As electrical products continue to evolve, so does circuit protection. An array of fuses on the market have each been developed to protect electrical circuits — an electric current’s closed path. Fuses work to safeguard circuits from excessive currents, which are also known as “overcurrents.” An overcurrent is defined as an electrical current that surpasses what an electrical item can handle. A fuse can halt a current’s flow when it becomes excessive, averting damage to the item and circuit itself.
It is important for licensed electricians and other professionals who work with fuses to stay versed and updated in fuse types. This can include knowledge about their construction and operation characteristics, as new kinds and classes are developed.
The following UL® standards below are accepted and used as requirements by which manufacturers must design their fuses. These standards are continuously being updated. It's important that anyone who works with fuses refer to the latest version when designing or constructing their next project.
- • UL 248-1, Low-Voltage Fuses – Part 1: General Requirements
- • UL 248-2, Low-Voltage Fuses – Part 2: Class C Fuses
- • UL 248-3, Low-Voltage Fuses – Part 3: Class CA and CB Fuses
- • UL 248-4, Low-Voltage Fuses – Part 4: Class CC Fuses
- • UL 248-5, Low-Voltage Fuses – Part 5: Class G Fuses
- • UL 248-6, Low-Voltage Fuses – Part 6: Class H Non-Renewable Fuses
- • UL 248-7, Low-Voltage Fuses – Part 7: Class H Renewable Fuses
- • UL 248-8, Low-Voltage Fuses – Part 8: Class J Fuses
- • UL 248-9, Low-Voltage Fuses – Part 9: Class K Fuses
- • UL 248-9, Low-Voltage Fuses – Part 9: Class K Fuses
- • UL 248-9, Low-Voltage Fuses – Part 9: Class K Fuses
- • UL 248-12, Low-Voltage Fuses – Part 12: Class R Fuses
- • UL 248-13, Low-Voltage Fuses – Part 13: Semiconductor Fuses
- • UL 248-14, Low-Voltage Fuses – Part 14: Supplemental Fuses
- • UL 248-15, Low-Voltage Fuses – Part 15: Class T Fuses
- • UL 248-16, Low-Voltage Fuses – Part 16: Test Limiters
- • UL 248-19, Low-Voltage Fuses – Part 19: Photovoltaic Fuses
The information below is provided to analyze several of these fuse classes, along with pertinent performance characteristics and ratings.
The standard UL 248-1 provides general manufacturing requirements and guidelines for construction, performance, markings, and testing for low-voltage fuses. Each part follows for specific low-voltage fuse types.
CLASS C FUSES
These non-renewable cartridge fuses fall under the UL 248-2 standards. This kind is suitable for AC and DC circuits. These were designed for general purpose applications that have moderate current rating, providing overcurrent protection in various electrical circuits and systems.
Current ratings range from a few to 600 amperes (A) for this fuse variety, while the voltage rating can vary between 124VAC to 600 volts alternating current (VAC). Class C fuses are additionally available with direct current (DC) ratings. The interrupting rating often ranges between 10,000 and 200,000A.
Their structure is not usually current limiting, which means they are created to quickly interrupt high fault currents, limiting their energy. A current-limiting fuse is designed that it melts its internal element to interrupt the current.
No time-delay is specified in Class C fuses, with a time-delay fuse also called a “slow-blow” fuse that often permits a temporary overcurrent to flow through a fuse for a certain period. The fuse can instead handle inrush currents and temporary overloads without tripping.
CLASS CA AND CB FUSES
These two fuse types fall under UL 248-3. Class CA fuses have a higher interrupting rating because they are time-delay fuses that provide moderate overcurrent protection in general purpose circuits. The current rating range for these fuses varies from a few hundred milliamperes (mA) to several hundred amperes. The current ratings are commonly 1A, 2A, 5A, 10A, 15A, 20A, 30A, and 50A, though there are higher ratings as well. It is important to choose a class CA fuse with a current rating that meets or exceeds the expected current within the circuit. The voltage rating can range from a few volts to a few hundred, with a few common ones 125VAC, 250VAC, 300VAC, and 600VAC. They come in AC and DEC ratings. The interrupting rating can vary from a few hundred to a few thousand amperes. This fuse has current-liming protection but not a specified time delay mechanism.
