Hi Jim,
MOV from phase to green wire dumps current onto green, IZ drop on green
back to the panel raises potential of that chassis. It has a low voltage
(signal cable) to another box, plugged into another MOV strip at a
different location, more current on green, probably not the same, probably
not the same IZ drop. The difference appears on the low voltage
interconnection and fries I/O for that interconnection. Same issue
happens if the interconnection is to equipment with a different ground
connection. I've seen MANY reports from engineers of destructive failures
in small wired Ethernet systems in homes and small offices with no radios
or towers involved. Likewise, large audio and video systems with equipment
at widely separated locations have this issue. I worked in that field for
many years, and we solved it by using series-mode protection on branch
circuits.
I understand your concern now, but that problem is really rooted in a design
issue with the equipment. The system should not be that sensitive to common
mode issues on ports.
It is foolish or poor planning to think, by simply not clamping a distant
line for surges, we somehow protect poorly designed input systems. A design
sensitive to common mode places a burden on everyone else in the world to
protect the poorly protected or designed ports. It's like saying "I'm going
to do a bad job, so watch out for me".
It is like the unrealistic dream that unbalanced audio lines from
chassis-to-chassis, grounded at each chassis, is a good system and that all
that needs to be done is be sure that all of the world's chassis are at
virtually zero potential to each other. That just sets everyone up for
problems. It is unrealistic to expect every piece of gear and system to
ensure all non-clustered pieces of gear are at the exact same chassis
potential. If something as simple as a MOV clamp causes a problem, you can
bet the same gear will have problems from dozens of other causes.
I suspect the real root of this is cheapness, where they want to avoid
isolation and protection on the port and pass the blame or burden off on the
rest of the world.
MOVs are fine IF the bonding of grounds and equipment is properly done,
and if everything is at a single outlet. I have long advocated a scheme
for AC power in shacks whereby all power comes from outlets that share the
same green wire, or from outlets whose green wires are bonded together.
Likewise, I have long advocated a scheme whereby every equipment chassis
is bonded to every other chassis by short fat copper, and to station
ground, and to all other grounds. That works well both for lightning
protection and for the prevention of noise coupled by leakage currents
into unbalanced interconnects, and into Pin One Problems.
That's true, and that's good advice. Although proper grounding for lighting
belongs at the entrance and not on the desk.
Now, when we make a signal interconnect between gear plugged into
different outlets, we have different IR drops due both to differences in
the relative strength of the harmonics on those outlets, and to the
lengths of the green wires, and the difference is the familiar power line
"buzz" that we have long called "ground loops." I prefer to call it what
it is -- noise coupled by leakage current -- because we can now understand
the mechanism, and knowing the mechanism, know how to prevent it.
The easiest way to prevent it, because there are dozens of causes, is to not
have port designs sensitive to common mode. In broadcasting, we would have
nearly been taken out and put in front of the firing squad for running audio
lines with shields grounded at each end, or unbalanced low level lines
between equipment that was not on the same rack when the shield was grounded
at each end.
Sometimes we need to rethink the point of ingress, and not make the rest of
the world responsible for our cheapness or lack of planning. :)
73 Tom
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Topband Reflector
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