I often deal with homes that were wired or rewired with the old style armored cable without the bonding strip. Most of them have had 3 wire receptacles added at one time using self grounding receps or pigtails to the box. Of course I know this cable is not approved for grounding, but now when I am called in for a new buyer I have to be the bearer of bad news and tell them the receps should go back to 2 wire.
There are probably millions of homes wired this way and whats more, the boxes are already grounded by the BX and there's nothing that can be done about that. And it seems that the most likely place for a H-G fault is going to be in that box, with all its old dried out insulation and perhaps a sharp edge on the BX to boot!
So am I doing the right thing here by telling homeowners they can't have grounding receptacles on this type of wiring? After all, it only extends the grounding six more feet from what's already there.
Or is the main risk that an uncleared ground fault will transfer to the user through the grounding pin?
As I understand it the risk is two-fold: 1) During a fault the grounded receptacle has the potential to energize grounded metal parts of an appliance, and there is nothing to clear that fault. 2) With BX the fault current will try to flow over the metal armor. Because the BX does not have a bonding wire, the armor will heat up much like a light-bulb filament. This is a potential fire hazard.
I'm not really sure if grounding each receptacle to the box is illegal. It doesn't appear to violate 406.3, especially if the receptacle yoke is in solid contact with the box.
However, at the panel, I would GFCI protect all the BX circuits with grounded receptacles, to comply with 406.3(D)(3)(c)
Jon, I think what Larry means is to isolate the grounding at the receptacle so as not to extend an ungrounded circuit which, technically, it is an ungrounded circuit since the BX is not recognized as a grounding path. Just like replacing a 2 wire recep with a 3 wire GFCI protected, no equipment ground but legal because of the GFCI. The IG recep will prevent the unrecognized grounding from extending to the user equipment.
reno....this is old old BX, not flexible metal conduit. And the lengths are considerably longer than 6 feet anyway.
[This message has been edited by BigB (edited 02-11-2006).]
Here's what I mean, specifically what is highlighted in bold:
From 2002 NEC:
406.3 General Installation Requirements. Receptacle outlets shall be located in branch circuits in accordance with Part III of Article 210. General installation equirements shall be in accordance with 406.3(A) through (F).
(D) Replacements. Replacement of receptacles shall comply with 406.3(D)(1), (2), and (3) as applicable.
(3) Nongrounding-Type Receptacles. Where grounding means does not exist in the receptacle enclosure, the installation shall comply with (a), (b), or (c).
(b) A nongrounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit-interrupter type of receptacle(s). These receptacles shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.
(c) A nongrounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the ground fault circuit interrupter shall be marked “GFCI Protected” and “No Equipment Ground.” An equipment grounding conductor shall not be connected between the grounding type receptacles.
[This message has been edited by Larry Fine (edited 02-11-2006).]
Got it. It gets a little contradictory, since the reason that we are considering using the GFCI is that the old BX is _not_ considered suitable to be an equipment ground, but we don't want to connect it because it might be an equipment ground
I guess the problem is that the code section being considered discusses installation _without_ equipment grounds, but doesn't really address situations where you have _insufficient_ equipment grounds.