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Old Armored Cable Ground


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Have a house built in 1939 with the original armored cable wiring. The breaker box is relatively new Square D. Everything looks to be wired tight to the box as far as the sheaths etc. Recently someone installed 3 prong receptacles in a lot of outlets and grounded with wires screwed to the boxes. Testing with a pocket LED tester looked OK. Haven't had a chance to test with a meter yet. From what I've read it may or may not be OK to use the boxes/sheaths for ground. Can't tell yet if there's a bonding wire in the cable. Any ideas? Would replacing the breakers with GFCI breakers and keeping the grounds wired to the boxes work?

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That '39 armor couldn't even approximate today's grounding standards. I can't say without examining it how big a safety issue it is but replacing the 3 wire receptacles with GFCI devices certainly will not fix everything. For example, surge devices that folks install to protect their sensitive electronic equipment won't do much good without a solid ground path to the earth. Also, I don't know if today's GFCI devices will even fit in '39 steel receptacle boxes.

Ain't no replacement for the real thing.

Marc

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Okay thanks for the reply. Looks like there are a couple GFCI outlets installed in the bath and kitchen. What about installing combo AFCI/GFCI breakers and keeping the 3 prong outlets grounded to the boxes? Maybe the ground may not be ideal but would the AFCI/GFCI breakers cover safety issues?

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Ain't no device on the market that's gonna allow a surge protector to work in the absence of a solid path to the earth. Surge protectors work by bypassing the lightning current to the earth via the grounding system where it can safely dissipate. No ground connection, no protection.

GFCI and AFCI are both marvels. I always recommend them.

Marc

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Actually, the spiral sheath on AC cable was, indeed, intended to be a grounding conductor. It'll have greater resistance than a copper wire, but it should do the job as long as the spirals remain interlocked and the terminations are solid. Check out this page from my 1936 GE catalogue:

Download Attachment: icon_adobe.gif bx002.pdf

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That said, if the spirals become unwound, then you have a problem. As I understand it (any anyone who truly understands this stuff, please speak up), if there were to be a fault on such a cable, the unwound portions of the sheath would form a choke coil that would impede the flow of current, allow the metal to heat up and, actually, glow in some cases. There's lots of lore about this out there, though I've never personally seen it. It's the reason that the bonding wire was added to this kind of cable in the early '50s. (The bonding wire prevents the choke coil from forming.) (Someone on this site once saw the glowing in action and posted a picture, but I don't remember who it was and I can't find the picture. Seems like it was one of the Lewis guys, Eric? Neal?)

Neither GFCIs nor AFCIs will protect against this. It might be a good idea to add them, but be aware that they might trip as soon as you add them to some of your circuits because wiring from this period often had a lot of comingling of neutrals.

If you want to do a better test of the grounds at each location, use a Wiggy. It'll actually put some load on the wires and it might detect problems that an electronic meter might miss.

By the way, while the unbounded sheath was considered to be an adequate grounding conductor at the time, it's not anymore. So it might or might not have been an acceptable practice to install new grounded receptacles on a circuit wired with an obsolete wiring method. That'd be a question for your local electrical inspector.

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Choke coils don't heat up because they're choke coils. The power developed in them is sometimes referred as to 'reactive' power and is just energy bouncing back and forth in magnetic form. It does not manifest in the form of heat.

I don't know why the unwound steel armor glowed and the armor still wound did not. Perhaps it was because of 'eddy' currents generated in the steel by the large magnetic fields. The same thing happens in transformer cores but laminations in the magnetic cores keeps eddy currents to a minimum. The armor steel is without any laminations and magnetic fields expand enormously when two conductors carrying a current are separated.

What Jim said.

Marc

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The length of the spiral sheath is multiple times longer than the actual cable lenght. Older steel sheath could rust and become a higher resistance path than it already was. I have seen it glowing red and still not trip the breaker. Without the bond wire it does not count as a grounding means.

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... There's lots of lore about this out there, though I've never personally seen it. It's the reason that the bonding wire was added to this kind of cable in the early '50s. (The bonding wire prevents the choke coil from forming.) (Someone on this site once saw the glowing in action and posted a picture, but I don't remember who it was and I can't find the picture. Seems like it was one of the Lewis guys, Eric? Neal?)

I think the attached photo came from Joe Tedesco and that it was staged.

