Arcing in Rigid Conduit - Class I Div 2

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
I have seen conductors ground out and blow fuses due to vibration in rigid conduit installed in class I div 2 areas. As per code, the conduit is not sealed because the entire run is within the classified area, and terminates in enclosures with devices acceptable in CID2 areas. When examining the damaged conductors, it is obvious that significant arcing takes place. Of course if in the middle of a conduit run there was, by design, open contacts that spark when opening or closing, conduit seals would be required. And of course the grounding fault is NOT by design. Yet it could still be expected. (And occurs often enough to BE expected). Forgive me for not going into more detail. I have pics I don't feel I am at liberty to post.

Soooo .... , I will be involved in installing conduit and wire in a similar situation. Doing so is acceptable and expected, but I don't feel good about it. Am considering pushing for sealing these conduit runs. If I do, I expect there to be push back because if (when) the same fault occurs, there will be a much longer period of down time to repair the fault. another option is to install some sort of hardened cable, instead of ordinary conductors.

Oh, there will not be detailed specs given that need to be followed.

Comments?
 
The rule in 501.15(A)(1) which applies to seals at explosionproof enclosure in both Division 1 and Division 2 locations permits the use of standard threaded couplings between the enclosure and the seal.
I did a Public Input to say that standard threaded coupling would not contain the hot gasses and could ignite the flammable gasses outside of the conduit system. I sat in on the task group when this was discussed by CMP 14 and the UL rep that is involved with the testing of explosionproof equipment stated that their testing shows that a standard conduit coupling provides sufficient cooling of the escaping gasses so that any exterior flammable atmosphere would not be ignited.
 
retirede said:
Are the enclosures mentioned explosion proof?
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No. But the devices inside the enclosures are rated CID2.
don_resqcapt19 said:
The rule in 501.15(A)(1) which applies to seals at explosionproof enclosure in both Division 1 and Division 2 locations permits the use of standard threaded couplings between the enclosure and the seal.
I did a Public Input to say that standard threaded coupling would not contain the hot gasses and could ignite the flammable gasses outside of the conduit system. I sat in on the task group when this was discussed by CMP 14 and the UL rep that is involved with the testing of explosionproof equipment stated that their testing shows that a standard conduit coupling provides sufficient cooling of the escaping gasses so that any exterior flammable atmosphere would not be ignited.
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But you need to have, what is it, 4-1/2 threads engaged? I go with "Tool Tight"
 
Use tray cable instead of standard single strand. That will buy you a longer life expectancy of the circuit.

Or GFCI protect the circuit so it will trip without all the welding going on.
 
Dsg319 said:
Use tray cable instead of standard single strand. That will buy you a longer life expectancy of the circuit.

Or GFCI protect the circuit so it will trip without all the welding going on.
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I have never seen the type of failure described in this post with single conductors installed in raceways in classified areas, and about half of my career was spent in as the preferred outside contractor for an industrial plant where over half of it was Class I, Division 2
 
don_resqcapt19 said:
I have never seen the type of failure described in this post with single conductors installed in raceways in classified areas, and about half of my career was spent in as the preferred outside contractor for an industrial plant where over half of it was Class I, Division 2
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OK, I have multiple times. It is on equipment that vibrates constantly.
 
At a different facility, we had a four shaker tables on the second floor of a steel structure. The motors for those has SO cord and the motor junction boxes had to be packed with duct seal for the vibration at the motor it self, but the vibration from the shaker tables caused the MCC bolted to the floor move back and forth between 1/4" and 3/8", and that was after they had added about 30,000 pounds of additional steel to stiffen the structure. Was about 2" of movement before the additional steel. That was a hard piped conduit project, and even there with that amount of movement, we did not have conductor failure in the rigid conduit.

The only place I ever came across in pipe conductor failures was in either concrete encased PVC or rigid installed under ground where water fills the conduit and the freeze thaw cycle crushes the insulation.
 
Since it hasn't happened to you, then it shouldn't happen to me, either? Oh the shame and heartbreak of having my wires ground out in conduit when others don't! I must be doing something wrong! Is there a 12 step plan or something? I really can't afford a conduit counselor.
 
TwoBlocked said:
Since it hasn't happened to you, then it shouldn't happen to me, either? Oh the shame and heartbreak of having my wires ground out in conduit when others don't! I must be doing something wrong! Is there a 12 step plan or something? I really can't afford a conduit counselor.
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Sharing and comparing our experiences is a valuable part of this forum. Mockery isn’t necessary.
 
Was any real root cause investigation done? Are there other factors that could contribute to the issue such as excessive heat or chemical fumes in the area that has the issue?

You did mention heat. How hot? Even if you are below the 90°C temperature thermoplastic insulation (THHN/THWN) will soften and deform at pressure points. I could see a combination of excessive temperature and excessive vibration causing this if the insulation is thermoplastic.
Area there other areas in the plant with excessive vibration where this is not happening?
 
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