Lightning
> radiates strong signals across a very wide part of the electro-magnetic
> spectrum. (Notice how
> 'bright' a nearby bolt of lightning is way up in the visible light portion
> of the spectrum. It has
> similar 'brightness' or power all the way on down to DC.) A nearby strike
> will induce currents
> across a wide range of frequencies on the metal, wiring and electrical
> equipment contained in that
> nearly-invisible-to-RF house... and the strength of those currents at each
> frequency can vary widely
> depending on the impedance & resonance at each frequency.
>
> The low-impedance earth bonding system helps in two respects:
> a) keeps adjacent chassis floating at similar electrical potentials,
> reducing the current flow
> between the pieces of equpment.
> b) provides a path to earth for most of the induced current which is
> of lower impedance than
> other, more damaging routes (e.g., via signal cabling).
>
> (a) above also reduces the danger of electric shock to operators when
> a fault occurs within a
> piece of equipment for whatever reason, since fault current flowing into
> the chassis will lift its
> potential and that of the neighboring bonded equipment similarly. (Fault
> current should be
> short-lived if the protective systems [fuses, etc] have been properly
> designed.)
>
> Note that no bonding system can keep equipment at the SAME potential
> to its neighbors, to the
> 'entrance panel' (which is also floating in space), or to the earth.
> There is always SOME
> impedance, greater at higher frequencies, and therefore SOME potential
> difference (voltage). A
> well-designed and -implemented system will keep these impedances low and
> the differences small
> across a wide range of frequencies down to DC ... but it can never be
> zero.
>
The bright flash that is seen from lightning is the plasma trail that the
current travels on; it is not the visible frequency of lightning.
If you want the best isolation of equipment, a separate ground lead from
each piece of equipment or rack of equipment would be run to the single
point ground plate. That would eliminate currents being fed from one piece
of equipment to another. Ideally ALL connections to each piece of equipment
should be directly referenced to the single point ground and not directly to
another piece of equipment. The Motorola grounding specs call for this type
of grounding of equipment. However this can't always be done due to
interconnections of signal paths between equipment.
You don't particularly care if all equipment is maintained at the exact same
potential during a lightning strike. What you do care about is that all
connections to a particular piece of equipment have equal potential. This is
within reason of course as great differences between equipment can result in
flashover between them.
As far as shock hazard is concerned you do want all equipment at the same
potential as earth.
Grounding equipment to keep it at earth as far as RF is concerned normally
should not be a concern. If there are RF problems on equipment chassis there
are usually other problems that should be addressed.
As far as induced currents into equipment in the shack, it is important to
keep lengths short between the single point ground and the equipment. It is
also very important to not have the protected side of the single point
ground panel wires close to or parallel with the unprotected side wires.
All unprotected lines should be kept well away from protected lines that go
to equipment.
73
Gary K4FMX
_______________________________________________
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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