UPS/SPS testing re: AC protective ground

[Stephen Friedl]

In article <35147 at cup.portal.com>, thad at cup.portal.com (Thad P Floryan) writes:
> A few weeks ago a discussion concerning UPS and SPS (Uninterruptible and
> Standby, respectively, Power Systems) appeared in this newsgroup.
> 
> I commented that one should NOT pull the wall plug (of the UPS) to test it,
> and was asked "why?".  I commented "because you lose the protective ground."

Thad's comments on safety are well taken, but there are other reasons
why the pull-the-plug test is not so hot: it's not a fair test and it
may lie to you because pulling the plug is not the typical way that
power goes out.  

If you just pull the plug, the system just sees an open circuit
on the input, and it has the normal notification of the power
loss.  If the power goes out for real, however, the entire
building is connected to the power cord of the supply,
effectively providing a dead short.  For some makes of UPS (Safe,
Datashield, Tripplite, and Topaz, for instance), the backup
supply is connected to this dead short for a brief moment before
the disconnect switch kicks in to break the connection with the
outside world.  This can introduce an additional delay of what is
claimed to be from 20 to 50%.

Other brands use different switching mechanisms such that the
pull-plug test will give an accurate representation of how the
whole thing will work.

Disclaimer: I got this information from a technical manual
provided by American Power Conversion, a vendor of UPS products,
so they are not unbiased.  I like their products, their technical
documentation is superb, but they could be faking these
photographcs of scope traces.

For what it's worth, though, my experience has been that
Datashield has a BIG difference between the two modes of power
loss -- they're pretty miserable.

     Steve

-- 
Stephen J. Friedl, KA8CMY / I speak for me only / Tustin, CA / 3B2-kind-of-guy
+1 714 544 6561  / friedl at mtndew.Tustin.CA.US  / {uunet,attmail}!mtndew!friedl

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