On Fri, 10 Mar 2006 07:37:31 -0500, Pete Smith wrote:
>Never did figure out what the mechanism could have been to account
>for it.
Not speaking of your problem or the original post directly, but there
are some power wiring errors that could account for the sort of
results being described. Consider outlets or fixtures that are
miswired so that the green wire (ground) and the neutral are
interchanged, or the neutral and the hot are interchanged, or even the
hot and the phase.
And consider a fixture where the neutral is improperly bonded to the
green wire. In a properly wired system, the neutral is bonded to
ground at one, and ONLY one, point -- the place where the system is
established. That point is either the service entrance, or, if there
is a local transformer, at the transformer itself.
When there is only ONE bond, as is required by code, all of the load
current flows "out" on the phase and "returns" on the neutral. This
causes the magnetic field to be almost entirely confined to the space
between the two conductors, and external fields nearly cancel. If
there is a second neutral bond, the "return" current divides between
the paths permitted by every "grounded" object in the building. Now,
the cancellation is quite poor, so the external field is both a lot
larger in magnitude and spreads out over a wide area.
Now, consider an electronic device that has a capacitor between
neutral and ground, and between the phase and ground. In this
condition, just as in the double-bonded neutral, the high frequency
return currents are not equal (because in real equipment the
capacitors, especially the strays, are rarely even close to being
equal). And just and in the case of the double-bonded neutral, the
magnetic field does not cancel, and spreads out over a wide area to
cause interference.
I am raising this issue to alert the EMC folk on this list to the
issue, because I am seeing systems where the "willy-nilly" use of
bypass capacitors on multiple circuits in large systems (like large
lighting or motor control systems) is shoving huge currents onto the
green wire and causing serious interference to audio systems. Last
year, I measured 60A on the green wire in the distro panel of a
theatrical lighting system in a "megachurch" in Maryland. When we
asked the Electrical Contractor to find and fix the problem, he told
the client the condition was "normal." Huh?
Also think about these mechanisms in the context of RF. Bypassing RF
to "ground" is not necessarily a good way to reduce radiation of noise
if the "ground" is also an antenna! Remember that although WE call
that green wire "ground," Mother Nature calls it an antennna. And she
ALWAYS wins.
Jim Brown K9YC
http://audiosystemsgroup.com
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