Will GFCIs Make Grounded BX Safe?

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Molo

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Hello,
- When an outlet is wired with armored cable and the armor is being used as the ground (2-wire cable without grounding strip), is installing a GFCI a safe alternative to avoid the armor from being used as the ground conductor? I've heard the armor can get hot without tripping the breaker.


Thanks,
Bill
 
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Jadnashua

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For the armor to get warm, one of two things must happen (in the common sorts of things): enough current being drawn by the circuit it is supplying or, there's a wiring fault, and the armor has become part of the circuit. Since a GFCI monitors the current going out the hot and in the neutral, yes, any imbalance would cause the GFCI to trip but you'd have to use a GFCI breaker rather than a receptacle unless that cable run was on the load side. Connected to the line side, it wouldn't have any idea if there was a problem.
 

Molo

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Would a GFCI outlet upstream of the rest of the circuit work or does it need to be a GFCI breaker?
 

Reach4

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I've heard the armor can get hot without tripping the breaker.
I presume you mean "electrically energized".

GFCI in breaker or outlet can prevent dangerous shocks, An outlet GFCI would not prevent the armor from becoming "electrically energized due to a short before that outlet.
 

Molo

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I presume you mean "electrically energized".

GFCI in breaker or outlet can prevent dangerous shocks, An outlet GFCI would not prevent the armor from becoming "electrically energized due to a short before that outlet.


Yes I mean electrically energized. I've heard that when the armor is used as the ground (no internal grounding strip) this can cause the armor to get hot and is a potential fire hazard. I am wondering if using a GFCI would prevent the armor from becoming the ground and therfore avoid the possibility of it getting hot.

Thanks in Advance
 

JWelectric

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The only way the sheathing would get hot is for there to be a short to ground in which case the fuse or breaker would open and the heat stops.

There is no danger to the sheathing starting a fire if everything is installed properly.

Warm to the touch is nowhere near the ignition point of what is in the wall.

GFCI is always good when a person is interacting with electrical current.
 

Jadnashua

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Would a GFCI outlet upstream of the rest of the circuit work or does it need to be a GFCI breaker?
As long as the cable in question is supplied by a GFCI, it doesn't matter if it is a GFCI receptacle or CB. If you have that cable coming out of the panel, I'd go with a GFCI breaker, otherwise, if you can intercept it somewhere and install a GFCI receptacle, that would do the same thing for less money.
 

hj

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If the "armor" is getting hot, then it has become the "neutral" wire and current is flowing through it. GFCI, or no GFCI, it is a defect and MUST be corrected. Since the circuit is balanced, a GFCI would not notice any problem. I once was at a residence where the "ground bonding" was a piece of BX armor from the water piping to a ground rod. Once the armor rusted off the water piping, and specifically the hot water piping which is where the ground clamp was, became the ground, and THEN the neutral when the neutral feed to the house failed. When the union on the cold water side was unfastened, the house lost its neutral and EVERYTHING that was turned on became 240 V and burned out.
 

Reach4

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Yes I mean electrically energized.
I think "Getting hot"normally implies temperature. "Becoming hot" would have been better to imply electrically energized. It's idiom. "Electrified" would have been even clearer.
 

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IF the sheathing of a BX wire becomes electrically energized it would IMMEDIATELY trip the circuit breaker because it would create a short circuit.

You are presuming that the BX sheath is effectively grounded. We know it should be. We know it probably is. But if that could be counted on, BX sheath might be accepted as an adequate protective ground. Molo is worried about it.
 

Molo

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If the "armor" is getting hot, then it has become the "neutral" wire and current is flowing through it. GFCI, or no GFCI, it is a defect and MUST be corrected. Since the circuit is balanced, a GFCI would not notice any problem. I once was at a residence where the "ground bonding" was a piece of BX armor from the water piping to a ground rod. Once the armor rusted off the water piping, and specifically the hot water piping which is where the ground clamp was, became the ground, and THEN the neutral when the neutral feed to the house failed. When the union on the cold water side was unfastened, the house lost its neutral and EVERYTHING that was turned on became 240 V and burned out.

