200A Fuse in a 400A FDS

[7carisfast]

It may be a bit embarrassed asking this question, but this is where engineering meets real life questions IMO. Is this 200A fuse allowed to be installed in the shown 400A FDS or are fuse reducers required (Eaton Part # J-42)?

 

[dpc]

You would have to review the documentation for the switch to see what type fuses are allowed based on its UL listing. It's not inherently a safety concern to have a fuse sized smaller than the maximum rating of the disconnect - that's common. But you can only use fuses that the fuse holder is designed to work with.

 

[AG-PPI]

400A is likely the absolute maximum rating for the FDS. Using a smaller fuse could be a simple way of derating the FDS dependant on load size.
But yes, as dpc says, ensure the fuse that has been fitted has been suitably selected.