Click to Enlarge
tn_20151218232436_333%20REDHOTBX.jpg

16.23 KB

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... There's lots of lore about this out there, though I've never personally seen it. It's the reason that the bonding wire was added to this kind of cable in the early '50s. (The bonding wire prevents the choke coil from forming.) (Someone on this site once saw the glowing in action and posted a picture, but I don't remember who it was and I can't find the picture. Seems like it was one of the Lewis guys, Eric? Neal?)

I think the attached photo came from Joe Tedesco and that it was staged.

Click to Enlarge
tn_20151218232436_333%20REDHOTBX.jpg

16.23 KB

That looks kind of like someone hooked up each end to a car battery. If so, pretty much any wire would react the same way.

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Jim Port has a point.

A car battery would heat the armor evenly...unless the cable is bent in such way that the continuity between the spirals is interrupted at some or several points along the cable. Resistance goes up when the current must follow a spiral path instead of taking the shorter and lower resistant path gained by jumping continuously between successive spirals, in which case the current flows pretty much parallel to the cable instead of in a spiraling direction.

Where the current takes a spiral path, the voltage between spirals is extremely small so it wouldn't take much manual flexing and stretching of the cable to alternate the path between linear and spiraling.

Wish I had some of that old cable. I could experiment.

Marc

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I gotta ask.... How big a deal is this? Reason I ask....

I've looked at (probably) thousands of miles of AC/BX cable...no NMC in Chicago....in every possible jacklegged mess imaginable, and I've never seen anything remotely like anything described here.

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Personally, I would make sure there are GFCI devices or breakers on circuits that need them and run new dedicated grounded circuits for the entertainment system and desktop (does anybody still use these?) And leave the rest alone. AFCI only if the new work requires it, far too many nuisance trips for my liking.

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I gotta ask.... How big a deal is this? Reason I ask....

I've looked at (probably) thousands of miles of AC/BX cable...no NMC in Chicago....in every possible jacklegged mess imaginable, and I've never seen anything remotely like anything described here.

I wouldn't call it a big deal. It's discussed here out of curiosity, I guess.

Marc

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... There's lots of lore about this out there, though I've never personally seen it. It's the reason that the bonding wire was added to this kind of cable in the early '50s. (The bonding wire prevents the choke coil from forming.) (Someone on this site once saw the glowing in action and posted a picture, but I don't remember who it was and I can't find the picture. Seems like it was one of the Lewis guys, Eric? Neal?)

I think the attached photo came from Joe Tedesco and that it was staged.

Click to Enlarge
tn_20151218232436_333%20REDHOTBX.jpg

16.23 KB

That looks like it was propped up so that he could safely heat it with a propane torch. Fake, alright.
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Well, some minor diffence of opinions. Seems like without a bonding wire if the sheaths get pulled apart into a spiral or there's a poor connection with a lot of resistance it becomes like a toaster element if there's a short from hot to the sheath and a breaker fails. Guess I'll put a load circuit analyzer and at least get some impedance/resistance numbers for the ground and check for weak connections in the old wiring and install those GFCI/AFCI breakers.

I found this info on another board so I'm quoting someone else. Appears to be as accurate a reason as I could find.

It is my understanding that when testing labratories pushed the elements of the flex of BX cable the presence of arcing during a short circuit condition caused a likely hood of setting a fire while the interupting rating caused the breaker to trip due to the present short circuit condition too high to be within the maximum levels allowed for that flex to be approved for use as an equipment grounding conductor. The way that I remember the report that I read it stated that the arcing on the outside of that metal flex of a BX cable was present the entire length of the test material to the extent to raise a high possibility of the arcing causing a fire.

The results of that test seemed to be the birth of type A/C cable and Metal Clad cable. Metal clad cable is an offspring of the old BX cable. This metal clad cable looks like the old BX cable but a green equipment grounding conductor was added to the manufacturing process.

The type A/C Cable also showed up about the same time frame as the Metal Clad Cable. The type A/C cable does have intent to use the flexible metal of the cable as an equipment grounding conductor. This flexible metal of the cable of A/C also looks like the metal sheath of the old BX cable. The major difference here with type A/C cable is that a metal strip is required to be installed the entire length of the type A/C cable and is designed to augment the metallic flexible part of the type A/C cable, thus using both the flexible metal of the type A/C cable and the thin metal strip combined to make up the designed use of that flexible metal of the cable as the equipment grounding conductor. The thin metal strip is designed to limit the excess arcing of the flexible metal part of the type A/C cable.

Also see below the NEC requirement requiring all electrical products to be used as approved for the purpose.

So since there grounded outluts on the old BX are not to code do I need to replace with 2 prong or label the three prong GFCI-No equipmanent grouind?

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