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 Post subject: Table and Arc Clearing Time
PostPosted: Tue Feb 19, 2008 4:49 pm 
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With reference to the Notes in Table 130.7C9A. Notes 1&2 cite a 2 cycle clearing time. That to me is pretty fast for what they model as an Industrial model. The table category is MCC's, so to me that means fed via Switchgear and Not molded case. Switchgear breakers have two basic time components: 1st is the relay time to react (lower and upper bands on the TCC) but then you also have the mechanical reaction time of the breaker. Usually this alone is 2 to 3.5 cycles.

With Instantaneous upper band time being up to 4 cycles, you get more of a maximum of 7.5 cycles. I've always used 5 cycles on average.

But this is way off from the Table, unless the Table is only looking at the Relay Reaction time.

Can anyone help with this, or am I missing something.


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PostPosted: Wed Feb 20, 2008 7:11 am 
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Haze,

The upper part of the time current curve's instantaneous is the total clearing i.e. breaker sensed, opened and cleared the fault. The fault is generally cleared at a zero crossing (momentary arc extinction) so it clears in 1 cycle, 1.5 cycles, 2 cycles etc. Yes 2 cycles is very fast indeed. A motor circuit protector or molded case breaker could probably do this but a lot of switchgear breakers take around 5 cycles. I guess this is just one more reason to calculate the incident energy instead of using the tables. There are lots of simple methods in 70E surrounded by lots of exceptions, boundaries and limitations. A medium voltage system with relaying would be a whole different animal. Relay operation, safety margins, induction disk overtravel (for older relays) breaker opening time.....
Hope it helps!

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PostPosted: Wed Feb 20, 2008 12:54 pm 
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Jim,
In my question I am referring to 480V Switchgear. The drawout, open air kind say like GE AKR class or Siemens RLE.

When you look at the TCC curve for the Trip Unit, and fully understanding that the width of the bands indicate min and max reaction times - are you saying that the curve times 'include' the breaker cycles to full open. So that in effect the curve is a combination of the 'Relay Reaction Time' plus the 'Breaker to Full Open" time.

I wasn't sure about this as today you see a lot of third party trip units on gear. The TCC curves are usually by the trip manufacturer, so how would they know the 'Breaker Full Open' time from the gear maker, unless they were just using industry norms.

Further explanation would be helpfull. I have a location where a clearing time of 4 cycles versus 7.5 cycles makes a big difference.


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PostPosted: Wed Feb 20, 2008 5:12 pm 
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If you have GE or Siemens' trip curve for the entire breaker/trip unit, it should have everything included in the band. That is typically how they are published. You do need to know the details like the old Westinghouse breakers used to be very specific if you had an Amptector 1A, or Digitrip RMS 310/810 etc. The curve had it all included. If it is a retrofited trip unit you are correct it would be a function of the breaker operation and the retrofit trip unit. You would probably have to contact GE or Siemens to see what they say at that point.

The other problem is if the time difference of a few cycles makes a large difference in the results, when was the last time the breaker was tested? If it has been a while and the breaker ends operating a little little slower "than advertised" that could likely add to the problem.

Lots of good postings out here. Sounds like your digging pretty deep into this. Keep it up!

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PostPosted: Wed Feb 20, 2008 7:59 pm 
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Thanks Jim, this is pretty much what I thought but confirmation is comforting. Think I mis-spoke, the difference is not so much a significant change in Incident Energy as it is in PPE. The 3 cycle difference was taking me from 7.8 cals to 10 cals. So Level 2 to 3.

This is a difficult application as the Main CB on the Switchgear has only LT, ST, and GF - not unusual. But all the branch CB's also have only LT, ST, and GF. So with the Short time set to its lowest setting, and I2T switch OUT, the max time band is saying 0.18 secs. I'm far into the horizontal portion of the curve and have plenty of Ibf. 0.17 sec is just the absolute fastest point on the curve. It is a band from a low of 0.10 to 0.17 secs but guess I have to use the upperlimit of the band as is customary.

I'm getting some quotes for new Programmers that have LT, ST, INT, & GF - and changing out the programmer is a piece of cake. But they won't be cheap and then they have to be field tested. All of which will be some $20K. But it would mean the guys could do all work in Level 2 and not Level 3. I can't see the practicality of changing fuses or doing voltage tests on a Size 2 FVNR bucket in Level 3. I don't care what the bus Ibf would be, that level Ibf would never travel through the MCC vertical bus, bucket stabs, and the line side leads.

I guess a person could include the added Z of these components, but with no Instantenous the time element would still be long and IE high. Seems like we need a practicality multiplier of something less than one. Using the Matrix would be okay, as I would only need Level 3 for bucket insertion, but that 2 cycle limit is a bear. I wish they would have permitted some combination like on the Flash Boundary less than 300KA cycles. At least that would allow some flexibility for a few more cycles with a lot less than 65k Ibf. Seems NFPA chose an very fast clearing time and a very high Ibf. I don't see many industrial installations with Ibf that high. If you stop and think that must equipment rarely exceeds 65K and 65k Is actually a high rating for a lot of manufacturers, there is not going to be many installations that actually develope 65K. Maybe 50K but for most transformers greater than 2000KVA I think the more expected solution is to request a higher %Z.

Any ideas.


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PostPosted: Fri Feb 22, 2008 11:18 am 
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Sounds like the addition of the IP will do the trick but too bad about the cost. No IP on the main is fairly commong but any time I have see a system designed without an IP on the main AND feeder, it usually means reliability / coordination is at a premium. Will the addition of the IP be acceptable to the operations people? Other than that, I think you have it covered.

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PostPosted: Fri Feb 22, 2008 11:24 am 
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A few sympathetic observations:

2 cycle clearing time in the tables seems to indicate fuse protection, not typical of what I have encountered and therefor the tables are not usefull.

Instantaneous trip is nearly mandatory to control arc flash exposure. I have proposed replacing trip units with new that include instantaneous, though that project has not been constructed yet. It is good science to do this, not terribly expensive, but does cause potential coordination problems with downstream protective devices if those don't also have instantaneous.

I have not felt there is a huge advantage in reducing exposure from RC 3 to RC 2. Most of our challenges have been to reduce exposure from "unworkable" to anything workable, or down to RC 1 because these plants already have a requirement of wearing one layer of nomex.

When you are calculating clearing times and setting your instantaneous pickup to minimize clearing times, do not overlook that the calculation will include "arc resistance" and your clearing time will not be driven by max available fault current, but something perhaps 50% of that value.


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