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Maynard A. Philbrook Jr.

In article , To-Email-Use-
says...

Odd Breaker Situation...

Changing out the cheapy light fixtures that came with the house, I
turn the switch off, remove the old fixture, accidentally bump common
(white) against frame (green) and breaker popped.

Measure white with DMM... AC-level is a few hundred mV (noise?).

Measure black with DMM... AC-level is even smaller.

Likewise black to white.

Touch black to green... nothing.

Touch white to green... pop.

Do these new breakers have the equivalent of GFCI built-in?

...Jim Thompson

Yes, you most likely have GFCI breakers.

joining ground and N at that location should cause a trip, and it seems
it's working..
that should work even if the Black wire is dead, due to your light
switch being off because the other wires N are still connected to the
system.
Appliances are suppose to balance the N line or use a transformer, this
method will detect a N hitting ground directly when it's not suppose at
the user end.

The breaker still has both L1 and L2 alive in it so when N hits ground
you have a imbalance.

Sounds like it's working fine.

Jamie