Arc Flash - The real thing not the "urban myth"

[BobLfoot 301]

Dear Members - I am starting this thread because those of us that work with electricity have all been through the "horrow show" that is Arc Flash Training and if our employer is worth anything they are at least discussing the topic. Responses are as varied as the weeds and flowers in my yard. What I hope to share and discuss in this topic is actual experiences and our observations and learnings from them. Please resist the urge to "story tell" or share a story your heard. INCIDENT DATE : 5/13/2009 Location of Incident - Motor Control Center and Ammonia Compressor Engine Room Voltage, Amperage, Power of Incident - 480 VAC breaker rated at 400A. NFPA 70E Arc Flash Analysis = Category 0 closed Category 2 Open. One Line Description of Event - Electrically Qualified Individual performing a routine safety inspection noted several Motor starter cabinets were improperly closed and partially open. Employee donned gloves, face shield and arc rated smock and using insulated screwdriver rated at 1000 V began closing cabinets. Upon turning the locking screw of this cabinet the energized motor lead was pinched in the door and created a flash incident. Door was blown open due to improper latching and starter and associated gear was damaged. Substation 400A breaker tripped, Observations and Comments - Employee was unharmed but shaken. Use of proper safety gear was the main reason. This employee had completed Arc Flash Training in the past 30 days and followed our NFPA 70E based procedures to the letter. Employee reported to the work the following night with a renew healthy respect for electricity. We will be reviewing with all Electrical Personnel the habit of not fully closing and locking cabinets in Motor Control Rooms. MORAL OF THE STORY - SAFETY WORKS

[paulengr 44]

Can you narrow the location down further? Was this in Indiana? Specific company is not important. But I'll probably use this on Monday and I know I'll get asked at least where it was if not who.

[BobLfoot 301]

Not authorized to say who, but it I did participate in the after action review in Southern Indiana.

[TimWilborne 108]

My only experience with a semi-serious arc blast came in 1999 before I even knew what arc flash protection was. I was working on a startup and we were having some issues with a 100 hp motor. We had the panel doors open and were running the motor through some test when the drive or the motor (depends on if you ask the drive or motor manufacturer) internally short circuited causing the drive to burn up. During the short circuit there was a flash of light that probably didn't extend 6" past the drive. But the explosion launched shrapnel out of the drive at a rate fast enough to blow the front cover off of the drive and got through a light cotton shirt at a distance of about 6'. No serious harm done besides my ruined shirt and the mess in my pants. No burns or temporary blindness due to intense light, just shrapnel. Most interesting was the report from the drive manufacturer. It read like a modern arc flash evaluation talking about incident energy and reaction time. The drive was on a breaker instead of fuses. The drive manufacturer blamed the failure on a short circuit in the motor windings. They said that when this happened the circuit breaker didn't react quick enough and let too much energy through the drive. They said fusing the drive would have prevented this from happening. At the time I felt the drive manufacturer was just trying to dodge responsibility, but today if I were to read the report I think I would believe them. While not as glorious as most arc flash experiences that you hear about, I'd say this is more typical of an experience that we will run into.

[BobB 58]

If ever any of you get an opportunity to see a switchboard fault tested please go. It is a revelation as to what an arc can do. The last one I attended was to test circuit breaker chassis. A board was built with busbar in the bottom feeding up to chassis from 3 manufacturers. Testing was done to determine the 'claimed' fault level of all chassis. Surprisingly all the chassis passed. One well known brand of miniature circuit breakers were unable to be re-closed and failed that test. Then it got interesting. A decision was made to destroy the board. There were a number of apprentices and new tradesman there that had never seen a decent fault go off in a switchboard. Wire was tied around the busbars of the circuit breaker chassis shorting all three phases together and to earth. An unlimited fault was then applied - 120kA transformer - for 1 1/2 seconds. We were in a room looking out through a 25 mm thick Lexan sheet about 10 metres away from the board. Molten copper ran down the Lexan sheet from the explosion scoring it heavily. The explosion was deafening and we were in a sound insulated room. I have not put this here as an experience as such but rather to emphasise that if the opportunity ever arises to see one of these tests performed in a testing laboratory, make sure you go - you will surely have a whole new respect for what electricity can really do in a high fault situation. By the way, the stud mounted conical insulators that are supposed to be kA rated to what the test was for failed and the copper busbars slammed together - that was the start of the big bang. Lot to be said for the 'old' way of doing things with Permali (pressure densified and varnish impregnated ply). The Permali kept the busbars apart. Good post Bob - should get all tradesman online to read the report.

[TConnolly 13]

I'm interested in how an energized lead can become pinched in the enclosure door. On the surface, that seems to be the primary problem. I hope that the safety culture is of a nature that the underlying cause of the problem was corrected and that steps are taken so that particular cause can never occur again. Have other panels been inspected to make sure the same problem cannot occur in them? What other corrective measures have been taken besides patting everyone on the back for wearing the proper PPE and having performed training?

