Older industrial equipment may contain fuses labeled Shawmut, Gould Shawmut, or Ferraz Shawmut, while current replacement research is typically performed through Mersen. The brand history is useful for locating documentation, but it does not mean that every older fuse automatically maps to a current Mersen product with the same amperage or a similar name.
The correct process is to identify the exact legacy part, use Mersen's current cross-reference and product documentation to identify a candidate, then verify the candidate's class or family, voltage, AC/DC ratings, interrupting capability, response type, dimensions, fuse-holder compatibility, and application before ordering.
Quick Answer
To cross-reference an older Ferraz Shawmut or Gould Shawmut fuse, capture the complete part number and label first. Search the part through Mersen's current Cross Reference tool and then open the exact Mersen product page or data sheet for the suggested part.
Mersen explicitly states that its cross-reference results represent the closest equivalent based on form and function, and that the user is responsible for reviewing the detailed specifications to determine suitability for the intended application. That means a cross-reference result is a candidate, not automatic proof of interchangeability.
Why You May See Shawmut, Gould Shawmut, Ferraz Shawmut, and Mersen
The names found on older fuse labels reflect the history of the product line.
Mersen's North American quality documentation traces the lineage from Shawmut Fuse Wire Company, through Chase-Shawmut, I-T-E, Gould Shawmut, and Ferraz Shawmut. In 1999, Carbone Lorraine acquired Gould Shawmut and combined it with the Ferraz business under the Ferraz Shawmut name. In 2010, Carbone Lorraine adopted the Mersen name.
For maintenance teams, the practical takeaway is simple: an older Shawmut-family fuse may now be supported through Mersen documentation, but the historical relationship is not itself a technical cross-reference.
Always verify the exact part.
1. Capture the Complete Legacy Fuse Marking
Before searching, record:
- Brand shown on the fuse
- Complete catalog or part number
- Ampere rating
- AC voltage
- DC voltage if shown
- Fuse class or family
- Time-delay or fast-acting marking
- Interrupting rating if shown
- Physical format
- Indicator system
- Any suffixes
- Fuse-holder or block part number
Photograph both sides of the fuse if possible.
Older labels can be faded or partially hidden. If the number is incomplete, do not fill in missing characters from a similar-looking online listing.
2. Search the Exact Part in Mersen's Cross-Reference Tool
Mersen provides a current cross-reference search for competitor and legacy fuse part numbers.
The tool can be used to narrow a search to a current Mersen product family. Mersen describes the result as the closest equivalent based on form and function.
That wording matters.
A "closest equivalent" still needs to be checked against:
- Electrical ratings
- Fuse class
- Protection characteristics
- Dimensions
- Holder compatibility
- Application requirements
Use the cross-reference result as a research path, not as a blanket substitution approval.
3. Confirm the Current Mersen Series
Mersen's current fuse portfolio includes multiple families that can look similar to a buyer unfamiliar with the product naming.
Examples include:
- ATMR Class CC fast-acting fuses
- ATDR Class CC time-delay fuses
- TR and TR-R Class RK5 time-delay fuses
- AJT Class J time-delay fuses
- A4J Class J fast-acting fuses
- OTS Class K5 fast-acting fuses
- OT Class K5 fast-acting fuses
- TRM midget time-delay fuses
- OTM midget fast-acting fuses
- High-speed semiconductor fuse families
- Medium-voltage and specialty fuse families
The series designation is part of the fuse identity.
Do not reduce the request to "Mersen 30A fuse."
4. Verify Fuse Class or Product Family
Fuse class can change the physical footprint, rejection system, interrupting performance, and intended application.
For example, a current Mersen ATMR is a Class CC fast-acting fuse, while an ATDR is a Class CC time-delay fuse.
The class may be the same, but the protection behavior is different.
Likewise, Mersen's OTS series is a Class K5 fast-acting family, while other Mersen products may be Class J, RK1, RK5, CC, midget, high-speed, or specialty fuses.
Verify the exact family before comparing amperage.
5. Verify AC and DC Voltage Ratings
Do not assume the AC and DC voltage ratings are the same.
Mersen publishes both ratings where applicable.
For example, Mersen currently lists OTS30 as:
- 30A
- Class K5
- Fast-acting
- 600 Vac
- 300 Vdc
By comparison, Mersen's OT30 is also a 30A Class K5 fast-acting fuse, but it is a 250 Vac and 250 Vdc product.
