Don, scott has a good point: if you use a single point grount you might as
well use an isolation transformer to put the room on its own ground. This is
a great way to set up yur room, but it can lead to the problems scott
mentioned and one other big one:
if your room is on its own ground (i.e. has its own power) and the rest of
your lab runs off the building's power, you can ruin a receiver or spectrum
analyzer when you try to connect it to a feed thru on the room's wall. In
our lab, we took extreme measures to avoid this.
You would have to REALLY TRY to power an instrument from anything
other than the shielded room's power. You would have to move a safe or a
refrigerator before you could do it wrong. We have had lots of clients thru
our lab in the ten plus years we've been in there, and have never had a
problem.
Most of these are young "hands on" type engineers too. They like to
twist knobs and try things, vs. just watching us work. We encourage them to
do that but brief them when they come in, then we make it almost impossible
for them to mess up.
So far, so good! :)
lou
At 10:49 AM 11/3/1999 -0500, you wrote:
>
>Don,
>
>One other point that needs mentioning re. chamber grounding is the safety
>issue. When our chamber was installed, the electrician connected a #6 ground
>wire to an existing bus block that is wired to a driven rod in the basement.
>While working above the chamber he got a nasty shock when he came into
>contact with a metal air-conditioning duct. A piece of metal conduit with
>one end pressing against the chamber wall would exhibit a fat blue spark
>when the other end touched any grounded metal. It turned out that the ground
>rod clamp had become corroded. The leakage current from the line filters
>supplying the chamber is several amperes, which certainly could be lethal.
>
>Your single-point ground should be carefully checked (and periodically
>rechecked). I would recommend connecting the wire to the rod using TWO
>clamps of the one-piece type for redundancy. Conductive grease helps prevent
>corrosion.
>
>Scott Lacey
>
> -----Original Message-----
> From: [email protected]
>[SMTP:[email protected]]
> Sent: Wednesday, November 03, 1999 9:25 AM
> To: [email protected]
> Subject: FW: Chamber Grounding
>
>
>
> > Our chamber is grounded/isolated per the instructions of the
>vendor. We
> > have one copper clad ground rod installed through a hole drilled
>in the
> > slab adjacent to the corner of the semi-anechoic chamber. Other
>grounds
> > are isolated from the chamber (conduits, air pipes, water pipes,
>service
> > entrance safety ground, etc.). The ground comes from the ground
>rod, not
> > the service entrance.
> >
> > 1) Is "single-point-ground" as described above for Tempest? Is
>the
> > degree of isolation useful for typical commercial work? The
>chamber spec
> > is 100 dB isolation. For our immediate work, 60 dB of isolation
>is
> > adequate. Is there any correlation between chamber isolation and
> > effectiveness of the ferrites for the uniform field required for
>immunity
> > testing? Is there some other observable sensitivity such as
>degraded RF
> > measurements that would result from not observing the isolation?
>What
> > would the manifestations be?
> >
> > 2) Using the isolation as described above, has anyone experience
>ground
> > loop problems between the service entrance power and the local
>chamber
> > power distribution due to the "single point ground" concept
>defined above?
> >
> >
> >
> > Don Umbdenstock
> > Sensormatic
> >
>
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