Relationship between Cut-Out Fuse & MCB PSCC

[kitleach]

Good morning all,

Apologies if you have gone over this a number of times before (I have tried searching for a thread, and have found some that have touched on what I'm after but not quite explicitly enough).

If the measured PFC is greater than 6ka (for example) with circuit-breakers' breaking capacity of 6kA, I need to check the breaking capacity of the fuse supplying the board.

How do I work out if the rating of the service fuse makes it acceptable?

Is it as simple as just making sure the capacity of the service fuse > the measured PFC ?

Which is how I read 434.5.1 "...the rated short circuit breaking capacity of each device shall be not less than the maximum protective fault current at the point at which the device is installed.

"A lower breaking capacity is permitted if another protective device or devices having the necessary rated short-circuit breaking capacity is installed on the supply side."

Or do you need to check the relationship between, how long the service fuse would take to blow at 6kA (ok fraction of a second) or do you need to prove the fuse would blow before reaching 6kA even in a dead-short situation?

And if so, where in BS7671 is the information I'm looking for/ how do I work it out?

Thanks, Chris