The circuit protection market is filled with such a wide variety of fuses today that the previously accepted method of specifying fuses is just not enough. Instead, the proper application of fuses in today’s sophisticated power systems requires much more knowledge of fuse types, construction, and operation characteristics. The sheer amount of information provided by manufacturers today will verify this. It’s important to stay informed on the latest developments as new types, constructions, characteristics, and classes are established.

The following tri-national (UL/United States, CSA/Canada, and ANCE/Mexico) standards are widely used as requirements by which manufacturers must design their fuses. For those applying fuses, the information included in the standards is of great significance. Since these standards are continuously being updated, it’s important to refer to the latest version when designing or constructing your next project.

• 248-2, Standards for Safety – Class C Fuses
• 248-3, Standards for Safety – Class CA and CB Fuses
• 248-4, Standards for Safety – Class CC Fuses
• 248-5, Standards for Safety – Class G Fuses
• 248-6, Standards for Safety – Class H Non-Renewable Fuses
• 248-7, Standards for Safety – Class H Renewable Fuses
• 248-8, Standards for Safety – Class J Fuses
• 248-9, Standards for Safety – Class K Fuses
• 248-10, Standards for Safety – Class L Fuses
• 248-11, Standards for Safety – Plug Fuses
• 248-12, Standards for Safety – Class R Fuses
• 248-13, Standards for Safety – Semiconductor Fuses
• 248-14, Standards for Safety – Supplemental Fuses
• 248-15, Standards for Safety – Class T Fuses

Fuse classes, types, and ratings. The following discussion will analyze several of these fuse classes, along with pertinent performance characteristics and ratings.

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.

Class G fuses. Developed for use in lighting and appliance panelboards with a special fusible-switch unit, these non-renewable cartridge fuses are intended for use only in AC circuits where interrupting ratings to 100kA rms symmetrical are required. Class G fuses are available with DC ratings.

These fuses conform to 248-5, and are rated for 600VAC (0A to 20A) and 480VAC (25A to 60A). They’re made in four ferrule sizes: 0A to 15A, 16A to 20A, 21A to 30A, and 31A to 60A. The dimensions for this class of fuse were selected so as to prevent the interchangeability with any other fuse clas.

Class G fuses are current-limiting and are able to be so labeled per UL 248-5, which prescribes the maximum peak let-through and I2t let-through values permitted for this class of fuse.

An optional time-delay test is included in this standard for Class G fuses. It is important to realize that the test calls for a minimum opening time of 12 sec at 200% of the fuse ampere rating. This is quite different than the optional time-delay test for larger body size Class H, K, and R fuses, where the minimum opening time is 10 seconds at 500% of fuse ampere rating (8 seconds for 0A to 30A, 250V, Class K and R fuses).

Class K fuses. These non-renewable fuses are subdivided into three individual classes, defined as Class K-1, K-5, and K-9. All are available in 250VAC and 600VAC ratings, with current ratings from 0A to 600A. Class K fuses are available with DC ratings. It should be noted that Class K-9 fuses are obsolete and have been discontinued by major fuse manufacturers.

The interrupting ratings may be 50kA, 100kA, or 200kA rms symmetrical. Class K-1 fuses provide the best degree of current limitation. On the other hand, Class K-5 fuses provide a lesser degree of current limitation.

The standards requirements for Class K fuses prescribe maximum peak let-through current and maximum I2t let-through energy for each class. As such, all Class K fuses are current-limiting fuses. However, the standard doesn’t permit Class K fuses to be so labeled because they are interchangeable with Class H fuses, which are noncurrent limiting.

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.

Class J fuses. These non-renewable fuses are current-limiting and conform to 248-8. Current ratings of UL Class J fuses range from 0A to 600A, and the voltage rating is 600VAC. Class J fuses are available with DC ratings. The interrupting rating is 200kA rms symmetrical. Fuses with 300kA interrupting ratings are available. Time-delay labeling is available if the fuse meets the UL optional time-delay test requirements of a minimum 10-sec opening time at 500% of fuse current rating.

Class L fuses. These non-renewable fuses are current-limiting and conform to 248-10. They’re designed for the protection of feeders and service entrance equipment.

Current ratings for UL Class L fuses range from 601A to 6000A, and the voltage rating is 600VAC. Class L fuses are available with DC ratings. Available case sizes are 800A, 1,200A, 1,600A, 2,000A, 2,500A, 3,000A, 4,000A, 5,000A, and 6,000A. The interrupting rating is 200kA rms symmetrical. (300kA fuses are available.)

