I am currently working on a shipboard study. The ship has four 850 KW gen sets 450 Volt, two gen sets provide power to two switchboards which are coupled by bus ties.

Software limits the number of generators that can be connected at any given time to three.

The main gen sets are protected by ACBs which also have protection that will cause instantaneous trip at currents greater than 24 Ka.

Here is my problem, the arcing current with three generators is such that an instantaneous trip will occur, however if only one breaker trips, and this happens in or close to post the subtransient portion of the generators the current will then fall below the instantaneous level; so I could have a sustained arc as the remaining generators will now take up to .4 of a second to respond. In the instantaneous portion the Incident level is 8 Cals in the non instantaneous it rises to 27 cals.

My question do I need to consider this possibility or caculate the fault as though all breakers opened or the fault cleared.

This one is going to take some effort. It is more of a dynamic simulation and will require multiple calculations (although it sounds like you may have already performed them).

Should you include this type of simulation? I believe the simple answer is: If it can happen, it should be considred.