steel so it would have similar high frequency
resistance to solid copper, but the steel core is harder to melt and may hold up
longer than pure copper would. in most cases even relatively small 10-12ga can
survived a normal lightning stroke because the time period is so short, where
they would vaporize if dropped across a car battery terminals.
Jan 14, 2016 01:24:16 PM, jim@audiosystemsgroup.com wrote:
On Thu,1/14/2016 8:49 AM, Kelly Taylor wrote:
> The important thing to remember is electricity doesn’t follow the shortest
> path somewhere, it follows the path of least resistance.
NO -- it follows the path of least IMPEDANCE, which is the path of least
INDUCTANCE, which is the SHORTEST path. Resistance matters ONLY to the
extent that a low resistance takes longer to vaporize, so the path able
to carry the strike for more of its duration. THAT'S why we want BIG
conductors.
73, Jim K9YC
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