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 Post subject: Effect Of Underrated Breakers On Arc Flash
PostPosted: Fri Jan 27, 2012 9:37 am 

Joined: Fri Jan 27, 2012 9:27 am
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What are your opinions and practices on the effect of Short Circuit Study/Equipment Evaluation results on Arc Flash?

Example: an Arc Flash study has determined the main breaker for an MCC is Category 3 and all the feeder breakers/MCPs are Category 1. However, the Short Circuit study has determined that while the breakers are 25kA rated, there is actually 32kA fault available.
Does this affect the results of your Arc Flash study in any way?

Thanks!
Nathaniel


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PostPosted: Sat Jan 28, 2012 6:04 am 
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Absolutely! Breakers are rated to indicate the maximum fault current (plus X/R ratio concerns) that they are designed to interrupt. As the current increases, two major concerns arise.

First off, the breaker may not be capable of interrupting the current. At this point, you have to look at the next upstream protective device to determine the fault time, if it faults at all. You may also want (have) to consider the "2 second rule" from IEEE 1584 and whether or not it applies in this circumstance to determine the worst case scenario. Whatever software you are using to determine "category 1" is now invalid because it is based on assuming that the breaker trips. That's about it from an arc flash point of view. The reality is that the arcing fault is most likely to be quite a bit lower so that the breaker SHOULD open (arcing current = 27 kA using IEEE 1584) but there is a small risk that it doesn't open as expected.

Second, there is the short circuit protection point of view. The bus bars and/or wiring are subjected to a magnetic force proportional to the square of the current in a fault condition. Under a bolted fault condition (not arc flash), if the phase angle is right so that you see full assymetrical current flowing during a fault, the bus bars are going to fail and come flying out of the cabinet since the structural design will be almost half of what it needs to be.

Making the mistake of underrating for short circuit protection is one of the most expensive problems to "fix" because unlike various options available to somewhat mitigate arc flash there's not much that can be done about it without replacing major equipment. It's time to either upgrade the switchgear or sometimes you can simply buy a new transformer with a higher than standard impedance to lower the risk. This does NOT automatically result in reduced arc flash. Sometimes it increases it.


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PostPosted: Thu Feb 16, 2012 7:05 am 

Joined: Fri Jan 27, 2012 9:27 am
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Thanks Paul, I understand your points.
Now how do you proceed if you're doing a large scale arc flash study? Obviously the gear is underrated and needs to be replaced. But until it's replaced, how do you label it?
Does your software automatically
1) compare the breaker rating to the arcing fault current, and then
2) if the breaker rating is lower than the arcing fault current take the breaker out of service and
3) calculate the trip time for the nearest upstream device
Or do you do that manually?
Do you indicate on your labels that this is a special case where the arc flash results are modified by the equipment evaluation results?


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PostPosted: Thu Feb 16, 2012 7:51 am 
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Location: Wisconsin
AS far as I am concerned this is not a software issue.

The arc flash calculation determines the amount of arc flash incident energy available at a location. The short circuit current values which result in the worst case AFIE may bear little resembelnce to the currents used in a device evaluation.

Device short circuit evaluations might need to be made based on a 'design level of fault current' provided by the POCO which usually takes into account future system upgrades. These design levels may bear little resemblance to the 'actual' installation analyzed in the AF study (for example, the installed transformer may be a 300kVA unit, but the utility reserves the right to install up to a 500kVA if that is the only replacement unit available).

Then there are other issues such as series-rated combinations of devices as well as the industry's AIC rating break points.
Say a breaker is tested at 17kAIC, this is not a 'recognized' break point so it will probably be called a 14kAIC device. However if it is tested in a series-rated combination it might be rated be at 22kAIC. So is this breaker misapplied if the bolted fault current is 15kAIC?


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PostPosted: Thu Feb 16, 2012 9:06 am 
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JBD wrote:
Say a breaker is tested at 17kAIC, this is not a 'recognized' break point so it will probably be called a 14kAIC device. However if it is tested in a series-rated combination it might be rated be at 22kAIC. So is this breaker misapplied if the bolted fault current is 15kAIC?


I thought the interrupting current of a CB was a pass/fail test, so I don't see why it would be tested at 17kA if the listed IC is 14 kA. And if it was tested (internally) by a manufacturer, as a end user, I wouldn't know about that value either.

What I mean is in such a case, either you wouldn't know about the 17 kA and only about the 14 kA (so no mean to know 15 kA is good for that CB), or you know about a higher value (series-rated or not) and then 15 kA is good to go.

I have a datasheet about Eaton breakers. A KD frame breaker alone is rated 25 kA, but series protected by 400 A Class J fuses, it can be exposed to 200 kA. Would I use it alone exposed to 27 kA? No, I'd upgrade it to a HKD frame breaker (35 kA alone), which is exactly what I'll do in a few days.


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PostPosted: Thu Feb 16, 2012 10:23 am 
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Hi Nathaniel,

I wonder if the installation design relies on cascading of protective devices whereby the upstream device is designed to limit the energy let through to the downstream circuit breakers. Apologies if you have checked that point out.

Mike


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PostPosted: Thu Feb 16, 2012 11:10 am 

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Well it looks like we've gotten both sides now, with PaulEngr modifying the AF study based on the equipment evaluation and JBD saying they are irreconcilably different. I thought there would be a variety of very valid yet very different perspectives on this, which was why I also posted a poll on this forum (I can't post a link, as I am a new member, and you don't trust me not to spam you). If you haven't already voted, please do!

