Hi What is the best way to calculate the short circuit fault current using SKM- model the infinite bus or use the available fault current from the utility. Which will give the worst case results? How to model the infinite bus in SKM?
You really should start with a calculator and text book on power system analysis before using the software.
The BEST way starts with what assumptions you are making. The “best” software of course is some variation of EMTP. SKM is a far cry from the accuracy of that system. But that’s for transients and brings us to the fundamental issue: what are you trying to do?
ALL short circuit calculation standards began back before calculators existed, when the calculations were done mostly with slide rules. And this there are a LOT of simplifications and assumptions. You need to know what’s going on or you will produce garbage results as well as waste a lot of time and money. There are TONS of engineering firms out there doing just that. What you are paying for is a trained monkey and a keyboard. All of their results are utter garbage. And as far as software goes SKM in particular is extremely buggy so you better be prepared to check over whatever results it produces.
Reason I’m saying this is you are asking an engineering question that is basic to understanding short circuit analysis on a forum for electricians, not engineers.
Answer: it depends. Utilities often give you crap data that is just plain wrong. So you need to be able to evaluate their claims. Often it’s just an infinite bus bases result. So before going further the “utility” number is often clearly wrong. Second is that inductive loads and shielded cable or overhead lines can often have as great or greater influence on the results than just transformers and the loads with long lines. But in general if you are modeling for short circuit rating purposes then infinite bus and ignoring impedance gets you to a quick answer and doesn’t even need SKM and you can adjust it for cables, motors, and transformers with some simple rules of thumb. Chances are though this results in way over sizing a lot of equipment. Once you get above certain AIC ratings costs explode. There is a big jump from 35 kA to 65 kA for instance in breakers.
If however you are trying to size for breaker settings or arc flash overestimation is a huge FAIL. It will result in underestimating everything. This is a huge safety problem because arc flash results can easily be many times the estimated result. However inductive effects will have little effect except from a simple impedance point of view (we don’t care about transient conditions).
Not going to write a book in this post but just saying if you are asking this question you shouldn’t be using SKM. Pay the money and take the 1-2 week training classes out there or take a semester class at a local college. Do NOT waste everyone’s time being another trained keyboard monkey, garbage in is garbage out.
I would say 90% of the engineering firm cal a out there aren’t worth the paper they’re written on. Even the Fuse Handbook point to point method is easier.
