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ekstra   ara
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PostPosted: Wed Dec 16, 2009 9:25 pm 
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Location: washington
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TX-I used to agree with the same practice you follow,pro maintenance,least disruptive to operations.

A close call with a Motor Starter Blowing up,an underated Circuit Breaker 35 ka in a 65 ka assembly,250 amp breaker set @ inst trip of 2500 amps,all due to poor insulation on a motor junction box .

The probability of this happening again is high until all associated motor connections are inspected and inst.trip settings adjusted as well as the AIC rating increased on the buckets.

These particular buckets are Siemens mfg. with a lot of open grill work in the front panel,a lot like a barbecue grill!You know most disasters are a combination of events,In this case I realized the contained explosion could have migrated to a full blown MCC meltdown,due to a combination of design factors.

In the end,companies can say the employee was not following the rules.I guess one really needs to study the design of the system and weigh the risk against the likelihood,I now feel as though deenergized is the way to go.It may be a pain but so is having your skin peeled off in a burn center!

Is it really worth the risk? How long has this been going on? Maybe too long..be safe..Thanks for the response


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PostPosted: Wed Dec 16, 2009 9:28 pm 
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Joined: Fri Dec 04, 2009 8:15 pm
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Location: washington
Statistics

I have heard that 40% off all Arc Flash incidents are due to equipment malfunction,In other words no one working on the equipment.Does anyone have information on these percentages?


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PostPosted: Thu Dec 17, 2009 6:01 am 
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Joined: Fri Jan 16, 2009 1:07 pm
Posts: 67
Location: North Florida
Brodie,

I completely agree with your comment that you must analyze the equipment and installation you have and make the best/safest decisions you can based on that information. My comments are based on the minimum requirements as I read them in the various codes. That becomes your starting point for making safe decisions.

When dealing with older equipment, the minimum is often not enough. That's why I'm in the process of a 3 year upgrade of some of my low voltage switchgear. At other places I've worked, the MCCs couldn't even be locked out properly. They were so old, they didn't even have locking mechanisms on the starter doors and we had to modify the starters to add locking mechanisms. It just points out that you must take the age and condition of the equipment into mind when establishing safe procedures.

Good luck and stay safe.

TxEngr


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PostPosted: Tue Dec 22, 2009 6:56 pm 
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Location: New England
My comments related to the breaker being designed to an acceptable AIC rating. As stated, it did not refer to Arc Fault IE. You do not need to 'derate' to 80% the manufacturers AIC. If you read the UL method for determining the AIC you will see that it is done at an extremely low power factor which would result in very high peak asymmetrical amps. The power factor in most applications will be much higher with lower peaks. There are enough safeguards built into manufacturers rating already, you don't need to add more.


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PostPosted: Tue Dec 22, 2009 7:31 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Haze,

I assume your 80% post is in regard to my previous since there have been no others about the issue, if not please disregard. Distribution systems are dynamic and that is why I opt for the margin. I make a living working on these systems, and have for the past 30 years, so I know the variances. Otherwise, do you really want to expect a 20 to 40 year old piece of equipment to have to do the max. I don't.

I have seen cabinets and 3" steel angle legs buckled on old oil filled breakers that were not exposed to faults in excess of their initial rating, and much less than my 80% max.

I know that it is arbitrary, but we do need to do some due diligence.

If your employer stresses you to the max at every crucial point, I expect you will end up needing some margin as well.


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PostPosted: Thu Dec 24, 2009 10:42 pm 
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Location: Ohio
haze10 wrote:
Brodie,
A couple of things:

The CAT 3 is for the MCC as a whole, which is meaning the bus. In the bucket you have small wire leads that come off the stabs, and that short length will limit your fault current more than you realize. It would not take much to calc the IE on the load side of that wire, even if you just model it for the largest one, assuming no more than size 3 or 4.

In a combination starter that employees a breaker, that breaker usually only has an 'instantaneous' function to prevent false trips. The motor 'overloads' (also called heaters) provide the long time trip functions. Table 430.52 of the NEC sets the max limit for the size of the circuit protector versus the motor FLA. For instantaneous trip only breakers it is 800%. If its more than that then its not code compliant.

As far as the AIC of the breaker. You said they were 'overdutied' not sure what that means. They have to have a minimum AIC that will handle the fault current at that point. Again, the fault current 'at that point' is going to be less than the fault current at the bus. The breaker has to have some AIC rating you just have to find it or look it up. So tell the owner that there is a chance he may be able to solve two problems by doing the bucket calc, which may lower the IE for live work on an open bucket, and make his existing breakers now code compliant. Maybe!

Cat 3 on a typical industrial MCC, other than something huge, would imply to me that your overcurrent device has too much time delay. You are operating in the short time band of the trip. There are only limited reasons, like small MCC lineups with big big across the line starters - that you can't employ an Inst trip set on your MCC. I was running 800A feeders to a MCC lineup, that had Inst trip, and was starting a 300HP motor on a wye-delta starter. It never came close to drawing the 6500 amps needed to hit the Inst trip point.

A lot of times the company doing the analysis does just that - the analysis. The numbers fall where they may. A good analysis will also point out opportunities to reduce the IE. I've taken MCC's with a Cat 3 to a Cat 1, by changing a $1200 programmer on a switchgear breaker to one that only had 'short time' to one that had 'short time' plus Instanteous. That was two years ago and I talk to the electrician regularly - they've never tripped the breaker. And I knew they wouldn't. So there is lots that can be done.

As for the policy, written versus what you are instructed verbally, a hardbound log book that is used regularly and consistently can be submitted as evidence. So if you are placed in a situation that the owner commands you to violate his own policy - I would snap to attention, say "yes sir", then write down the request, date, name, reason, etc in my log book, don my FR, and do the work. You show up with your log book with 50 entries after someone is hurt, and you'd be glad you had it.

There is a good chance that the 'infeasible' definition may be applied to this company's MCCs. They just need to perform the analysis. There is nothing like having 3 or 5 engineers, ESHa, production people in a meeting, and have the notes of the meeting documented and added to the appendix of the Arc Flash Analysis Handbook to show why the conclusion was made. A lot of this is about following procedure and conforming to the intent of the code.

My 2 cents.


Another engineers view:

1. There is no such thing as a secondary calculation on the bucket. The fault can propagate to the line side of the bucket and even to the line side of the incoming main device. I have pictures of two such incidents, one was a death. Secondly, the lower fault current can increase the IE.

2. In addition, an MCC is a prime example of a barrier or horizontal case, this increases the the IE beyond the typical 1584 calculation. Refer to Mike Lang's paper on the Ferraz website for clarification.


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PostPosted: Fri Dec 25, 2009 3:19 pm 
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Joined: Fri Dec 04, 2009 8:15 pm
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Location: washington
Pictures

Fash I would like to view your photos,I would like to make others aware of the possibilities and consequences of arc flash migration.In my personal case an overdutied breaker in the MCC bucket AIC 32 in a 65 KA MCC could have led to such an incident.
The facility I work in has several of these breakers.The Short Circuit Study only reccomended replacement,In my opinion these breakers must be replaced and until they are unsafe work conditions exist within the plant :mad:


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