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 Post subject: CON ED Spot Network IE levels
PostPosted: Wed Jun 24, 2020 9:12 am 
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Joined: Tue Feb 17, 2009 8:38 am
Posts: 51
Location: Westminster, MD
Hello fellow practitioners,

Attached is a one-line diagram of a spot network by CON ED in Brooklyn, NY. The system consists of up to five 2500 kVA transformers connected on the 480V secondaries.

The gear that is owned by the customer is powered from five 3000-amp takeoff fuses in CON ED's vault, to separate sections in the service-entrance gear. The resultant incident energy level is less than 1 calorie directly downstream of the 3000-amp current-limiting fuses.

I can't in good conscience label a piece of equipment powered from a 480V, 200kA source at less than 1 calorie, but given the upstream 3000-amp fuses, that's where it ends up.

Granted I will develop multiple scenarios that look at less than 4 or 5 of the transformers being connected. That gets me up to around 30 calories for the case where there's one 2500 kVA transformer, and I have a request in to CON ED to see if they ever get to that low level of available fault current.

But I'm curious to see if anyone else has run into this situation.

Thanks


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Thu Jun 25, 2020 9:17 am 

Joined: Thu Apr 18, 2019 11:42 am
Posts: 30
If there is a way to get the actual utility data...get it. Using infinite bus is usually not worse-case (for AF calc's).

Mike


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Mon Jun 29, 2020 9:36 am 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 191
Location: Maple Valley, WA.
You should definitely contact the utility and find out what the maximum and minimum secondary fault current would be. Then you will want to run one scenario at Maximum Fault current with all motors on and another scenario with Minimum Fault Current with all motors off. Then use the worst case (maximum AF energy) to put on the AF warning label.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Mon Jun 29, 2020 9:38 am 

Joined: Mon Aug 20, 2018 6:03 am
Posts: 10
I seemed to have lost my previous reply. the IEEE model does not apply at 200kA. Should not be used!!!!

I would not trust the calculation at all, nor would I expect the fuses to be current limiting. Not aware that there is a model to predict arcing current with more than 106kA Ibf... and I am not 100% sure I would trust it at 106 kA either as that was, itself, and extrapolation by the team that the created the model.

At high fault currents the Iarc/Ibf ratio decreases in LV systems.

See attached chart based on the 2018 model. The Iarc values past 106kA are my own extrapolation using a straight line! I would think Iarc will actually be lower than what this shows. But even with this the minimum I arc for 200kA Ibf, at 32mm gap could be <45kA, HCB!

Short circuit calculations, for fault values this high could be very inaccurate. Small, normally ignored impedance, can have a big effect!

I would suggest this is not a system to which you can assign AF values.


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Mon Jun 29, 2020 10:01 am 
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Joined: Tue Feb 17, 2009 8:38 am
Posts: 51
Location: Westminster, MD
These are actual reported CON ED contribution values. CON ED normally runs the system with 3 of the 5 transformers connected.

I will run alternate scenarios to determine a worst-case arc flash level, none of which will apply to buses with bolted fault currents in excess of 106 kA in accordance with IEEE 1584-2018. Alternate methods within SKM's capabilities will be investigated.

Not labeling the buses with incident energy levels is not an option.


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Tue Jun 30, 2020 3:26 am 
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Joined: Tue Feb 17, 2009 8:38 am
Posts: 51
Location: Westminster, MD
mayanees wrote:
These are actual reported CON ED contribution values. CON ED normally runs the system with 3 of the 5 transformers connected.

I will run alternate scenarios to determine a worst-case arc flash level, none of which will apply to buses with bolted fault currents in excess of 106 kA in accordance with IEEE 1584-2018. Alternate methods within SKM's capabilities will be investigated.

Not labeling the buses with incident energy levels is not an option.


Edited to add: SKM advised that the program defaults to the Ralph Lee equations for anything outside of the 1584 current-range parameters.

I believe this shows that downstream of a 480V, 3000A current limiting fuse the incident energy level is low.


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Tue Jun 30, 2020 6:42 am 
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Location: Scottsdale, Arizona
mayanees wrote:
Edited to add: SKM advised that the program defaults to the Ralph Lee equations for anything outside of the 1584 current-range parameters.


The 2002 Edition of IEEE 1584 stated to use "Theoretical Methods" when outside the boundaries of the equation's validity. (only meaning the equations were not validated above 106 kA)

The 2018 Edition of IEEE 1584 makes no such statement. Mostly for legal reasons, we decided to state the range of validity and not make any recommendations if a study is outside that range. i.e. up to the user and that typically means switching to the Lee Equations. In years past, I have seen some use the IEEE equations above 106 kA and then have a footnote stating the calculations are outside the range of validity.


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 Post subject: Re: CON ED Spot Network IE levels
PostPosted: Mon Jul 13, 2020 8:27 am 

Joined: Mon Aug 20, 2018 6:03 am
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The statement was made that not posting Ei levels on buses is not an option. That implies, to me, that live work on the buses is a potential necessity. I am aware that there are methods to estimate EI such as Dr. Lee's methods. However, I would caution that at the time of that Dr. Lee created his method I do not know how well the phenomena of Iarc values versus high fault current availability at low voltage was known. Dr. Lee worked for an industrial company and 200kA at 480V is not a commonly found situation in industrial environments.

The fact remains what the arcing current may be is very difficult to predict and fuses are very much dependent on the prospective arcing fault current to drive their response. As initially stated in the original posting, a low Ei result would be, in my opinion, highly questionable. So IF one must implement Ei values on a label the advisable course of action is to develop a conservative estimate of those values which will probably result in relatively high Ei values.

As Mr. Fuhr indicate this probably requires evaluating multiple scenarios for connected topology. And, though there may be probable typical scenarios, there may also be atypical improbable scenarios... but which of these will accompany the contemplated exposed at risk task to occur?

The Ei numbers are an element in risk analysis... I would suggest you need to ensure that all applicable elements are being considered.


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