Fuses furnished to this specification and listed by Underwriters Laboratories Inc. are intended for use in accordance with the National Electrical Code (NFPA 70). These fuses are designed for use on circuits which may be subjected to high short circuit currents, and these fuses are capable of limiting the peak let-thru current and maximum clearing I²T (see 3.1 and 3.3).

Class G fuses.

These fuses provide safe protection for circuits in homes, commercial buildings, and industrial plants of 120/208, 120/240, and 277/480 volts (not to exceed 300 volts line to ground).

Class J fuses.

These fuses provide safe protection on circuits of 600 volts or less that have available short circuit currents up to 200,000 amperes. These fuses are fast (no time delay) and are current limiting.

Class K fuses.

These fuses have the same National Electrical Code dimensions as class H fuses but differ from class H in that they have a high interrupting rating. The class K fuses have varying degrees of current limiting ability but may not be designated as current limiting. Class K fuses are divided into subclasses as follows:

a. K1 - Highest degree of current limitation characteristics; interrupting capability up to 200,000 amperes.

b. K5 - Medium degree of current limitation characteristic. Time delay - shall not open in less than 10 seconds at 500 percent of rated current, interrupting capability up to 200,000 amperes.

c. K9 - The lowest degree of current limitation characteristic. Same time delay as the K5, but an interrupting capability up to 100,000 amperes.

Class R fuses.

These fuses have the same overall National Electrical Code dimensions as classes H and K fuses but differ by possession of a "rejection feature" which prevents them from being used in a fuseholder for a class H or K fuse. These fuses are current limiting and are divided into subclasses as follows:

a. RK1 - Highest degree of current limiting characteristic; interrupting capability up to 200,000 amperes.

b. RK5 - Medium degree of current limiting characteristic; interrupting capability up to 200,000 amperes.

Class L fuses.

These fuses consist of the larger current ratings (600 to 6000 amperes) and are used on circuits of 600 volts or less that have available short circuit currents up to 200,000 amperes. These fuses provide safe current limiting protection when properly bolted to bus bars. They have no intentional time delay.

Interchangeability of classes H, K, and R fuses.

Fuses which have the same amperage, voltage, and time delay ratings are one-way interchangeable between classes as follows:

a. RK1 may be used in lieu of RK5, K1, K5, K9, and H.

b. RK5 may be used in lieu of K5, K9, and H.

c. K1 may be used in lieu of K5, K9, and H.

d. K5 may be used in lieu of K9 and H.

e. K9 may be used in lieu of H.