When running my arc flash study, in the table it shows an arcing fault of 5ka, but incident energy is high because trip time is over 2 seconds. When I look at the protective device the INST setting is below 5kA. I thought the breaker would trip but it doesnt. After looking further into it the I see that the protective device arcing fault is 2kA. How can these be different? If the downstream bus sees 5kA then why doesnt the upstream protective device see this?

Does the bus in question have motor loads that are contributing to the fault current at that bus? Not having the benefit of seeing your one-line so speculating here but it could be motor contribution from one direction and utility contribution from the other direction creating higher fault current at the bus than at the protective device.

Here are some screen shots illustrating this for bolted fault current. I don't have a datablock that shows arcing fault current with current direction and am too lazy to make one at the moment but I think it illustrates the point.





The other possibility is that you have two sources feeding the bus through different breakers but that is not that common and paralleling breakers is a code violation unless it is part of an OEM assembly.

Also, from IEEE 1584-2018: