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 Post subject: Manual and PTW arc flash results differ for 1000V 50Hz arcing fault
PostPosted: Mon Apr 14, 2014 7:49 pm 
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Location: Stratham, SW Australia
I have used the following two methods to calculate arc flash hazards for an arcing fault at 415V, 50Hz and also at 1000V, 50Hz :

  1. Arc flash formulae from IEEE 1584-2002
  2. [font=Calibri][color=#000000]PTW arc flash program[/color][/font]

The arc flash results for both above methods agree for an arcing fault at 415V, however the results differ for an arcing fault at 1000V.



The following have been used for both calculation methods :

Arc in box K = -0.097

Arc in box K1 = -0.555

Busbar gap G = 25mm

Distance factor x = 1.641

Ungrounded system K2 = 0

Cf = 1.5

Arc duration 0.03 sec

Working Distance 457mm



The 1000V results of the spreadsheet are :

  • AF_Bolted Fault 19.931 kA
  • [font=Calibri][color=#000000]AF_Arcing Fault 19.119 kA[/color][/font]
  • [font=Calibri][color=#000000]AF_Boundary 733 mm[/color][/font]
  • [font=Calibri][color=#000000]AF_Incident Energy 2.60 Cal/cm2 at Working Distance of 457mm[/color][/font]





The 1000V results of the PTW arcflash program are are :

  • AF_Bolted Fault 19.931 kA
  • [font=Calibri][color=#000000]AF_Arcing Fault 19.119 kA[/color][/font]
  • [font=Calibri][color=#000000]AF_Boundary 574 mm[/color][/font]
  • [font=Calibri][color=#000000]AF_Incident Energy 1.74 Cal/cm2 at Working Distance of 457mm[/color][/font]


I have uploaded the manual calculation sheet.

Which results are correct ?


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PostPosted: Tue Apr 15, 2014 2:12 pm 
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Hi Murray,

Always great to hear from you. Reviewing the spreadsheet, one thing jumped out at me right away. The difference between the spreadsheet and the PTW answers is (conveniently) 1.5. The IEEE 1584 2002 edition requires using a calculation factor of 1.5 "at or below 1 kV" and 1.0 "above 1 kV". I wonder if the software just made the 1.5 applicable "below 1 kV" and not "at or below 1 kV".

Another thing I noticed is you had the x distance factor for medium voltage equipment as 1.641. For MV equipment it changes to 0.973. I'm not sure you have control over this in the software, maybe you do and forced it to 1.641 just for comparison.
Either way, that did not explain the difference in answers. I played around with the spreadsheet briefly and the only number that really sticks out and gives you the difference in answers is the 1.5 calculation factor.
Hope it helps!

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Tue Apr 15, 2014 6:32 pm 
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Joined: Sun Jul 22, 2007 5:00 pm
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Location: Stratham, SW Australia
Hello Jim

Thanks for your prompt and helpful reply.

The PTW software links the distance factor x to the 'gap' of IEEE 1584-2002 Table 4 by default where for a voltage of 1kV and gap of 25mm, the distance factor x is 1.641. Distance factor x can not be forced by the PTW user.

As you noted; the ratio of manually calculated incident energy / PTW calculated incident energy = 2.60/1.74 = 1.49 indicates that
PTW may have made the 1.5 applicable "below 1 kV" and not "at or below 1 kV".

I will post confirmation when I have had a reply from SKM/PTW.

Regards
Murray Newman


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PostPosted: Wed Apr 23, 2014 1:28 am 
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So of course this begs the question. The IE step change at 1 volt on either side of 1kV is obviously not happening. Why do we (1584) not attempt some "transition" function across this boundary (or any other where this type of issue would exist)?

Malcolm Hebert
AbearEngineering.com
San Diego


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PostPosted: Wed Apr 23, 2014 5:38 am 
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Malcolm Hebert wrote:
Why do we (1584) not attempt some "transition" function across this boundary (or any other where this type of issue would exist)?

You'll have to choose a width for that transition. From 900 V to 1100 V? 950 V to 1050 V? Without tests, your guess at a width is as good as mine.
Then, why study really hard something (IE around 1 kV) where it's not as useful as elsewhere (choose among 208 V, 240 V, 277 V, 347 V, 380 V, 400, 415 V, 460 V, 480 V, 550 V, 575 V, 600 V, 690 V, 3 phases and 1 phase, and that's just for common low voltage values)? I'm not saying that it must not be studied, just that other voltages are more prevalent so should be better studied to better protect the maximum of people.


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PostPosted: Fri Apr 25, 2014 11:28 am 
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Remember...this is an empirical equation. At the edges where the validity of the equations comes into question, arcing current can actually be higher than bolted fault current, which is clearly invalid. Calculations at the edges of the "valid region" produce artifacts, and the 1 kV transition boundary is one of them. These are some of the criticisms of IEEE 1584 but to date, there are no good alternatives available so IEEE 1584 gets accepted, warts and all.

The test data does not include tests near the transition (at 1 kV). I can only think of two situations where we would see voltages at the transition for standard voltages. The first was a 1.2 kV system that I've only seen once that was used to feed a large motor while avoiding shielding requirements on 2 kV or larger cable. The second is a lot of underground coal mining equipment operating at "995 V" because for a long time, MSHA would not allow utilization voltages above 1 kV due to safety concerns. Even though taking reasonable boundaries around the nominal voltages (say +10%/-15%) would result in crossing the transition, the 1584 equation uses nominal voltages so the discrepancy would not normally be noticeable.

Finally, IEEE 1584 only claims 95% confidence, and an examination of the actual test data for arc flash PPE suggests correlation is only around 0.85-0.9 using the current equations. Similarly a careful examination of actual ASTM data on PPE testing typically shows that the range of pass/fail cases around the 50% passing point (ATPV) is between around 0.5 and 1.5 cal/cm^2. A discrepancy of the size you found does not sound all that bad with this in mind.


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