The ampere rating and fuse class alone do not make those two products equivalent.
6. Verify the Interrupting Rating
The interrupting rating must be suitable for the application.
Mersen states that OTS one-time fuses can interrupt available short-circuit currents up to 50,000A within the series ratings and specifically warns that replacement fuses should not have lower interrupting ratings than the original.
That is an important cross-reference rule.
Do not select a candidate replacement without confirming the exact current data sheet and the required fault-current capability of the application.
7. Match Fast-Acting or Time-Delay Behavior
Response type is part of the fuse's function.
Current Mersen examples include:
ATMR: Class CC, fast-acting.
ATDR: Class CC, time-delay.
TRM: Midget, time-delay.
OTM: Midget, fast-acting.
A buyer can easily find two fuses with the same ampere rating and similar dimensions that have different response characteristics.
Do not change from time-delay to fast-acting, or vice versa, simply because a fuse physically fits.
8. Check Current-Limiting and Application Characteristics
Where applicable, compare:
- Current-limiting status
- Time-current curves
- Peak let-through data
- I²t data
- Motor-circuit suitability
- Transformer applications
- Control-circuit applications
- Semiconductor protection
- Coordination requirements
A cross-reference should preserve the protection objective of the original design.
If a proposed substitution materially changes fuse behavior, treat that as an engineering decision, not a routine procurement substitution.
9. Verify Dimensions and Mounting
Check:
- Overall length
- Diameter
- Ferrule size
- Blade dimensions
- Bolt spacing
- Terminal style
- Rejection features
- Indicator clearance
- Holder fit
Mersen's product portfolio includes ferrule, blade, bolt-in, and other configurations.
One useful warning appears in Mersen's Class C FES documentation: those products use British-standard dimensions and are specifically not dimensionally or electrically interchangeable with other North American fuse types.
Physical similarity is not enough.
10. Verify the Fuse Holder or Block
Record the installed holder:
- Manufacturer
- Catalog number
- Fuse class
- Ampere range
- Voltage rating
- Number of poles
- Terminal style
- Rejection feature
The holder can determine which fuse classes and dimensions are acceptable.]
Do not defeat a rejection feature to install a different fuse.
If the holder is obsolete or damaged, review the holder separately rather than forcing a new fuse into an old block.
11. Legacy Branding Does Not Guarantee an Identical Modern Part
A fuse may carry an older Ferraz Shawmut or Gould Shawmut label while the current product family is documented by Mersen.
In some cases, product numbers or family names may continue. In other cases, a current Mersen cross-reference may point to a different family that Mersen considers the closest equivalent.
Do not assume:
- Same legacy manufacturer = same modern fuse
- Same amperage = same replacement
- Same dimensions = same performance
- Same family nickname = same ratings
Use the current product record.
12. A Useful Current Mersen Cross-Reference Example
Mersen's current cross-reference search illustrates the workflow well.
A search for a competitor KTK family can return Mersen ATMR Class CC fast-acting candidates.
That does not mean every KTK fuse maps to every ATMR fuse.
The exact amperage and specifications still need to be matched.
For example, Mersen ATMR30 is a current Class CC fast-acting 30A product. Its exact voltage, interrupting, dimensions, and other specifications should be verified against the original fuse before substitution.
This is the correct way to use a cross-reference tool: narrow the family first, then verify the exact part.
13. Treat One-Time and Non-Rejection Fuses Carefully
Mersen's current OTS Class K5 documentation deserves special attention.
OTS fuses are described as one-time general-purpose fast-acting fuses. Mersen also states that they are not rejection fuses and warns users to ensure that replacements do not have lower interrupting ratings than the original fuse.
This means the holder may physically accept more than one product type, but physical acceptance does not establish equivalent protection.
The technical record still controls.
14. Do Not Confuse Midget Fuses With Branch-Circuit Class Fuses
Midget fuses can resemble other ferrule-style fuses but may have different listing and application limitations.
For example, Mersen's OTM midget fast-acting series is identified for general-purpose applications and its product documentation states that it is not for branch-circuit protection.
That distinction matters when an older fuse has lost its label or when a buyer is ordering by dimensions alone.
Identify the product family before selecting a replacement.
