Sparks and flames from low voltage conduit through a concrete floor

[gadfly56]

Looks like there is some spray foam on the underside that's blackened.

-Hal

Wax residue, spray foam, or is that just the jagged blowout from a sloppy penetration?

 

[hbiss]

Had to look too. Looks like the same stuff they used to caulk the wall/ceiling corner opening.

-Hal

 

[oldsparky52]

I think I'd carefully put a voltmeter between that rebar and the conduit.

 

[tortuga]

My guess it the data cable is newer than the conduit and a data cable installer might hasve tried to widen that hole and somehow the vibrations combined with age shorted a wire in the conduit, and the conduit is now hot.

 

[Jerramundi]

My guess it the data cable is newer than the conduit and a data cable installer might hasve tried to widen that hole and somehow the vibrations combined with age shorted a wire in the conduit, and the conduit is now hot.

Interesting thought, but that would take (1) some really hefty vibrations + (2) some really old, fragile wire. Like we're talking 100 yr old + cloth wire for that to happen in my opinion.

Probably more likely that the wire in the conduit was scarred during installation and vibrations shifted it around and caused contact. That, or for whatever reason there is a poor splice mid-wire in the conduit that came loose.

Definitely not enough voltage on the data cable to arc across an air gap to the conduit, unless it's direct contact.

I would do what the others have suggested and do a voltage test on the conduit itself to ground, such as the rebar. But the rebar would have to be bonded properly for that to work. I'd temp a wire from your system ground so that you know for certain what you're testing to is solidly grounded and perform the voltage test that way. If the rebar isn't proper bonded, you could get a false negative.

If a potential difference exists, you have (1) a grounding / bonding issue and (2) an insulation issue on either (a) the data cable or (b) the wires in the conduit or (c) both.

 

[hbiss]

Or the rebar isn't bonded and has become hot because of a fault someplace in the building.

I hesitated to mention it because I would hate to see it become a permanent "fix", but you could cut a length of PVC pipe or conduit of a suitable size and slit it in two. Then place it around the conduit and slide it down into the hole to insulate the conduit from whatever it may be contacting. Secure it with electrical tape. That should stop the problem until you can determine the cause. It would also allow you to probe with some stripped THHN down between the PVC and slab so that you can connect your meter between it and the conduit to see if there is any voltage.

-Hal

 

[LarryFine]

I would start by determining exactly what is energized. A non-contact tester would be a good start, followed by an extension cord and a solenoid tester.

 

[tesi1]

I would check to see if they cut thru an another unknown power conduit when drilling thru the floor & half cut thru the existing conduit with one of the wires grounding out to the new conduit, the wax simply just created a better short . a lot of floor waxes have mineral spirits which is flamable.
i have seen this many times in commercial buildings

 

[Cow]

Put an amp clamp around the conduit to see if it's carrying current.

I had one many years ago that would spark when bumped, it turned out it was carrying neutral current.

 

[xformer]

The first thing I would check is that data cable to see that it isn't damaged where it goes through the hole. There absolutely should not be any kind of voltage on that cable that would cause sparks and flame but you never know what somebody might have done.

The other thing (more likely) is that the conduit is contacting the rebar or reinforcing wire in the slab on the inside of the hole. Both the conduit and the rebar or reinforcing wire should be bonded to ground but It's been my experience that there still can be a considerable potential between the two. So, make sure the conduit is bonded back to ground but I'm open to suggestions on how to bond the reinforcing in this situation, short of breaking up the slab to expose it.

I don't think this had anyhing to do with the cleaning people spilling cleaner down the hole other than in trying to stop it from running down and cleaning it up they may have stuffed a rag down the hole which moved the conduit. So really, you should thank them for exposing what might be a serious situation.

-Hal

Would a bond directly to building steel in the area work?

 

[mwm1752]

If they had protected that rated ceiling properly it might be a different issue - I would guess work done without inspection as you can see the attempt to fire caulk the plumbing penetration in one of your pics

 

[WasGSOHM]

Big sparks when a circuit is opened means heavy current and could mean a large inductor in the circuit. And/or at least a few dozen volts.