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ekstra   ara
 Post subject: SKM Default values
PostPosted: Wed Jun 26, 2019 3:39 pm 

Joined: Fri Jun 21, 2019 7:51 am
Posts: 3
Hello!
I was wondering if anyone could answer a question I have. SKM default values, how accurate are they? If I have an ARC flash curve, the Td" and Td' values are defaults. If my trip happens somewhere in between those values, which would be more accurate? Using the exact trip value or the default values? Any insight at all would be extremely helpful.


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 Post subject: Re: SKM Default values
PostPosted: Mon Jul 01, 2019 6:20 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
I'm not sure what you mean by "an ARC flash curve". Please elaborate.


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 Post subject: Re: SKM Default values
PostPosted: Mon Jul 01, 2019 6:34 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
I guess this is related to your June 21 post with attached graph. I have no idea what the attached graph represents. It seems to be a combination of a current sine wave and a generator impedance decrement curve or something. Please explain.

You should keep your posts on a single subject in one thread.

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 Post subject: Re: SKM Default values
PostPosted: Mon Jul 01, 2019 2:09 pm 

Joined: Fri Jun 21, 2019 7:51 am
Posts: 3
jghrist wrote:
I guess this is related to your June 21 post with attached graph. I have no idea what the attached graph represents. It seems to be a combination of a current sine wave and a generator impedance decrement curve or something. Please explain.

You should keep your posts on a single subject in one thread.

Image



For the graph, the second half is a TCC curve. I'm mostly just wondering if it would be more accurate in my calculations to use the defaulted trip time SKM gives you, or use the trip time you know to be true, since I've seen them be different numbers


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 Post subject: Re: SKM Default values
PostPosted: Mon Jul 08, 2019 11:11 am 

Joined: Fri Apr 22, 2011 1:51 pm
Posts: 6
The two Td points on either side of the trip point are what appears to be the relative clearing times for the change in reactance for the generator/system while transitioning from the sub-transient to transient and ultimately to the steady state impedance of the machines. Using the sub-transient offers the maximum fault current based on the impedance models with the decaying time constant (X/R etc.). The trip time is nearer to reality in that the combined time for OCPD + Breaker clearing time is well beyond the Td".


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