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

[Ross Bernheim]

Rick,

I forgot to mention that in addition to putting MOV’s on all three lines, they 
should be placed after a heavy duty multi-stage low pass filter.

Ross


> On Feb 16, 2015, at 6:34 PM, Rick Moen <rick at linuxmafia.com> wrote:
> 
> 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.
> 
> 
> _______________________________________________
> conspire mailing list
> conspire at linuxmafia.com
> http://linuxmafia.com/mailman/listinfo/conspire