Understanding Industrial Fuses – Types, Protections, and UL Standards
Industrial power fuses play a critical role in electrical systems, providing reliable overcurrent protection that prevents equipment damage, fire hazards, and system failures. Choosing the right fuse depends on the specific application, performance requirements, and relevant safety standards like UL certifications. See our full Fuse Selection Guide for complete context.
In this article, we explore key aspects of industrial fuses, including the differences between supplementary and branch protection, current limiting characteristics, fuse response types, UL classifications, and more.
Supplementary, Branch, and Main Feeder Circuit Protection
When selecting the right industrial fuse, it’s essential to understand the three core levels of circuit protection: Supplementary, Branch, and Main Feeder. Each plays a distinct role in safeguarding electrical systems and components.
Supplementary protection fuses are designed to protect individual components within a device or panel. These are not intended to protect the branch circuit wiring but instead serve as a second layer of defense for specific loads such as motors, transformers, or sensitive electronics.
Supplementary fuses are ideal for applications where component-level isolation is critical, especially in control panels or electronic equipment.
Branch circuit protection fuses are intended to protect the wiring and connected equipment in a branch circuit from overcurrent conditions. They serve as primary protection and are required to meet stricter interrupting ratings and safety standards.
This type of protection is often used for protecting individual loads fed from a main panel. This fuse should be coordinated with the main feeder fuse to provide selective coordination protection.
Common fuse classes for branch protection include Class CC, Class J, Class RK5, and Class T.
Main feeder protection is used to safeguard the entire electrical system or panel, typically at the main service entrance or distribution board. These fuses must also meet rigorous safety and interrupting standards but differ from branch protection by their higher current ratings and broader system coverage.
Due to their high amperage ratings, Class CC fuses (which max out at 30A) are not suitable for feeder protection. Instead, larger fuse classes such as Class J, Class T, or Class RK5 are typically used.
The Role of Current Limiting in Overcurrent Protection
Current limiting fuses are specially designed industrial fuses that open the circuit before the fault current reaches its peak during overcurrent or short-circuit conditions.
By minimizing the let-through energy (I²t), these fuses significantly reduce thermal and mechanical stress on electrical equipment, enhancing system safety and preventing costly damage. Current limiting fuses are essential components in protecting motors, transformers, and sensitive devices in industrial and commercial electrical systems.
Response Time: Fast-Acting vs. Time-Delay
The right fuse response time depends on your application. Choosing between fast-acting and time-delay fuses ensures optimal performance and reduces unnecessary downtime.
Fast-acting fuses are designed to respond quickly to overloads and short-circuit faults. These industrial fuses are commonly used to protect sensitive equipment that cannot tolerate dangerous inrush current or sudden power surges.
Time-delay fuses react quickly to short-circuit faults but feature a built-in delay mechanism to allow temporary inrush currents—common in motors, transformers, and inductive loads—to pass without blowing. This delay prevents nuisance trips while still providing reliable overcurrent protection.
Types of Industrial Fuses: Overview of 5 Key Classes Offered by OptiFuse
OptiFuse offers five main classes of UL-listed industrial fuses, each designed to provide reliable protection tailored to specific applications and electrical requirements.
Discover our full product guide to find the perfect fuse for your industrial needs.
Midget Fuses
Midget fuses are industrial fuses tested to the UL 248-14 standards, ensuring safety and performance in a variety of applications. They are available in both fast-acting and time-delay styles.
- Fast-acting midget fuses are commonly used in control panels and lighting circuits where quick interruption of faults is critical to protect sensitive electronics.
- Time-delay midget fuses are designed to handle temporary inrush currents and are often used in motor control transformers and circuits with designed power surges.
Midget fuses primarily provide supplementary circuit protection for individual components within electrical devices.
Class CC Fuses
Class CC fuses are tested to the UL 248-4 standards. These fuses are rated for branch circuit and are offered with both fast-acting and time-delay styles.
- The fast-acting versions are often used in control panels, lighting circuits, and control transformer circuits where a fast-acting trip speed is required.
- The time-delay versions are often used for protection of motor branch circuits and power control transformers.
Class T Fuses
Class T fuses are tested to UL 248-15 and designed for branch circuit protection, with higher amperage models often used in main feeder circuit protection. They feature a fast-acting response and are known for their high current-limiting capabilities and compact, low-profile design. Class T fuses are popular for battery and inverter protection in the marine and RV industries. In the industrial industry, these fuses are often seen in load centers, panelboards, switch boards, metering centers, and variable load drives.
Class J Fuses
Class J fuses are tested to UL 248-8. These fuses are rated for branch circuit protection and higher amperage models are often used as main feeder circuit protection. They are offered in both fast-acting and time-delay styles.
- Common applications for the time-delay versions include motor circuits, feeders, lighting, heating, transformers, control panels, and load centers.
- The fast-acting versions are often used in load centers, lighting, heating, feeder circuits, capacitors, and panelboards
Class RK5 Fuses
Class RK5 fuses are tested to UL 248-12 standards. These fuses are rated for branch circuit protection and higher amperage models are often used as main feeder circuit protection. These fuses are offered in a time-delay style and are often used for motor overload protection in industrial equipment, air conditioning units, pumps, and other motor driven machinery.
Note: OptiFuse Class RK5 fuses do not reach the higher amperage ratings and typically are not recommended for main feeder protection. For assistance with sizing fuses to protect your system, please consult a qualified electrical engineer.
Samuel Kenny, Product Manager at OptiFuse, leads the development of advanced circuit protection products for automotive, marine, and industrial sectors. He designs custom solutions and shares expert fuse selection tips on the OptiFuse Blog.