Arcing conduit-to-HVAC – no shock when touched – what cause?

What could cause conduit to arc where it touches another conduit? How can this happen without the conduit being charged with 110V?
A few months back, I added two AFCI breakers and a new house ground wire in a home improvement project. A licensed electrician installed the breakers and made the box connections. I assume he did it correctly.
Here’s the problem:
I installed a basement HVAC duct touching an old metal flexible conduit. A few weeks later, I noticed arcing between the conduit and HVAC duct where they touched. Neither of the two shocked me when I touched them, but sure enough – an orange arc was making a little noise. Upon examination, the arcing had welded holes in the HVAC duct and the conduit.
I snipped & dissected the conduit. The wire sheathing inside were fine – one was a bit darkened. Over a few weeks, I expected much worse. Oh well, problem solved. Maybe it was HVAC static discharge.
But then yesterday, I heard the same noise inside a kitchen light fixture on a different circuit. The box was hot to the touch. Upon dissection, however, nothing in the box was damaged.
HOWEVER – that box’s conduit was touching another conduit, and sure enough – a little hole had been welded where they had been touching.
What could be causing this? I am a little spooked.
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What could cause conduit to arc where it touches another conduit? How can this happen without the conduit being charged with 110V?
A few months back, I added two AFCI breakers and a new house ground wire in a home improvement project. A licensed electrician installed the breakers and made the box connections. I assume he did it correctly.
Here’s the problem:
I installed a basement HVAC duct touching an old metal flexible conduit. A few weeks later, I noticed arcing between the conduit and HVAC duct where they touched. Neither of the two shocked me when I touched them, but sure enough – an orange arc was making a little noise. Upon examination, the arcing had welded holes in the HVAC duct and the conduit.
I snipped & dissected the conduit. The wire sheathing inside were fine – one was a bit darkened. Over a few weeks, I expected much worse. Oh well, problem solved. Maybe it was HVAC static discharge.
But then yesterday, I heard the same noise inside a kitchen light fixture on a different circuit. The box was hot to the touch. Upon dissection, however, nothing in the box was damaged.
HOWEVER – that box’s conduit was touching another conduit, and sure enough – a little hole had been welded where they had been touching.
What could be causing this? I am a little spooked.
*My first thoughts are that a neutral wire is touching something or is broken and the conduit or duct is being used as the return. My other thought was that maybe there are some grounding (Or lack of) issues and the conduit or duct is some how energized. However you did mention the installation of a house ground wire. Was a new ground rod installed? Are all of the pipes bonded? How old is this house?
When this is happening, put an ammeter on each of the ground wires in the circuit breaker panel to see if any are carrying a load. Then check the loads on the neutrals and compare the readings to the respective hot leg for each. You could also try putting the ammeter around the metal conduit and see if you get a reading.
You could also use a voltmeter and check the volts by touching one lead to the metal conduit and the other to a water pipe (Assuming the water pipe is grounded properly).
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If you paid the electrician, was he licensed?? If so, I would call him up asap and tell him there is a problem relating to the recent work and demand (politely that )he come out and at least look at it. If he finds a problem unrelated to his work, then you have to pay $ to get it fixed. If it is related to his work, he should fix it, if he resists suggest that you might be in touch with whomever licenses electricians where you live. THIS IS POTENTAILLY LIFE-THREATENING!!!
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wrote:

Umm Bob, it is quite possible, even likely that the arcs the OP is posting about are happening on circuits other than the ones the licensed electrician worked on...
@OP:
The arcing between the HVAC duct and the flexible metal conduit is one of the reasons why code compliant electrical runs should not be in contact with other metallic objects like HVAC ducting or copper plumbing lines or black iron gas lines... When connections in the circuit open or were not correctly installed to begin with those other metal utility pipes or ducts can become the return side of the circuit and cause arcing/fires/electric shocks...
I would install a proper run of flexible metal conduit which does not physically come in contact with your HVAC trunk line possibly even using a non-metallic sheathed type of wire or raintight (vinyl coated flex) flexible conduit for the run from point to point where it passes by your HVAC duct if you can not reroute the run so it doesn't physically touch the metal ductwork...
As far as the other problems you mentioned in the boxes with the conduit, you definitely need a qualified electrician to come out to your house and completely trace all of the circuits which are displaying this arcing behavior to find out where the wiring is damaged or where the incorrect connection is located so that it can be properly repaired so as not to arc anymore and not be a shock/explosion or fire hazard any longer...
You might be looking at pulling new wires to fix some mystery problem somewhere but doing that is cheap compared to having to deal with some sort of electrical fire...
~~ Evan
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You have a reference where it says that in the code? If so, there must be a hell of a lot of illegal installs. A typical example would be a furnace where metal conduit was used to run power to the furnace and the the conduit is fastened with metal clamps directly to the furnace housing along it's run and to the metal connection box. That in turn means that the entire metal duct system is connected to the metal conduit. So is the metal gas pipe connected to the furnace and everything connected to that piping system.

