Generally, those fuses are used by OEMs, such as VFD mfrs, INSIDE of their units in order to pass UL listing requirements, not as part of a disconnect and I have never seen them required outside of a VFD before. A-B and ABB use them on a lot of their newer larger drives, but they are inside of the drive cabinets as they come from the factory. In your case I think someone got lazy and/or cheap when supplying the VFD, pushing it off into you to figure out how to do it.
