On Sun, 24 Dec 2006 21:31:36 -0500 email@example.com wrote:
| On Sun, 24 Dec 2006 16:32:37 -0900, "Gerald Newton"
|>Does anyone have anything on this?
|>Also, does the GFCI reptacle listing permit use on a AFCI circuit?
| I don't see anything mutually exclusive in either listing. Nor can I
| think of a technical reason why their could be a hazard.
| GFCIs are commonly downstream of other GFCIs with no problem.
Does this ever include a GFCI _receptacle_
downstream from a GFCI _breaker_?
One small problem I do see where a GFCI _receptacle_
is downstrealm from a
GFCI -or- AFCI _breaker_
is the risk of electrical shock from the neutral
conductor. A GFCI receptacle interrupts both hot and neutral. A breaker
generally does not. There are such animals as "switched neutral" breakers
typically used for fuel pump circuits where the issue expands to triggering
fuel vapor explosions. However, from just an electrical perspective, there
is some, small as it is, risk from electrical shock from a neutral. This
generally a case of _two_
(or more) things having to go wrong at the same
time for this to happen. Not only does someone who is well grounded have
to be contacting a neutral (the probability of this being higher than that
of contacting a hot wire because of legacy appliances when frames connected
to the neutral, assuming a polarized plug), but some situation must also
exist that places significant voltage on the neutral. One example is an
open neutral on an upstream feeder while opposing poles are way out of
balance. Another example is that this contact of the neutral just happens
to be when some other device or appliance has a line-to-neutral fault.
If a current imbalance causes the GFCI receptacle to open, both line and
neutral conductors will be opened and a safer condition then exists. If
this happens with a GFCI or AFCI breaker, then the neutral remains connected
there is no power available to the GFCI receptacle for it to take any
action. The (small) extra protection of the GFCI receptacle is thus disabled.
The issue to consider is just how much of a risk this is. I'm sure we all
agree it is definitely smaller than the risk of contacting a hot line wire
under similar conditions of the person being grounded. But I believe it is
certainly a non-zero risk. Your home would be a lot safer from electrocution
by just not having electricity at all, though it would now be arguably less
safe due to things like fires from errant candles, etc. There is no zero
risk scenario; risk has to be managed.
My position is that if you compare the risk between having a GFCI receptacle
on an AFCI (with GFCI protection) breaker protected circuit and having just
a regular receptacle, you'll find that not having the GFCI receptacle is the
greater risk. So even with AFCI breakers serving most of the home, I will
continue to have GFCI receptacles protecting at least certain areas like the
I happen to dislike the style of GFCI receptacles for my kitchen. So even
before the NEC 2008 decision to expand use of AFCI protection, I have been
looking at ways to protect the kitchen without GFCI receptacles. One option
is a bank of dead-front GFCI "receptacles" away from immediate view which can
interrupt any or all circuits in the kitchen. Since I also expect to have
and use 240 volt plug-connected appliances, that approach is not complete.
One solution I have is this. I will use GFCI (or now AFCI with GFCI in it)
breakers in a subpane near the kitchen. The 2-wire 240 volt circuits will
not have a neutral and won't need any more protection since the breaker will
cut off both line wires. The 120 volt circuits can become fully protected
by one of two means: either the dead-front GFCI "receptacles" in the circuit
or a 2-pole contactor providing supplimental protection by opening neutral
and line wires when L-N power is lost. A 3-pole contactor could do this
for 2 circuits on one yoke (neutral shared to the contactor and powered by a
240 volt coil fed from a 2-pole breaker).
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
Click to see the full signature.