Glowing AC/MC/BX

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lostoptimism

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I have only ever heard and seen of a "glowing connection" in improperly wired houses, but I have heard and never seen a small length or segment (even a couple inches) of Metal Clad cable that was glowing due to a fault, is this a common occurrence? One should assume even a standard circuit breaker would halt this activity.
 
Although the OP is not an electrician, I am allowing the thread since there is no question of a "how to" nature.

With that being said, the old BX used the outer sheath as the EGC (equipment grounding conductor). It was never a good EGC and most times it would be compromised even further with age and often being broken. If the ungrounded (hot) conductor shorted to the compromised sheath it would be a high resistance connection, therefore a "glowing" connection. There normally wouldn't be enough current to trip a breaker. It's just a very hot and dangerous heater hanging in the wall.
 
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lostoptimism said:
I have only ever heard and seen of a "glowing connection" in improperly wired houses, but I have heard and never seen a small length or segment (even a couple inches) of Metal Clad cable that was glowing due to a fault, is this a common occurrence? One should assume even a standard circuit breaker would halt this activity.
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Your statement is riddled with false assumptions. You apparently are talking about old, what was called BX cable that was commonly used for decades to wire buildings up until the 50's. BX was constructed with a steel armor spirally wound around the conductors to protect them. It also provided a continuously grounded system (EGC) so that all boxes and fixtures are grounded. This was a big step up from the open conductor knob and tube system that it replaced.

Problem with it was that over time it was discovered that the steel armor tended to corrode which caused an increase in resistance from one turn to another along the spiral wrap. When that occurred, if a fault were to happen from a hot conductor to, say a box which is grounded by the BX armor, it was possible that the high resistance of the corroded armor would allow enough current to flow to heat the armor (sometimes red hot) but not blow the fuse.

So, I don't think it would be possible for this to happen with as you say "a small length or segment (even a couple inches)" of cable. The resistance wouldn't be that high so yes, the fuse would blow or breaker trip. Usually this happens over a longer length. One could also surmise that red hot armor would burn up the insulation on the conductors within causing a short that would blow the fuse or trip the breaker.

Was this a common occurrence? Well it has happened but it obviously takes a number of conditions to fall into place for it to occur. I don't think it was all that common and certainly with the elimination of BX you won't see it with current cables.

As was said, BX was replaced with AC cable around the mid 50's that uses the same spiral armor design BUT has a continuous bare bonding wire under and in contact with the armor to provide additional continuity to eliminate this problem. AC cable is widely used today and is approved for use with grounding receptacles, something that BX was not.

MC or metal clad cable, like AC is also an armored cable but takes it a step further by incorporating a green grounding conductor along with the current carrying conductors. The armor itself is grounded but is not used as an EGC.

As to "improperly wired houses", that really has nothing to do with this as BX was an approved wiring method. It was just that over time it deteriorated.

-Hal
 
Thanks

Thanks

Thanks, Hal

That is exactly the answer I was looking for. It amazes me that issue such as those are only barely touched in literature, I'm an apprentice at the moment and am trying to move up the rungs some time, and there are some gaps in information in which only a seasoned professional may be able to answer, which you have done. Much Appreciated.
 
Glowing connections can also happen with terminations with any wiring method (NM, EMT, etc) or wire type (Cu, Al). I've seen numerous hotel 6-20R PTAC receptacles melt from corrosion of the termination, which provides high resistance and accompanying heat and eventual destruction. Happened most frequently there with leaky exterior walls allowing water/condensation to get in the box, or in receptacles mounted face up in the PTAC sub-base. Breaker panels, especially certain ones like Zinsco, can also burn up the buss stabs and/or breakers from high resistance connections.