If the armor becomes the neutral, then a GFCI breaker or outlet would trip, correct?
 

Jadnashua

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The issue is balance...the hot and neutral must be in balance. The only difference that is allowed is <5ma before the GFCI will trip. Depending on the length of the run and the bonding of the ground and neutral in the panel, there would almost certainly be enough difference between the two sensing leads running from the load side back - if for nothing else, the steel in the armor is higher resistance than that of the copper wire hot feeding the circuit. If the neutral and ground were connected, adding your body resistance to that parallel circuit would then likely also create enough balance and trip it. Nothing is absolute, but you will be safer with a GFCI feeding that circuit than if you did not have one. A GFCI breaker provides two different levels of protection: the standard circuit breaker functionality caused by an overload of the wiring, and monitoring of that balance between the hot and neutral; a GFCI outboard of the panel only provides monitoring of the balance.
 

Jadnashua

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If the neutral and ground are shorted together somewhere away from the panel by the neutral contacting the sheath, current would return to the panel via parallel paths, and that going out and returning at the GFCI breaker would not be equal through that breaker.

If the hot side ended up shorted to the sheath, it would likely trip the breaker immediately unless the armor was not intact or the clamps were so corroded that the path couldn't carry enough current to trip things.

On an open circuit, it is likely that the voltages would measure the same until something came in contact, creating a current path, and a likely imbalance, tripping the GFCI. Same idea of water pressure in a piped system until there is some flow.

FWIW, steel has about 3x the resistance of copper, so with current flowing, you should see a voltage drop which indicates some differences in voltage. This should cause a GFCI to trip when under load.
 
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Molo

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Interesting note: Regarding the insulation on the wires of 50+ year old armored BX cable.
Where ceiling lights were ceiling mounted the heat generated into the box, the electrician observed that the cloth insulation was very brittle and flaking off of the copper conductors. Where ceiling lights were hanging 2' lower than the ceiling, the insulation was still flexible and intact. The heat from the lightbulbs deteriorated the insulation.
 

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Interesting note: Regarding the insulation on the wires of 50+ year old armored BX cable.
Where ceiling lights were ceiling mounted the heat generated into the box, the electrician observed that the cloth insulation was very brittle and flaking off of the copper conductors. Where ceiling lights were hanging 2' lower than the ceiling, the insulation was still flexible and intact. The heat from the lightbulbs deteriorated the insulation.
This has nothing to do with the wiring method (BX/AC/NM/etc). It has to do with the fixture design as you explained.
 

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IF the sheathing of a BX wire becomes electrically energized it would IMMEDIATELY trip the circuit breaker because it would create a short circuit.
This is not at all true in all cases.
The sheathing of older cables, without the thin aluminum bonding strip, CANNOT be used as an effective grounding path. A direct short to this sheathing CAN, and has, created a heating element effect due to the higher resistance which can cause the sheathing to glow red hot. I have see it with my own eyes.
 

Jadnashua

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My parent's house was originally wired with a lot of BX, and they relied on the armor for a ground. In the plastered walls, over the years, there was so much corrosion in the boxes, it was a very high resistance compared with the neutral back to the panel. Since the receptacles were nearing 60-years old, I ended up replacing all of them with new ones, and found the first in each series with a GFCI unit to protect the rest. Rewiring and trying to repair all of the plaster verses drywall just wasn't in the scope, not counting trying to match the paint. This does provide improved safety, and new receptacles mean a more reliable connection. Many of the well-used originals, the plugs would just slip in (no history of high load heating units, just lots of in-out). Ideally, the whole house would have been rewired. Will leave that to the next owners if they feel it is necessary.
 

Molo

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This is not at all true in all cases.
The sheathing of older cables, without the thin aluminum bonding strip, CANNOT be used as an effective grounding path. A direct short to this sheathing CAN, and has, created a heating element effect due to the higher resistance which can cause the sheathing to glow red hot. I have see it with my own eyes.

Would a GFCI trip in this instance?
 
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