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Why an apparently slower upstream breaker is reported in Arc Flash

In the given project, faulting the CLCA bus reports the upstream breaker CB-USS3-UPS INPUT instead of the CB-UPS3 BYPASS as the protective device in Arc Flash.




Comparing the plots of the two devices, it may appear that the upstream breaker CB-USS3-UPS INPUT is slower than CB-UPS3 BYPASS.



However, in order to compare the device coordination properly in Arc Flash, the curve needs to be plotted in which it faults the same faulted bus and use the branch current shifted to a common base. The actual curve for the CB-UPS3 BYPASS breaker is shifted 4x (24968A/6242A = 4) in reference to the fault current on the CLCA bus. Due to the shift, CB-USS3-UPS INPUT is seen to be tripping faster.