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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: 840
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: 33
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: 45
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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