UPS/SPS testing re: AC protective ground

[Gil Kloepfer Jr.]

In article <1349 at westmark.WESTMARK.COM> dave at westmark.WESTMARK.COM
(Dave Levenson) writes (in response to my article):
>Did you get this shock immediately as the plug came out of the wall
>socket, or some time later?

I don't remember...

>I just unplugged my Tripp-Lite BC450 from the wall.  (I'm posting
>this article on backup-power!)  I find that I can touch the plug and
>feel nothing.  A volt-meter across the power pins on this plug reads
>zero.

Just to be a little more specific about the posting, after reading this
one, I put a meter on the plug of the Tripp-Lite with my system on
backup power.  The (AC) voltage across the hot and neutral are about
1.9 volts (nothing really terrible) *BUT* the voltage from the hot to
ground and neutral to ground are around 60 volts (each!).  That would
definitely be enough to give me the tingle I felt when I grabbed the
plug!

>Could your Tripp-Lite be a different, or older, or defective
>model?

Let's compare notes, okay--  I have a Tripp-Lite BC-450-b rev 790.  I
just bought it about 3 weeks ago, so my feeling is that it should be
pretty well along in the testing stage from Tripp-Lite.

I can think of one additional failure mode not mentioned -- I'm running
the following devices from this supply:

	AT&T 3B1 computer
	Telebit Trailblazer-Plus
	AT&T UNIX-pc expansion box
	Tape drive on a HD case/supply (turned-off)

Could one of these devices be feeding garbage back through the ground?
How does the Tripp-Lite UPS handle "ground"?  I betcha that one of the
devices could conceivably be creating an electrical path from neutral
to ground, as I have been bitten by that once already.

This is an interesting discussion - please take a look at your supplies
with a meter and post your findings.  I'd like to fix a problem before
it fixes me!

Gil.
-- 
Gil Kloepfer, Jr.              gil at limbic.ssdl.com   ...!ames!limbic!gil 
Southwest Systems Development Labs (Div of ICUS)   Houston, Texas