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 Post subject: Validity of NESC Table 410-1 for open wire distribution?Posted: Wed Feb 20, 2008 11:47 am

Joined: Wed Dec 12, 2007 6:54 am
Posts: 9
NESC Table 410-1 may be used to determine the arc rating of clothing required and by extension if the fault current and or clearing times are outside the parameters of the table then an arc hazard analysis shall be performed.

For my utility, it turns out because of the high fault currents and long clearing times (that are outside the table parameters) we will be required to perform a large number of arc hazard calculations not only within substations but also along the distribution system.

Here is my question.

If the table was generated based on an arc in a box and relatively small
conductor gaps then how can the table also be valid for open air large phase separation events (e.g. overhead distribution lines)?

Does use of this table force arc hazard analysis when in reality a table based on overhead distribution line parameters may suffice for many situations?

Kevin Medley
203 926 5230

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 Post subject: Posted: Wed Feb 20, 2008 6:32 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Table 410-1

Medleyk wrote:
NESC Table 410-1 may be used to determine the arc rating of clothing required and by extension if the fault current and or clearing times are outside the parameters of the table then an arc hazard analysis shall be performed.

For my utility, it turns out because of the high fault currents and long clearing times (that are outside the table parameters) we will be required to perform a large number of arc hazard calculations not only within substations but also along the distribution system.

Here is my question.

If the table was generated based on an arc in a box and relatively small
conductor gaps then how can the table also be valid for open air large phase separation events (e.g. overhead distribution lines)?

Does use of this table force arc hazard analysis when in reality a table based on overhead distribution line parameters may suffice for many situations?

Kevin Medley
203 926 5230

Kevin,
Table 410-1 in the NESC IS based on single phase arcs in open air....not in a box. Are you thinking of 70E? If you have newer style relays, you could consider adding a toggle switch to the breakers to lower the settings (especially the inst.) when maintenance is being performed. This could be done at the same time reclosing is disabled for maintenance.....assuming that your practice is to place the breakers on non-reclose for maintenance. All of the calcs I have done result in energy values at 15 kV and 10 kA with inst. active that are below 2 cal. At 25 kV and 10 kA the value is just over 2 cal.

We are using Arcpro for our calcs, and it matches the NESC numbers exactly! You would have to adjust the values up if you were to use Arcpro for arcs in a box.

Hope it helps,
Alan

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 Post subject: Posted: Thu Aug 14, 2008 7:27 pm

Joined: Thu Aug 14, 2008 5:00 pm
Posts: 10
Location: Wisconsin
Acobb, how do you intend to use a 50 element to control arc flash energy? Are you able to guarantee that any arcing fault will exceed your pickup value for the 50 element such that it will always cause a trip during an arcing fault?

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 Post subject: Posted: Thu Aug 14, 2008 8:09 pm
 Sparks Level

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

Actually none of us can guarantee anything except that if you make a mistake there will be consequences. As with many of my posts, you will see that my major concern is when the fault current is low and we have to rely on the time elements. I believe that the best we can do is to modify the settings in certain situations. As for arcing faults and time, at some point we must believe that the worker will take some action to mitigate the damage to himself. Hence we need to apply a cutoff time for the calcs. What that is still remains to be seen and debated I guess.

Alan

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 Post subject: Posted: Sun Aug 17, 2008 12:15 pm

Joined: Thu Aug 14, 2008 5:00 pm
Posts: 10
Location: Wisconsin
It seems 1EEE 1584 provides guidance in cutting off calculations at 2.0 seconds of arc exposure. I provide training as such and caution the workers that for certain job tasks/approaches this assumed time constraint may not be valid.

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