On 10/8/2013 8:11 AM, Tom W8JI wrote:
Why would that be true? I can't understand any mechanism that would
consistantly increase damage because an MOV protected stip is added.
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.
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.
The mechanism for the leakage current side of it is quite similar. We
know that the AC line is full of the harmonics of 60 Hz because current
is drawn by capacitor-input supplies in pulses at the peaks of the
cycle, and that the triplen harmonics add both in the neutral and in the
ground of 3-phase systems. Few of us have 3-phase in our homes, but a
LOT of power distro to us uses "high-leg delta" on the street to feed
us. High leg delta is 240V delta, where one side of the delta has a
center-tapped transformer to feed single phase customers, and 3-phase
customers get all three phases. These single phase customers have no
neutral, so much of their harmonic current shows up on our neutral.
In our homes, we have equipment with intentional capacitors (line
filters) and stray capacitance (mostly transformers mounted to the
chassis) between line and neutral, and between line and green. This
capacitance dumps the line voltage, including the harmonics present in
the voltage waveform, onto the neutral, increasing linearly with
frequency. So when we stick a scope between the two ends of a green
wire, we see those harmonics.
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.
With unbalanced interconnects, this buzz is added to the signal. And if
the gear has Pin One Problems, it's also coupled into gear by that
mechanism.
The power distro scheme (same green wire, or bonded outlet boxes)
typically reduces the buzz by 20 dB (by taking the drop in the long
green wires to the panel out of the equation). Bonding the gear is
typically good for another 20-30 dB.
73, Jim K9YC
_________________
Topband Reflector
|