4160 and 480 voltages don't mix

I had an issue with a refiner at our plant yesterday and I was surprised to see the cause. We have a refiner that operates 2 800 HP motors and the control voltage is 120. There was a bundle of wires that ran in the starter cabinet that were used for various limit switches and such. Whoever installed it thought it would be a good idea to secure the bundle to the top of the cabinet with the sticky back zip tie blocks. What we found was the bundle had come loose and fallen onto the top of the mains connection. After a period of time one of the wires rubbed and from our best guess, shorted between 2 phases. I was standing in front of the cabinet working in the 120V section of the cabinet trying to see why the amp signal was not going back to the PLC. Just before I hooked up my process meter there was an explosion and a bundle caught fire. After everything was said and done we had to replace 3 output cards, several din rail fuse blocks, a UPS, and lots of wire. If we didn't have the UPS it could have been much more expensive. The reason for this post is I have a few question;

1. The wire that caused the failure tripped the breaker in our substation. The wire was a #16 THHN. The breaker in the substation feeds 2 Refiners, one has two 800HP motors and the other has one 1000HP motor. How can the #16 wire cause that kind of trouble? I would think the wire would have melted and the motor would have kept running.

2. The rating in the #16 wire is 600VAC I think. If that is the case, did the insulation wear through or could it be caused by the wire just touching the lug.

3. At the same moment we had another breaker trip that is feed from a separate transformer and runs at 480vac. It is a 250hp motor on a VFD. The VFD had a fault of "over current". Are these events related?

Got a pic of the bundle if anyone is interested...

Thanks,

Kevin

I had a tech using a voltmeter and didn't realize he had it on amps when checking a motor starter on the output side. It took out all 3 250 amp fuses upstream on the disconnect. It seems incredulous that meter lead wire that is smaller gauge than Walmart speaker wire can do that. Once that arc no matter how small creates ozone combined with the vaporized copper, party's over. Fortunately the only things hurt were his meter, his dignity and 6 hours downtime finding replacement fuses because we only had 2 on hand.

There are several good lessons here to think about.

All energized electrical equipment is dangerous. Hopefully, you were at least wearing clothing that was rated for the arc flash energy available in that starter. If your plant hasn't done an arc flash study, they should immediately.

Sticky back wire ties are not a permanent installation method. I have outlawed them here for just that reason. I had a thermocouple wire installed with sticky back on a 13.8KV-120 V xfrm. The vibration cause the same thing only our primary protection was not existant and we lost a transformer and a static exciter in a huge fireball.

Your wire created a phase to phase and possibly a phase to ground short. The wire was just a bridge that started the arcing (and as todster stated) once you created the arc, it no longer needed the wire, just something to interrupt the arc - you substation breaker.

The insulation most likely wore through. Even though 600 V wire doesn't seem to be rated that high, it is test with 8,000 Volts as a proof test at the wire manufacturer - of course, you can not rely on that for a rating.

The reason the other breaker tripped is because the voltage at the facilty went way down during the fault. This is not uncommon in a facitly that doesn't have a prefect coordinated power system. I would bet that you don't or haven't done a coordination study or an arc flash study.

Go through and make sure there is no other place that control wires are run about high voltage mains (or any other mains). I would also remove all sticky back wire ties.

Thank God your OK.. Wondering what type of PPE were you wearing when this occured. Did you have a fire rated uniform, facesheild?? Please let me know as I am trying to change the culture at our plant to make sure we use the proper PPE when troubleshooting. Your story points out how a routine troubleshooting call can go wrong. Your feedback would be greatly appreciated.

Bruce what do you use in place of stickbacks? We have that same problem in many panels here. It is something i would like to correct.

The Plc Kid said:

Bruce what do you use in place of stickbacks? We have that same problem in many panels here. It is something i would like to correct.

