Jim, first of all, thank you for your response. You
are a valuable asset on the list.
I would also stress that there is one more reason for
'grounding' of equipment, esp. anything that has
exposure to the outside (e.g. antennas in our case,
but this could include remote wx sensors with wiring
back into the house, etc.)
If you recall, a change in voltage on one plate (say,
the 'cloud' for the purposes of discussion) attempts to
induce a 'displacement' current (via charge induction)
in those conductors (the other 'plate') or conducting
mechanisms within its view, again, usually an antenna
in our case BUT could include metal railings, fences,
etc.
If _no_ leakage path (a connection of device to earth
ground) is provided, then, the potential on the surface
(maybe connected via coax to a chassis down below,
e.g. a converter box) can rise to .... well, as many as
volts as the 'impressing' or charged element (cloud here
again) is able to develop (ignoring certain limiting factors
like what the 'view' is, distance between them, etc.)
(This would be separate from corona discharge which
is the breakdown of air with subsequent flow of
electrons/ions.)
Many of these consumer devices are 'double-insulated'
and the 'mains' input hipot tested to 5 KV or so; this
represents quite a potential shock hazard, never mind
a direct lightning strike, if this proposed scenario
develops without some conductive 'leakage' path back
to earth (earth ground) to equalize potential between
the 'object' and earth (and possible human touch or
interaction).
(I wonder if that rationale is ever expressed by the NEC
or its annotated version with the above in mind as the
reason for grounding ...)
Background: Demonstration of concept (19 MB):
"Charge induced in ground plane by overhead conductor"
http://ocw.mit.edu/ans7870/resources/zahn/video/demo-4-7-1_300k.mp4
I want also to make mention of a volume set that I
once upon a time thought invaluable and covered
some of the 'systems issues' you addressed in the
linked pdf file from the previous e-mail; one volume
in this particular set was quite instructive, and perhaps
accounts for my unwillingness to lightly accept anything
in the nature of noise, grounding, signal termination, etc,
without a thorough examination of the asserted subject.
Title: "Physical design of electronic systems" (in 4 volumes)
by Bell Telephone Laboratories
Much of this is a little more then slightly off topic for
the RFI list, so this will be my last address on this
subject.
Regards,
JimP // WB5WPA //
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <rfi@contesting.com>
Sent: Wednesday, December 24, 2008 11:10 PM
Subject: Re: [RFI] DirecTV to ground or not to ground
> On Wed, 24 Dec 2008 21:04:26 -0600, jvpoll@dallas.net wrote:
>
> >b) History, practice and more importantly, mistakes in
> >practice (accidents) have made important contributions
> >to the NEC than simple theory would or could predict;
> >take for iinstance grounding practices. What _is_ the
> >rationale for grounding the secondary circuit (the
> >service to the home) of a power transformer for instance?
>
> >This isn't strictly neccessary for that service to
> >function. (I am asking this for the purposes of bringing
> >forth an example; I can cite cases where, in practice, this
> >has saved property and/or life, but that is different than
> >a statement or the treatment a textbook might give.)
>
> >Could you cite a textbook wherein that rationale is brought
> >to light?
>
> One reference is the tutorial cited below. While it's written
> for sound and video contractors, the fundamentals apply to
> hams.
>
> http://audiosystemsgroup.com/SurgeXPowerGround.pdf
>
> It all comes down to safety. The secondary is bonded to provide
> both lightning safety and, with the equipment (safety) ground
> conductor (green wire) provide a solid path to blow a fuse or
> breaker if a fault develops that could shock someone or start a
> fire. The lightning protection comes from because the bond to
> earth limits the potential that can exist in the event of a
> strike. The concept behind lightning protection is to have the
> whole building rise in potential by the same amount.
>
> NEC can be purchased in annotated form -- that is, with
> explanations and commentaries interspersed with each paragraph.
>
> 73,
>
> Jim Brown K9YC
>
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