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

Benjamin Scott dragonhawk at iname.com
Fri Jun 17 10:00:01 EDT 2005


On Jun 16 at 11:47am, Bill Sconce wrote:
> Ben didn't mention that shunt TVSS isn't the only way.

   Actually, I did, it was just buried in the paragraph about shunt TVSS.  I 
don't understand much of the technology involved in series mode protectors, so 
all I can do is describe it in very general terms, which only takes a few 
words.

> Note also that 330v is a LOT - 3 TIMES what a device sees in normal service. 
> A good suppressor can be designed to clamp at a 10v rise (or at 2v) if 
> you're willing to pay for it.

   Actually, no.  Remember, we're talking alternating current.  In the US, AC 
goes from zero to (IIRC) 180 volts (absolute) twice per cycle, or 120 times 
per second.  In other words, AC is *constantly surging*.  Simply clamping at 
any voltage increase isn't possible, because AC does this all the time.  I've 
read a technical justification for the 330 V figure but didn't understand it.

   Now, I would expect it is possible to build a device that clamps if the 
voltage rise is too fast or out-of-phase, but I'm really just guessing here.

   One of the things to consider here is that wall current is not precision 
signal quality and never has been.  Any device expecting to perform precision 
electrical signal work is going to have extensive filtering built into it. 
It flat out wouldn't work otherwise.  So the job of a TVSS should not be to 
attempt to make the power "perfect", but to keep *damaging* surges from 
getting through.

   For example, on the www.brickwall.com site you linked to, they post some 
oscilloscope traces.

   Now, since they don't post scales, I'm already a little weary.  A dishonest 
company might use different scales for different measurements to make their 
product appear better.  But they seem like an honest company, and you're 
obviously satisfied with them, so I'll give them the benefit of the doubt, and 
assume all the scales are the same.  But we still don't know what the scales 
*are*.  In other words, just how big are those surges they are graphing?

   I can deduce the time scale.  I know some of those products they list are UL 
Listed to clamp in less then one nanosecond.  So they must be using a scale of 
no larger then about 500 picoseconds per vertical grid line.  That's damn 
small.  Remember, US AC is 60 Hz.  A single AC cycle is about 16.5 billion 
picoseconds long, if my math is right.  Is a fluctuation during 0.000003% of 
the cycle really something to worry about?

   I'm sure if you spend $200 on a really elaborate device from a quality 
manufacturer, you're getting something for the money.  The question is, does it 
really matter?  Are you paying for protection that simply does nothing, due to 
the fundamental nature of AC?

   Given the choice, I suspect it would be better to put (less) money towards a 
quality voltage regulator, which not only protects against surges, but sags as 
well.  I know from personal experience that the brief under-voltage conditions 
often encountered in a factory (when big machinery kicks in) are far more 
problematic to a lot of equipment then surges are.  I know this because the 
test equipment kept malfunctioning when the 100 amp electric motor on the big 
mill kicked in -- until we put a voltage regulator on the TE.

> MOVs are junk.

   It does seem to be the case that simply slapping a MOV or six between the 
line and grounded conductors is not a good way to build a TVSS.  That being 
said, I've seen tech stuff that alleges that MOVs can be designed for the 
application, and that coupled with other components, will do a quality job of 
shunting over-voltages.  As I keep saying, I don't understand the EE theory 
enough to make a judgment either way.

   For example, here's one analysis I've found that goes into all sorts of math 
about this kind of thing.  Alas, it's way over my head.  Link:

   http://www.iaei.org/magazine/04_b/04_b_brown.htm

   Now, to me, the big question is, "Just how bad is shunting power to the 
grounded (neutral) and/or grounded (safety ground) lines?"  It doesn't matter 
if you have really great MOVs (or gas tubes for that matter) if the whole 
design concept is flawed from the start.

   I've seen lots of vague comments about how bad this is.  In effect, "The 
surge is just going to go from line to ground, in through the ground to the 
equipment, and cause Bad Things To Happen!"  While that seems like a 
reasonable concept to me, what little I know about electrical engineering 
includes the knowledge that "common sense" is often misleading.  I'd like to 
some unbiased (i.e., not trying to sell me something), technical analysis on 
this.

   As I said, surprisingly complicated.  :-)

-- 
Ben <dragonhawk at iname.com>



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