20-amp NON-type fuses will open in roughly 5 minutes at a 30-amp {150%} load. “Fuse curves” are generally referred to as inverse-time-current characteristics. See if the suspect fuse has it’s rating also stamped on the ferrules.
Not sure if it’s always the case, but {before “deregulation”} one used to be able to take a personal meter into the local-utility metering cal lab and if not busy they would compare your meter to their “bench standards” without any written report. A local met lab charges $45 to “sticker” a Fluke handheld, without any “as found” documentation. [If you have an accessory current probe, that costs extra.]