And a followup question: what happens to a breaker that experiences a fault that is greater than it's rating?
Does it
A) Attempt to open and is unable due to magnetic forces, etc? I.e. the breaker turns into a pretty decent conductor but a pretty poor protection device.
OR
B) Does it self-destruct in the process of clearing the fault? I.e. the circuit is off and not going to be back on until you clean up this mess!

Essentially the same question for fuses that are under-rated: do they self-destruct in the process of opening, or do they fail to open?


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PostPosted: Thu Feb 16, 2012 11:47 am 
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Vincent B. wrote:
I thought the interrupting current of a CB was a pass/fail test, so I don't see why it would be tested at 17kA if the listed IC is 14 kA. And if it was tested (internally) by a manufacturer, as a end user, I wouldn't know about that value either.


I too would go to the larger rating.

My point is that the breaker would perform correctly at 17kA even though it failed its evaluation. In this case the arc flash incident energy would not be impacted by the 'failed evaluation' of the breaker. But the breaker needs to be replaced based on its labeling.


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PostPosted: Thu Feb 16, 2012 11:58 am 
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diakonos1984 wrote:
And a followup question: what happens to a breaker that experiences a fault that is greater than it's rating?
Does it
A) Attempt to open and is unable due to magnetic forces, etc? I.e. the breaker turns into a pretty decent conductor but a pretty poor protection device.
OR
B) Does it self-destruct in the process of clearing the fault? I.e. the circuit is off and not going to be back on until you clean up this mess!


Short: it depends.
Even if the breaker is able to open (part the contacts), doesn't mean the arc between the contacts is quenched if the current is high enough for the arc to sustain. Lower current will pass through, as it's an added impedance in the circuit.
The breaker can be destroyed by the sheer magnetic forces, too. Especially if the fault is way higher than the IC.
Or it can open, but with the contacts surface so burned out that when you'll close it, everything will look fine, except it will generate heat by contact resistance. And you can have another problem shortly after.

Even properly sized breakers (IC-wise) should be checked after interrupting a fault comparable in magnitude to their IC level, before returning to normal service.

diakonos1984 wrote:
Essentially the same question for fuses that are under-rated: do they self-destruct in the process of opening, or do they fail to open?


They can fail to extinguish the arc, even with the fuse element melting, in which case they just add impedance, lowering the fault level but not clearing it completely.
They can also blow open, sending shrapnel all over the place.


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PostPosted: Thu Feb 16, 2012 12:06 pm 
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diakonos1984 wrote:
... JBD saying they are irreconcilably different...

I did not say that.

I said they are not absolutely interdependent.

Another example, say a 10kA protective device is located in a panel board rated as >40cal/cm² with an available bolted fault of 20kA.
This protective device feeds a piece of equipment, through sufficient impedance, such that its incident energy calculates to <4cal/cm² with a bolted fault of 6kA. How does the protective device being overdutied impact the arc flash results?


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PostPosted: Thu Feb 16, 2012 3:21 pm 

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Sorry JBD, I didn't mean to mis-represent you. I struggled with how to summarize, but you did well saying
Quote:
they are not absolutely interdependent
.

Your example is a good one. NEC 110.9 says
Quote:
Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the...current that is available at the line terminals of the equipment.

which would be 20kA in your case making the breaker overdutied. However, the fault current at the equipment, and hence the amount of "thru current" at the breaker is only 6kA (for a bolted fault, so an arcing fault would be even less) which is well within the equipment ratings.
My conclusion would be that the equipment evaluation has no impact on the arc flash calculation in this case.

But let's modify your example slightly.
Let's say the panel board was determined to be 36cal/cm^2 with an available bolted fault of 20kA. The main breaker is also rated for 10kA, but the 20kA (or something extremely close to it) is available at it's line side terminals, so it's overdutied per NEC 110.9. If we were to determine that the protective device arcing fault current (as a function of the 20 kA bolted fault) was something greater than 10kA (I don't feel like defining the problem sufficiently to calculate it), would you feel comfortable with the accuracy of the 36cal/cm^2 rating?


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PostPosted: Fri Feb 17, 2012 8:54 am 
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diakonos1984 wrote:
Let's say the panel board was determined to be 36cal/cm^2 with an available bolted fault of 20kA. The main breaker is also rated for 10kA, but the 20kA (or something extremely close to it) is available at it's line side terminals, so it's overdutied per NEC 110.9. If we were to determine that the protective device arcing fault current (as a function of the 20 kA bolted fault) was something greater than 10kA (I don't feel like defining the problem sufficiently to calculate it), would you feel comfortable with the accuracy of the 36cal/cm^2 rating?


If the 36cal/cm² comes depends on the proper operation of the overdutied breaker, I would say it is not valid.
However if the 36cal/cm² comes from the upstream protective device, then I would say it is valid, but that normal PPE may not be sufficient. The breaker could fail violently during operation and normal arc flash PPE is not designed to protect against flying parts.

Because there is not always a true interdependency, each portion of my reports always includes a reference to review the other portions.


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PostPosted: Tue Feb 21, 2012 5:01 pm 
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C. It self destructs in the process of attempting, and failing, to clear the fault. The fault is cleared by an upstream device and then you clean up the mess.


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