Maintenance and procurement teams often need a Mersen replacement for an Eaton Bussmann fuse because of availability, lead time or approved-source requirements. The safest method starts with the fuse's functional family. Two fuses can share the same ampere rating and body size yet behave differently during transformer inrush, motor starting or a semiconductor fault.

Corrected series-level cross-reference

The table deliberately separates FNQ-R from LP-CC. FNQ-R and ATQR are commonly associated with control-transformer and other high-magnetizing-current duties. LP-CC and ATDR are dual-element/time-delay choices for small motors and general branch protection. Treating all Class CC time-delay products as one interchangeable group can change overload behavior.

Exact model examples: what the suffixes mean

Eight checks before approving a substitution

1. Read the complete installed part number, including suffixes for indication, packaging, mounting or special performance.

2. Match UL class. A Class J candidate cannot be installed in a Class RK1 or RK5 holder even if ampere and voltage ratings look similar.

3. Match the operating characteristic: fast-acting, time-delay, dual-element, control-transformer or high-speed semiconductor.

4. Use an equal or higher AC voltage rating and separately verify the published DC voltage and DC interrupting rating if the circuit is DC.

5. Match current rating only after checking the equipment manufacturer's permitted fuse range and applicable code sizing rules.

6. Confirm interrupting rating and the equipment's short-circuit current rating (SCCR) remain valid with the proposed fuse.

7. Compare time-current curves, peak let-through current and clearing I2t where selective coordination or component protection matters.

8. For listed equipment, confirm the replacement is permitted by the equipment documentation, certification file or manufacturer engineering approval.

High-speed fuses need a different workflow

Do not cross a Bussmann FWP, FWH, 170M or 170T high-speed fuse to a Mersen A70P, A70QS or another semiconductor series using current and voltage alone. Compare total clearing I2t at the operating voltage, pre-arcing I2t, peak let-through current, time constant, body size, mounting centers, terminal orientation, watts loss and the protected semiconductor's withstand limit. The final candidate may require manufacturer application engineering.

Frequently Asked Questions

Is Mersen ATDR the equivalent of Bussmann FNQ-R?

Not as the preferred functional-family match. FNQ-R is commonly cross-referenced with Mersen ATQR for control-transformer duty. ATDR is closer to Bussmann LP-CC and Littelfuse CCMR for motor/general time-delay Class CC applications.

What is the Mersen equivalent of Bussmann LPJ-30SP?

AJT30 is the same-class, same-current, time-delay candidate. Confirm voltage, DC requirements, indication preference, time-current curve and equipment approval.

Can AJT replace LPS-RK because both are time-delay?

No direct swap. AJT is Class J and LPS-RK is Class RK1; they use different physical dimensions and holders.

What is the Mersen candidate for JJS-100?

A6T100 is the 600 V Class T, 100 A fast-acting candidate family. Verify the exact AC/DC ratings and application data.

Bottom line

A reliable Bussmann-to-Mersen cross-reference begins with the functional family: FNQ-R/ATQR for control-transformer duty, LP-CC/ATDR for motor/general Class CC time delay, KTK-R/ATMR for fast Class CC, LPJ/AJT for Class J time delay, and JJS/A6T for 600 V Class T. Exact approval still belongs at the individual part-number and application level.

Send ASBEAM the installed part number, system AC/DC voltage, load type, available fault current, holder model and equipment nameplate for a model-level cross-reference review.

Technical references

• Eaton - Bussmann Low-Peak upgrade and competitor family mapping

• Mersen - ATQR Class CC time-delay

• Mersen - ATDR Class CC time-delay

• Mersen - ATMR Class CC fast-acting

• Mersen - AJT Class J time-delay