5 Sep
2014
5 Sep
'14
6:59 p.m.
On 2014-Sep-05, at 6:00 PM, drlegendre . wrote:
Eric & All,
On Fri, Sep 5, 2014 at 6:29 PM, Eric Smith <spacewar at gmail.com> wrote:
Yes, that's what a GFCI does.
Inside the GFCI, the hot and neutral are sent through dual primaries (a turn or two each
on a toroid core) of a current sense transformer, connected such that the currents are in
opposition. A third - sense - winding feeds an an op-amp and thence the circuit-trip
relay. If the currents in the hot and neutral are in balance (equal) they cancel and
nothing is induced in the sense/secondary. If out of balance the difference induces V in
the sense winding, which trips the circuit open.
The same principle works for three phase, with all three (or four) current-carrying wires
going through the transformer.
On Fri, Sep 5, 2014 at 5:16 PM, Chuck Guzis
<cclist at sydex.com> wrote:
Your statement made me think for a moment.. and it became clear that I must
not fully understand the function of GFCI hardware.
Now if a GFI can operate on an outlet with n/c to the 3rd prong (read: no
ground wire) that +must+ mean that the GFI can sense a situation where a
first current I is flowing in the hot lead (black) but it's not equal to a
second current I' that is flowing in the neutral (white).
An example would be a radio that draws 500mA under normal conditions, where
500mA flows in both the hot & neutral. But if the radio falls in the
bathtub, now we might see a current of 750mA flowing in the hot, but only
500mA flowing in the neutral and an additional, anomalous 250mA flowing
"somewhere" - which in this case, would be through the water / human to the
cold-water pipe ground (via a metal tub, or metal drain hookup, etc).
So a GFI doesn't respond to currents flowing through the ground circuit,
but to current differentials in the hot & neutral circuits - is that
correct? If that's true, then it doesn't really matter what's going on in
the ground circuit - so long as the Ineutral < Ihot there's a fault
condition.. there's a current going somewhere it's not intended to flow, eh?
A bit of advice--when you do get it right, please
also install a ground
fault interrupter also. We want you around for a few more years.
Note that code allows for ungrounded three-prong outlets with GFCI.
Having a proper ground is still better, even with the GFCI.