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 Post subject: Flash study for 1ph systems - labeling substations
PostPosted: Tue Jun 16, 2009 9:57 pm 
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Gentlemen,

I working on a arc-flash study for a system which comprises of a single phase emergency distribution network. This network is fed from a 4160V switchgear where the 3 phase bus is tapped in 3 different cubicles to feed 3 different single phase networks, with the phase to neutral voltage being 2400V. These single phase cables feed couple of 112 kVA, 50 kVA and a whole bunch of 3 kVA mid-tap transformers which step down voltage from 2400V to 240V (which feed lighting panels). My question now is,
1. Is there any software out there which can help me calculate the HRC on the lighting panels? I want to make sure the HRC is 0 before sticking labels with 0 PPE cat.

2. How does the single phase cable impedance compare to three phase cable impedance i.e. is it higher or lower? There is a 600V neutral running along with 2400V 1P cable. The guy in the field collecting data is having a tough time verifying the cable manufacturer. Would anyone know of any tables where I can get the cable impedance info? Is there an approximate value I can use? I have the PTW single phase module which does not help me model the cable correctly since the cable library basically refers to NEC table for 3 phase cables.

3. Part of the job involves labeling 69 kV substations. Our team was planning on hanging a metal plate around the substation fence. Our plan is to display a PPE 2 requirement to enter the substation and a statement asking linemen or maintenance personal to refer to NESC 441-1 table for arc-flash boundary distance. Displaying the entire table on the plate isn't an option. What are your thoughts on this? Is there a better approach to labeling the 69 kV substations?

Thanks for reading all this. Appreciate any comments or suggestions.

regards,
Aleen


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PostPosted: Wed Jun 17, 2009 5:58 am 
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I don't know how much help I will be but I will put in what I can.

1. One thing you can do is model the same configuration that you mentioned but treat it like it was a three phase system. This would give you conservative results and you can use engineering judgement after that. One reason that "208V and below unless fed by 125kVA" is part of the 1584 is because they found that they could not sustain an arc at those levels, keep in mind that testing was three phase so I seriously doubt that single phase could sustain one either.

2. Single phase cable is no different than three phase as far as impedance goes. Impedance is based upon Wire size, material, insulation, and configuration with other wires, and what it is run in....conduit, tray, + material of raceway. All these things affect the inductance of the cable. The resistance is also affected based upon operating temp. All this should be in your software you just have to choose the right options........on a side note how you use the impedance in calculations would be different between single phase and three phase but the inherent characteristics of the cable would not be.

3. Sounds like a good plan to me. You are referring to table 410-1 and 410-2 though rather than 441. Also thre was a Tentative Interim Ammendment to table 410-2 you might check into that if you have not already.

Hope some of this might be of benefit to you.


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PostPosted: Wed Jun 17, 2009 6:31 am 
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Another Note....unless your 69 kV equipment meets the exceptions in 70E...ie utility owned, maintained, and controlled, you should be using 70E not the NESC tables.

Alan


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PostPosted: Wed Jun 17, 2009 7:51 am 
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Quote:
Our plan is to display a PPE 2 requirement to enter the substation and a statement asking linemen or maintenance personal to refer to NESC 441-1 table for arc-flash boundary distance.

Why not post the maximum arc-flash boundary in the station instead of making the linemen look it up? If you require PPE 2 (I assume that this is the maximum for any equipment in the station) to enter that station anyway, it doesn't matter what the arc-flash boundary is. The arc-flash boundary is only relevant if you are not going to wear the PPE (to let you know how close you can get).


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PostPosted: Sat Jun 20, 2009 8:43 pm 
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acobb wrote:
Another Note....unless your 69 kV equipment meets the exceptions in 70E...ie utility owned, maintained, and controlled, you should be using 70E not the NESC tables.

Alan


Thanks Alan...missed out on this one. The substation is indeed customer owned.

I have noticed that the 69/4.16kV transformers do not have current limiting fuses. In fact, they are protected by overcurrent relays like GE's IAC or SEL's 351S. Is this set-up normal? If yes, why avoid current limiting fuses for protection?

Regards,
Aleen


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PostPosted: Sun Jun 21, 2009 7:33 am 
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It is not unusual for the high side protection to be a breaker or circuit switcher with overcurrent relays. Unlike fuses that can cause a single phasing condition when one blows, the 3 pole device will not have that problem. Sounds as though all of your load is single phase? Also if it is a small installation, fault duty might not have been considered as an issue during design.

Alan


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PostPosted: Sun Jun 21, 2009 7:33 am 
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acobb wrote:
Another Note....unless your 69 kV equipment meets the exceptions in 70E...ie utility owned, maintained, and controlled, you should be using 70E not the NESC tables.

Alan

NFPA 70E does not prohibit using NESC tables. The appendices of NFPA 70E show different analysis methods, but these are not requirements of NFPS 70E. I believe that for overhead high voltage systems, the NESC analysis method is based on later, more applicable data than the methods shown in the NFPA 70E appendices. Using the tables, or the program used to develop the tables, is appropriate.


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PostPosted: Sun Jun 21, 2009 2:08 pm 
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I agree that the NESC method is better for overhead HV systems, just have not seen any documentation that allows that for non-utility equipment.


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PostPosted: Mon Jun 22, 2009 9:32 am 
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acobb wrote:
I agree that the NESC method is better for overhead HV systems, just have not seen any documentation that allows that for non-utility equipment.

NFPA 70E 130.3(B)(1) gives the requirements for incident energy analysis. The NESC tables and the ArcPro program used to develop the tables can meet these requirements. NFPA 70E 130.3(B)(1) does not say what method to use, although an FPN references Appendix D. FPNs are informational only, and Appendix D is not part of NFPA 70E and is informational only.

The NFPA 70E Handbook says "Annex D illustrates how the Arc Flash Protection Boundary and incident energy might be calculated. These illustrations are not intended to limit the choice of calculation methods."


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