On Sat, 14 Aug 2004 16:11:30 -0400, "Jack Painter"
wrote:

Hi Richard - can you please explain this "duty cycle of 0.1%" ?

Hi Jack,

Duty cycle is a simple ratio of on time to off time. It would be
presumptuous to offer that the strike off time for any particular spot
on earth is hundreds of years, so the choice of one second is suitably
long enough given strike components have long since faded, but are
easily recent both.

The shape of the pulse complicates the estimate because Duty Cycle is
often expressed in the expectation of a square wave. What is the on
time? When you get into pulse work, most in the field arbitrarily
assign the width of the half-power, or half-voltage points. For
Dave's numbers, this would be some 100µS which, when compared to 1
second is actually 0.01%. Being generous, and given the shape of the
decay I simply threw in a 10X fudge factor.

I thought it was expressed as a 10% duty cycle

10% of what? Lightning current flows for 100µS and is off for 1mS?
Dave clearly expresses lingering current flow out to 300mS so that is
clearly wrong.

- but then a 50kva strike would be
expressed as 5kva sustaining current in the lightning channel, not 500
amperes.

Where did the volts come from? If the pre strike fields are running
10KV then your strike has only 5A in it. This is the pencil whipping
that comes with lightning: the voltage description. ALL of that
voltage is dropped across 10000 feet of discharge length, not in the
last 3 inches from the tip of the bolt to ground.

In normal, settled air, in the most benign weather without any
disturbance, the potential gradient from earth to sky is 180V/M. That
is to say, your head is at an elevated potential of 300V with respect
to your feet. At an altitude of 10000 feet the potential is
1,800,000V without any inducement to discharge.


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.

You need to look at those surge protection ratings again. My
experience is that they are rated in Joules capacity which is NOT the
same thing as v-amperes. The two may be equivalent, but your
reference for volts is missing altogether. As I offered in other
postings: where did the voltage come from? If your tower is of the
standard design, you are not going to develop any appreciable voltage
unless you introduce resistance or impedance to develop it from the
current flow. 100,000A through 0.001 Ohms may give you 100V at best,
and only if you can reach the top of tower where the strike hits.

For those who want to multiply this voltage with Z, calculate the
impedance of a 12" diameter wire 50' tall at a frequency of 1MHz.
Perhaps this will make an NE-2 glow, if its leads are long enough.

It just happened less
than half a mile up the beach from me last month.

No doubt, but what exactly was "It" that happened? Stick 50 feet of
tower up into the air, and interrupt it with insulation and YES! arcs
will spark. No one needs an insulated $$$$$$ tower - thousands of
commercial installations typically discard that feature in favor of
simple $$ lightning protection.

73's
Richard Clark, KB7QHC