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Which bus to fault? Switchgear + Generators
https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=4615		 Page 1 of 1
Author:  mbk.2k3 [ Wed Apr 04, 2018 9:26 am ]
Post subject:  Which bus to fault? Switchgear + Generators
Hi guys!

See attached sketch to help better visualize my situation.

I have a utility connection via a fuse that connects to the first single cell switchgear.
And then this connects to a generator via another single cell switchgear.

Now when doing arc-flash studies (for non-generator applications), I've always used the source (aka line/utility) side of the protection device (PD) to get the worst case fault scenarios and thus, worst case arc-flash results.

But when I have a generator application, there is no fault clearing device between the single cell switchgear / breaker and the generator.

How do you deal with this scenario? Where do you insert the busses to model the maximum clearing times and incident energies used for detering arc-flash labels?

Attachments:
Modelling Question.pdf [277.03 KiB]
Downloaded 236 times
Author:  bbaumer [ Wed Apr 04, 2018 11:50 am ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
I'm not really sure I understand your question but can offer this. I usually find the worst case scenario / highest incident energy is when the generator is the source due to the lower available fault current and longer clearing time.

Since you say you have no OCP device at the generator or down stream from the generator then your clearing time will be whatever you set your maximum time to be in the software, if any. Generally set at 2 seconds max in my experience for incident energy calcs. A 2 second clearing time will generally skyrocket the incident energy so your thinking that the higher the available fault current and arc fault current the higher the incident energy is not necessarily true. What is worse, 14000 amp arc for .1 seconds or 6000 amp arc for 2 seconds in terms of total energy?

I'm surprised the generator does not have any overcurrent protection nor do I see a transfer switch. Are the two gear sections interlocked somehow to prevent you from closing the generator into the utility or do they have paralleling controls?
Author:  mbk.2k3 [ Wed Apr 04, 2018 12:18 pm ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
what you see in the sketch in the original post is what the system is.

its a CHP / cogeneration generator exporting all the power to the grid. there isn't any ATS.

breaker 2 in the sketch is the generator breaker that is controlled by the generator controller. but ETAP can only do arc-flash analysis for busses that have an overcurrent protection device between the source and the bus.

so if i place the bus on point C in the sketch, ETAP has no trouble calculating the incident enegy. it does it based on the TCC curves of breaker 2 (its got a micrologic trip unit with 50/51 protection).

but if i place the bus on point D in the sketch, ETAP can't determine clearing time since there is no protection device between it and the generator.
Author:  bbaumer [ Wed Apr 04, 2018 12:27 pm ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
I've only used ETAP a little and it's been a few years.

In SKM you can select to calculate incident energy on both sides of a protective device. Not sure if ETAP has that ability or not.

If not, you could try adding a bogus device whose curve is outside the limits so it won't clear and let the 2 second rule kick in. If you can find one.
Author:  bbaumer [ Wed Apr 04, 2018 12:32 pm ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
A couple screen shots from SKM. Maybe you have this in ETAP too?

Attachment:
skm line load.JPG
skm line load.JPG [ 83.19 KiB | Viewed 3464 times ]


Attachment:
skm line load.JPG
skm line load.JPG [ 83.19 KiB | Viewed 3464 times ]


Attachments:
skm 2 seconds.JPG
skm 2 seconds.JPG [ 83.46 KiB | Viewed 3464 times ]
Author:  mbk.2k3 [ Mon Apr 09, 2018 6:10 am ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
Hi Bbaumer,

So when you calculate the arc flash levels / incident energies on both sides of a protective device, which one do you use for the label? the worst case scenario / i.e. the higher incident energy?
Author:  bbaumer [ Mon Apr 09, 2018 7:11 am ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
mbk.2k3 wrote:
Hi Bbaumer,

So when you calculate the arc flash levels / incident energies on both sides of a protective device, which one do you use for the label? the worst case scenario / i.e. the higher incident energy?


Generally, yes, I select the higher value for the label. If the values are the same I select the line side of the main so the upstream protective device on my label is accurate. Here is a screen shot of a model with several panels that all have main breakers.


Attachment:
skm line side main.JPG
skm line side main.JPG [ 103.21 KiB | Viewed 3453 times ]
Author:  vicdog [ Tue Apr 10, 2018 11:39 am ]
Post subject:  Re: Which bus to fault? Switchgear + Generators
You can double click the bus and go to the arc flash tab. Select fixed fault clearing time of 2 seconds (or whatever) and it will calculate IE.
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