[time-nuts] Lightning and grounds...

Don Latham djl at montana.com
Sun Oct 4 22:43:21 UTC 2009


Hi Burt and all:
Only a reminder: lightning is a pulse, and especially at the ground
terminus can have frequency components in excess of tens of MHz. Think in
terms of impedance rather than resistance. Many of the "freaky" things
lightning pulses do are more clearly seen this way. A sharp bend in a
ground lead, changes in wire size, spacing from other conductors, etc.
etc. should be considered.

Use of "brush" or radioactive lightning rods, as has been pointed out, is
pointless and even dangerous. If you feel you have to experiment with
such, at least just use a sort of ball of barbed wire rather than paying
someone a small fortune for junk. Any "brush discharge" lightning rod only
serves to protect itself, and not even that in the presence of wind.
Research points out :-) that rods of 1/2 or 3/4 inch, with hemispherical
ends and well grounded with low impedance leads, are the right thing. The
single plate ground point is of course correct, as is non-tapped leads
from each piece of equipment.
Don Latham
www.lightningforensics.com

Burt I. Weiner
> Something I should've mentioned is that the connections are all
> essentially in series.  For example.  If you have a wire going to a
> ground rod and you connect several devices to it, they are all
> meeting at the top of a "resistor".  If a device attaches part way
> down a wire or you have a wire running along the back of a bench and
> the various pieces of equipment attach somewhere along he main ground
> wire, this is like a tapped resistor.  Each piece of equipment is
> connected a some point along the resistor.
>
> Ground plates are usually insulated from walls, inside or out because
> the wall can become part of the path.  Lightning is tricky stuff and
> the only rule it follows is that of finding the quickest and most
> direct path to ground.  However, that doesn't mean that it won't take
> many paths.  Make it convenient for it to follow the least damaging path.
>
> Again,
>
> Burt, K6OQK
>
> At 12:44 PM 10/4/2009, time-nuts-request at febo.com wrote
>>One of the ways that I've always explained this is to draw a
>>schematic, actually more of a pictorial of the ground system and each
>>piece of equipment in the overall system as a block.  In place of the
>>wires to ground, I draw a resistor symbol, including the ground and
>>ground lead itself.  In fact, each chassis or cabinet is also one of
>>the resistors.  By looking at it this way you can see that the ground
>>system path is nothing more than a low resistance voltage divider,
>>actually a current divider between a strike and the good earth.
>
> Burt I. Weiner Associates
> Broadcast Technical Services
> Glendale, California  U.S.A.
> biwa at att.net
> K6OQK
>
>
>
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-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com




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