While it may or may not be a violation of this installation, the fact that the middle fuse is a Class H fuse with only a 10,000 amp interruption rating while the other two are Type K fuses with a 50,000 amp interruption rating provides me with an opportunity to discuss the importance of knowing the available fault current.
Section 110.9 requires fuses, circuit breakers, and other equipment intended to interrupt current at fault levels to have a rating equal to or greater than the fault current available at the line terminals of the equipment. If the fault current provided here is 10,000 A or less, there is no code violation or safety concerns with fuses of different AIC ratings. However, if the available fault current is greater than 10,000 amps, there are code violations and serious safety concerns. Installers must know the available fault current in order to select the correct equipment to install. If the fault current provided here is 50,000 A, a fuse with a rating of 10,000 AIC could fail violently and catastrophically if exposed to this high fault current. Anyone operating this disconnect switch during this fault condition could suffer serious injury or worse if this fuse blows. Many code sections require the available fault current to be marked or documented for equipment including Sections 110.24, 408.6, 409.22, and others.
