what is RK1 vs. RK5 Fuse?

RK1, RK5: Popular Fuses for Motors and Transformers

by Greg Carter

Power fuse safety and test procedures have been established by the U.S, Europe and Asia as performance standards. Power fuses standards of low-voltage (600 volts or less) have evolved into distinct classes. The most common classes are RK1, RK5, G, L, T, J, H and CC.

Time Delay Dual Element Fuses interrupt over-currents to 500%-600% of their nominal rating. Common applications include inductive loads such as motors and transformers.

Inductive loads in motors and transformers are common applications for Class RK Time Delay Dual Element Fuses. These fuses interrupt over-currents to 500%-600% of their nominal rating.

Previously we’ve discussed Class J, L and T power fuses and the importance of selecting the correct fuses for an application. Not only when you first specify them, but also when you replace them. To aid in selection, manufacturers produce fuses designated by class.

Let’s now focus on Class RK1 and RK5 fuses which are high performance fuses rated 1/10-600A in 250Vac and 600Vac ratings. Many are dual rated and can be used in dc applications.

All are current limiting and carry a 200kA interrupting rating. Some are self-certified up to 300kA IR. Most RK fuses are time-delay dual element design. Fast acting RK fuses are also available for sensitive or critical component protection applications. Class RK fuses are commonly used in mains, feeder and branch circuits, motors, transformers, solenoids and general purpose circuits.


Class RK1 vs. RK5: What’s the Difference?

Class RK1 and RK5 fuses are the same physical size, but Class RK1 fuses are more current-limiting than Class RK5 fuses. This means Class RK1 allow less damaging energy to pass through them while opening (blowing).

Class RK1 Dual-Element Time-Delay Fuses are interchangeable with RK5 according to UL equipment listing conditions and are specified as practical and economical for upgrading systems.

Recommended applications for RK1 fuses are the same as RK5 in most cases and as mentioned above, they offer greater current limitation.

Class RK5
Dual-Element Time-Delay Fuses are the most prevalent fuses in industry for their economy in most applications, especially in motors and transformers.

What is Time Delay Dual Element?

Time Delay Dual Element Fuses have internal construction with separate time-delay overload dual elements that interrupt over-currents to approximately 500%-600% of their nominal rating. This allows fuses to be sized more proportionate to inductive loads such as motors and transformers.

Why Choose Fast Acting?

Based on the equipment being protected you can choose fast acting RK fuses for application specific protection of sensitive devices and critical components. Fast acting fuses are recommended for circuits having a high percentage of non-inductive loads such as heating and lighting.

Monster Fuses’ website lists more fuse class choices for Eaton Bussmann, Eaton Cooper Power Systems, Eaton Cutler Hammer, Eaton Edison, General Electric, Littelfuse, Mersen and S&C Electric.

Specifying and Selecting Fuses for Circuit protection

The fusing market is filled with a wide array of choices, requiring more details in correct fuse sizing and selection. Fuse applications today involve many types of power systems that require an understanding of fuse types, construction, and operational use.

Understanding the fuse characteristics of the various fuse classes simplifies specifying and selecting fuses to avoid mistakes that could jeopardize equipment and component systems. Proper fuse selection depends on many factors, so consult a professional engineer for your application.

The detailed information manufacturers publish today demonstrate this. Stay up-to-date with ongoing developments in types, constructions, characteristics, and classes. Listed below are links to manufacturer datasheets and other educational resources.

Monster Fuses and OEM RK Fuse Resources
OEM Monster Fuses Catalog OEM Specifications
EATON Bussmann RK1 & RK5 RK1 KTN-R
EATON Bussmann RK1 & RK5 RK1 KTS-R
EATON Bussmann RK1 & RK5 RK1 KWN-R, KWS-R
EATON Bussmann RK1 & RK5 RK5 FRN-R
EATON Bussmann RK1 & RK5 RK5 FRS-R




Monster Electrical is a “knowledge-based” independent distributor of fuses, fuse hardware and control equipment. We don’t just ship boxes. Our product and application knowledge goes into every transaction.


Monster Fuses provides immediate shipment of power fuses and power fuse mountings from our extensive inventory. Clients can access more than 14,000 power fuse solutions provided from a stock of 100,000+ fuses, eliminate long factory power fuse lead times, and supply real time solutions for urgent fuse challenges in utility, industrial, institutional and commercial applications.


Monster Controls provides immediate shipment of 5kV Class control components. Clients can access thousands of 5kV class control parts from an extensive inventory, eliminate long factory lead times and supply real time solutions for electrical control challenges.

Specializing in parts for 2.3kV, 5.5kV and 7.2kV motor starters, contactors and switches, Monster Controls also stocks complete starters, contactors and switches.

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