The use of plug fuses of the Edison base type is permitted for replacement when there is no evidence of over fusing, or tampering.
In the image below the two 30 amp fuses will be considered unsafe, and MUST be replaced with the Type S adapter, and Type S fuse of the proper size in accordance with the size of the branch circuit conductor.
One hint I like to share, and that is when the correction is made be sure that the Type S adapter was installed properly, I mean so that it cannot be removed unless use of the special “can opener” type of tool is used.
If the steel wire on the outer part of the adapter is cut off, the adapter can be removed, and the larger fuse can be put back. This was a problem for us in the field as inspectors in furniture stores where so many lamps were used creating overloads.
As an electrical inspector, I used a short piece of a broom stick that was tapered to fit into the open adapter that sometimes revealed that the adapter could be easily removed.
However, I guess that the HI cannot remove a fuse though?
PS: No. 14 AWG open knob and tube wiring was permitted to be protected with 20 amp fuses! There was a note in Article 310 that verified that before it was removed a few code cycles ago, maybe in the 1981 edition?