Modern GFCIs fail safe.

That's true. AND the protect against many wirkng errors.



They open when deprived of power and will
therefore open when the neutral fails upstream.

MAYBE some GFCIs do open when there is no power but that certainly isn't
true of most of those out there.

Think about it: if it opens when the power goes away then it must re-close
where the power is restored (or folks would be running about resetting all
the GFCIs in the house after a 1 second power outage.)

I don't think the contacts are rated to "switch" the load every time there
is a power glitch.

In the "interest of science" I arranged an experiment of a "out of the box"
GFCI that "Meets All New UL943 Safety Standards" to include: Ground Fault
(duh!); Correct line/load wiring; and GFCI Electronics Integrity.

As received it was "tripped" so there wasn't any continuity between line and
load.

Using a cord set salvaged from an old and forgotten lamp, I connect 120
volts to the "line" side. I hit the reset button and it "clicked."

When I removed the power there was still continuity between corresponding
"line" and "load" wires.

One interesting thing was when I hit the "test" button with the power off,
it "clicked" and broke the connection between "line" and "load."

The GFCI does protect against a LOT of stuff. BUT it doesn't protect
against a neutral that is connected AFTER the unit is reset.

How could ths happen?

It might happen if the "romex" neutral is accidentally cut by a missplaced
nail. The nail might even maintain the circuit for a time before
something shifts and the neutral is open.

Most likely, however, is that someone wiring another circuit accidentally
forgets to connect a white wire. He might even even mistake a neutral for
a "switch loop" white wire.

Or, someone might have been careless and placed two white wires under the
same screw in the CB box (happens all the time.) That's a "no-no."
Worse than putting two grounds on the same screw (which wasn't a 'no-no' but
it might be now.) Down stream someone else replacing or re-arranging a
circuit will end up taking off another neutral in the addition to the one
associated with his target.



The newer ones can also
detect a neutral to ground fault without a load on the circuit by
injecting a detecting current onto the neutral.

Quite true. That particular feature has been around for quite a time.

--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison