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Bud--

wrote:

Ferrite rings are for EMI, so the buzz in the box stays there and not
in your radio or stereo. Switching power supplies can generate a
whole lot of hash and that's the type that's gradually replacing the
old-syle wall warts. The ring is acting as a choke for RF, also
generated by the computer itself. Different deal than surge
protection, but also needed these days.

Look up "surgistor" or MOV, that's what's in those surge protectors.
They're rated in joules, the amount of energy they can pass. The
higher, the better, and more costly they get. The better surge
protector strips will say how much energy they can handle on the
package. I assume the panel versions do the same.

There is (in the US) apparently no definition for how joules are
measured. As a result, some manufacturers measure jules in a deceptive
manner, which puts honest manufacturers at a disadvantage. As a result,
some good manufacturers are not providing joule ratings anymore. Instead
they give surge current ratings (which are comparable).

The IEEE guide warns against comparing devices based on joule ratings
unless the test method is the same. Too bad - I like joule ratings.

What none of the
ad copy says is that MOVs have a distinct lifespan. They WILL wear
out after snubbing "x" number of spikes and become useless.

Certainly can happen.

In my first post I quoted an investigation by Martzloff that found only
about 35J max at a plug-in suppressor with even the strongest surge that
can be reasonably expected on power wires. One reason is arc-over at the
service panel. The other is the impedance of branch circuit wiring. Both
are mentioned in other posts. If the branch circuit is very short -
outlet under the panel, the energy can be higher.

I recently bought a major brand plug-in suppressor (about $30) with
ratings of 590J and 30,000A per MOV, 1770J and 90,000A total. I don't
expect the suppressor will ever fail. The manufacturer apparently
doesn't either - there is a connected equipment warranty.

The current ratings, 30,000A, are in some ways odd - there is no way you
could get that current to the suppressor. It is higher than what will
come in on the service. But the high current ratings go along with the
high joule ratings.

The joule rating for a MOV is the single event energy hit the MOV can
take and be at the defined end of life (but still functional). If you
look at MOV ratings curves, you find that if the individual energy hits
are a small fraction of the rated energy, the cumulative energy rating
is far higher than the single event rating. If my 590J MOVs only see 35J
or far less events, the cumulative energy rating will be far over 590J -
another reason for connected equipment warranties.

High ratings are useful in the same way for service panel suppressors.

Some of
the power strip units tie the neon switch light to the MOVs. If no
light when switched on, the MOVs have expired and it's time for a new
strip. But nobody tells the consumer about it. So there's a whole
lot of dead protectors out there that are just power strips now.
Usually there's MOVs between ground and each supply wire and between
the supply wires. Not rocket science.

The normal failure mode for MOVs is to start to conduct at lower
voltages until they conduct at normal voltages and have thermal runaway.
Any suppressor you buy in the US should be listed under the appropriate
UL listing - 1449. UL1449 has required, since 1998, a thermal disconnect
for overheating MOVs.

For plug-in suppressors, the IEEE guide goes on at length about how the
protected load can be connected across the MOVs, and be disconnected
when they fail, or can be connected across the incoming line. In the
former case, the load is 'protected' even if the MOV fails. Another
reason why manufacturers can have protected equipment warranties.

As far as lightning protection, they'll do part of that, up to the
energy rating. Which is why you need the tiered approach. Arrestors
on the line in, surge protectors on the panel and on each high-value
electronic item.

A good idea, particularly in high risk areas. It is suggested by
Martzloff in another post.
Plug-in suppressors are particularly useful if the protected equipment
has both power and phone/cable connections - equipment that is
particularly at risk.

Note that all interconnected equipment has to be connected to the same
plug-in suppressor and external wires (including phone and cable) have
to go through the suppressor. I think this is not as well understood as
it should be.

The max probable current on hot service wires is 10,000A. Service panel
suppressors with much higher ratings are readily available.

My sister is always getting hits, they blow the
phones off the walls, but since she's gotten decent surge protectors,
those get fried instead of the computer or video equipment. They have
to be replaced, but she gets the sort with insurance attached, so not
that costly.

Phones off the wall sounds odd. I would look at the phone entrance
protector and length of ground wires from phone and cable entrance
protectors to the ground at the power service. The entrance protector
and earthing should protect the phone (using US installation practice).
And look at earthing system?

There are other approaches to surge and spike protection, an MG set is
pretty much immune to any such up to direct lightning strikes. A
ferro-resonant transformer used to be a big part of the innards of one
line of power conditioners, pretty much immune to spikes, but the
transformer itself was noisier than a whole switch yard. Had one in a
corner of the shop for a mini-computer, had to go outside to talk to
anyone. None of those will snub spikes on LAN, phone or video cables,
for that you have to go to power strips with built-in protection or
stand-alones.

All good information.
Never been around a ferro-resonant transformer - interesting.

--
bud--