On Feb 18, 10:56 am, Nate Nagel wrote:
Heh... all right, I feel dumb now.

When I started to replace the receptacle, I plugged the UPS and the
associated extension cord into a receptacle in the hallway which was on
a different circuit.
...
Despite the fact that I thought I'd unplugged every appliance upstairs,
....a printer plugged in to a wall receptacle. It, of course, was
connected (by a USB cable) to the computer that was connected to
the UPS. ... I was just explaining yesterday why it was important to
connect all peripherals of a computer to either the same power strip
or the same UPS so there's no issues with floating grounds etc. and
voltage imbalances that end up going through USB or other ports. ...

I dug out another extension cord ... and plugged the printer into the
back of the UPS on one of the "surge only" receptacles. All appears
to be good now.

I'm still not sure why the breaker tripped when I simply tried to move
the UPS' power feed from one receptacle to another, unless the fact that
the UPS now had a proper ground while the printer still had a bootlegged
ground was enough to cause a problem.

I ASSume that this little exercise does indeed confirm that the Siemens
AFCI is also a GFCI...

Yes the AFCI is a GFCI (plus). What the USB printer is plugged into
should make no difference. Implied is that something is leaking via
the printer. Printer on another circuit is not desireable only
because backup protection is missing. But if working properly,
powering from both circuits should not have contributed to AGFI
tripping.

Meanwhile, temporarily disconnect neutral wire and safety ground
wire of the AGFI circuit in mains box. Infinite ohms should exist
between those two wires. Also infinite ohms should exist between that
disconnected neutral and mains box. All tests best performed with AC
electric off so that leakage and other problems do not distort that
ohm meter reading nor harm meter.

Disconnected safety ground might or might not conduct to mains box -
that is acceptable. But disconnected neutral must not measure
conductivity to disconnected safety ground nor to mains box (test
obviously done with appliances unplugged).

One final test. Ohm meter test (use extension cord to measure this)
should measure conductivity from receptacle safety ground prong to
disconnected safety ground wire in mains box. If this conductivity
does not exist, then that test explains everything seen previously and
suggests problem still exists.

Testing appliances (computer and printer) on another GFCI circuit,
and tested with computer and printer split between GFCI and non-GFCI
circuits should still be performed as posted earlier. Information
from that test is necessary - and would provide more useful
information for many reasons including because GFCI trips at only 5
ma.

AGFIs have been required for years now in New Jersey on bedroom
circuits.