Arc Flash Include/Exclude Main 1

[Arentsch]

When can an Arc Flash propogate from the feeder bus of a panel to the line side of the Main? What constitutes a barrier between the Main section and a feeder section of a distribution panel? I'm curious how other firms deal with these questions. It's real easy for the Arc Flash firm to just exclude the main on all panelboards, switchgears, and MCC's. However, is this the best solution for the customer?

Example, we're working on a switchboard where the main is in a separate panel from the feeder breakers. Each of the cabinets were shipped separately and bolted together during installation. Obviously there is an opening between each of the cabinets for the bus to get from the Main to the feeder cabinets. In order for the main breaker to be "included" or be considered "isolated and barriered", does this penetration need to be sealed around each of the buses with no air gaps? Here is a layout of the panelboard from the example above:

Would your firm include or exclude this main from Arc Flash calculations? Do you have a concise rule for this application?

 

[dpc]

Speaking only for myself, I would exclude the main breaker for this and all other low voltage switchboards, panelboards, and MCCs. In other words, calculate on the LINE side of the main breaker. Unless this equipment is arc-rated, it is not tested for internal arcing faults. The only low voltage equipment I would consider including the main breaker would be low voltage power switchgear built to ANSI standards. Even then, it isn't arc-rated, so it's a judgement call. Excluding the main breaker is always conservative, but as you're aware, sometimes the resulting incident energy is quite high.

This decision is left up to the person responsible for the study. IEEE 1584 gives only general guidelines, basically stating the obvious issue of potential propagation between sections.

Hope that helps.

Dave

 

[Arentsch]

Thank you Dave. That's our current analysis strategy, but wanted a reality check to make certain there wasn't a more scientific approach to isolation and barriers. Thanks for your response!

 

[unclebob]

I'm in the process of installing insulated barriers exactly for this scenario. The plant has given us all the barriers we needed, that will block most of the openings, including where the bus cross the cabinets. No other 'sealing' material was added, be it rubber bushing, tape or silicone. The switchgear isn't arc flash rated. It appears that only barriers are sufficient to isolate the main breaker section. With these barriers, incident energy goes from 67 to 18 cal/cm2, even less now that the arc reduction device on the main breaker is usable. Now switching the upstream 25 kV breaker is no longer needed to work on the smaller breakers on the switchboard because of that high incident energy.

On these safety issues, my firm is quite conservative. I guess they have good reasons to accept that.

 

[Arentsch]

Unclebob, Thanks for the insight. I'm sure the insulated barriers will be a cost effective AF mitigation technique vs. MV breakers or other expensive methods. I'd love to see some before and after pictures upon completion of the barrier install. Thanks again!

 

[unclebob]

Sadly, the contractor did a bad job and a bunch of cables cross the cabinets (we only have two side by side). So, the biggest feeder cables enter on the top of the main breaker and then cross into the switchboard, to their respective feeder breaker. Moreover, the line cables that go on the lugs of the main are too long, and the C phase cables enter the switchboard side. Putting a barrier in there will be hard. Contractor told us that we can use a 10 x 10 in opening in the barrier for the cables. That opening is bad and defeats all what we wanted to do.

Waiting the decision the plant will take. I wouldn't accept that and forget about the barriers until the contractor redo his job correctly.

 

[Arentsch]

That's too bad. Sounded like a good opportunity to reduce feeder side Arc Flash IE. Good Luck and thanks again for the reply.

 

[Coco_HueHueHue]

Personnaly, i always exclude the main unless the equipment is arc-rated as it was mentioned above.
To me, added insulating barriers is not enough.

If the fault occurs upstream of your main device, but inside your equipment, what tells you that you will not receive the blast in the face?
Even if the sections are segregated, you still stand in front of the same equipment.

 

[davidbeach]

I've seen a case where an arcing fault in the cable compartment of standard switchgear punched though the sheet metal and got into the bus area, completely by passing the feeder breaker. Worst location in the gear should govern the whole gear, trip something upstream.

I’ll see your silver lining and raise you two black clouds. - Protection Operations