15. Record the Evidence Behind the Cross-Reference
For every Mersen cross-reference decision, record:
- Original brand: [Shawmut / Gould Shawmut / Ferraz Shawmut / other]
- Original part number: [part number]
- Mersen candidate: [part number]
- Mersen cross-reference source: [URL]
- Current Mersen product page: [URL]
- Fuse class/family: [class]
- Amp rating: [rating]
- AC voltage: [rating]
- DC voltage: [rating]
- Interrupting rating: [rating]
- Response type: [fast-acting / time-delay / high-speed]
- Physical format: [confirmed / review needed]
- Holder compatibility: [confirmed / review needed]
- Application review: [confirmed / review needed]
- Reviewed on: [date]
- Reviewer: [name]
- Notes: [limitations]
This gives procurement teams a traceable basis for the purchase.
16. What to Send JW for a Mersen or Ferraz Shawmut Fuse Quote
Send:
- Complete legacy part number
- Brand shown on the fuse
- Clear label photographs
- Ampere rating
- AC voltage
- DC voltage if applicable
- Fuse class or family
- Fast-acting or time-delay marking
- Interrupting rating if known
- Fuse holder or block number
- Equipment manufacturer and model
- Application or load type
- Quantity
- Required condition
- Ship-to location
- Required-by date
If the legacy marking is incomplete, include dimensions and photographs of both the fuse and holder.
Related JW Fuse Resources
Buyers can review JW Electrical Supply's Fuses and Fuse Holders inventory while preparing a sourcing request.
JW's Industrial Fuse Cross-Reference workflow provides the broader verification process, and the Bussmann Fuse Cross-Reference guide covers the same evidence-first method for Eaton Bussmann products once those articles are live.
For general industrial condition terminology, JW's New vs. Reconditioned vs. Surplus vs. Used Circuit Breakers guide explains why condition and technical compatibility should remain separate questions.
Important JW Inventory Note
Do not link the current JW OTS-30 product page from this article until its product data has been corrected.
Mersen's current OTS30 documentation identifies the part as a 30A, Class K5, fast-acting fuse rated 600 Vac and 300 Vdc. The JW product record should match the verified OEM data before it is used as an internal product link.
Common Mersen Cross-Reference Mistakes
Using the brand history as a technical cross-reference: Ferraz Shawmut's relationship to Mersen helps locate current documentation, but it does not prove that every legacy part has a direct modern replacement.
Matching amperage only: The same ampere rating appears across multiple Mersen classes and product families.
Ignoring AC versus DC ratings: OTS30 and OT30 are both 30A Class K5 fast-acting products but have different voltage ratings.
Ignoring response type: ATMR and ATDR are both Class CC families but one is fast-acting and the other is time-delay.
Ignoring holder restrictions: Physical fit, rejection features, and fuse class still need to be verified.
Treating Mersen's cross-reference as final approval: Mersen explicitly requires users to review the detailed product specifications for suitability.
Using an unverified JW product page as technical evidence: The OEM source should control whenever an internal product record conflicts with current Mersen documentation.
Frequently Asked Questions
Ferraz Shawmut became part of the business now operating as Mersen. Mersen's own history documents the transition from Gould Shawmut to Ferraz Shawmut and then to the Mersen name. For replacement work, use current Mersen product documentation rather than relying on the brand history alone.
Record the complete legacy part number, search it through Mersen's current cross-reference tool, then verify the suggested Mersen part's class, voltage, interrupting rating, response type, dimensions, and application.
No. Mersen states that the suggested part is the closest equivalent based on form and function and that the user must review detailed specifications to determine suitability.
Both are Class CC families. ATMR is fast-acting, while ATDR is time-delay. They should not be treated as interchangeable solely because amperage and class match.
Mersen currently identifies OTS30 as a 30A Class K5 fast-acting one-time fuse rated 600 Vac and 300 Vdc. Verify the current Mersen data sheet before using any distributor listing.
Need Help Cross-Referencing a Ferraz Shawmut or Mersen Fuse?
Contact JW Electrical Supply with the complete legacy fuse number and clear label photographs. Include the fuse holder, equipment model, quantity, and required-by date.
JW can help identify the current Mersen product family and available sourcing options. Final substitution should remain supported by current Mersen specifications and the application's electrical requirements.
For fuse sizing changes, protection redesign, coordination changes, or uncertain fault-current conditions, involve an appropriately qualified electrical professional.