[robh 12]

About 8-10 years ago I was a first year apprentice tagging along on a 75 hp delta-wye motor that would not stay running. It would ramp through the delta then just after it would switch to the wye shut down. The guys I was with called the local rep out to give us some ideas. When he got there he said "oh yeah, all we need to do is move this jumper over here". I watch as the jumper is moved and stood about 10' to the side of the open bucket door. One of our guys gets the ok and hits the start button. BOOM!, FLASH!, BANG! Watched as a 200 amp breaker blew up because of a phase to phase to phase short. Wasted everything in the bucket, 2 size 4 starters, one 200 amp breaker, and some mechanical timers and relays. Nobody was hurt and everybody was clear of the bucket when word was givin to start. Rep had some parts to buy. I learned a few things that day. Never stand in front of a bucket while testing or switching (even in my arc flash gear), what a delta-wye start motor was, what an arc flash looked like, and to never, never let an outside person get near anything I am working on. I found the real problem later to be a pressure switch that was not confirming pressure within a set amount of time, thus shutting down the motor, just so happens that timer was about 3 seconds after the motor went to wye.

[paulengr 44]

Oh come on, you've never defeated the disconnect (flipped the latch) and reclosed it with the door open? How do you check for problems in those situations where you have to have it live to do the test, such as when you have your amp clamp hanging out the door? I've seen many electricians do this as a matter of COURSE instead of closing the door first, THEN re-energizing. It becomes force of habit when you are working with stuff live...I said habit, not a good one at that. As to why the door wasn't closed properly in the first, it's simple. Nobody is getting fired or severely reprimanded for taking shortcuts, and supervisors aren't following up. It only takes once or twice before everyone has an understanding of expectations. Sounds like though that at least one guy in the crew will start closing the doors properly from now on.

[plastic 0]

Please resist the urge to "story tell" or share a story your heard. No "hearsay", "what happened to me", or "story tell", but a link to another documented incident. http://www.hss.energy.gov/csa/csp/aip/acci...AC_20041011.pdf

[Ken Moore 23]

Good link, thanks

[robh 12]

X2

[TConnolly 13]

I'm not asking why a door was opened or closed. I'm wondering why the wire was routed in such a way that it was possible for a conductor to get pinched in the door. Why wasn't the wire secured in the cabinet? Why was there that much wire slack? If it is possible that closing a door will make contact with energized wires then something is wrong either with the design or with the quality of the installation. Its not unreasonable to expect that there might be an electrician on staff who routinely leaves wires extra long and unsecured and thinks he has a perfectly good reason for doing so. Now that a potential hazard has been identified, other panels on site should be inspected for the same hazard. Whether the panel was energized at the time the door was closed or de-energized first and then subsequently re-energized is irrelevant. The short from the pinched wire would have occurred either way. However the accident is completely preventable through correctly routing and securing the wires. No one is pointing any fingers, this is about improvement. Safety is as much or more a culture than it is a stack of regulations and training sessions. We will never get better if we don't ask and answer the hard questions. Why did something that should not have been able to occur happen? Have other panels have been inspected? Have corrective measures have been taken? Edited 21 May 2009 by Alaric

[JeffKiper 9]

In 1999 I was a second year apprentice. Got a call to look at a motor that had tripped the OL small 25hp 3ph. I found a bad B phase contact on the contactor. We did not have a repair kit for this old GE. I pulled the contacts out and filed them they looked good. I placed them in the frame and put the 2 clips on the NEMA 12 panel not tight. First time I hit the start button BOOM!!! I didn't clean the filings off. The plunger of the contactor was blown across the 10' walk path and hit another machine. That day my respect for that energy source became ten fold. The power that we take for granted everyday by harnessing it can break free and turn on us at anytime so we must be ready.

[paulengr 44]

http://www.mikeholt.com/newsletters.php?ac...mp;letterID=727 Just got this one in the old E-mail today.

[plcdp 11]

There is a company that manufactures fire resistant clothes that does testing using actual arc flashes. The company is called Westex and on their website is a video library of arc flash demos. Amazing!

[paulengr 44]

Slight correction. They make the coating, and/or I believe they will also coat the cloth. As far as I know as long as it's 100% cotton, they can use it on almost anything. They don't really make the clothing themselves. Almost all the stuff I've seen out there has the Westex coating in it when I went shopping around not too long ago. The only other prevalent choice is Aramid fibers which will be either Nomex or Kevlar depending on the weave and intended use. Trouble with Nomex is that it is heavier, hotter, more expensive, and gives you about the same amount of protection. I don't know the name but I know that there is definitely a third coating out there. The standard light weight welding "greens" are coated with something different. Unlike the Westex stuff, it will wash out of the clothing so that after about 50 washings, you have to throw the whole thing away, and it can (or should) carry warnings about this. When I worked in a foundry, we had several guys that got burned because they were wearing older "greens" that caught on fire because the coating had washed out. So let it be a warning to you...whenever you buy anything other than Nomex, be sure to ask how many washings that the clothing will tolerate, and KEEP TRACK. Winter fire resistant Carhartt winter jackets are one thing (where it might get a washing 4-5 times a season), but regular every day shirts and pants are an entirely different story!