In a nutshell, my recommendation (for what little it's worth) is this:
(1) a secondary SPD on a branch circuit should only be used when a primary
SPD is used at the utility company's meter or at the premises main panel;
and (2) assuming condition 1 is met, then the secondary SPD should divert
surge current only onto the neutral, and never the grounding conductor.
Paul,
The issue I have with this is ***thinking*** ( incorrectly) that unwanted
surge is on the hot or neutral, and is diverted to the safety ground because
we call that "ground". That's a big stretch, and probably mostly never true.
The suppressor doesn't pluck something off a hot lead and drop it there just
because we call it "ground", or because we want it all to be collected and
go there.
A suppressor only clamps or limits voltage across its terminals. It goes low
resistance where a transient comes along. I can have two suppressors, one
each from hot and neutral to safety ground (although that is a poorly
thought-out system) and it will simply clamp the three wire ends together.
It does not back-feed the safety ground or neutral with any grossly
upsetting currents, because almost certainly any **power line** sourced
surge already came from the direction of the breaker panel. The clamps would
divide currents by wire impedance and clamping voltage differential, so the
neutral and other two wires would be part of it any way we wire the clamping
system.
To illustrate how misplaced the concept of protection devices increasing
damage actually is, the safety ground is already connected to the chassis!
It is a thinner bare wire that closely parallels the other two wires for
some significant distance, and already connects to the neutral at the box.
Suddenly we are supposed to believe that clamping it to wires it already
parallels (and already has a low impedance to) will cause or increase
damage. Additionally, there is a bunch of stuff inside devices that lowers
impedance or voltage breakdown between all conductors anyway.
There isn't anything wrong with clamping the lines at a common point in the
shack, or at any equipment cluster, no matter what else we have in the
system.
Most of the damage we get is common mode from our antenna grounds and
antennas. The potential from that sees a voltage differential from the
mains. That differential already occurs between the antenna and chassis
common connection to all three wires almost equally. If you DON'T have the
MOV device, the safety ground and chassis is actually the best path to the
box. After all, that safety ground already is directly tied to the chassis,
and to out antennas, or in consumer gear to Telco lines or CATV lines. The
only thing the MOV's do for nearly all of the problems is keep the "snap"
out of the power line wiring and components inside the cabinets.
I'm just baffled why any theory would propose allowing two wires of a three
wire line float from the chassis is an improvement, when the bulk of the
problem is a ground loop from the antennas or cable/Telco grounds to the
power mains, and most mains surges are the same. A secondary but lesser
mains issue would be between the neutral and hot, and that surge would
primarily flow back on the heavier neutral rather than the lighter safety.
This entire thing defies common sense. The next thing I expect to hear is
them telling us to cut the safety ground off to protect their ports.
73 Tom
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Topband Reflector
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