Is it possible that the downstream breakers are series rated with the immediate breaker upstream?

Is it possible that the downstream breakers are series rated with the immediate breaker upstream?

The upstream breaker is a 65 kA breaker, meanwhile the downstream are 25 kA breakers.

We need to know the exact upstream and downstream breaker manufacturer and model numbers in order to determine if is a tested series combination between the two. I do not know what Bielomatik is or what some of your other labels are.

If Bielomatik is a manufacturer I am unfamiliar with them and will be unable to help. Now, if the manufacturer is Siemens, Square D/Group Schneider, GE or Eaton/Cutler Hammer/Westinghouse we can look those up easily enough.

The arcing fault current is less than the short circuit rating so it may turn out that the combination would work. Otherwise just model the downstream breakers as if they aren't there and then the upstream breaker handles both short circuit and arc flash.

Long term either you need to consider upgrading the breakers or else look at adding current limiting to get the higher rating either by adding line reactors to the line side somewhere or current limiting fuses somewhere in the circuit (usually downstream).

At extremely high currents there is the TCL (triggered current limiter) option but let's just say that this sounds good on paper but not so great in practice. A chemical plant customer of mine has 3 cogen turbine generators and sits right across the street from a power plant. At some point in their past they went to closing both mains and the ties on their two main substations forming a ring due to the lowered impedance which allowed them to do what they are doing...most of the time they are a net power producer harvesting energy off of what is apparently a fairly exothermic reaction. The problem is that on their main buses they are pushing something like 40-50 kA short circuit but the breakers they have are rated in the 20-25 kA range (typical for draw out 15-25 kV medium voltage breakers). With two 8000 HP compressors adding line reactance is not practical due to the reduced voltage conditions it would create. So that left them with TCL's as the best option. They put two of them in to split the two buses between the main subs. The remaining issue is that every time they fire the replacement cartridge (it's like a really expensive fuse) is $8,000 each.