UL fuse classes define tested electrical and dimensional characteristics. They help prevent substitutions with an unsuitable class, but the class label does not replace a series datasheet or a circuit coordination study. This guide separates the common branch-circuit choices from their actual selection work.

What the classes tell you

Class R includes RK1 and RK5; “R” refers to rejection features, not renewable construction. Eaton lists LPN-RK and LPS-RK as time-delay RK1 series and other fast-acting RK1 series. Do not assume that one listed prefix has the same time curve as all others.

Select from the installation, not the class name

First establish voltage, load and startup profile, conductor protection and available short-circuit current. Then compare the manufacturer's curve, interrupt rating and current-limiting data. In a panel, check the required short-circuit current rating and any documented tested combination. The fuseholder must accept the class, ampere frame and voltage duty.

Example: If a small control circuit requires a Class CC rejection fuse, a physically similar 10 × 38 mm supplemental fuse is not a substitute. If upgrading an RK5 feeder to RK1, examine motor starting, coordination and equipment instructions; a different curve may change nuisance operation and selectivity.

Brand catalogs may list ranges and features by series. Verify the specific part number before ordering, particularly where a product code appears in multiple markets or AC/DC variants.

Sources for verification

Eaton fuse classes  •  UL fuse and fuseholder services  •  UL short circuit rating guidance