about grounds

[ned+xtension at mrochek.com]

On the subject of GFCIs, it is a common misconception that GFCIs depend on
having a proper ground reference.

This is not the case. GFCIs operate not by checking for a flow of current to
the ground, but rather by comparing the flow of current on one conductor (hot)
to the other (neutral). If these aren't equal thSuch a flow is not  returning
by another path and the device trips. Note that this is not necessarily a fault
to ground - it could be a fault to a different neutral connection - and so the
term "ground fault" is something of a misnomer. The better term for such
devices is "residual-current circuit breaker".

The circuit is actually very simple: Both conductors are run through a sensing
coil, forming a differential current transformer. If the current in both
conductors are equal they cancel each other out, if not they induce a current
in the coil. The rest of the circuit detects this current and trips the breaker
if it exceeds some threshhold.

Note that a common application of GFCIs is as a way to upgrade to 3 wire
outlets without having to upgrade the in-wall wiring. It is perfectly legal to
install a GFCI in a box without a ground wire because the GFCI doesn't need it
to operate correctly. Such outlets are supposed to be labeled as not providing
a true ground, although in practice I rarely if ever see such labeling done.

As for panel grounds, the rules about those are primarily intended to prevent
ground loops since having current always flowing on ground wires is something
of a no-no. The basic approach is that grounds should be strapped to neutrals
at main panels but not at subpanels, but see the NEC for specifics, and note
that there can be state, county, or even local requirements that amend the
NEC.

				Ned