|To:||firstname.lastname@example.org, "TowerTalk" <email@example.com>|
|Subject:||RE: [BULK] - RE: [TowerTalk] ground strap width: 3" vs 6"...|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Wed, 18 Aug 2004 11:45:42 -0700|
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.
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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