The other fuse type that is under UL 248-3 is the Class CB fuse. These are frequently used in electronic equipment and control panels. They have a lower interrupting rating than Class CA fuses and a quicker response time to safeguard against short circuits. These are quick-acting fuses with a current rating at 0.5A, 1A, 2A, 5A, 10A, 15A, 20A, and higher. The voltage ratings for these fuses varies from a few to several hundred, with common ratings 32VAC, 60VAC, 125VAC, 250VAC, and 600VAC. The interrupting rating for Class CB fuses is often less than other classes but ranges usually from several hundred amperes to several thousand. Class CB fuses are considered current limiting — and have a time delay, generally in the range of several seconds to tens of seconds. The delay may depend on various factors for the fuses.
CLASS CC FUSES
These non-renewable fuses are current-limiting and intended for the protection of components sensitive to short-time overloads, non-inductive loads, and short-circuit protection of motor circuits.
This fuse type conforms with UL 248-4. 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.
CLASS G FUSES
These fuses, which align with standard UL 248-5, were specifically created for use in lighting and appliance panel boards that are equipped with a special fusible-switch unit. These are non-renewable cartridge fuses that are for use only in AC circuits where interrupting ratings to 100kA rms symmetrical are required. These can also be available with DC ratings.
G Class fuses conform to standard 248-5. They are rated for 600VAC (0A to 20A) and 480VAC (25A to 60A). Class G fuses are available in four ferrule sizes: 0A to 15A, 16A to 20A, 1A to 30A, and 31A to 60A. The innovative design of this class of fuse was selected to prevent the interchangeability with any other fuse class.
Class G fuses are current-limiting and can 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.
The time delay test feature is optional. It is important to realize that the test calls for a minimum opening time of 12 sec at 200% of the fuse ampere rating which differs Class H, K, and R fuses.
CLASS H FUSES
This is a cartridge fuse and is intended for general purpose branch circuit, lighting circuit, and the protection of non-inductive equipment like electric ovens and resistance heaters. You can select from renewable (UL 248-7) and non-renewable Class H fuses (UL 248-6). The renewable fuses allow the user to replace the internal fusible link after the fuse operates.
All Class H units are tested for short-circuit requirements and 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 respect to the time delay feature of fuses, the renewable types cannot perform the time delay but the non-renewable can.
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 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 K FUSES
These non-renewable fuses fall under UL 248-9 and are available in 250VAC and 600VAC ratings, with current ratings from 0A to 600A. Class K fuses are available with DC ratings.
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 lower 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 and all are current-limiting fuses. Interestingly, these fuses cannot be labeled as current limiting because they are interchangeable with Class H fuses, which are noncurrent limiting. There is not a specific time delay for Class K fuses, which are quick-acting to provide short-circuit and overload protection, while time-delay fuses have a deliberate delay. These fast-acting fuses can avert trouble quickly by isolating faults, preventing damage to electrical systems.
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.
This type of fuse is renewable or non-renewable and falls under the UL 248-11 standard. It is an old-style one that might be found in residential and commercial buildings with older electrical systems. The fuse body has a cylindrical shape with an element that melts when confronted with short circuits and overcurrents. This type, which is primarily used for AC systems, is often replaced by modern circuit breakers. They may still be found if the building has not been upgraded to a circuit breaker system.
Current ratings for plug fuses range from 15A to 60A. The most common plug fuse sizes in residential applications are 15A, 20A, and 30A. For commercial or industrial, one would often find 40A and 60A plug fuses. The voltage rating for plug fuses averages 125VAC, for both residential and commercial applications. The interrupting rating often ranges from 10,000A to 200,000A.
