Short Circuit Equpment Evaluation Bus Method vs Protective Device Method

[philly]

When performing short circuit equipment evaluation analysis I've seen study results that either show equipment evaluation based on a "Bus Rating" and others based on "Protective Device ratings" Obviously the intent of the equipment evaluation is to compare the available fault current against the equipment bus rating and rating of protective devices but I curious to see how others convey this information with the various analysis software package.

For the Protective Device rating analysis the software simply lists each protective device and compares their device ratings vs the available fault current. This I guess is the most straightforward way however this can lead to cumbersome results since it will essentially list every protective device (could be 100's) and lead to a quite lengthy results table. Many of these devices will be redundant such a having the same breaker type in a given panel or panels, so you are almost listing all these breakers unnecessarily.

The "Bus Rating" method involves a little more input but seems to provide a cleaner output in my opinion. When modeling a piece of equipment (panelboard, switchboard, etc...) most software programs have fields that can be entered manually for bus rating values such as ampacity, interrupting rating, and X/R value. With this method the bus interrupting rating is based off of the lowest rated device in equipment (as well as taking the bus bracing into consideration) So essentially you look at all the devices in the panel and give the bus an interrupting rating and X/R ratio corresponding to the lowest rated device. For example if there is a panel with all 65kA rated breakers but there is one breaker in the panel rated at 14kA for some reason, then the entire bus rating would be based off of the 14kA breaker rating and its tested X/R value. With this method the output report simply lists each bus and its corresponding value and is much cleaner in my opinion and avoids some of the redundancy with the other method.

I was curious to hear the opinions of others and pros / cons for each method. Is there and instance where the protective device method may be beneficial?

 

[topgone]

Either way, the documentation provided by the software program maker explains what figures were assumed as well the algorithm of the program. If the results of both softwares differ by a few decimals, it's useless comparing the two methods, IMHO.

 

[philly]

Either way, the documentation provided by the software program maker explains what figures were assumed as well the algorithm of the program. If the results of both softwares differ by a few decimals, it's useless comparing the two methods, IMHO.

I wasn't necessarily referring to comparisons between two different software packages but rather two different methods of evaluating equipment within the same software package. For instance in SKM you can run a protective device evaluation or a bus evaluation based on the methods I described above. They more or less give the same fault results and evaluation results but have different methods of conveying the information.

 

[charlie b]

I haven't done it either of those two ways. My input information has never included anything related to the panelboard itself (i.e., its short circuit current rating or anything about its breaker ratings). Rather, I create a model of the one-line diagram, and input the utility information, the transformer information, the sizes and lengths of the feeders, and the locations of any large motors. The software then calculates the fault current available at each panelboard's location. I use that to select the SCCR ratings that I show on each panelboard's panel schedule, and that is what the contractor uses as the basis for purchasing the equipment.

If the project calls for a selective coordination study or an arc flash study, then I would need to work with the equipment supplier to select breaker types and settings that will satisfy the project requirements. But most often those two studies are assigned to the equipment supplier, rather than performed up front by the engineer.

 

[philly]

I haven't done it either of those two ways. My input information has never included anything related to the panelboard itself (i.e., its short circuit current rating or anything about its breaker ratings). Rather, I create a model of the one-line diagram, and input the utility information, the transformer information, the sizes and lengths of the feeders, and the locations of any large motors. The software then calculates the fault current available at each panelboard's location. I use that to select the SCCR ratings that I show on each panelboard's panel schedule, and that is what the contractor uses as the basis for purchasing the equipment.

I usually perform an analysis after the above work that you mentioned is complete. So I am either verifying specified panel/breakers ratings that were specified for the job or I am evaluating existing facilities equipment. Either way it involves the same software model that you used above except the actual equipment details are filled in the buses that are crated and the software performs an equipment evaluation based on the available fault current (from utility info, motors, etc...)