[BULK] - RE: [BULK] - RE: [TowerTalk] ground strap width: 3" vs 6"...

[Steve Katz]

I agree with Jim. 

The traditional "fabricated" pulse used for testing against lightning
transients, and used by most industries worldwide, is an 8x20uS pulse,
repetitive.  This is much closer to RF than to DC.

-WB2WIK/6

-----Original Message-----
From: Jim Lux [mailto:jimlux at earthlink.net]
Sent: Wednesday, August 18, 2004 11:46 AM
To: keith at dutson.net; TowerTalk
Subject: [BULK] - RE: [BULK] - RE: [TowerTalk] ground strap width: 3" vs
6"...


At 01:19 PM 8/18/2004 -0500, Keith Dutson wrote:
>What frequency?  I have always looked at lightning as a low/no frequency
>flow of electrons, i.e. like a DC current.
>
>Keith NM5G
>
>-----Original Message-----
>From: Steve Katz [mailto:stevek at jmr.com]
>Sent: Wednesday, August 18, 2004 12:37 PM
>To: 'keith at dutson.net'; TowerTalk
>Subject: RE: [BULK] - RE: [TowerTalk] ground strap width: 3" vs 6"...
>
>Isn't most impedance in a length of copper inductance?  The resistive
>component should be very, very small.....-WB2WIK/6

Lightning impulses are fairly fast pulses (rise time of a few microseconds, 
fall times of several tens of microseconds).  Lightning impulses induced 
from nearby hits will be a bit different, of course.

Since what cooks your gear is the high voltage between input and ground, 
you'd want to be concerned about the L*di/dt thing, so minimizing L is a 
"good thing".

If your gear were all floated, using the exact same ground as the lightning 
impulse is travelling through, then everything would go up and down 
together, and you'd be safe.  HOWEVER... there are always stray paths 
(capacitive, for instance) and for microsecond rise time pulses, the 
impedance of that stray path might be a bit low for comfort (i.e. the 
current through the path is high enough to produce voltage drops that will 
zap a junction).

There's also the whole induced voltage problem.  A wire carrying a current 
that changes quickly (high di/dt) can induce a voltage in an nearby 
conductor, thereby causing damage.  For your basic lightning impulse, 
you're looking at 10kA in a microsecond, or a di/dt of 1E10 A/second.  That 
will make a pretty healthy magnetic field transient to couple to the victim 
loop.


In any case, far from a DC current. 

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