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 Post subject: Determining maximum clearing time for use with NESC Table 410-1Posted: Mon Dec 21, 2009 2:46 pm

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
I'm evaluating a 12.47 kV distribution sytem. I know the maximum available fault current for each feeder and my system voltage. The last thing I need to use table 410-1 is the maximum clearing time.

How is the maximum clearing time determined for each feeder? Is it the maximum clear time at the maximum fault current?

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 Post subject: Posted: Mon Dec 21, 2009 3:02 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
The way I look at an overhead system is with the view that an employee can be working anywhere on the circuit. That will usually take them from max available down to as little as 1000 amps available on some parts of the system.

Hence we look at 150%, 200%, 500% of relay pickup since these are usually times we have from testing and then add breaker clearing time using Arcpro.

Then we go in increments of 1000 amps up to max available and consider the inst. if one exists on the relays.

Usually the energies are low at 12.47 until you get to the slow portions of the curve, then I have seen them get as high at 6 to 8 cal, but that takes many seconds. We also consider a cutoff level on time when appropriate.

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 Post subject: Posted: Mon Dec 21, 2009 3:40 pm

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
The NESC says to use table 410-1 OR perform an arc hazard analysis.
What you are describing sounds like doing an arc hazard analysis.

If choosing to use the "table method", I want to understand what value to use for the maximum clearing time.

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 Post subject: Posted: Mon Dec 21, 2009 5:16 pm
 Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 596
clearing time is relay time plus breaker trip time, or the fuse clearing time. Breaker trip time can be found in the breaker specifications. Relay time and fuse clearing time depend on current magnitude and can be picked off the TCC graphs, or sometimes calculated.

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 Post subject: Posted: Mon Dec 21, 2009 6:23 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
What I was intending to convey is that I for at least one do not believe in the table concept for a distribution line that can be worked at such varying levels of fault duty and to offer another method for your consideration. It is my opinion that the comittees missed that concept with the tables, and that it is incumbent on the engineer to perform some due diligence.

In many cases, if you did the calcs you would find with an inst setting that the IE may well be less than 1 cal. However if you move a couple of miles away from the sub, you are working on fault duties below the inst and are on the relay time curve.

In fact I believe there was a post here some time back where a legal case was lost because the lower fault currents were not considered.

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 Post subject: Posted: Tue Dec 22, 2009 6:27 pm

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
Good feedback.

I'm on the same page as you with regards to staying away from the table method. I am doing the arc hazard analysis in detail, but I wanted to report the difference between my analysis and what would have been required by the table method.

I think I would be better off summarizing, as you did, the reasons why I chose to do the detailed analysis and why the table method is not adequate, rather than spend time trying to do both methods.

Thanks!

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 Post subject: Posted: Tue Dec 22, 2009 7:47 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Glad to hear. I just tell my folks that the tables are extremely coarse and will most likely end up with the wrong results for your system. Plus the tables cannot account for working downtream on the feeder. And the calcs are so easy to do.

Thanks and Merry Christmas to you and everyone,
Alan

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