Very true abotu the high inductive resistance, why AC now comes with a shorting wire. But if the AC is old, the wiring method should have been per code when it was installed, so why not just pigtail the metal boxes, if they exit.
Heck, I think the NEC allows for not pigtailes if you can establish a ground off the yoke in contact with a grounded box. Not sure, since I think the cost of ground screw and wire is cheap to ensure good ground connection.
later,
tom @
Ohms is a measure of resistances, but typically measure using DC. The old AC has inductive resistance, which is the problem. So I've been told.
later,
tom @
Also don't confuse the "pushwire" outlets which grab the inserted wire with a crummy spring tab with the "backwire" outlets that clamp the wire inserted in the hole when you tighten the screw. The "backwire" type are just fine while the "pushwire" type are junk.
Pete C.
About a year back when this issue was raised before, I ran the numbers can came up with a value of a hundred microhenries of inductance in 100' worth of AC. That can be ignored at 60hz. Further, as much of the turns will have shorts to adjacent banding, that will kill most of the inductance (even a single winding-to-winding short in a coil makes a huge difference).
The sheath on AC has been made in a variety of ways over the years. Aluminum, cut ribbons of galvanized steel, and other things.
Cut galv. ribbon has gaps in the coating - moisture => rustout. Aluminum surface oxidation. Corrosion on connectors/boxes. Etc.
I can imagine that WWII military buildings were made with the good stuff, and installed rather better than average.
I personally would hesitate to use AC armor as ground in old systems where it was more of an incidental box-to-box ground rather than something more actively involved in direct grounding of devices via third wire grounding systems which didn't exist at the time these circuits were laid.
My co-author of the electrical wiring faq has seen AC armor participating in a dead short where it was a poor enough connection to _not_ blow the breaker, but a good enough conductor to glow red hot.
There was a major fire in a Los Vegas casino several years back which turned out to be just this sort of thing.
Heck, another close friend found an AC armor segment to be fully live, yet, as far as he could tell from the wire segments he could see, was fully bonded back to the panel. Not.
According to Tom The Great :
The NEC does not permit 3 prong outlets on ground-less circuits _unless_ they're downstream of a GFCI (and labeled as such).
The CEC used to permit you to plug the third prong with silicone goop, but not for some time.
You have very very good points, but I'm thinking they reflect on the older BX cable that didn't have the shorting wire. Infact AC is so recognized as a good ground, if they toss in an insulated ground wire you get HFAC. Sure there more qa done, but the jacket becomes the normal ground fault, and the isolated ground is used for the third prong for sensitive medical equipment.
Now I doubt this 1953 house has AC with the quality control of HFAC, but just making a point that AC is good, damn good when properly used, and installed.
;)
later,
tom @
Yeah that is what I said, but I was wondering if you put in a GFCI breaker, does that open you up for using 3 prong receptacles. Haven't looked it up yet.
later,
tom @
Yes it does
According to Tom The Great :
Frankly, I don't think the teensy little shorting wire is of much use, if you have termination problems, for example, the shorting wire won't help.
Besides, in a system old enough to have two prong outlets (as per the OP's original posting), the cable is not going to have the shorting wire anyway. It's going to be a highly tarnished, pitted, gritty, dirty grey color, where the connections are going to be highly doubtful and the banding may be fairly brittle. Especially since it wasn't originally intended to be a ground path for 3 wire devices.
Heh. In Canada, AC armor is so NOT recognized as a good ground, that you're not permitted to use it as a ground. Period. Here, AC armor is only considered physical protection. The only armored cable commonly available here for normal residential purposes is the stuff called "MC" in the US - contains a full size bare ground wire.
[There are other armored cables of course, but not ones that you'd use in a home except in exceptional conditions. Eg TECK cable - suitable for power feeds in corrosive/high physical damage risk. Eg: mines. Power feeds for portable carnival rides.]I suspect there's rather more to HVAC than simply a "well Q/A'd AC".
According to :
If they're labeled.
The intent is to ensure that every three prong outlet is either _really_ grounded, or, provides warning to the user that it's GFCI'd.
According to Chris Lewis :
phoo! HFAC. I couldn't find any specs for it online.
What I called HFAC is "Hospital Facility Armored Cable".
It's called different things, HCF AC, etc.
Just do a google search.
Enjoy,
tom
Thank you!
I got a sec to check, 2005NEC 406.4(d)(3)(c) Says GFCI, not GFCI receptacle.
Thanks,
tom
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