4 Independent FTs Value Added in PLC to Create Trip on Total


I am designing an architecture for a SIF that monitors the total combustion air intake flow trip in a BMS. As per the SIF description, when the sum of 4 flow transmitters (on four different lines) is less than the trip set point, or if any of these FTs goes into fault etc. the trip will take place. So in a sense, it is four 1oo1 added into one value and then compared for the SIS trip.

I understand that these 4 FTs are measuring separate flows, and then being summed in logic solver before being compared with SP. Or in other words, this SIF has four initiators but instead of a being independent, there is a twist of summation inside PLC. What should be the voting configuration (1oo1 or 4oo4)? More importantly is it allowed in IEC61511/NFPA to design BMS SIF using 4 x 1oo1 sensors? I have never seen such design of BMS trip. Please guide.

Thank you in advance.