[conspire] Fwd: Another motherboard was _not_ burned out today

[Rick Moen]

Quoting Ruben Safir (ruben at mrbrklyn.com):

> Well, I had a dozen of those dropped on me and I tossed them for lack of
> room.... sign

Ross sent me offlist some additional details about local line power, and
also some bits about MOVs (metal oxide varistors).  Some folks seem to
send lots of -- usually unintentional -- offlist comments, which is a
shame because it deprives the public thread of their participation.  In
some cases, it appears to be because they've not yet trained their
fingers to use the mailer's reply-all command by preference, and
reply-sender only when you want to specifically reach one person.

(No, I will not indulge the perversion of the Reply-To header still
indulged by some mailing lists whose listadmins ought to know better.)


Ross opined that an MOV is 'fairly fast'.  My experience is that they
just aren't fast enough to stop the spikes that matter.  This is less
the theoretical specs than the in-the-field disappointing behaviour of
real-world devices -- the very worst being the terrible and in fact
actively dangerous ones often built into consumer electronics and into
power strips.  

_Theoretically_, MOVs respond in the 40-60 ns range.  In practice, they
just don't, and the spike goes through and fries equipment.

Ross says you actually need 'a lower level MOV and a higher voltage
MOV'.  Well, that'd be for starters.  One of the several reasons why
typical MOV devices suck is that often only hot-to-neutral gets a
varistor, while the other two (hot-ground, neutral-ground) don't, which
as my old boss used to say is 'almost useful'.

But there is much worse:  There were so many cases of sustained
overvoltage causing MOVs to go into thermal runaway and literally catch
fire that the National Fire Protection Association issued a series of
warnings about them, and Underwriters Labs created the safety UL1449
spec (3rd edition revision released in 2009) that MOV devices are now
_supposed_ to meet but very often don't bother to meet -- on account of
price pressure and buyers being obvious and not caring.

And, as Ross mentioned offlist, once an MOV has been stressed by,
y'know, being exposed to out-of-range voltage, it is now invisibly
damaged and needs to be replaced -- except, gosh, unless it's no longer
passing current at all, you probably have absolutely no idea it's
damaged.

In short, I'm staying far away from MOVs, especially the ubiquitous
low-end devices.