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Arc Flash Calculations and Motor Control Centers
https://brainfiller.com/arcflashforum/viewtopic.php?f=33&t=4940		 Page 1 of 1
Author:  Jim Phillips (brainfiller) [ Sun Mar 24, 2019 11:56 am ]
Post subject:  Arc Flash Calculations and Motor Control Centers
This topic continues to surface from time to time. It is about performing arc flash calculations and labeling motor control centers.

Some will perform only one calculation for the entire MCC with the arc flash duration based on the clearing time of the next device upstream from the entire MCC. Label(s) have only this value.

Others perform two calculations – one for the main section based on the next upstream device and one for the branch sections based on the MCC main device. 2 different labels are used – main and branches sections.

When performing arc flash calculations for MCCs, do you perform:
> One calculation - for entire MCC
> Two calculations - main section and branch sections
> It depends
> I don’t perform studies
Author:  tish53 [ Mon Apr 01, 2019 6:44 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
I always looked at an MCC as open internally to allow an arc flash to travel between sticks or sections. So I calculate worst case with the incoming protective CB or fuse as the clearing device. Most of our MCC's are main lug only so this is very appropriate. better to be on the conservative side of the calculation in MHO
Author:  jmoore284@gmail.com [ Tue Apr 02, 2019 6:36 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
Unless the customer chooses otherwise I provide one analysis (worst case) at the line side of the main disconnect unless it is an Arc Rated gear that provides true isolation between compartments. Given that most of the MCC's we see do not have isolation between compartments I assume the shared airspace very well will allow for propagation of an arc. I then apply one label on the main disconnects compartment as well as an additional label at the top of every section of the MCC.
Author:  Robertefuhr [ Fri Apr 05, 2019 11:30 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
I have been told that when an arc flash occurs, the plasma and heat tends to run in the opposite direction of the source. If this is true, then I am not sure how the arc flash would jump across a main breaker.

A question for the forum, "Has anybody seen where an arc flash started in a bucket and then propagated upstream, then jumped across the main breaker?" If the answer is yes, was the main breaker in good working order (i.e. had it been tested) before the accident?

Another question, "Has anybody seen an arc flash that occurred on the main bus of an MCC and jumped across the main breaker?"
Author:  GaryPC [ Sun Apr 14, 2019 10:16 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
IEEE-1584 2018 discusses a fault jumping to the line side of a main and that this is reason to use the line side available fault current in the main compartment. I appreciate discussion here on this as I was considering two labels for MCCs. But I see it is best to bring up the question with the manuf. first.
Author:  NeelsfAMSA [ Mon Apr 22, 2019 11:57 pm ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
Hi, It depends on the network layout mainly whether to use upstream breaker to clear the fault especially when the supply source is from a Transformer with a large ratio (Prim to secondary) then in most cases the upstream protection will 'miss' the arc current fault on the inst O/C set.
If the Board is directly fed from a upstream feeder then of cause it is easy to use this breakers trip time to clear the fault! What the bests scenario properly will be is to do the calculation to the incomer Breaker protection time to trip if equipped with protection AND to fit an Arc flash protection device on the Incomer breaker's compartments which will trip the upstream feeder for this case only. yes it means additional cost but it will be worth to do so!
Can somebody assist me with the formula to calculate the AFPBoundary for DC network calcs please, those used in the Doan, Ammermann and Wilken Dc calculation (EFCOG)sheets please.

Regards NeelsF
South Africa
Author:  engrick [ Wed May 01, 2019 7:27 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
I have not ever seen evidence of an arc jumping across to the main of a breaker, probably as was noted above the arc travels away from the source. The bigger concern would be ionized gas or molten metal droplets bridging the gap. I would not worry about closed or sealed (like switchgear) but my concern is with open MCC designs. I have seen many large breakers with bus completely exposed where an arc event could produce gas or flying material that could enter the main compartment - possible - yes, probable - ???. Two labels if the possibility is small enough, one label if I am not sure.
Author:  stevenal [ Wed May 01, 2019 8:29 am ]
Post subject:  Re: Arc Flash Calculations and Motor Control Centers
Arcs tend to move away from the source, but also have a tendency rise with the heat produced. An inverted Jacob's ladder will not direct the arc downward. https://www.youtube.com/watch?v=MTY1Kje0yLg.

My experience with a fault moving from the load side to the source side of a breaker was with outdoor equipment. Bird got into the load side bushings tripping the breaker, but the fault continued on the source side until cleared upstream.
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