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Posted: 2020-11-23 09:21 AM
For completely selective coordination (sometimes referred to as 0.01 seconds) you will first need to know the fault current at the point(s) in the system where you need to selectively coordinate. If you do know this value, you can start with an infinite source calculation at the source transformer which is calculated by dividing the transformer's full-load amperage by the percent impedance (%Z). From there, you can now use the manufacturer's guide book such as Square D / Schneider Electric's Data Bulletin 0100DB0501 titled "Short Circuit Selective Coordination for Low Voltage Circuit Breakers". Or, you might be able to find the same type of information via an on-line tool such as the one found here: Selective Coordination Analysis (squared.com)
This on-line tool will also help you to calculate the fault currents in your system.
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