Goedjn wrote:
On Wed, 29 Nov 2006 16:16:40 -0500, Jeff Wisnia
wrote:


Pop` wrote:

snipped


GFI, Ground Fault Interruptor is a misnomer. "Ground" in the original case
may have been ground, but now, because of poor terminology, is considered
the Neutral conductor. How a GFCI operates has nothing to do with the
ground. Since it senses differences in current between the hot/neutral
conductors, any third wire earth ground is irrelevant to its opeartion. Hot
amps = Neutral amps, all OK. Not equal, it trips. It's literally that
simple.

Pop`



And, don't forget that the more recent GFCI's will detect a leakage path
between the neutral and ground as well, and thus disconnect power when
there's a neutral to ground short.

Jeff


Ok, I thought I knew how GFCI outlets worked, and now I find out
that I didn't know how the test button worked. Fine, I'm ok with
that, but this doesn't make any sense. It seems as if
a nuetral-ground leak should look exactly like a hot-ground
leak, except probably smaller, as there's less resistance downstream
of the correct current path.

How would you tell the difference even if you wanted to?

--Goedjn

I didn't SAY that they tell the "difference". Since their only "output
means" is to open the circuit between the supply and load side of the
breaker for either kind of fault, hot to ground leakage or neutral to
ground shorts.

They will detect and "pop" if you make a connection between their
neutral output and ground, even with nothing at all connected to their
hot output.

Before you worry too much about how they can sense a connection between
two leads which should be at the same potential (neutral and ground) if
there isn't any current flowing in either of them, I'll tell you that
there will be, because the GFCI contains a current transformer which
adds a small in-phase voltage to both the hot and neutral output leads.

That voltage on the neutral lead will cause a small current flow if the
neutral gets connected to ground. The normal differential current
sensing portion of the GFCI responds to the current flowing in the
neutral which has not come from the hot output and "pops" the GFCI.

Trust me it works... Or test it yourself with a neutral to ground short.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.98*10^14 fathoms per fortnight.