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Fault

RE: Fault

Lets see here, what does not look right.
Overpacked conduit, sharp edges on cable, maybe some over heating caused by dense packed wires, or maybe over current.
The insulation seems a little rough, so they maybe a little old style.
Slag on the base of the conduit, could be arcing, or leftover from welding.
Seems to be clean around the conduit, was it swept, or natural blown?
Open weep holes, likely nothing, but not positive.

Gee looks like age and abuse.

RE: Fault

I've seen sharp conduit edges cut through many a conductor before. When they are first installed, the cable sheath comes out of the conduit, as it should, but over time the cable shield gets pulled down, and your conductors end up being exposed to the sharp edges on the conduit.

That's one possibility...

edit: changes cable 'shield' to cable 'sheath'

RE: Fault

First thing that stands out to me is over filled conduit. If the load is high enough on each conductor they will over heat bundled like that. Understanding conduit fill and de-rating are essential.

RE: Fault

Agree with mbrooke, looks like long-term thermal effects on the insulation. W-a-y too many conductors in that conduit.

Agree with comments about the general state of the installation too - something was going to go wrong, it was just a case of which fault happened first.

RE: Fault

(OP)
@cranky: this was the first view so no one cleaned the place.
We had the same conclusion, overpacked conduit, so it was just a metter of time after construction for the failure to happen and the fire that destroyed many small market stores. We found other cables still intact but damaged by sharp edges of other conduits, so it was just a matter of time...
On reconstruction we will change many design details and will supervise the construction as the codes state. Thanks for your observations!

http://albanesmolina.wix.com/home

RE: Fault

Miniature Circuit breakers do not open based on the temperature of conductors, they open based on current. What protects those wires from over heating are de-rating and conduit fill. Which boils down to fewer circuits per conduit, and up-sizing the wire need be. This is not an electrical issue, its a thermodynamics issue.

RE: Fault

It was not my intent to dump on anyone, but it is part of my job to think in a negative way about what can or has gone wrong.

At least there was no dead rodents, oily rags, standing water, snakes or the skins of, burnt foam fill, insects, or a nest in the mix.

And it is possible that old molded case breakers can fail, or operate out of normal test values (Who has ever tested these).

RE: Fault

(OP)
There was overheating for a while, but once the insulation broke there was a short circuit and the slag shows that. You are right cranky, those holes should had never been left open, that won´t happen again

http://albanesmolina.wix.com/home

RE: Fault

The code does not require sealing of weep holes. Punch outs, yes. But none the less if the environment is such things are getting inside you might want to consider a different NEMA enclosure. If someone can get conduit fill wrong good chance they got the enclosure wrong as well.

RE: Fault

Weldinspect; It sounds like you're saying what we see here "burned down a bunch of businesses"?

How?? That whole bundle should've been able to burn inside the panel/conduit without resorting to a burned down structure. Are we missing more input?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Fault

Derating according to the code that I work to.
The maximum size breaker allowed for #14 AWG wire is 15 Amps.
For 25 or more conductors the ampacity is 60%
A 20 amp breaker would protect a wire rated at 20/.6 = 33.3 Amps.
That would be a #10 AWG at 75 C. The ampacity of a #14 wire at 75 C will be 12 Amps.
For 43 or more conductors the ampacity is 50%
A 20 amp breaker would protect a wire rated at 20/.5 = 40 Amps.
That would be a #8 AWG at 75 C.
For 25 or more conductors the ampacity of #14 AWG is 12 Amps
For 43 or more conductors the ampacity of #14 AWG is 10 Amps

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Fault

(OP)
@itsmoked: yes, probably the EMTs heated up so high and there was a flamable material close by that started the fire.

@waross we are using NFPA 70 too, and the derating factor is considered now in the new design that it was not done before. Thank you

http://albanesmolina.wix.com/home

RE: Fault

it cause ampacity reduction

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