As with the other classes of fuses conforming to 248 (G, J, R, and T), Class L fuses must be labeled “Current Limiting.” They may be labeled “Time Delay,” although the standard doesn’t have a requirement for their time-delay characteristics.

Class T fuses. These non-renewable fuses are current-limiting and are designed for protection of feeders and branch circuits in accordance with the NEC. Current ratings range from 0A to 1200A for 300V rated fuses, and 0A to 1,200A for 600VAC rated fuses. Class T fuses are available with DC ratings. The interrupting rating is 200kA rms symmetrical.

As mentioned for the Class L fuses, Class T fuses also must be labeled “Current Limiting,” and may be labeled “Time Delay” if they pass time-delay requirements.

Class CC fuses. These non-renewable fuses are current-limiting and intended for the protection of components sensitive to short-time overloads, noninductive loads, and short-circuit protection of motor circuits.

Current ratings range from 0A to 30A, and the voltage rating is 600VAC. Class CC fuses are available with DC ratings. The interrupting rating is 200kA rms symmetrical.

Class CC fuses must be labeled “Current Limiting,” and may be labeled “Time Delay.” The optional time-delay test requirements for this class of fuses (a minimum 12-sec opening time at 200% fuse current rating) is different than that of other larger body fuses.

Until recently, most motor controllers and similar devices in the United States were manufactured to NEMA standards. Today, IEC type devices have become increasingly popular because of their smaller size and lower cost. One drawback to the use of IEC devices is that they have a lower short-circuit withstand capability than NEMA devices. Because Class CC fuses are the most current limiting, rating for rating, as compared to other types of control power fuses, they’re particularly suited for use in IEC devices.

North American and IEC standards define two types of motor controller protection in the event of a short circuit.

• Type 1 protection permits extensive damage to the contactor and overload relays. Starters may require comprehensive repair or complete replacement.
• Type 2 protection permits no damage to the contactor or overload relays. Easily separable light tack welding of contacts accompanied by minimum contact burning is allowed.

As more specifiers and end-users are requiring Type 2 protection levels for both NEMA and IEC devices, they’re also using CC current-limiting fuses to satisfy this requirement.

Supplementary protection. This information is given to complete the picture of common fuses available for protection of individual pieces of equipment or internal components and circuits of large “packaged” equipment.

Standard 248-14 covers micro fuses, miniature fuses, and miscellaneous fuses available for the protection of equipment or internal components and circuits of large packaged equipment. Micro fuses. Also known as sub-miniature fuses, these are the smallest fuses made and are available in tubular, cylindrical, or rectangular prism shapes. They may be provided with pigtail leads of terminal pins for fuse terminals.

The maximum current rating is 10A, and the maximum voltage is 125VAC. Micro fuses are available with DC ratings. The short-circuit rating is 50A rms symmetrical. Micro fuses can be labeled “Time Delay” if they met the UL optional time-delay test requirements for supplementary protection fuses of a minimum 5-sec opening time at 200% fuse current rating for fuse sizes 3A or less. The minimum opening time is 12 sec for fuse sizes above 3A.

Miniature fuses. Tubular in shape, these fuses have a terminal at each end. The end terminals may be standard-type ferrule fuse terminals or pigtail leads securely fastened to the ferrule end for special installation. The maximum current rating is 30A, and the voltage ratings can be 125V or 250VAC. Miniature fuses are available with DC ratings. The interrupting rating is 10kA rms symmetrical. Miniature fuses can be labeled “Time Delay” if they meet the same optional time-delay test requirements noted for micro fuses.

Miscellaneous fuses. These fuses can’t be installed in fuseholders intended for Class G, H, J, K, L, R, or T fuses. They’re usually of the ferrule-type construction.

The maximum current rating is 30A, and the voltage ratings range from 125V to 600VAC. Miscellaneous fuses are available with DC ratings. The interrupting rating at 125V is 10kA. Optional interrupting ratings of 50kA and 100kA are available in higher voltage ratings.

Fuses aren’t what they used to be. It’s not enough to just specify that “fuses shall be one-time, renewable, or time-delay type of ampere and voltage ratings as required.” The resultant installation simply won’t provide the selective protection required of modern electrical distribution systems. If you understand the multiple characteristics that make specifying fuses so much more important these days, though, your decision will be that much easier.

Additional Reporting by Cooper Bussmann, St. Louis, Mo.