If wiring were done that way, there would sure be some strange looking conglomerations out there.

From what I've heard so far, this is his only real problem.

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wrote:

As far as the code cite: I don't know chapter and verse, but the basics of the code not allowing wiring to be in physical contact with ductwork is that the ducting can move around as the HVAC system operates and duct pressure rises and falls and cause mechanical damage to the wiring over the course of time... All wiring runs MUST be installed so that they are protected from mechanical damage... It goes more to that specifically and the general phrase "all work must be done in a workmanlike manner" which is something that looms over all electrical installs...
I have forgotten how much clearance must be afforded for wiring runs near other utility lines but I know for a fact that there are different rules for clearances depending on whether you are running the wiring parallel to some other utility line or perpendicular to another utility line...
I am not a practicing electrician who does wiring in new homes every day, I do maintenance on existing installs in commercial buildings and trust me, I know that there are quite a few code violations out there in otherwise normal looking buildings because it is sealed up in the walls or way above a ceiling where an inspector never looked when signing off on the permits...
I agree with the theory that there is an open or misconnected neutral somewhere in the OP's house which is now using the HVAC ductwork and the arcing conduit as a return path which is very dangerous and could cause a fire... I don't think it is in any way connected to either of the circuits he had the electrician install the AFCI breakers on because those breakers would be tripping if the amount of current going out on the hot didn't match up with what was coming back on the neutral -- it is what such breakers are designed to detect and open the circuit to prevent...
~~ Evan
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Which the metal conduit that he has that you objected to provides.

Maybe workmanlike means to you that metal conduits or raceways can't be in contact with any other metal like an HVAC duct, but it doesn't mean it to me, nor does it say it anywhere in the code I am aware of. I can show you new installs of HVAC that passed inspection here where conduit is touching metal or fastened directly to it. In fact, NEC allows metal conduit to be used INSIDE HVAC ducts. So, it's hard to imagine they have a prohibition against a metal conduit touching metal like an HVAC duct anywhere along it's run.

When you figure it out, let us know.

I suspect some of the things you think are violations are in fact not, as is the case in point. You;d have the guy rewiring his conduit runs, when there is no need to do so. The only problem in evidence is that current is flowing in the ground path and that is what needs to be fixed.

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OK - so an update -
The same guy came out and looked at the kitchen fixture. He found some spots where the ancient cloth insulation had been gradually worn away by a tiny arc. Then, when he moved the box, it shorted and the circuit tripped.
We rebuilt the box and fixture, and made sure there were no further problems.
We're waiting-and-seeing and inspecting everything else on that circuit. But for now, I am hoping it's OK (famous last words).
In a couple weeks, I'll have another 2 circuits put in my box. We'll have another chance to inspect everything then.
Am I being too hopeful?
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*Ancient cloth insulation tells me that this is an old house. Anything is possible as a result of age, remodeling, repairs and the people that did them. Just keep an eye out.
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Time to consider rewiring the entire house. Cloth insullation not been made for many decades, that stuff has to be ancient. Will be more problems.
--
Christopher A. Young
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My brother lives in the house in which I grew up, built in 1916. Still has the original fusebox in the basement, pushbutton light switches throughout, and none of that silly ground plug or GFCI nonsense. Plenty of 100 yr. old houses that don't have electrical problems.
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Ah, but for whatever reason *this* OP's old house with the old cloth covered wires has experienced something which is indicating that it is getting close to the time to remove ALL of the old wiring and replace it with conductors whose insulation is not compromised and unsafe... Whether this degradation was the result of the *lottery winning* history of weather which dried out the insulation over time, due to past overloading of the circuits or poor workmanship in the original installation is as yet unknown and may never be known...
However in this situation the most prudent thing for the homeowner to do is replace all the original cloth covered wires in his conduits with new conductors...
~~ Evan
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I seriously doubt his ancient cloth covered wire, if indeed that is what he has, was run inside conduit.
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