"Richard Clark" wrote
wrote:
Richard Harrison wrote:
Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts
at
a bajillion amps)."
It's not a monster under the bed. But it is a heaven of a blast!
Hi Dave,
The point of the monster is that it is NOT under the bed, but in the
heavens. Let's look at the numbers you provide:
First peak 100,000 amperes with a second strike of
50,000 amperes. Full Width Half Maximum [approximately 50% pulsewidth]
of 100 useconds for each peak.
Expressed as power into a section of tower where the cumulative
resistance is 1 mOhm (not unreasonable) and giving the stroke a full
second sustained current flow (I've never seen such a long one) so we
can round the numbers into watt-seconds (never mind KWH); and figuring
a duty cycle of 0.01% based on your pulse width, but let's get
extravagant and say 0.1%; then both strikes express all of 150
milli-watt-seconds of power.
500 amperes sustaining current in the lightning channel for 300
milliseconds.
Again, expressed in watt-seconds (sneering KWH) this probably doubles
the power burden another 150 milli-watt-seconds. Total power: less
than half a watt-second or as much heat as 1/10th of a Christmas tree
bulb held for the same time as the strike. For those who remember
NE-2 bulbs in their radio's front end, these are rated at 1/4 Watt.
The paranoid may wish to parallel several, but such devices exhibit
what is called current hogging - one will fire to destruction before
the others light up.
To demonstrate the catastrophe that is so often associated with a
strike, divert the same strokes to a nearby tree that shows all of 10
Ohms resistance in its sap: 3 KW-Seconds. Try holding three clothes
irons for 1 second. :-)
Even this is barely remarkable given the heat is spread over a
considerable bulk. What makes the difference so destructive? That
same time element. The heat does not have the leisure of dissipation
in 100µS and concentrates. This accounts for the scoring of a strike
on metal, or the steam explosion in a tree trunk.
4) Electric field intensity prior to prestrike is greater than 10,000
volts per meter.
Conclusion: Lightning has lots of energy.
Energy is a strange thing, sunlight has vastly more energy than radio
waves at HF (or VHF or UHF or SHF or....) No one worries about their
radio at the beach, but they put sun screen on their skin. Walking
across a wool carpet generates far more energy than a pre strike, but
hardly enough power for a pinwheel. Separating two sheets of typing
paper is about the same risk.
This does not diminish the liability to sensitive components. The
electric fields created by the casual separation of paper can destroy
a transistor IFF it is not in a circuit. The power absorbed by
common, resistive components in relation to that same transistor
protect it simply. There are some circuit designs that seek a high
input resistance that easily fail to this assault. I should note that
in this day and age of surface mount that there are also resistors
that can be destroyed by these casually generated potentials.
Systems have been designed to
not only survive a direct strike but to operate through the direct
strike. All it takes is $$$$$$.
Well, for the amateur (not working through a strike) perhaps $$.
The risk is: "Do you maintain 0.001 Ohm or better strike paths?"
73's
Richard Clark, KB7QHC
Hi Richard - can you please explain this "duty cycle of 0.1%" ? I thought
it was expressed as a 10% duty cycle - but then a 50kva strike would be
expressed as 5kva sustaining current in the lightning channel, not 500
amperes. Dave - was that 500 amperes a typo or the figure used by the USAF
for protection design? My internal surge protection is designed for 10kva
max, and the rooftop downconductors would certainly be expected to carry at
least 5x that much for a short time from a direct attachment. Even internal
AC wiring is designed to carry 6kv/1kva before dialectric breakdown. Which
does incidentally happen from those 100kva strikes. It just happened less
than half a mile up the beach from me last month.
Thanks,
Jack Painter
Virginia Beach VA
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