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Posted by w_tom on July 11, 2005, 12:10 am
MOV data sheets provide no spec for becoming an open
circuit. The charts for MOVs relate three parameters: number
of transients, transient current, and time. All factors that
determine MOV *degradation*. Degradation means no open
circuit failure. Properly sized MOVs degrade - do not
vaporize - with use.
A datasheet from one Taiwan MOV manufacturer even defines a
number for degradation. A 10% change in the Vb voltage. They
provide examples of how an MOV can degrade by 10%. For the 18
series MOVs, a 200 amp (classic 8/20 usec) transient is
applied 10,000 times. No open circuit (vaporizing) condition
in these tests. 18 series MOV degrades after about 10,000
pulses. Degradation - not vaporization - not open circuit
failure - is how MOVs fail when properly sized. An MOV
becoming an open circuit (as Pop recommends) is a violation of
what MOV manufacturers intend.
In the late 1980s, PC Magazine published two articles about
power strip protector failures. MOVs were so undersized as to
vaporize - some actually spitting flames. When MOVs became
open circuits, then MOVs created a serious human safety risk.
A previous post provides numerous pictures of the fire
danger. Power strip protectors vaporizing MOVs to create
potential house fires.
Since those 1980s articles, the UL created a standard:
UL1449 2nd Edition. Urban myth promoters cite UL1449 as proof
that a protector is effective. But UL does not care whether a
protector protects anything. In fact, the protector can
completely fail during testing - and the protector still gets
a UL approval. Why? UL's only concern is that a protector
does not harm human life. UL does not care whether the
protector is effective. They worry about the MOV going open
circuit - vaporizing - endangering human life.
How is this UL rating obtained? MOVs are placed in series
with a tiny thermal fuse. Fuse that (should) blow before an
MOV vaporizes - so that human life is protected. IOW the
undersized protector disconnects even quicker - leaving
adjacent appliances connected longer to a destructive
transient. It blows a fuse so that the MOVs do not go open
circuit, do not create fires, protect even less, and get a
UL1449 approval.
Pop insists that vaporization is how protectors are suppose
to work. Who do we believe? Pop? Or do we believe the UL,
MOV manufacturer datasheets, the West Whiteland Fire
Department, government laboratories, and the reason for
thermal fuses?
Number of joules inside a protector determines it life
expectancy. To fail catastrophically, power strip protectors
are routinely undersized - too few joules. Therefore humans
who don't have technical knowledge will insist vaporization
(or blowing thermal fuse) is a normal failure mode, recommend
those ineffective protectors to friends, and buy more grossly
overpriced, undersized plug-in protectors.
An open circuit MOV even endangers human life. Best
solution for effective protection is a properly sized and
properly earthed 'whole house' protector. A protector that is
not located in dust balls, on a carpet, or on a desk full of
papers. A protector sufficiently sized so that it remains
functional after every surge. The important number here is
joules so that MOVs do not vaporize. Essential is a 'less
than 10 foot' connection to earth ground.
Pop wrote:
> "INSTALL JOULES"? Do you even know what a joule IS,
> or what an equivalency might be? You don't "install"
> joules.
> ...
>
> Sometimes. And sometimes everything works fine, but
> the MOVs have done their job and BECOME OPEN CIRCUITS,
> which will no longer have a knee voltage at which they
> begin to turn on at.
> That does NOT say they open the ckt; it says the
> MOVs become an open ckt. Learn to read if you're going
> to give advice. It would also help if you knew what
> you were talking about.
> ...
>
> So will knowledgeable and experienced electrical
> engineers and technicians and those with horizontal
> experience records. "Naive" appears to be a word you
> like, but not one that is descriptive in the context
> you're using it in.
> ...
>
> NOT if the surge protection clamped the surge down to
> usable levels. It's also possible after such an event,
> that the "protection" inside the computer (it's
> actually in the power supply and telephone connection
> ckts, by the way) could concievably be no longer in
> existance. The MOVs could easily have also done their
> job, and been blown before the "power strip" clamped.
> You'd have to know the knee voltages and the clamping
> times to make such a statement as you' ve tried to
> argue here.
> ...
>
> HOW was it undersized? Are you aware of the joule
> ratings used in most PC supplies? And those in the so
> called "power strips"? I am, and I've evaluated and
> repaired a LOT of them. THEN you have to go further
> and consider CMOS damage, whether it's lost ITS
> protection, and so on.
>
> ...
> You don't PUT JOULES INSIDE a protector!
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