I might be being a bit thick here (has happened before, will happen again), but doesn't that mean that the MCB can't safely break more than 1KA and with a Pfc of 1.8KA you have a protective device which is incapable of safely interrupting a fault current?
The device upstream with the higher rated fault current will break. The MCB with the 1kA rating would then need to be replaced as it could have been damaged with the higher than rated fault current.
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