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 Post subject: Ungrounded Systems vs NESC-2007
PostPosted: Thu Sep 11, 2008 12:38 pm 

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Hi,

some utilities have ungrounded (DELTA) systems. The tables of NESC are based on Line-Ground faults. Since the ungrounded systems have a theorically 0 A LG faults, I'm wondering how can we use the NESC tables for these kinds of systems.

For example, I have a system of 4.16 kV on the secondary side. This system is a single-phase 2 wire ungrouded.

Another example would be a transmission line (69kV) ungrounded.

Does anyone know how to treat those situations.

Thanks

John


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PostPosted: Fri Sep 12, 2008 8:27 am 
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
John,

With a true 2 wire ungrounded system the first ground will obviously not cause a problem, it is just that now you have a grounded system and the second ground will cause a 4.16 kV single phase fault.

Haven't really looked at anything above 25 kV so far, but would ask if they are gloving the 69 kV hot? If not and they are using sticks, I would think that at that distance, the energy would be very low. In any case, our focus will be at 35 kV and below, but maybe someone else can help at the 69 kV level.

Alan


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PostPosted: Fri Sep 12, 2008 9:05 am 

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Yes, Alan,

the first Line to ground fault will not cause any problem. The second will cause a Line-To-Ground fault.

But how should we approach this case with NESC standard. Should we calculate the second Line-To-Ground fault and compare with NESC tables?

In a Arc Flash point of view, it seems that the concern should be the line-to-line short-circuit. NESC seems to only cover the Line-To-ground short-circuit.

It is ok to say that NESC doesn't support this kind of configuration?

My question is then, how to perform an Arc Flash calculation, determining the Incident Energy exposure for this kind of system?

thanks


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PostPosted: Fri Sep 12, 2008 9:25 am 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Line to Line

John,

you are correct, but in this case the line to line is also the line to ground. I would assume a line to ground fault with the available L/L fault current. Question is what arc gap to use? Tables go down to 2" for 15 kV and no less. You will have to assume that the second one has low enough impedance to cause the full L/L values. We have adopted the practice of looking at the entire circuit since you cannot assume that all of the work will be at the bus, and will be in areas where the fault duty is lower. For relayed circuits, we use values from 150% of pickup to max available.

Amazing how these people thought 3 or 4 pages of standard would solve all of the issues!

Alan


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PostPosted: Fri Sep 12, 2008 10:09 am 

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Alan,

thanks for the replies, I will continue to meditate on the question and on the answers that you gave me.

John


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