Jim,
IF using a 130 V rated MOV, where does it get wired ? Do you
wire between 120 vac and neutral.... or 120 vac and chassis of
equipment in question ?
Between 120VAC phase and neutral. NOT to the chassis!
IF a spike / transient / glitch etc occurs.and MOV
conducts..which path do you want the fault current to flow through,
the ground wire or the neutral ?
You want it to flow through the neutral line, so that the chassis of
your equipment does NOT get a dangerous voltage on it. If a high fault
current was injected into the chassis, all the inductance and resistance
of the ground wire might cause enough voltage drop to become dangerous.
The only place the neutral and ground are bonded together is at the
main 200A panel. On my 100 A sub panel, the neutral + ground are
NOT bonded, per the electrical code.
That's fine. And there should be a GOOD, real earth connection there.
That bonding matter is controversial, though. In my country the law
requires that the neutral and protective earth lines are NOT bonded, but
instead each has its own earth connection. The neutral at the
distribution point (pole pig), the protective earth right at the house.
Each of the two systems has advantages and disadvantages.
Either way, the MOV is fused, so if it does fail shorted... the
fuse opens up. Also I use a neon / led wired between output of
fuse..which is also the input to the MOV...and neutral. In normal
operation, the neon / led is illuminated. IF fuse opens up, the
neon / led goes out.... telling me the MOV is doa.
It would be better to use just the main fuse of the equipment, so that a
dead, shorted MOV blows the main fuse, instead of allowing the equipment
to keep powered and eating the overvoltage that blew the MOV!
If just the MOV is fused, the sequence is this: BIG surge starts - MOV
clamps the surge - MOV fails shorted - MOV fuse blows - now unclamped
surge goes on to equipment - equipment blows up. Instead when using just
the main fuse, the sequence is: BIG surge starts - MOV clamps the surge
- MOV fails shorted - main fuse blows - equipment loses power but
remains unharmed except for MOV and fuse.
MOVs are intentionally designed to fail in short circuit, TO PROTECT
EQUIPMENT, not to be disconnected by fuses, defeating that protective
function!
Manfred
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