SWBD Short Circuit Rating

[RonShap]

480V, 3Ph, Paralleling SWBD rated at 100kAIC.
Using SKM the 3 phase bolted fault calculated at 91.8kA with x/r=15.4
L-G fault calculated at 121.8 with x/r=12
Does a normal 3 phase switchboard short circuit rating of 100kA have any associated L-G rating, or do I use the 3 phase rating?
Does this SWBD fail a short circuit evaluation?
I realize the breakers need to have a multiplier since the x/r is higher than the test value, but I'm trying to focus on the SWBD first.
Thanks

 

[rbulsara]

You need to use higher of the L-G and the 3-Phase values. What does the program evaluation say?




Rafiq Bulsara

http://www.srengineersct.com

 

[RonShap]

I'm only refering to hard copy printouts of the A_Fault report. The study was done a few years ago by another consultant that did not use the Evaluation module of SKM.

 

[dpc]

The switchboard short circuit rating will be a max of 100 kA, regardless of fault type.

You might want to look at the data that was used in the model. It is true that the SLG fault current from a local generator can be greater than the 3-phase fault current, but most generators are only braced for the three-phase fault current, with the assumption that the ground fault current will somehow be limited externally to at most the three-phase fault current. If there is any external neutral grounding impedance, make sure that is included in the model that was used.



David Castor

http://www.cvoes.com

 

[RonShap]

I was considering adding some impedance that is typically omitted from the short circuit model, as related to typical contact resistance for the breakers and conductor termination resistance.
Are there references out there of typical values for that type of normally ignored impedance?

 

[dpc]

Contact resistance is generally measured in micro-ohms. I don't think that is going to help much. If you are trying to get from 122 kA to 100 kA by tweaking the model, that probably won't help you sleep well. I'd look for something substantial that may have been overlooked, and double check all of the generator reactances that were used, especially the zero sequence impedance. If this was a "typical" value the software plugged in, you should really try to get a real value from the generator manufacturer.

David Castor

http://www.cvoes.com

 

[burnt2x]

what dpc said. It's just like garbage in - garbage out. Your software will spit out results based on what was inputed, so track back and see what was "wrong' in the input data, if there were any.

 

[rcw retired EE]

An old GE guide to short circuit calculations has a table listing impedances for typical air-frame low voltage breakers and bus. I was going to include those values in short circuit calculations but I realized the data was probably based on thermal-magnetic trip units and not modern CT driven solid state trips. The spiral copper bus on older breakers had to have a lot more impedance than a straight bus and CT.

As stated above, don't attempt to include the breaker impedances. The results won't change much, unless you are using obsolete data.