At a sister plant of ours they had a arc flash and it melted the 3/4" conduit almost in half.The wires were fed from a cable tray that went over to a 3/4" conduit that fed a 75KVA transformer the wires were not junctioned at any point in between.There is no vibration to speak of in this area. I have pictures but not sure how to load to this forum but I was trying to help with what happened and I am still puzzled. The job was not even 2 yrs old and the transformer was not even 50% loaded. My only conclusion is that the wires were nicked on installation and finally went to ground or to phase to phase, voltage is 575v. My question is if eveything was properly grounded would a wire going to ground( conduit)melt the conduit to where it looked like a torch was put to it. I am not sure of how it was fused. Would like to hear comments if someone has one. Thanks in advance .
conduit arc flash
- Thread starter zmikc115
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al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: conduit arc flash
You can get a free place to park a copy of your photo at places like www.photobucket.com that will allow you to use the IMAGE button in the "Instant UBB Code?" section while you post a reply. The What is UBB Code ? , in the same location is a help page that will provide some pointers.
You can get a free place to park a copy of your photo at places like www.photobucket.com that will allow you to use the IMAGE button in the "Instant UBB Code?" section while you post a reply. The What is UBB Code ? , in the same location is a help page that will provide some pointers.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conduit arc flash
Such a failure could be explosive in nature, and could severely damage the local area. This might explain the blow-torch appearance of the damage. Also, if the conductors were really packed in that tight, then there would be very little air space to take up the force of the arc fault.
Can you verify the contents of the conduit (i.e., number, size, and type of the conductors)?
I didn't pick up on that. Good point. At 575 volts, the primary feed to a 75KVA three phase transformer would be rated at 75.3 amps. You would need at least a #4 copper to handle that current. You can only get 2 of those conductors in a 3/4 inch conduit. It they really had 3 or more #4AWG conductors in a 3/4 conduit, then it was destined to overheat and fail sometime. Perhaps it failed by way of a 2-phase or 3-phase short that would result when the high temperature caused a melting of the insulation.
Such a failure could be explosive in nature, and could severely damage the local area. This might explain the blow-torch appearance of the damage. Also, if the conductors were really packed in that tight, then there would be very little air space to take up the force of the arc fault.
Can you verify the contents of the conduit (i.e., number, size, and type of the conductors)?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conduit arc flash
OK. That makes the rated current only 40 amps, and a #8 conductor would serve. You can get enough #8's into a 3/4 inch conduit. So we are left still scratching our heads.
By the way, I thought the standard transformer sizes went from 7.5 to 15 to 30 to 45 KVA. I didn't know there was a 40 KVA. But then, I am not familiar with 575 volts as a primary feed voltage either.
OK. That makes the rated current only 40 amps, and a #8 conductor would serve. You can get enough #8's into a 3/4 inch conduit. So we are left still scratching our heads.
By the way, I thought the standard transformer sizes went from 7.5 to 15 to 30 to 45 KVA. I didn't know there was a 40 KVA. But then, I am not familiar with 575 volts as a primary feed voltage either.
peter d
Senior Member
- Location
- New England
Re: conduit arc flash
1. Canada.
2. An old industrial facility still using 600 volts.
3. An industrial facility that purposely chose 600 volts rather than 480.
I have a few guesses:
1. Canada.
2. An old industrial facility still using 600 volts.
3. An industrial facility that purposely chose 600 volts rather than 480.
Re: conduit arc flash
Agasin just going by what electrician is telling me that it was 40 KVA. We are an old paper mill with 575voltage. Alot of people not familar with 575 v or even heard of it.Makes it nice sometimes because of wire size and other things but difficult when you want a motor in a hurry. I guess my question still goes back to would the conduit have melted if the grounding was in place properly?
Agasin just going by what electrician is telling me that it was 40 KVA. We are an old paper mill with 575voltage. Alot of people not familar with 575 v or even heard of it.Makes it nice sometimes because of wire size and other things but difficult when you want a motor in a hurry. I guess my question still goes back to would the conduit have melted if the grounding was in place properly?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: conduit arc flash
OK. Let's make some suppositions:
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I can not say that this is what did happen. But one question that you "asked but didn't exactly ask" was, essentially, "Could it happen"? So my answer to that unasked question would be, "Yes, it could happen."
Furthermore, I will add that if there were a GEC within the conduit, and if there were proper bonding connections between the conduit and the GEC (at one end of the conduit, or at the other, or at both), then the fault current (from phase to conduit) would be much higher than 100 amps. It would also have lasted a far shorter time period, as the breaker would have tripped on the fault current. But could the conduit have seen the damage you described, under this set of circumstances? I am not certain, but I think it could have. So to address your specific question, "Could a proper EGC have prevented this damage to the conduit," my inclination is to say that it might not have prevented the damage.
OK. Let's make some suppositions:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Suppose that the feeder to the primary did not include an Equipment Grounding Conductor, or that it was not properly connected.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Suppose that the conduit could not serve as an EGC, either because it was not intended to be a low-impedance path back to the source (i.e., it had non-metallic connections) or because it was not properly connected.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Suppose that the conduit did have a low-impedance path that led to planet Earth, perhaps via bonding wires to building steel to the Grounding Electrode System.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Suppose that there was both primary and secondary protection for the transformer, and that the primary protection was set at 250% of the rated primary current.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Finally, suppose that the resistance of the Grounding Electrode System to planet Earth was 5.75 ohms (obviously, I am looking for a limiting case here).</font>
I can not say that this is what did happen. But one question that you "asked but didn't exactly ask" was, essentially, "Could it happen"? So my answer to that unasked question would be, "Yes, it could happen."
Furthermore, I will add that if there were a GEC within the conduit, and if there were proper bonding connections between the conduit and the GEC (at one end of the conduit, or at the other, or at both), then the fault current (from phase to conduit) would be much higher than 100 amps. It would also have lasted a far shorter time period, as the breaker would have tripped on the fault current. But could the conduit have seen the damage you described, under this set of circumstances? I am not certain, but I think it could have. So to address your specific question, "Could a proper EGC have prevented this damage to the conduit," my inclination is to say that it might not have prevented the damage.
Re: conduit arc flash
Thank you for your answer and with out knowing some things about this situation I think this is the best we can do. I thank you again for your answer and I think you are probably right. I will pass this along and maybe it will prompt someone else to look harder at doing a better job in the future. I would have hated to been working in that area when that went on. Every days is a holiday and every meal a feast every night like the fourth of July. Well thats what my drill instructor use to say. LOL
Thank you for your answer and with out knowing some things about this situation I think this is the best we can do. I thank you again for your answer and I think you are probably right. I will pass this along and maybe it will prompt someone else to look harder at doing a better job in the future. I would have hated to been working in that area when that went on. Every days is a holiday and every meal a feast every night like the fourth of July. Well thats what my drill instructor use to say. LOL
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