Chris Lewis wrote:
According to Jeff Wisnia :

An inbalance in the current flowing in the black and white wires is not
*all* they detect Doug.


Actually, it is all they detect.


Most folks who understand that much about GFCIs don't know that nearly
all GFCIs will also detect a neutral to ground fault downstream from them.



How do they do that?


Because the current flowing in the black and white wires isn't balanced,
some of the current that should be going thru the neutral is going down
the ground wire. Even if _nothing_ is turned on in the circuit in question,
there'll often be enough current flow (due to slight differences in voltage
between neutral and ground) to trip it - an implicit ground loop.

Chris, my question was rhetorical, leading into the link I posted to
Goldwasser's page:

http://www.codecheck.com/gfci_principal.htm

If you read that link, you'll note that the GFCI actually induces a
small voltage onto both the hot and neutral leads with a *second*
toroidal transformer. Even with "nothing turned on", it's that voltage
which produces an inbalanced current through the principal ground fault
detection toroid if the neutral lead is shorted to ground "downstream"
of the GFCI.

Pretty clever I think.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"