SKM and EasyPower Results for Arc Flash

[wbd]

I am replicating a project that was originally done in SKM about 4 yrs ago and I am using EasyPower Ver 9.5. There are some differences in conductor impedances but nothing major. I have found one item that is curious. For this particular panel:

SKM: HRC 0, 1.2 cal/cm^2, Ibf 16.06kA, Iarc 8.35 kA, Trip time 0.044 sec

EasyPower: HRC 1, 2.4 cal/cm^2, Ibf 16.4 kA, Iarc 8.515 k, Trip Time 0.086 sec

The protective device is a Siemens Sentron NXD6 1200A breaker with a 1200A trip and Inst at 5000A. Now what is interesting is when the TCC is plotted, SKM's Iarc current would result in a trip time of ~0.091 sec. When I cross a trip time of 0.044 on the curve, the corresponding current is ~12.4 kA.
This is at 480V, 60hz system
Any thoughts or comments? -

 

[dpc]

For me, Rule #1 for arc-flash calculations is that it is not an exact science. The two programs appear to giving similar arcing fault currents, with the difference probably entirely due to the difference in the calculated bolted fault currents.

I would compare the two time-current curves (SKM vs EasyPower) for the breaker and see if they match. With the thousands of TCCs stored in the program libraries, there are sometimes problems with the TCCs. Also, manufacturers periodically update their TCCs so there can be different vendor curves for the same device.

Bottom line is that I am not at all surprised to see differences like this and it would not overly concern me. See Rule #1.

 

[wbd]

I did look at the TCC from SKM and it is identical to the one in EasyPower so I am not sure why SKM thinks the breaker would trip in 0.044 sec. This is in the area where it may trip but is definitely not the clearing time.

 

[dpc]

In SKM it is possible to specify fault current calculations be done over a range of certain variables such as transformer impedance or utility impedance. I'm not sure if they allow for a range of clearing times - I don't have our SKM dongle right now, so can't check.

There are two different values of arcing current that are of interest - the total arcing current and the arcing current through the upstream device. But based on your description, I don't think that is a factor here. If you can post the SKM arc-flash report, maybe someone will be able to spot something. I use both programs, but I'm more familiar with EasyPower.

 

[wbd]

dpc, I am working off of a report that has all the data used and results of the study done in SKM. I have not used SKM and use EasyPower.