6 Sep
2014
6 Sep
'14
7:51 p.m.
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).
YEs, that is exactly what they do. If you think about it, it's tbe most
sensible thing to do. Sensing the currenti n the (green) earth wire would
detact a leak via that wire, but current wil lonylflow there if (a) the
earhth wirie is earhted and ()b) the leakage aoccurs to some thign that
is conencted to the earth wire.
By detectign an imbalance in live nad neutral currents the GFI [1] will
trip if the leak occurs through some other path. Say if you accedientally
touch a live part. The current frols to warht not via the earth wire of
htt device, but as the live an neutral currents are now imbalanced a GFI
would trip, hopefully protectign you.
[1] We tend to call them RCD's (Residual Current Device). The old name
here was ELCB (Earth Leakage Circuit Breaker.
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?
Exaactly.
Hwoever, I would still recoemnd having the earth (ground) wiring
conencted up). Firstly becasue if the GFI fials (and they can do) it's
another protection system. Secodnly, samll leakage currents which will
not trip the GFI can, under some circumstances cause enoguh voltage
differencebetween 2 devices that you will ahve problems with TTL level or
RS232 conenctions between them.
-tony