Littelfuse, Mersen Class J 600Vac Fast-Acting, Time Delay and High-Speed Fuses

Littelfuse, Mersen Class J Current Limiting Power Fuses
600 VAC – 200,000 Amp Interrupting Rating



In previous articles we’ve discussed Class T and Class L fuses. In future articles we’ll continue the discussion covering RK1, RK5, G, H and CC class fuses.

Here we’ll discuss various Littelfuse and Mersen Class J current limiting fuses. Class J is one of the more common classes of fuses. For a discussion of current limiting fuses, see the Monster Blog article on how current-limiting fuses work.

To develop their fuse safety test procedures, the U.S, Europe and Asia established classes with defined performance standards. In the case of power fuses, these standards have evolved into distinct classes. Understanding fuse characteristics of these classes simplifies specifying fuses.

Class J fuses are commonly used in branch circuits, motor circuits, mains, feeders, control panels, electronic motor controllers, drives, soft-starters, circuit breaker back-up, transformers, load centers, lighting, heating and general loads to name a few.

Class J Fuse Safety Specifications

Class J Fuse safety is guided by UL standards identified by classes of low-voltage (600 volts or less) fuses.  All U.S. fuse makers below certify their Class J fuses at a 200,000 amp interrupting rating (I.R.) at 600Vac with the exception of the Bussmann LPJ series, which is certified with a 300,000A interrupting rating.

Many of the manufacturers have self-certified their Class J fuses up to 300,000A I.R. at 600Vac. In addition, some Class J products carry a DC rating and a corresponding interrupting rating. Interrupting rating is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration.

Class J fuses are available in time delay, fast acting or high speed types, and they are non-renewable. This means they can’t be returned to service after operation. They have no renewable elements.

Below is a summary of Class J fuses from Littelfuse and Mersen with links to their datasheets and complete specifications (including AC and DC ratings and I.R.) for use in typical applications.








Littelfuse JTD_ID & JTD POWR-PRO® Time Delay Power Fuses

Littelfuse POWR-PRO® JTD_ID Indicator Class J fuse provides visual blown fuse indication and maximum protection in a compact package. The current-limiting time delay JTD_ID offers a patented design which reduces nuisance fuse openings.

The Littelfuse POWR-PRO® has IEC Type 2 Protection. This means increased safety, as the contactor or starter with Type 2 Protection is meant to cause no danger to persons or installation.

The JTD series is also available in a non-indication versions. Just like the JTD_ID, They are ideal for protecting series rated molded case breakers, fused combination motor controllers, transformer protection and other general purpose circuits.

Littelfuse JTD_ID & JTD Data Sheet

Littelfuse JLS Fast Acting Power Fuses

The Littelfuse UL Listed Class J JLS fast acting fuse saves space and offers terrific protection for overloads and short-circuits. Perfect for industrial and power conversion applications.

This fuse interrupts all over-currents with protection up to 200kA. By reducing heat and jls-fuse magnetic conditions, the JLS Series fuse protects devices and components sensitive to surges.

You will find them in variable speed drives, rectifiers, solid-state devices and where resistive loads generate excessive heat.

Littelfuse JLS Data Sheet

Littelfuse POWR-GARD® LDFJ Drive Fuses

Littelfuse LDFJ POWR-GARD® fuses are designed specifically for short circuit protection of AC and DC drives used to control motor speed and power. They are considered “Drive Fuses”.

Littelfuse LDFJ fuses have current limiting features with an extremely low I²T for protecting semiconductor devices in electronic circuits sensitive to heat.

Littelfuse POWR-GARD LDFJ Drive Fuses Data Sheet

Mersen AJT Amp-Trap 2000® Time-Delay Power Fuses

Mersen Amp-Trap 2000 ® SmartSpot AJT fuses include a visual indicator of fuse condition. AJT time-delay fuses can withstand typical motor and transformer inrush Untitledcurrents and offer excellent current-limiting performance. They’re suitable for all load types while taking up minimal mounting space.

Mersen AJT fuses are made with a fiberglass body for harsh industrial environments and carry a 500Vdc rating with a 100,000A interrupting rating.

Mersen Amp-Trap 2000® Data Sheet

Mersen A4J Fast Acting Power Fuses

A4J Class J fast acting current-limiting fuses provide protection for a variety of applications. Their dimensions are uniquely designed to prevent replacing them with fusesa4j-fuse having lower voltage ratings, interrupting ratings, or current-limiting capability.

Mersen A4J fuses are made with a fiberglass body for harsh industrial environments and carry a 300Vdc rating with a 100,000A interrupting rating.

Mersen A4J Fast Acting/Class J Data Sheet

Mersen HSJ High Speed Power Fuses

High Speed (HSJ) fuses use a combination of low I2t semiconductor fuses and the branch circuit performance of a Class J UL fuse. This means they are good at handling the spontaneous and random transfer of heat like that found in semi-conductors.hsj-fuse

HSJ fuses are designed for the starting abilities of solid state motor controllers and provide protection of power semiconductors such as Diodes, SCRs, GTOs and SSRs.

Mersen HSJ fuses carry a 500Vdc rating on their 15-600A units. I.R is specified L/R =10mS or less.

Mersen HSJ High Speed/Class J Data Sheet

Correct Fuse Sizing for Safe Installation
and Reliable Circuit Protection

The above Class J fuse summaries should help you choose the right fuse to ensure safe installation and optimal circuit protection in today’s electrical distribution systems.

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.



Monster Fuses is a Factory Authorized Provider of Eaton Bussmann, Eaton Edison and Littelfuse Class J Fuses. Monster Fuses also provides Mersen Class J Fuses and the industry’s largest factory authorized of Medium & High Voltage Power Fuses and Power Fuse Hardware from all the major fuse lines.


Monster Electrical is a “knowledge-based” independent distributor of fuses, fuse hardware and 5kV Class 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, hardware 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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