On 2009-09-11, Twayne wrote:

Wayne Whitney wrote:

A balanced surge on the hot returning on the neutral will have no
effect on the GFCI. Only if it leaks to ground will it trip a GFCI.

More accurately, if the Hot and Neutral currents are not equal within a
tolerance range, a gfi will trip. It doesn't matter where the current
leaks to; it can be to other than ground. It's just that usually it will
be ground. So, it's "Only if it leaks current from one conductor more
than another, regardless of whether it's to ground or not.". Ground is
irrelevant to the operation of a GFCI.

You are quite right that the current can also leak to a conductor of
another circuit. And GFCIs will work on an ungrounded circuit and
increase safety. But I wouldn't go quite so far as to say the ground
is irrelevant; grounding still improves safety. For example, if an
ungrounded appliance on a GFCI circuit has a high resistance short
from hot to the case, the GFCI won't trip until you complete the
circuit from the case to ground or another circuit's conductor. While
if the appliance is grounded, the GFCI will trip as soon as current
through the short exceeds the 5ma threshhold.

Cheers, Wayne