Standard breakers will NOT detect any kind of glowing connection unless in its trip range, and neither will AFCI or GFCI breakers if there is no ground fault. Think of running a 600W space heater that is dumping all that heat into a few square inches of space, or in the case of old BX, a giant toaster coil in the wall.... which would be easily found with an IR scanner.
 
lostoptimism said:
Thanks, Hal

That is exactly the answer I was looking for. It amazes me that issue such as those are only barely touched in literature, I'm an apprentice at the moment and am trying to move up the rungs some time, and there are some gaps in information in which only a seasoned professional may be able to answer, which you have done. Much Appreciated.
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The problem with the old style AC cable is that it's armor with a spiral wound design would cause the fault current to travel in a circular motion along the armor creating in effect a heating coil (hence the term glowing). The thin aluminum strip added in the 50's eliminated that problem by shorting each section the spiral together and removing the circular current path.

Although this glowing effect is theoretically possible I highly doubt that it was or is a widespread problem. We have tens of thousands of homes around here wired prior to the 1950's with the AC cable without the bonding strip and the design of the cable has contributed little to electrical problems in these homes. Just my 2 cents. :)
 
How much current has to flow in that old armor before it gets hot enough to glow? I would think more then it takes before a 15 or 20 maybe even 30 amp overcurrent device trips on thermal protection. I could see glowing happening at a fitting with lower current levels because of contact resistance between armor and fitting.
 
infinity said:
I agree, IMO that would be more problematic than the armor itself.
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Just some further comment on this - the armor itself can add enough resistance that it limits current enough to prevent overcurrent device operation, but the armor wouldn't have a high watt density at any particular point in continuous portions of that armor. Yes that resistance will have heat losses, but how much heat is going to develop in continuous armor without going over 15 or 20 amps necessary to even make the overcurrent protection to start to think there might be a problem and @ what is likely 120 volts supply? Me thinks well below the amount necessary to start glowing, which makes the biggest hazard here any weak connection at terminations and the fact the armor is at some voltage potential above ground leaving potential shock hazards.
 
lostoptimism said:
I have only ever heard and seen of a "glowing connection" in improperly wired houses, but I have heard and never seen a small length or segment (even a couple inches) of Metal Clad cable that was glowing due to a fault, is this a common occurrence? One should assume even a standard circuit breaker would halt this activity.
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Welcome to the Forum, lostoptimism.

Your OP (opening post) asks only about Metal Clad cable. Metal Clad cable is Type MC and has its own Article in the NEC. Metal Clad cable is not Armored Cable, Type AC. BX is a jargon name for Armored Cable and is not, and never has been, used in any version of the NEC all the way back to 1913 (I can provide images of original Code pages). Most Type MC does not use the metal sheath as the sole Equipment Grounding Conductor (EGC), although there are a few exceptions. Most Type MC has an green insulated EGC that is bonded on both ends to the boxes that the MC cable is connected to. Fault current originating at the boxes, devices or utilization equipment (if bonded), is going to flow in both the sheath and the green EGC.

Think about that. Green wire type EGC in parallel to the conductive path that is the MC sheath. . . Then think about applying a current to these "normally non current carrying paths" of a high enough amperage to get the MC sheath to "glow". . . The Green EGC is the same guage as the Hot conductor. . . the hot conductor is designed to carry the circuit current (let's say, 20 Amps) without heating so as to endanger the insulation on it. . . so, the Green EGC can handle 20 Amps with minimal heating, AND it is in parallel with the MC cable sheath which means the sheath and the Green EGC DIVIDE the 20 Amp current into lower values.

Bottom line, to get a current HIGH enough to get an MC sheath to "glow" means there is something REALLY wrong somewhere else, maybe several places, to get that current.

Now, with regard to Armored Cable, there are reports of "glowing sheath". Many are apocryphal. . . some, few, are based upon good analysis.

I, like Infinity, work in a large area (a different one than Infinity's) with tens of thousands of buildings and homes wired before the Late 1950s with non-bonding strip "BX" Type AC Armored Cable, and it has been and still is a basically stable wiring method.
 
Thanks again

Thanks again

This filled the gap in my knowledge, sometimes you can only get the answers on the issue appropriately and from professional(s) sources. I guess speculation about safety is not one of the tenets for designing textbooks. The EE i am apprenticed to said pretty much the same thing, but I of course cannot rely on only one source, I needed a general consensus on the matter. Much appreciated!
 
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