PeterD wrote:
On Fri, 27 Nov 2009 17:40:49 -0700, D Yuniskis
wrote:

PeterD wrote:
On Mon, 23 Nov 2009 18:38:47 -0700, D Yuniskis
wrote:


This is how GFCI breakers work -- they watch for current "leaking"
off to ground someplace other than in the "return" conductor.
No they don't. They look for imbalanced current flow between the two
conductors.
Gee, isn't that what I *said*?

No, you didn't.

How do you get an imbalance if
current isn't *leaking* off to ground someplace other than
in the "return" conductor?

Re-read your post.

I don't have to re-read it -- I *wrote* it!

A GFCI is nothing more than a tiny transformer with
"sense electronics" as its "load". The primary to the
transformer is the circuit being monitored. *BOTH*
conductors pass through the transformer.

Since an alternating current generates a magnetic field,
that field couples *through* the transformer to the
*secondary* of the transformer -- which is the "sense
electronics". The field generated by the primary is
a function of the *net* current flowing through the
primary "winding" (winding can often be confusing in this
context as it is usually just a "single turn" -- as such,
it doesn't even go completely *around* the transformer's
core!).

All of the current flowing *to* the INTENDED load (remember,
the GFCI can also see an *unintended* load!) goes through the
supply/hot lead, through the transformer's primary. All
of the current *returning* from the load passes through the
neutral/return conductor *also* through the transformer's
primary.

If any of the supply current has "leaked" away via some
other path (to "ground") -- like through a person's body -- then
the current to and current from will not be equal. As
such, the magnetic fields generated by each conductor won't
*perfectly* cancel out. As a result, some energy will be
coupled across the transformer's core to its secondary.

You can have a *lot* of gain across the transformer since
the secondary doesn't need much power to function. As such,
you can look for very small "leaks" even in circuits carrying
very *large* currents! I.e., aside from the physical size of
the transformer and the primary conductors passing through
it, a GFCI for a 1000A circuit is essentially the same as one
for a 20A "household" circuit.

(N.B. this would not be the case if you tried to *directly*
measure the individual currents -- e.g., resistively -- and
form the difference -- i.e., comparison -- algebraically)

If you've ever examined a GFCI circuit breaker, you will note
that it isnt the simple "two terminal" device of a regular
(non GFCI) breaker. This is because the GFCI breaker has to
have an additional "ground" connection (which a regular
breaker doesn't need) usually implemented with a short pigtail
(that you mechanically fasten to the panel's ground).

GFCI *outlets* are dogs. They work the same way but are
usually built of lesser quality components. Also, they
are subject to more abuse (each time an appliance is
plugged/unplugged). And, are often exposed to more
environmental extremes than a GFCI breaker in a panel box.

Also, note that there is no way a (typical) GFCI can "test
itself" to determine that it is (likely) operational. So, a
failure in the sense electronics can cripple the protection
feature in a way that is not obvious to the user (i.e., the
circuit still supplies "unprotected power").

Note that a GFCI offers no protection against an unintended
load directly across the hot + neutral. I.e., if you wear
rubber soled shoes (which is advisable when working with
electricity) and accidentally touch hot *and* neutral,
the circuit will gladly deliver its full rated capacity
*through* your body -- as if you were a light bulb! :
OTOH, if you did NOT have rubber soled shoes on (or had
some *other* path to "earth"), the GFCI *would* protect.
(i.e., don't fall victim to the false sense of security
that a GFCI protected circuit is somehow *safer* than
a regular circuit!)

Anything else you need to know? Wanna check my spelling?
Or, my grammar??