Low voltage circuit breaker arc flash

[chairmounted]

I am using IEEE 1584 spreadsheet to calculate the arc flash energy downline from a low voltage circuit breaker. The maximum available fault current is 26kA and the Digitrip is set to 18kA instaneous trip. The cell in the spreadsheet turns orange noting 'out of range'. What range are they talking about? 18kA is well below the 26kA that is max available. Any help would be much appreciated.

 

[dpc]

The arcing fault current will be less than the "maximum available" fault current due to the resistance of the arc. For low voltage systems, the arcing fault current is MUCH less than the bolted fault current.

I suspect the calculated arcing fault current is less than 18 kA. In addition, IEEE 1584 recommends a second evaluation at 85% of the calculated **arcing** fault current, since arcing current is highly variable.

I haven't used the spreadsheet in many years, but you should check what the calculated arcing current is.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[JMatt]

Is 26kA the "bolted fault" current at the piece of equipment in question or is that just a number to tell the spreadsheet what your source Z is?

I'm guessing that you input 26kA as the source (1.5MVA transformer with Z=7%), but there's a large impedance between the LV breaker and the equipment you're analyzing.

All of this results in a much lower bolted fault current used in the equation. As dpc pointed out, the arcing fault current is much less than the bolted fault current (remember to check your conductor gap), so you're probably not getting the trip unit to pickup.

 

[chairmounted]

Thanks for responding. I'll try to explain better. At the LV breaker the max bolted fault current is 21.94kA and the calculated(by IEEE spreadsheet) arcing current is 12.83kA. 85% of 12.83 is 10.91kA. The existing instantaneous trip setting is 18kA, so I change the instantaneous trip setting to 10kA and the cell is still orange where I figured it would work. I have to change the instantaneous trip setting to 9.9kA which is 77% of the arcing current in order for the cell to not be orange. I hope this made sense. any help is appreciated.

 

[dpc]

It may be due to the tolerance band on the instantaneous pickup setting. Generally +/- 10% or so. The 85% arcing current must be greater than the high end of the inst. pickup band, not just the pickup setting. IEEE 1584 has a "generic" molded case breaker response curve they use.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[bacon4life]

The cell should turn orange letting you know that you need to enter the time curve data rather than having it use the instantanous energy curve. The standard does mention a factor of 1.3 to adjust to the top of the tripping band for equation 26, which is derived from equation 2 with a closed box and a gap of 25mm.

Perhaps the 1.3 factor is equivalent to a +10% breaker tolerance and an 85% fault? 1.294=1.10/0.85

 

[chairmounted]

Thanks for the responses. I didn't think of the tolerance and 1.3 factor. So if I enter the time curve data of the Short Delay curve, do I enter the trip time at the max 'arcing current' and then again at the 85% max 'arcing current' or do I have to enter many points to determine the worse case scenario?