The production floor supervisor calls maintenance to say the equipment is down. A competent engineer quickly diagnoses the problem: A loosened terminal screw caused the circuit to heat enough to blow the fuse. He tells maintenance to replace the fuse. Uh, oh, no more fast-acting KTKR fuses. Maintenance thinks, "We're out of KTKR fast-acting fuses, but I've got a bunch of KTDR fuses in the same current rating. They'll be fine."
In our example, if a time-delay fuse gets put where a fast-acting fuse should be, the fuse will allow 20 more amps through before melting. Improper substitution, even though the fuses are identical in shape and current rating (and the part numbers differ by only one letter), can cause machine damage, fire, and possibly human injury.
Who becomes liable? Is this foreseeable misuse? The codes governing electrical panel safety require panel manufacturers to have fuse replacement schedules prominently placed on the panel (along with terminal screw torque requirements). The trick is following them. This is something that concerns every machinery manufacturer.
This makes me think of the Darwin Award given to the gentleman who removed himself from the gene pool without actually killing himself. OK, it was just an urban legend. But improper substitution does happen. If we think Thurston Poole of the bullet fuse urban legend is an idiot, shouldn't we think the same of the maintenance technician who replaces a fast-acting fuse with a time-delay one? Or should we be looking at the machinery manufacturer and the fuse manufacturer as the dummies for designing circuit protection where fast-acting and time-delay fuses can be mixed and matched interchangeably?
CC is just one class. We also have classes J, T, L, and H, to name a few -- not to mention inverse time circuit breakers. The subject of circuit protection is not a simple one.