This type of fuse, also called an “Edison base fuse,” can interrupt excessive currents, protecting against overloads and short-circuits, but are not current limiting in that they can interrupt currents quickly. A plug fuse does not have a time delay but are quick acting in their responses to overcurrents.
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 fit the standards for UL 248-12 and have an interrupting rating of 200kA rms symmetrical. This fuse type is 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.
Class R fuses must be labeled "Current Limiting" and may be labeled "Time Delay," which means they meet the optional time-delay test by not opening in less than 10 seconds at 500% of fuse ampere rating.
This type of fuse, which is also known as a semiconductor protection fuse, is used to safeguard semiconductor devices and circuits where high-speed protection is needed, such as in inverters, motor drives, power electronics, and similar applications. These are the standard for UL 248-13, with current ratings from several amperes, to several hundred. Their common current ratings are 1A, 3A, 10A, 50A, 100A, and more. The voltage ratings, which can be both DC and VAC, also run from a few to several hundred VAC. Common ratings are 250VAC, 500VAC, and 600VAC. The interrupting rating can vary on this fuse type because it is designed to handle high fault currents, and may range from a few thousand amperes, tens of thousands, or even greater.
Semiconductor fuses are current limiting and come in both fast-acting and time-delay varieties.
Supplemental fuses are used in circuit breakers or with primary fuses, in applications where localized overcurrent protection is needed. These fuses, which fall under the UL 248-14 standard, are often marked “SUP” in their part numbers to make them distinguishable from primary fuses.
The current ratings can vary for this fuse type, from several millamperes (mA) to a few hundred. A few of the common ratings are 0.5A, 1A, 2A, 5A, 10A, 20A, 30A, and 50A. The voltage rating can depend on the electrical systems and applications, ranging from a few volts, to a few hundred, with typical ratings at 125VAC, 250VAC, 300VAC, and 600VAC, with some designed for DC — and some with AC/DC voltage ratings. The interrupting rating for these fuses is often less than primary fuses or circuit breakers, since they offer supplemental protection; and can range from a few hundred to several thousand amperes.
Like semiconductor fuses, this kind has current-limiting capabilities, plus can come in both fast-acting and time-delay types.
CLASS T FUSES
The Class T fuse conforms to UL 248-15 and is often made of a fuse body of ceramic or non-combustible material that encloses it. These fuses are usually located in industrial machinery, motor control center electrical panels, power distribution panels, switchgear installations, renewable energy systems, commercial buildings, data centers, power electronics, and marine and recreational vehicles. Once exposed to some current levels, it is designed that it can melt or open the circuit. Class T fuses have a blade or bolt-on terminal to install and connect them easily. Their current ratings can vary from a few to several hundred amperes; and they have common voltage ratings like 125V, 250V, 300V, and 600V. These fuses have high-interrupting capacity with the rating from several thousand to tens of thousand amperes.
Class T fuses are current-limiting, but they do not have a time-delay, responding quickly to overcurrent conditions.
Test limiters are fuses covered under standard UL 248-16, that are used to limit current while electrical systems undergo testing and verification. Standard 248-6 is based on the calibration limits of test limiters. The current rating can vary for these plugs from several milliamperes to a few amperes. A test limiter’s voltage rating can range from several volts to a few kilovolts. Since this type of fuse is used to limit current in temporary testing and verification scenarios, an interrupting rating is not usually connected to this type of device, because it is not made to handle fault currents. With the nature of test limiters, they were not designed to be current limiting. They also do not have a specified time delay, with some more faster acting than others.
Photovoltaic fuses, which are used in solar power systems, are covered under UL 248-19. They are made to sustain in outdoor environments and to operate with DC that solar panels generate. Their current, voltage, and interrupting ratings can vary, depending on their construction, size, and capacity of the system. The current ratings can be a few amperes to several hundred, with a voltage rating range from a few hundred to several thousand volts. The interrupting rating for photovoltaic fuses depends on the fuse model and manufacturer, as well as applications and requirements of a photovoltaic system.
This type of fuse has current-limiting qualities; and comes in both fast-acting and time-delay varieties.
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