Is it okay to plug a power-strip into a UPS?

Benjamin Scott dragonhawk at iname.com
Sat Jun 18 18:56:01 EDT 2005


On Jun 17 at 10:54pm, Bill Sconce wrote:
> I wouldn't say "constantly surging", though, just "alternating".  A "surge" 
> is an excursion beyond the normal alternating limits.

   I used the term "constantly surging" for effect.  Those of us who spend most 
of our time in the digital world like to think of nice, neat DC power, where 
any change in voltage is a big deal.  AC is a different beast, and it's easy 
to forget that.

> And good UPSs can do things in addition to voltage clamping, such as 
> cleaning up the waveform (related to your "rise too fast".)

   It's also worth pointing out that a UPS -- a battery back-up unit -- doesn't 
necessarily give you anything else.  The simplest UPS does only one thing: 
Switch over to battery power in the event of a problem.  Things like voltage 
regulation, noise filtering, even basic TVSS are all something else.  Of 
course, most decent UPSes include at least some of those, but it's something 
to be aware of.

>> Is a fluctuation during 0.000003% of the cycle really something to worry 
>> about?
>
> Oh YES.
>
> Consider a lightning strike.

   Okay, you're right, that is flawed (or, at the very least, woefully 
incomplete) thinking.  It stemmed from the fact that I thought I could deduce 
the horizontal scale but didn't know the vertical.  Now, though, looking at it 
again, the table provides the missing data.  For example, the Tripp-Lite 
Isobar let through 290V before clamping.

   Your figure is 165V for the nominal maximum of the AC wave.  That means, as 
part of normal function, AC goes from zero to 165 volts absolute 120 times per 
second.  290V would be 125V above the nominal maximum.  That's 75%.

   So, let me restate my question: Is a fluctuation of 75% above nominal 
maximum voltage, for a duration of 0.000003% of a cycle, really something to 
worry about?

>> The question is, does it really matter?  Are you paying for protection that 
>> simply does nothing, due to the fundamental nature of AC?
>
> I agree that it may NOT really matter.  Not because of the fundamental 
> nature of AC -- good, clean AC is fine.

   The point of "the fundamental nature of AC" is that AC is already cycling 
between 0 and 165 volts 120 times per second.  Anything using AC is already 
designed around that.  Is 125 extra volts for so short a time within the 
tolerances already built in to the equipment?  Everything I've read says it 
is.  Which makes paying for additionally "close" clamping voltage a waste of 
money.

   As I said, I'm a lot more concerned about the effects of shunt-type TVSSes 
"dumping" the surge into the neutral or safety-ground lines.  I haven't seen 
any unbiased information on that.

> It's MOVs which are junk.

   You keep saying that, but haven't specified your objection.  Are you 
objecting to the use of parts which will "wear out" sooner?  If so, what about 
other shunt-mode TVSS mechanisms (e.g., gas tube)?  Or are you objecting to 
shunt-mode surge suppression in general?

-- 
Ben <dragonhawk at iname.com>



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