Nate Nagel wrote:
Pete C. wrote:
Nate Nagel wrote:

I'm ASSuming that I already know the answer to this, but I'll try anyway
- assume a usual little surge protector with three MOVs, H-N, H-G, N-G.
Will it provide protection to a device connected directly by hard
wires to the receptacle that it's plugged into? I'm guessing that it
really doesn't care as to "upstream" or "downstream" only that the level
of protection depends somewhat on the distance from the protector. Also
if a surge destroys it it obviously will not disconnect a device not
"downstream" of it.

As Bill posted, the voltage at the outlet will be higher than at the
plug-in suppressor because of the significant impedance of wire to
surges. Minimizing the wire length in a service panel is very important
because currents are high. At your furnace currents will be much lower
because of the impedance of the branch circuit. If the surge current to
the suppressor was 500A and leads were 39" the voltage at the receptacle
would be about 325V higher than at the suppressor.


Reason I ask is, due to the fun and games I've been having with
repairing appliances due to a big surge about a week and a half ago, I
thought that adding surge protection to my furnace and air filter would
be a good idea. Problem is, now that I dig into it, the furnace is
hardwired to the back of a box on the side of the furnace. There is a
switch and a duplex receptacle in that box. The switch controls the
furnace and one half of the recep; the other is always hot. A
humidifier is plugged into the switched side, a condensate pump into the
unswitched side. I figured the best I could do, without adding some
kind of hardwired surge protection, was to plug it into the unswitched
recep for the condensate pump and it would provide the same protection
as if it were inline, with the caveat that if a surge destroyed the MOVs
in the surge protector, it is possible that it might zap the furnace
before the breaker tripped. Am I correct?

Suppressors are required (since 1998) to have a thermal disconnect that
disconnects failing MOVs. I believe the thermal disconnect is likely to
disconnect the MOV before any breaker. Second would probably be the
breaker on the plug-in suppressor. For the overvoltage you had, none of
these, or the panel breakers, are guaranteed to safely disconnect.


Are there any common, commercially available point of use surge
protectors designed to be mounted in, say, a 1900 box screwed to the
side of a piece of equipment? I am thinking that one at the dishwasher
might be advisable as well, as its only protection appears to be one H-N
MOV and we already established that that wasn't sufficient in at least
one instance

What you were hit with is temporary overvoltage, not a surge. It is not
easy to protect from overvoltage. But what you were hit with is quite
uncommon.

I would suggest reinstalling a service panel suppressor and letting that
protect the furnace and dishwasher (from surges, not major overvoltage).

It shouldn’t be hard to find a KO mounted suppressor. Should have a
ground wire in addition to hot and neutral.



The only relevance to upstream / downstream of a typical MOV type surge
suppresser would relate to a fuse or breaker that would be tripped when
the MOV(s) create their short circuit.
So, yes, a MOV containing suppresser would provide some protection to
other devices on the circuit, though in the event of a sustained over
voltage like you seem to have had from the transformer failure, if the
MOV trips it's own fuse before the fuse or breaker at the panel trips,
upstream loads would be unprotected.

Most of the suppressers intended for service entrance mounting could be
installed most anywhere, and many are designed to mount to a typical
1/2" knockout on any panel or box.

One of the main panel suppressers would in all probability clamped and
tripped the main breaker on your panel thus isolating your whole house
from the transformer fault.

Do you have any examples of a main panel suppressor like that? The only
ones I've seen either attach to or replace a double pole branch circuit
breaker not the main.

The closest ones would be the breaker type. Wired ones all probably
connect to a branch breaker. In either case it is a question whether the
thermal protection would disconnect before the main. I would bet on the
thermal disconnect.


The idea of using an add-on panel suppressor an excellent idea, although
possibly pricey. Is there anything to the average suppressor besides
three MOVs and a fuse?

Thermal disconnects that reacts to the heat of a failing MOV.

I certainly could handle fabbing something like
that myself, and there is a fairly decent electronics supply somewhat
nearby - not convenient, but not an insane drive either.

I made my own plug-in suppressor long ago. Later I decided it was not a
good idea to use because it does not have the thermal disconnect that
suppressors now have. If you make one, make it to be safe if a MOV
catastrophically fails.

--
bud--