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 Post subject: Dual Labels on Equipment with Multiple Sections.
PostPosted: Wed Mar 23, 2016 5:06 am 

Joined: Tue Dec 31, 2013 12:19 pm
Posts: 37
I need some advice with the IEEE 1584 Annex B.1.2 provision on not using the main protective device in an enclosure. We are having a debate on the interpretation of that provision relative to service rated MCC’s and multi-section switchboards with a main device. Some people want to make the interpretation that if there is a main device in an MCC, then load side adjacent sections are not in that enclosure. Therefore they should be allowed to use the line side of the main device incident energies for adjacent sections. Some people question the validity of one label for the whole line-up when you’re dealing with a 6 or more section of MCC line-up. It is hard to sell that an arc flash event working in section 6 has the same incident energy as the incoming lines 5 sections away. And if you can use that argument for 5 sections away, why not two or one section away?

Arguments advanced have been,
1) If the main is a current-limiting device, then it has been tested and listed to reduce the short-circuit current on the load side of the device and it's such a well established science that the manufacturers publish let-thru energy graphs and UL allows lower SC and AIC (series) ratings. Therefore you should be allowed to use to use the load side of the main device incident energy for adjacent sections of a MCC or switchboard.

2) You have to keep the high incident energy for the incoming main section. If you're working in adjacent downstream sections, what would be the incident energy of a failure at the line side of the main? Couldn't you use that arcing fault and the clearing time on the LINE side of the main, but adjust the working distance accordingly? So if you're working in section 2 of a MCC, the working distance would be sqrt(18^2 + 20^2) = 27" and section 3 would be 44" and so on. Run that against the LOAD side of the main for clearing time and the normal 18" working distance, and take the higher of the two. The only flaw of this is argument is if you're still within the arc flash boundary of the main.


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 Post subject: Re: Dual Labels on Equipment with Multiple Sections.
PostPosted: Wed Mar 23, 2016 1:00 pm 
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Location: North Carolina
Jeff S wrote:
I need some advice with the IEEE 1584 Annex B.1.2 provision on not using the main protective device in an enclosure. We are having a debate on the interpretation of that provision relative to service rated MCC’s and multi-section switchboards with a main device. Some people want to make the interpretation that if there is a main device in an MCC, then load side adjacent sections are not in that enclosure. Therefore they should be allowed to use the line side of the main device incident energies for adjacent sections. Some people question the validity of one label for the whole line-up when you’re dealing with a 6 or more section of MCC line-up. It is hard to sell that an arc flash event working in section 6 has the same incident energy as the incoming lines 5 sections away. And if you can use that argument for 5 sections away, why not two or one section away?

Arguments advanced have been,
1) If the main is a current-limiting device, then it has been tested and listed to reduce the short-circuit current on the load side of the device and it's such a well established science that the manufacturers publish let-thru energy graphs and UL allows lower SC and AIC (series) ratings. Therefore you should be allowed to use to use the load side of the main device incident energy for adjacent sections of a MCC or switchboard.

2) You have to keep the high incident energy for the incoming main section. If you're working in adjacent downstream sections, what would be the incident energy of a failure at the line side of the main? Couldn't you use that arcing fault and the clearing time on the LINE side of the main, but adjust the working distance accordingly? So if you're working in section 2 of a MCC, the working distance would be sqrt(18^2 + 20^2) = 27" and section 3 would be 44" and so on. Run that against the LOAD side of the main for clearing time and the normal 18" working distance, and take the higher of the two. The only flaw of this is argument is if you're still within the arc flash boundary of the main.


If you have ever seen what happens to an MCC after a fault on the buswork, typically the bucket where the fault happened is not where the arc flash occurs. The arc moves by magnetic force (like a rail gun) until it encounters a physical barrier to movement of the plasma cloud that follows the arc and then it stops and that's where you get the arc flash. In a vertical section it occurs at the bottom. I haven't seen one in the top yet. So out of perhaps a half dozen MCC arcing faults that I've had first hand knowledge of outside of the lab the failure was always within the bucket or at the bottom at the ends of the vertical bus.

There is an additional force at work though. As an arc heats up the surrounding air within a couple cycles it will reduce the critical flashover voltage to the point where an arc will occur across non-faulted conductors. For instance a single phase arcing fault in the standard IEEE 1584 test enclosure (3 vertical busbars) quickly becomes a 3 phase arcing fault even if it started as a single phase case. In a "big" arc flash, it can also transfer from the load to line side of circuit breakers in a panelboard for instance and this occurs because even "non current limiting" breakers are in reality somewhat current limiting...the impedance on the line side of a circuit breaker is less so the arc moves to the line side in some cases.

So when it comes to switchgear in which each enclosure is separate from the others, you won't get propagation from load to line side. In a panelboard it definitely does occur since there are basically no barriers between sections. In an MCC though there is a lot of debate on the subject with many individuals on both sides of the fence. I've never seen evidence of propagation from section to section in open literature or in internal company files I've seen. I've been told that it exists but I've seen no evidence. So personally I look at entire MCC sections are separate like switchgear.


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