SKM Arc Flash Analysis Question: TCC and Analysis Result

[FrankieAugust]

Hi all,

I am new to SKM and arc flash analysis. I am currently trying to convert a project from ETAP to SKM PTW. After did the conversion and run the analysis, the result puzzle me.
The result after transformer is 33.52 Cal/cm^2. After the LV-51ACB, the AF IncidentEnergy dropped to 1.32 Cal/cm^2. But, the last bus (CH-01A) result given is 7.32 Cal/cm^2. I expect the last bus result should be lower than 1.32 Cal/cm^2? I attached the TCC drawing and the 1 line diagram in this question.

Can someone help to clear my doubt?
1. After LV-51ACB, the AF incident energy dropped from 33.52 to 1.32, is it correct?
2. Why the last bus AF IE higher than 1.32 Cal/cm^2? What is the logic behind?

Many thanks.

 

[wcaseyharman]

When I have questions like this in SKM I use the arc flash study report. The two major parameters that tend to dominate the arc flash hazard is the fault current and clearing time.
Look at the arc flash report, compare the fault current and the clearing times. Make sure the clearing times make sense. It is not inconceivable that a downstream bus has a higher arc flash rating if the lower fault current causes a breaker to operate on the time delayed thermal curve rather than the magnetic instantaneous, for example.
If you have miscoordinated devices (downstream protection set higher than upstream) SKM only looks at the immediate upstream device unless you tell it otherwise, which could be another reason.
Yo have a motor, depend on size this can add to the bus fault current making it worse.
But look at the data in the arc flash report and that will help you troubleshoot your model.

 

[FrankieAugust]

hi wcaseyharman,

Thanks for your reply. I went to analysis report after have your reply. It is due the main device failed and use upstream device.
Another question, SKM can look for higher upstream device if immediate upstream device failed? I went through the configuration, seem there dont have such option.

 

[wcaseyharman]

In the arc flash study options you can check “Check Upstream Devices for Mis-coordination”. You can then select the number of levels.

 

[FrankieAugust]

hi wcaseyharman,

Thanks for your advice. I found it.
Seem like the analysis only can go up to 5 levels. I modified the on line diagram, and found the tripping device (SHV8 Relay). If base on the new report and TCC drawing, can I say the tripping point as what I circled. Or I understand wrongly? I am sorry, I am bad on it, still learning and try to understand.

 

[wcaseyharman]

Looks like you understand correctly, that’s what I would have done.

 

[mparenteau]

Double check your electrode configurations as I am doubtful of a 4000A breaker having VCB (more likely HCB), but could be wrong. Also SWGR instead of PNL?

Mike

 

[FrankieAugust]

Hi all,

Thanks for all the knowledge sharing. I found that I never key in the device interrupt rating of the protective device and cause the device's protection failed. The protection covered by higher device in this case.
Once I key in the interrupt rating, the result made more sense to me.

Again, many thanks.