Short circuit analysis
Short circuit analysis
(OP)
Why are both the maximum and minimum short circuit currents required for relay co-ordination? Isn't the maximum short circuit current sufficient? And while computing minimum short circuit current, do we neglect only the motor contribution or are there other parameters to be ignored?






RE: Short circuit analysis
RE: Short circuit analysis
- minimum source impedance
- all other infeeds present (motors, generators, etc)
Minimum faults occur for:
- maximum source impedance
- no motors, optional generators, etc.
Note that if you're doing short circuit calculations using the IEC spec there are a bunch of little adjustment factors you use including an assumption of higher line resistances for min fault conditions.
As jghirst mentioned, the simplest reason for considering minimum fault conditions is to ensure your protection can discriminate between minimum fault conditions and maximum load conditions.
Ian Dromey - www.dromeydesign.com
RE: Short circuit analysis
As jghrist said, generally you will probably loose separation at the high levels, but that is where you can usually count on the relay operating "dead on" on the curve.
Alan
Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin
RE: Short circuit analysis
RE: Short circuit analysis
1) equipment sizing - must capable to withstand or interrupt the maximum fault
2) relay setting - one of all aspects is you need to look into the minimum fault vs the maximum load
3) arc flash - minimum fault results longer clearing time, thus often leads to a higher arc energy
RE: Short circuit analysis