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 Post subject: SKM model with lots of erros (*N2 and *N9, help please!)
PostPosted: Mon Oct 29, 2012 2:26 pm 

Joined: Tue Sep 11, 2012 9:55 am
Posts: 5
Location: Portland, OR
Does anyone have experience with building an SKM model and then having tons of *N2 and *N9 erros in it? I have completed my model and been very careful to put the exact equipment settings found in the field, however, it has a lot of errors (as in 39 of 114 buses in one building). I feel like I'm missing something with SKM.

When I called SKM technical support the guy said I could change the time setting for the Max Arc Duration and I may get different results. However, I'm of the opinion that this might put personnel in even more danger by giving incorrect AF results. Furthermore, I notice that A LOT of my errors are coming in at the 208V level. Since this site has ZERO transformers larger than 125 kVA feeding a single 208V branch should I just get over it and exclude them from the study? Any advice would be greatly appreciated. Thanks!


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PostPosted: Mon Nov 05, 2012 8:45 am 

Joined: Wed Aug 31, 2011 5:26 am
Posts: 11
These are not errors, they are merily notes pointing out different methods used to obtain the result.


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PostPosted: Mon Nov 05, 2012 11:26 am 

Joined: Mon Apr 18, 2011 1:43 pm
Posts: 11
Location: Denver, CO
I agree with mdy and wouldn't call those errors as much as explanations of what the software is doing for the settings you have applied. For example the *N9 is for "Max Arcing Duration Reached" means the settings you have configured for your fault calculations are pre-maturely "extinguishing" the fault before it has run its complete duration. In this case you've probably set the Max Arching Duration to 2 seconds. We are allowed to use 2 seconds as a reasonable amount of time for a worker to remove themself from a hazardous situation (set as seen below).
N2 works similarly and if you look in the Report Options tab you'll see the "Cleared Fault Threshold" value you've set the program to run to.
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PostPosted: Tue Nov 06, 2012 10:10 am 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
As others have said, those are not errors.

If you are using the "2 seconds" as max trip duration, and reaching it [The *N9 ones] you should first be asking yourself, "is 2 seconds reasonable"? While I think in many cases 2 seconds is unreasonably short for big service entrance gear,I think it is probably reasonable for most 208 V equipment.

If it isn't reasonable, then increase it. The question I use is, if some one tossed a couple of flash bang grenades in the room slammed the door, and turned out the lights,..... could I get out in two seconds?

If you are setting the cleard fault threshold high (it is 80% by default) then per SKM, you will get more "*N9"s becasue you are waiting for more of the fault current to go away.
For Example, If you have two sources feeding a bus one ("A") which feeds 90% of the fault and the other ("B") can feed 10% forever (but it isn't enough to trip its breaker), if you set the cleared fault threshold to 100%, "A" trips, "B" doesn't, and you never reach the cleared fault threshold. As a result you "time out" at 2 seconds. In that case you will always "time out", regardless of your max trip time setting. If your cleared fault threshold is 80% however, you will consider that you have cleared the fault when "A" trips.


My guess would be that you have increased your cleared fault threshold to something > 80%, perhaps have changed your induction motor fault contributin time, are not excluding them if less than 50HP, and are not checking for miscoordination.


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