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 Post subject: Modelling Multi-section Switchboards in ETAP
PostPosted: Wed Apr 07, 2021 7:37 am 

Joined: Thu Jun 11, 2020 11:12 am
Posts: 2
I am working on an arc flash study for a system with a multi-section main switchboard. I have created the model in ETAP. I modeled each of the three sections as a separate busses linked in order by very low-value Impedances (since you can't link a bus to a bus directly in ETAP; I've also modeled it with an appropriately sized conductor of negligible distance). Each section is a separate compartment, with the first section with the main breaker from the utility being different dimensions than the other two sections.

The arc flash analysis in ETAP can't determine a FCT for the first section with the utility main breaker. I'm guessing this has something to do with not being able to isolate from the back-current coming to the bus from the second section of the MSB. However, it seems like that problem should apply to the other two busses for the MSB as well, but ETAP did find a FCT for both of those.

Is there a better way to model a multi-section main switchboard? Is there a workaround for this issue if anyone else has encountered it?


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 Post subject: Re: Modelling Multi-section Switchboards in ETAP
PostPosted: Fri Apr 09, 2021 4:31 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 842
Location: Rutland, VT
Unless there is some specific reason you are trying to create separate bus sections to represent each breaker compartment, there is no reason to do so for arc flash. Unless it is arc resistant switchgear, you cannot take credit for separate compartments. The incident energy value is going to be the same for the whole line up. The main breaker is excluded and the next upstream protective device is considered the clearing device.

Model it as one bus.

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 Post subject: Re: Modelling Multi-section Switchboards in ETAP
PostPosted: Mon Apr 12, 2021 7:40 am 

Joined: Tue Dec 31, 2013 12:19 pm
Posts: 36
wbd is technically correct. However, I don't think that's realistic. I think this needs more guidance/clarification from a standards standpoint.

Let's use the example of a 15 vertical section MCC with a Main Circuit Breaker in Section 1. I've run across several facilities like this. At Section 10, you're 16 feet away from the main circuit breaker. At Section 15, you're 23 feet away from the main circuit breaker. We know from experimentation and Lenz's Law, an arcing event will travel away from the source. To say the main circuit breaker cannot be counted on to take out an arcing fault in Sections 10 through 15 and therefore the whole lineup needs to be > 40 cal/cm2 doesn't seem to be realistic.

At some point in a multi-sectional lineup of switchboards or MCC's, that Main Circuit Breaker will be effective in clearing an internal fault. That's what it's for and what it's designed to do. The question then becomes, at what point does that happen? How many vertical sections or feet away is that?

I've had clients balk at this notion that the whole MCC/switchboard needs to be rated in this manner and it's tough to respond to their reasonable argument with "that's what the standard says". When we have to fall back on that as an argument, we lose respect for the standards which isn't good for anybody.


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 Post subject: Re: Modelling Multi-section Switchboards in ETAP
PostPosted: Mon Apr 12, 2021 7:48 am 

Joined: Thu Apr 18, 2019 11:42 am
Posts: 49
Jeff,

You bring up a good point...one which I have thought about also. But, there is no basis or experiments that I'm aware of that sheds light on this question. Logically, yes, we can make that claim but there is nothing to back it up other than the standard.

Kind of a pickle I know...

Mike


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 Post subject: Re: Modelling Multi-section Switchboards in ETAP
PostPosted: Fri May 07, 2021 11:41 am 

Joined: Fri Sep 28, 2018 7:32 am
Posts: 14
Location: Sioux Falls, SD
I've run into this several times with the most significant being a customer that was in the process of energizing a new utility feed to a brand new area of the facility. This included a 30 section MCC which received the utility feed. It had a main circuit breaker with ARMS that would reduce the >50 cal hazard to around 2 cal. In my review I determined they did not add isolating barriers in the main compartment and therefore the entire 30-section MCC would be deemed off limits as their internal policy stated they would not work energized on anything greater than 40 cal. To strengthen my assessment I fortunately found the MCC manufacture stated in their own documentation the importance of the barriers and that should they not be in place the entire assembly shall have only one arc flash hazard recognized and it must be the greatest hazard found. I've run across at least two of the large manufactures (Allen-Bradley and Eaton) that have this written in their equipment documentation.

The customer was obviously frustrated as they paid extra to meet the code requirements by adding the ARMS feature but now were unable to realize its benefits. I've put in a public comment to have an informational note about these barriers added to the NEC as its very important to be aware of when adding ARMS or similar options to OCPDs.


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