"Asimov" wrote in message
...
"w_tom" bravely wrote to "All" (07 Jul 05 22:29:12)
--- on the heady topic of " Lightning protection"
w_ From: w_tom
w_ Xref: aeinews sci.electronics.repair:52721
w_ Meanwhile TimPerry repeatedly refers to DC pulses. That
w_ would be DC analysis. Lightning pulse is AC electricity. It
w_ creates electromagnetic waves of same frequencies. You even
w_ have a chart for that frequency spectrum. What the chart does
w_ not show is how the energy content quickly tapers to zero as
w_ frequency approaches DC. That pulse called lightning is
w_ composed of electricity at RF frequencies.
He is not completely wrong, Tom.
alas: this implys i'm not completely right
.
There is a phenomenon of charge
separation which takes place in the cloud and there is a DC potential.
The initial discharge starts with DC but with distance and motion we
have the effects of inductance and capacitance which establish an
impedance and results in reflections where discontinuities exist for a
moving wavefront.
What we basically have is a square wave going from
one DC potential to ground but, like the Fourrier model, made up of an
infinite series of sine waves. So yes, there will be AC
but only as a theoretical concept. the electrons only travel on one
direction. cloud to earth or earth to cloud, or cloud to cloud. (mostly)
but the
initial natural process is all about charge separation in the clouds.
It can be no other way.
A*s*i*m*o*v
... Power is obtained by current meeting resistance
This week a master electrician asked me how well versed I was in lightning.
I looked him straight in the eye and said "there is no human on the planet
that's well versed in lightning". He cracked up and nearly fell down
laughing.
He then gasped out "that's exactly what I wanted to hear"
We then went on to discuss his problem. he had installed some motors with
solid state starter circuits in a grain elevator 2 years ago. this year a
series of storms has blown the starter/controller on multiple occasions. The
frustrated granary owner accused master electrician Frank of installing
faulty grounds.
I explained that from my viewpoint that the initial causation of equipment
damage can be mighty hard determine. Did the lightning come sown the
structure? Was it a hit on the power line or transformer? was is a spike
caused by brief power interruption? in many cases I am faced with blown
fuses, open breakers, transformers that are shorted and sometimes on fire,
and sometimes arc holes in equipment cabinets. At this time I am replacing
some arc gaps and lighting transformers that were melted by a stroke. The
project will cost about $15,000 USD as some aircraft beacons will need to be
replaced along with AC cable.
We then proceeded to plan the install of a 100 kW generator. I pointed out
the parallel solid state TVSS unit and the 4" ferrite toroids on the load
side of the 3 phase service. He said "wow does that prevent lightning
damage?" I replied when is comes to direct hits by lightning anything can
happen but over the long term this and similar arrangements reduce
maintenance / repair costs and downtime.
for those interested in reading a "white paper" on lightning protection this
page
http://www.nautel.com/support/files/...protection.pdf may be
informative.
It is geared toward a specific type of situation. it lists sources.
section 2.2 describes lightning characteristics. you may agree or disagree
with the data it presents but you may note
section 2.2.2 describes a median main lightning strike pulse as a
uni-directional near-exponential pulse of 20,000 amperes peak amplitude
lasting 40 microseconds to half amplitude.
I don't know about you but uni-directional pulse sounds like DC to me.