Re: MOVs and surge suppressors
- From: ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
- Date: Tue, 28 Aug 2007 02:13:34 GMT
bud-- wrote:
w_tom wrote:
On Aug 22, 10:38 am, "griz...@xxxxxxxxx" <griz...@xxxxxxxxx> wrote:
...
I thought the only way to test a MOV was to send a surge
through it, and then you will only know what condition it was in
before you sent in the surge. What is the circuit diagram for one of
these indicator LEDs, and is this a way to test any MOV or MOV device?
As others have noted, an MOV is 'protected' by a thermal fuse. If a
surge is so large as to cause MOV to vaporize, then a major human
safety threat exist (see scary pictures). A thermal fuse is placed in
series with MOVs in a desperate hope to disconnect an MOV before it
vaporizes Vaporizing MOV is a complete violation of MOV manufacturer
specs AND a human safety threat.
Vaporizing is a scare tactic.
MOVs have an energy (Joule) rating. They do not protect by absorbing surge energy, but in the process of protecting they absorb enengy.
Yes.
1) The mechanism by which they change from high resistance to
low resistance requires absorbing energy. 2) Staying in the low resistance mode requires energy absorbtion. 3) Any energy they
absorb cannot reach the device(s) they are protecting.
Absorbing surge energy is the only way an MOV can work.
It cannot provide protection without absorbing surge energy.
That does not mean it absorbs the entire surge energy.
Where does the surge energy go? Some is absorbed and
dissipated in the source path, some in the MOV and some
in the return path.
This is a point w_tom has missed in the past when he insists
that point of use MOV's don't absorb surge energy. They most
assuredly do. If they did not absorb, they would not switch to
low resistance. When they do switch, they absorb I^2R, per ohms
law. They clamp the voltage that the device "sees" to some level
by absorbing energy. They do not absorb the entire energy that
the surge contains - just the amount of energy they "see"
that falls into their operating specs.
When they have absorbed an energy equal to their rating, they will conduct at successively lower voltages, eventually conducting at 'normal' voltages and overheating. UL has, since 1998, required disconnects for overheating MOVs. Plug-in suppressors have their current limited by the significant impedance of the branch circuit.
So why is that appliance working while the protector failed? Surge
was too small to overwhelm protection inside the appliance. But MOV
protector was so grossly undersized as to be permanently destroyed.
For w_, all plug-in surge suppressors are "grossly undersized". In fact suppressors with very high ratings are readily available at rather low cost. And apparently a surge that can destroy a MOV won't damage protection inside an appliance? Hallucination.
By undersizing it, a plug-in protector manufacturer gets the naive
to recommend a grossly undersized protector. Effective protectors
earth surges AND remain functional - do not blow the fuse. Any
properly sized protector remains functional after a surge. So that
'failed' protector light says what about the protector? Grossly
undersized?
"Grossly undersized" red herring again.
Another problem when that fuse does not disconnect fast enough:
http://www.hanford.gov/rl/?page=556&parent=554
The hanford link describes overheating as being a problem with "some older model" power strips and says overheating was fixed with a revision to UL1449 that requires thermal disconnects. Overheating was fixed, for UL listed suppressors, in 1998.
IOW many plug-in protectors will fail even on smaller surges to
avoid those scary pictures. Failure also promotes sales among the
naive.
Competently manufactured suppressors engineer the fuses/thermal disconnects to open only when the MOVs fail. (They fail by conducting at too low a voltage and overheating.) w_, apparently, buys only cheap no-brand Chinese suppressors.
How to test an MOV? Apply a 1 ma current source to it and measure
its voltage.
I agree this is the way to test a MOV.
So again, if a surge was so large as to trip that indicator lamp,
then the protector was grossly undersized - completely ineffective.
The "grossly undersized" red herring again. Grossly undersized applies equally to service panel suppressors, which will also be disconnected if their ratings are exceeded.
w_ believes that plug-in suppressors to not work. Instead of using technical arguments, he doesn't have any, he is using scare tactics.
For accurate information on surges and surge protection read:
http://omegaps.com/Lightning%20Guide_FINALpublishedversion_May051.pdf
- "How to protect your house and its contents from lightning: IEEE guide for surge protection of equipment connected to AC power and communication circuits" published by the IEEE in 2005 (the IEEE is the dominant organization of electrical and electronic engineers in the US).
And also:
http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf
- "NIST recommended practice guide: Surges Happen!: how to protect the appliances in your home" published by the US National Institute of Standards and Technology in 2001
The IEEE guide is aimed at those with some technical background. The NIST guide is aimed at the unwashed masses.
The author of the NIST guide, who was the surge guru at the NIST, has said "in fact, the major cause of TVSS [surge suppressor] failures is a temporary overvoltage, rather than an unusually large surge."
--
bud--
Nicely stated.
Ed
.
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