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Here are some suggestions.
Panduct link:
http://www.panduit.com/stellent/groups/mpm-lp/documents/pdfcatalog/068548.pdf
Cable ties screw mounted:
http://www.ace4parts.com/Products/AL-SM1-9C--TY-WRAP-MOUNT-6-SCREW-100-PK__AL-SM1-9C.aspx

The Plc Kid said:

Bruce what do you use in place of stickbacks? We have that same problem in many panels here. It is something i would like to correct.

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We rarely have the need for anything like that now. On most panels where I have a cable outside of the wireway, I use a spiral wire loom ( http://www.grainger.com/Grainger/items/1LEZ4?Pid=search )to keep them together and if I need to put in a wire tie, then the screw terminal that jrwb4gbm showed in his links (page E12) or like this ( http://www.grainger.com/Grainger/items/4YF44 )is what I use. I also do not allow self tapping screws to be installed with anything either. Those will eventually loosen up and cause the exact same problem. Drill and tap for anything mounted in a panel.

Your goal should be to not have to use these in a panel or starter if possible. Most of the time during troubleshooting, the wire ties will be cut and thrown on the floor and nothing put back in its place.

As far as PPE, I was wearing leather gloves and safety glasses. I didn't think of wearing anything else because the cabinet is seperated into 3 sections with 3 doors. The bottom 2 are for each motor starter and the top is for the control equipment. As far as the zip tie blocks it is against our company policy to use them for electrical, it has been that way for a few years. We didn't see the bundle because where it was tied it was hidden very well. I had that cabinet open 2 weeks ago and didn't see that bundle. We are going to re-route all wires and find a different way to secure them. Our plant follows the arc flash rules but has not had the arc flash calculations done for each panel.

jrwb4gbm said:

Here are some suggestions.
Panduct link:
http://www.panduit.com/stellent/groups/mpm-lp/documents/pdfcatalog/068548.pdf
Cable ties screw mounted:
http://www.ace4parts.com/Products/AL-SM1-9C--TY-WRAP-MOUNT-6-SCREW-100-PK__AL-SM1-9C.aspx

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I use the plastic trunken as well even to carry wires to buttons on the door but the only problem i find is mice love to nest in them.Maybe its just the factories i work in animal feed plants.You could also use spirial wrap see link
http://radionics.rs-online.com/web/...matchallpartial&N=4294964553+4294955809&Nty=1

Liam

Attached is the pic. The wire is laying on top of B phase. If you look at the bolts on A&C phase it looks loke the acc went between all three phases. One other thing I saw was there was another bundle in another part of the cabinet that caught fire but didn't seem to have any wear damage. That is why I asked about the insulation rating. It was the only other bundle that was damaged and there were other bundles much closer to the arc.

Thanks for the replies!

Wow, thats an amazing picture, just immagine what would have happened if you were in contact with that low voltage wiring. There doesn't seem to be much damage to the wires though.
Did you go out and buy a lottery ticket?
Roy

many Years Ago I was "assisting" a commissioning engineer on a small crushing plnat when he went to check the voltage accross two phases of a very large motor, unfortunately he had the fluke meter set on mAmps at the time. There was a very loud BANG and the lights went out, he was unharmed and the Fluke's leads (silicon) were warm to the touch. The Fluke meter was a mess internally but looked OK from outside, apart from a craked screen. The only other equipment damaged was the main breaker for the switchroom, it had to be replaced at (from memory around $20,000). I went out and replaced my meter's leads with good quality silicon leads that day..

todster said:

I had a tech using a voltmeter and didn't realize he had it on amps when checking a motor starter on the output side.

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Last week the new manager here had his test leads already hooked up when he spun the meter selector switch so get to the volts setting. Somewhere along the way he must have passed through either the ohms or current settings...

Not too much of the meter was left when the smoke cleared, and his hand got blackened from the flash, but no real harm done.

The funny thing was that he just got through telling the electrician (Vince) that "he knew this electrical stuff".