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

L.B. MCCULLEY bmcculley at rcn.com
Wed Jun 15 23:40:01 EDT 2005


Ben, 

thanks for the most informative response in this thread to date (and there have
been some other good ones!).

I really appreciate your sharing the fruits of your research.  The explanation
about UL 1449 FUD was very useful!  And the explanation of direct lightening
strike consequences was, well, interesting.

This is the sort of post that really makes the list worthwhile!

THANKS again!

-Bruce

---- Original message ----
>Date: Wed, 15 Jun 2005 23:18:28 -0400 (EDT)
>From: Benjamin Scott <dragonhawk at iname.com> 
>Subject: Re: Is it okay to plug a power-strip into a UPS?  
>To: Greater NH Linux User Group <gnhlug-discuss at mail.gnhlug.org>
>
>
>   This is a surprisingly complicated topic.
>
>Power Taps
>----------
>
>   First, the easy one:  The technical term for the "octopus" that Bill Sconce 
>was describing is "relocatable power tap" or "RPT".  This just takes one plug 
>and turns it into multiple plugs.  No protection of any kind is implied. 
>Many "power strips" are nothing more then an RPT.  The presence of an on/off 
>switch, over-current protector ("circuit breaker"), and/or indicator light 
>does not guarantee any additional protection, either.
>
>   As everyone else has been saying, plugging a plain RPT into a UPS is fine, 
>as long as you don't cause an over-current (over-load) condition.
>
>   Beyond that, things get interesting.  I've done a bit of research on this 
>topic.  I am somewhat hampered by the fact that I'm not an EE and don't 
>understand the truly technical stuff about electrical theory.  Here's what I 
>know...
>
>Power Protection
>----------------
>
>   One of the few things I can say for sure is that most of the information 
>written for people without an EE background is usually trying to sell 
>something.  It often boils down to "Everybody else's technology is crap, but 
>we use $TECHNOLOGY which will solve all your power problems".
>
>   TVSS = Transient Voltage Surge Suppressor.  This is what most people are 
>talking about when they say "surge protector".  One of the most common types 
>of power threats is a brief but large over-voltage -- spikes and surges.  (A 
>spike is basically just a shorter-duration surge.)  TVSSes protect against 
>these.
>
>TVSS Specifications
>-------------------
>
>   The UL standard for TVSSes is UL 1449.  Note that exactly how useful this 
>standard is is open to debate.  UL 1449 is mainly concerned with clamping 
>voltage, which is the highest voltage that the TVSS will let though before it 
>kicks in and "clamps" the power.  So lower is better.  The best (in terms of 
>UL 1449) is 330V; almost any TVSS will meet this requirement.
>
>http://www.ul.com/consumers/surge.html
>
>   Other specs one sees are the response time and the energy absorption. 
>Response time is how quick the TVSS can clamp an over-voltage.  Pretty much 
>everything is <1 nanosecond, making this part uninteresting.  Energy 
>absorption tells you the largest power transient the TVSS can handle before it 
>fails.  Measured in joules, with larger numbers generally being better. 
>However, I've also seen literature stating that joule ratings alone are 
>misleading.  See above about selling stuff; I'm not sure who to believe at 
>this point.
>
>TVSS Mechanisms
>---------------
>
>   Next we can talk about TVSS mechanisms -- how they actually do the job.
>
>   It's a fundamental law that you cannot make the excess voltage that is a 
>surge just disappear.  You have to do something with it.  There are two 
>approaches to this.  One is to shunt the excess into the grounding conductor 
>("third prong").  In theory, this is fine, since all US power ends up grounded 
>to earth eventually anyway.  However, dumping all this excess power into your 
>grounding line might (or will, depending on who you ask) cause other problems. 
>The other approach used is to store up the excess energy, and then gradually 
>let it back out over time.
>
>   When it comes to the shunt method, there are multiple mechanisms for that as 
>well.
>
>   The most common type, as everyone has been pointing out, is the MOV 
>(metal-oxide varistor).  As used in a TVSS, MOVs are basically a switch which 
>triggers on voltage.  Once the voltage goes over the clamping point, the 
>switch closes ("turns on"), dumping the surge into the grounding conductor.
>
>   It is true that some kinds of MOVs will burn up if subjected to a prolonged 
>over-voltage.  However, it is possible to design it so this doesn't happen, 
>and UL 1449 requires this, so this is a kind of FUD.  It is also true that 
>MOVs wear out.  It is also true that *everything* wears out, given enough 
>time.  Just how much of a threat "MOV death" is again depends on who you ask.
>
>   Another method used to shunt over-voltages is the gas tube.  It does the 
>same thing as a MOV -- diverts excess power to the grounding conductor -- but 
>via a different mechanism.  They use a gas which is normally non-conductive. 
>During a surge, the gas ionizes and becomes conductive, closing the shunt.
>
>   There are other mechanisms as well.  All of these, shunt and store, MOV and 
>gas-tube, foo and bar, apparently have their advantages and disadvantages, and 
>their advocates and detractors.  I think the best advice I've seen suggests 
>using multiple mechanisms together.
>
>Other Protection
>----------------
>
>   All of that is just about TVSS -- protecting against over-voltages.  Beyond 
>that, you have noise filtering, voltage regulation, isolation transformers, 
>and other neat stuff.  You can worry about back-feed on grounded (neutral) and 
>grounding (third prong) conductors.  You can worry about data line surge 
>protection.  You can get a UPS for battery backup.
>
>   Some TVSSes are designed to fail safely -- they disconnect the load if they 
>are are overwhelmed or have an internal fault.
>
>   There is also non-technical protection.  Look at warranty, for example. 
>I've got a Tripp-Lite IsoBar that has a lifetime warranty.  If it ever stops 
>working, for any reason, Tripp-Lite will replace it.  They've done so once in 
>ten years, so far.  A lot of power protection equipment also provides 
>reimbursement if the device fails and your equipment suffers resulting damage.
>
>Lightening
>----------
>
>   Finally, let me state that just about nothing will protect against a 
>lightening hit.  I've the results of a direct building strike, and it ain't 
>pretty.  You worried about your MOVs?  This hit caused light switches, 
>outlets, light fixtures, various wiring, several TVSSes, miscellaneous 
>equipment, and a mains transformer to burn up and/or blow apart.  It fried all 
>the network hubs and several NICs.  It killed a couple of air conditioning 
>units.  It really, really sucked.  All the normal rules go right out the 
>window when you're dealing with lightening.  We're talking hundreds of 
>millions of volts.  At that kind of potential, *EVERYTHING* is a conductor.
>
>The Original Question
>---------------------
>
>   So, is it safe to plug a TVSS into a UPS?  The only sure answer I can give 
>is "I don't know".  Given that it's trivial to get an RPT without TVSS, I 
>would say you're safest going that route.
>
>-- 
>Ben <dragonhawk at iname.com>
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