Topband: Best Outlet sttrip

[Jim Brown]

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