What's the difference between a class RK5 and a class H 30 amp fuse?

Class H fuses. This cartridge fuse, once known as the NEC dimensional fuse, is suited for general purpose branch circuit, lighting circuit, and the protection of non-inductive equipment like electric ovens and resistance heaters. Class H fuses are available in renewable and non-renewable models. Renewable types allow the user to replace the internal fusible link after the fuse operates.

All Class H fuses are tested for short-circuit requirements on an AC circuit of sufficient capacity to deliver 10kA rms symmetrical. Class H fuses are available with DC ratings. The short-circuit power factors are relatively high: 0.45 to 0.50 for fuses rated 110A to 600A, and 0.85 to 0.90 for fuses rated 100A and less. The actual short-circuit power factors encountered in typical installations are on the order of the above test values, when the available short-circuit current is 10kA rms symmetrical or less.

With regard to the time-delay designation, renewable Class H fuses can't be so designated because the renewable link, can't be designed to perform the melting function required for time-delay characteristics. Class H non-renewable fuses, however, can be designated as time-delay and so marked on the cartridge or carton if they don't open in less than 10 sec when carrying 500% of their rated current.

Class H fuses, both non-renewable and renewable, are often misapplied in the electrical industry for a couple reasons. First, their minimal 10kA interrupting rating is easy to exceed in an industrial plant or commercial building. Second, workers unfamiliar with the differences between devices might be tempted to install more than one link in a renewable fuse.

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Class R fuses. These non-renewable fuses are made in 250VAC and 600VAC ratings, with current ratings from 0A to 600A. Class R fuses are available with DC ratings. They have an interrupting rating of 200kA rms symmetrical and are separated into two separate classes, RK1 and RK5. RK1 fuses are available with interrupting ratings of 300kA.

Both RK1 and RK5 fuses are current-limiting and meet standard-prescribed maximum peak instantaneous let-through current and maximum I2t let-through energy requirements.

Actually, Class RK1 and RK5 fuses are Class K-1 and K-5 fuses with ferrules and knife blades modified to conform to Class R rejection requirements. The grooved ring in the ferrule and the slot in the knife blade, when used in conjunction with special fuseholders designed to accept Class R fuses only, prevent interchanging of any other fuse class with the installed Class R fuse.

All Class R fuses must be labeled “Current Limiting.” They may be labeled “Time Delay,” indicating that they meet the optional time-delay test by not opening in less than 10 seconds at 500% of fuse ampere rating.

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copied from:
Specifying the Right Fuse for Circuit Protection (http://ecmweb.com/news/electric_specifying_right_fuse/)

Also note this thread:

https://www.askmehelpdesk.com/electrical-lighting/type-h-fuse-21509.html