In an arc flash calc at an MCC - do you "Include Main" or "Exclude Main"?

[bdn2004]

Seems like this question comes up a lot in Arc Flash. Just looking for a better more consistent answer.

How do you determine if you "Include Main" or "Exclude Main"? We deal with a Client that has a standard that clearly states: Exclude the Main in the arc flash at an MCC. But there are others exact opposite. The wires come in, land directly on the Main Breaker, seems downstream should be protected. I thought that's why we included the Main breaker.

I understand the barrier argument too. There's a physical barrier between the Main compartment and the rest of the MCC - then you can include the main. What makes a barrier? There's going to be an opening between the Main section and rest of the lineup.

 

[chuckd83]

Yes, essentially you have to determine if an arc occurs downstream, does it have the capability of migrating to the line side of the main breaker. Your client says yes, it does. Or they simply want the entire MCC labeled the same in order to prevent confusion for those selecting the appropriate PPE.

 

[Gr8blu]

bdn2004: What Chuckd83 said. Those are the layman's terms for it, I guess.

I will try to get you a better answer in a couple of weeks, after I sit down with the "movers and shakers" behind IEEE 1584, which is the "Guide for Performing Arc Flash Hazard Calculations". Consider this post a "place holder" until about 17 September.

Converting energy to motion for more than half a century

 

[dpc]

Unless the equipment is arc-rated, there is a risk that an arcing fault in one compartment could propagate to another compartment. Arcing faults inside MCC compartments frequently blow the door completely off. It is always conservative to exclude the main device. If you do decide to include the main device, it will be necessary to do a separate calculation to cover work within the main breaker compartment itself. So there would be two arc-flash labels - one for the main breaker and one for the rest of the MCC (as a minimum).

For our arc-flash studies, we always exclude MCC main breakers from the arc-flash calculation for that MCC.

Cheers,

Dave

 

[Coco_HueHueHue]

I used to exclude the Main Breaker from the calculation for the incoming bucket of the MCC.
But i used to include it for the buckets downstream of the Main Breaker inside the incoming bucket... all those buckets are protected by the Main Breaker actually.

All buckets usually have their specific arc flash label for some clients.

But if the client only want 1 label for the whole MCC, i exclude the main breaker.

@dpc, i am curious. Is there a specific reason for this conservative approach?

Coco

 

[dpc]

Maybe, maybe not. Do you think if you asked to MCC manufacturer to certify in writing that an arc-flash could not propagate from the Main Breaker section to an adjacent section that you would get such a certification? This equipment is not tested for that.

 

[nhcf]

Even if you include the MCB - under what scenarios would you be assigning different labels to different buckets?

 

[Coco_HueHueHue]

My memory was wrong yesterday. I did have different labels on all buckets 1 or 2 times. But,it was because I had some clients that their internal standards were to show the load end arc flash value on each specific bucket. Not saying it is the best practice, but that is how they wanted it. Those were the only time i did this. It was not because of the MCB, but because of every buckets' breakers.