Short Circuit calcs
Short Circuit calcs
(OP)
My boss asked me today to figure out what short circuit capacity a switchboard would need to be designed for given:
2 x gensets with 22kA fault rating each (1 MW 50hz 415V sets, got the kA from manufacturer)
Synced onto a common bus in the switchboard
Each genset will be connected through a circuit breaker to the bus via 3 parallel 100 ft long flex cable at 240sqmm, that according to the book each have an ac resistance at 50hz of aroung .09 ohms/km each.
Note, this won't be used as a definitive because it's been outsourced to a real engineer (I'm only a young foolish electrician) but it's along the lines of a learning experience.
Do I treat the 2 gensets as a 2MW with 44kA, figure out the effective impedance of that using ohms law, then add that impedance with the total impedance of the cables per phase, and again use ohms law to figure out amps = 415v / total z ?
Note - this isn't an assignment. I finished my apprentiship a few years ago now, just not needed to do this sort of thinking before.
2 x gensets with 22kA fault rating each (1 MW 50hz 415V sets, got the kA from manufacturer)
Synced onto a common bus in the switchboard
Each genset will be connected through a circuit breaker to the bus via 3 parallel 100 ft long flex cable at 240sqmm, that according to the book each have an ac resistance at 50hz of aroung .09 ohms/km each.
Note, this won't be used as a definitive because it's been outsourced to a real engineer (I'm only a young foolish electrician) but it's along the lines of a learning experience.
Do I treat the 2 gensets as a 2MW with 44kA, figure out the effective impedance of that using ohms law, then add that impedance with the total impedance of the cables per phase, and again use ohms law to figure out amps = 415v / total z ?
Note - this isn't an assignment. I finished my apprentiship a few years ago now, just not needed to do this sort of thinking before.






RE: Short Circuit calcs
You may be using the wrong word or the wrong number.
You should be using the impedance.
Bill
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"Why not the best?"
Jimmy Carter
RE: Short Circuit calcs
A proper resource to learn short circuit calculations is to start with IEEE Red Book, assuming you have other basic electrical engineering understanding. Then make friends with a good electrical engineer, who does this for living. It will be very difficult to learn through fora like this. You will get bits and pieces of information but not a whole picture.
There may be other less expensive books, but IEEE is more authentic.
Coming back to details, you need to work with per phase (single phase quantities). So in the end, you need to divide total Z per phase with V/ph or 230V in your case.
Also verify what 22KA is, it may be the AIC rating of the breakers they have provided and not the actual short circuit current deliverable by the generator. Or you can request the sub transient reactance. Use that as your source impedance. There a few other factors that need be considered based on actual system details.
Rafiq Bulsara
http://www.srengineersct.com
RE: Short Circuit calcs
Rafiq: I didn't even consider the single phase thing, damn. About the rating, I'll have to ask the boss on Monday, because he was the one who told me (in his exact words pretty much ) "The maximum short circuit current the generators can supply the switchboard is 22kA each".
I will certainly look into into getting hold of the Red Book, because I get the feeling it will be useful in other areas.
RE: Short Circuit calcs
Rafiq Bulsara
http://www.srengineersct.com
RE: Short Circuit calcs
RE: Short Circuit calcs
Rafiq Bulsara
http://www.srengineersct.com
RE: Short Circuit calcs