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ekstra   ara
 Post subject: New results for open air bus and end users perspective
PostPosted: Tue Jun 18, 2019 2:02 pm 

Joined: Fri Feb 22, 2019 8:02 am
Posts: 3
We've been recently working on 5-year studies updates. For open air buses over 15kV (69kV in our case), we now use the NESC table method (or CSA Z462 annex R) to evaluate the incident energy. In practice, we use EasyPower NFPA 70E 200E Annex D.8 option for calculation.

We get much lower incident energy levels compared to the earlier studies for the same plants, calculated with Lee. We know from all the litterature that those results are more representative of the reality.

What is your approach when presenting those results to your clients? For a technical end users usually it is easy to understand the reasoning behind the calculation method change, but for the service guys in the field going from extreme energy to 4 cal/cm^2, without any system changes, can cast some doubts.

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 Post subject: Re: New results for open air bus and end users perspective
PostPosted: Mon Jul 08, 2019 9:40 am 

Joined: Mon Nov 19, 2007 5:25 am
Posts: 30
Location: Titusville, Fl.
So not sure if you are asking for Single Line to Ground Faults, but I’ve supported a team to determine the Arc Flash Incident Energy values for a similar situation. Although, it was for underground MV cabling at 13.2/13.8kV, single line to ground faults. Testing was completed at the KEMA Labs for multiple scenarios, and an equation was derived using the resultant power of the arc incident converted to energy. This equation proved the same situation that you’ve experienced, whereby using the equations in NFPA70E Annex D, resulted in lower energy values than what was tested at the KEMA Labs. Now, I had a similar situation to yours, whereby I needed to calculate the Incident Energy of a blown HV Potential Transformer (PT), that occurred on a Single Phase Transmission Line (67kV). I used the Lee equation, and then bounced it off the power equation I mentioned earlier for several distances. All results came w/n 2% of each other. Unfortunately, single phase faults for MV/HV, have not been fully addressed by the IEEE 1584 for documented equations to be used, though I’m patiently waiting. In the end, when all else fails: My suggestion is to use the Theoretical Ralph Lee Equation to determine Incident Energy >15KV. OBTW, you can find such, in the “ Complete Guide to Arc Flash Hazard Calculation Studies,” by Jim, Ch. 10 “Incident Energy Calculations,” pg. 142.

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