Interesting enough, I always said there is no such thing as a real ground.
Ground loops are a serious problem, especially in research labs. The worse
case of
ground loop interference was sort of the inverse. Remember the reciprocal
principal of antennas. It is true
with ground loops. If can pick up a signal it can produce one.
A researcher working with collisions at very low energies, less than 100
eV had problems with things working from
time to time. At the time I did not know it, he had not yet asked for my
assistance and assumed it was an instrument
problem.
I walked into his lab and saw a computer CRT display slowly moving back
and forth. I asked the graduate student if
they were having a problem acquiring data, an he responded "how did you
know?". I said look at your CRT.
He said, yes , it does that sometimes. Well, my response was if that is
happening to your 30keV electrons in
your CRT imagine what is happening to your 50 eV electrons in your
experiment. This experiment was so sensitive
to magnetic fields that they had 3 pairs of Helmholtz coils with 3 sets of
power supplies to null out the Earths magnetic
field.
Ended up that they had a short in an heater element of a oil
diffusion pump on the return side of the line to the case.
The pump was water cooled and they had copper pipe most of the way to wall
and from there PVC pipe to the chiller on
the roof. However, There was another copper pipe that was gounded nearby.
And from time to time they would touch.
This pipe provided a very low resistance path back to the local power panel
and when these touched most of the
current flowed thru this pipe. What he had was a single loop electromagnet
carrying 20 or 30 AMPS around his
experiment, some of the time.
Most of the ground loop problems were found in the linear accelerator
lab which had kWs of RF power and another atomic physics lab
which had a pulsed N2 UV laser. When I first started working there there
were impulse noise problems that my predecessor could not
resolve, I would come in and start moving coax cables around and bundling
them until we resolved the problems.
In my previous life as an engineer before I was involved in designing
broadcast audio tape recorders. A small ground loop
of less than 2 inches in length caused some serious harmonics of 60 Hz to
be picked induced in the preamplifier by the field
from the power transformers. This was resolved by a change in the circuit
board where I replaced a radial electrolytic with an
axial one in such a way that a loop of equal area canceled out the
unavoidable one on the board.
Yes, ground loops do exist!
73
Bill wa4lav
At 06:08 PM 7/6/2012 -0700, Jim Brown wrote:
>On 7/6/2012 2:42 PM, Jerry Kaidor wrote:
> > It reminds me of the importance of ground loops.
> > Ground loops and me - we have a history.
>
>There is no such thing as a "ground loop" -- it is a totally false
>concept. The actual cause of the hum/buzz problems we blame on these
>imaginary "ground loops" are LEAKAGE CURRENT from the AC power system,
>the IR drop in the green wires puts that voltage between the chassis,
>and is added to the signal when unbalanced interfaces are used.
>
>The leakage currents are the result of capacitive coupling between the
>phase (hot) power conductor and the chassis, which, by law, must be
>connected to the green wire, and from there to the breaker panel. There
>are excellent reasons for that connection, all related to safety.
>
>The simple solution is to first get all power for interconnected
>equipment from outlets that share the same green wire or from outlets
>whose green wires are bonded together; and second, to bond from chassis
>to chassis of interconnected equipment with short fat copper.
>
>73, Jim K9YC
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