Replacing fuse with CB - equivalent ratings

I have determined that the existing fuse will only blow slightly above the rated current.
I therefore have to assume the submain load is 200A and this precludes my use of the 160A MCCB.

The next available MCCB is 250A which requires a 630A MCCB upstream to ensure discrimination/selectivity.

However, the supply TF is rated at 270 Ampere only.

In stead of an MCCB with a thermal-magnetic mechanism, try one with an hydraulic-magnetic action. They can have overcurrent protection very close to the actual overload setting (105% typical). We (in South Africa) also have curve 2 & curve 3 actions for our normal MCBs, whereby the one reacts faster than the other one (250% for 1 sec vs 400% vs 1 sec), typically to allow for motor starting.

Better to replace incomer fuse with 400A MCCB. Fuse will act fast on short circuit. Hence at S.C. condition incomer fuse will not grade with 250A MCCB

To get the grading I need, I will have to replace the existing 300A fuses with a 630A set 300 MCCB with electronic trip unit.

I can then grade with 2-off 250A thermal-magnetic MCCBs protecting my 2-off submains.

Obviously this particular arrangement is specific to my chosen manufacturer...

As per IEC standrd MCCB should not trip at 5% overload and at 130% load it should trip after one 1 hr. Normally at 200% overload mccb will trip between 24s to 240s. better to consult mccb manufacturer time vs over load current curve.