Short circuit study for commerical buildings
Short circuit study for commerical buildings
How do you pick an equipment AIC rating without a fault study? Arc-flash is more and more common.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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Short circuit study for commerical buildings
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RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
Specifically - Fault studies...
In a typical industrial application, I would map a fault onto every bus in the 480V system. Using ETAP, it takes a few hours at least to properly lay out the breakers, cabling, motors, and transformers. But then I could use the one line to do over current protection coordination, fault currents, and arc flash protection requirements.
In a commercial application, such as a small building fed by a 250KVA kva transformer, one 3 phase 208V distribution panel, and let's say 3 lighting panels. To do a full blown short circuit study would take some time. But to just look at the transformer available short circuit, a quick calculation tells you it really cannot produce very much, even with an infinite bus. If the available fault current does not exceed the ratings of the breakers in the distribution panel board, then why would you spend the time to model a one line of the building?
Other - Does anyone do arc flash, grounding studies, and protection coordination studies for such a building as described above?
thanks for your help
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
Whether you do it manually or by software, it is required for you to verify equipment ratings and perhaps, overcurrent protective device settings for proper coordination. You may use per-unit R and jX calculations to determine the symmetrical and asymmetrical faults or use the KVA method (as long as you tend to go to the conservative side). You may lump some motors loads (preferrably lower than 50HP); anyway, your LV non-motor loads will not contribute to the short circuit fault. If you neglect cable impedances, your symmetrical fault calculations may become conservative. You see, you have options on how to get it done as long as you have decent information or equipment estimation to get at least a conservative calculated fault. You need this as a basis for specifying equipment.
For arc flash on the other hand, if you intend not to verify it, then you should design your electrical distribution equipment to ensure shutdown during maintenance/inspection or locate them in such a way that they are readily accessible to public while energized. At lower voltages (220V, 208V, 120V etc), it is possible to get issues on them due to the use of MCCBs and possible levels of fault current that may generate enough arc to hurt somebody nearby.
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
For a pretty small commercial building, the same may apply. For a large commercial building, the power company might design a dedicated power supply for your building loads and they may require you to provide them more information of your electrical loads (check loading, estimate harmonics, etc.). In parallel, you may have to obtain the MVAsc and X/R from them and calculate the fault current
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
RE: Short circuit study for commerical buildings
I have to admit I have been quite confused at the replies in this thread, especially the 4 or 5 minutes required to do a fault study. I don't know anyone who can do anything of value in 4 to 5 minutes. But I get it now. The assumption here is that everyone already owns a copy of SKM and that the one line for the system has already been done in SKM, and the system impedances and available fault currents are already on the one line. This is an inaccurate assumption, but I do get your points. Thank you for your advice.
RE: Short circuit study for commerical buildings
I don't think you can make it a blanket statement that all dwelling units would have an AIC of 10,000 amps or less. There are utilities that require the service equipment to have a rating of 22,000 for services greater than 150 amps and less than 401 amps. Maybe that is just CYA on their part, but they will not turn the power on if you install 10,000 amp rated equipment.
RE: Short circuit study for commerical buildings