SSQ 4/5K Surface Mount Fuse - Equivalent & Substitute Parts

Part Overview

The SSQ 4/5K is a 4 A, 125 V AC/DC surface mount board fuse manufactured by Bel Fuse Inc. in a 2-SMD, Square End Block package. This component is classified as obsolete, making identification of functionally equivalent alternatives essential for ongoing design support and production continuity. Substitute parts must maintain electrical performance across current rating, voltage specifications, and thermal characteristics while accommodating modern supply chain availability.

Substiute Parts

SSQ 4/5K

Bel Fuse Inc.In Stock: 1109SSQ 4/5K Datasheet

Current PartRFQ

0679L4000-05

Bel Fuse Inc.In Stock: 976040679L4000-05 Datasheet

DirectRFQ

0476004.MR

Littelfuse Inc.In Stock: 51820476004.MR Datasheet

DirectRFQ

0476004.MRSN

Littelfuse Inc.In Stock: 20470476004.MRSN Datasheet

DirectRFQ

0451004.MRSN

Littelfuse Inc.In Stock: 20470451004.MRSN Datasheet

SimilarRFQ

Key Parameters

Substitute Part Grouping Explanation

Substitution eligibility is determined by strict alignment with the following electrical and mechanical parameters:

Primary Substitution Criteria:

• Current Rating: 4 A (exact match required)
• Voltage Rating - AC: 125 V minimum
• Voltage Rating - DC: 125 V minimum
• Response Time: Fast Blow
• Mounting Type: Surface Mount
• Package / Case: 2-SMD, Square End Block
• Operating Temperature Range: -55°C to 125°C minimum
• RoHS3 Compliance

Substitute Categories:

Direct Equivalents (Identical Electrical & Mechanical Specifications):

• 0679L4000-05 (Bel Fuse Inc.) - Active status, Tape & Reel packaging, identical voltage and current ratings

Compatible Substitutes (Enhanced Voltage Rating):

• 0476004.MR (Littelfuse Inc., NANO²®476 series) - Active status, 250 V AC rating (exceeds 125 V requirement), maintains 125 V DC and 4 A current rating
• 0476004.MRSN (Littelfuse Inc., NANO²®476 series) - Active status, identical specifications to 0476004.MR

Similar Substitutes (Reduced Voltage Rating - DC Only):

• 0451004.MRSN (Littelfuse Inc., NANO²®451 series) - Active status, 125 V AC rating maintained, DC voltage rating not specified in documentation, Cut Tape packaging

Parameter Comparison

Engineering Selection Recommendations

For Direct Replacement (Recommended Primary Choice): 0679L4000-05 is the preferred substitute. It maintains identical electrical specifications (4 A, 125 V AC/DC, Fast Blow) and package geometry. Active product status ensures long-term supply availability. Tape & Reel packaging supports automated assembly processes. Minor dimensional variation in height (2.79 mm vs. 2.54 mm) and melting I²t difference (4.6 vs. 5.1) require PCB layout verification for clearance and thermal performance validation.

For Enhanced Voltage Margin Applications: 0476004.MR and 0476004.MRSN (electrically identical variants) provide 250 V AC rating, exceeding the original 125 V AC specification. These are suitable for designs requiring higher voltage headroom. Littelfuse NANO²®476 series certification includes cURus approval. Dimensional differences (2.69 mm height vs. 2.54 mm) and lower melting I²t (2.82 vs. 5.1) necessitate thermal analysis and PCB footprint compatibility assessment.

For Cost-Optimized Alternatives: 0451004.MRSN (Littelfuse NANO²®451 series) maintains 125 V AC and 4 A specifications with active product status. DC voltage rating is not specified in available documentation. Cut Tape packaging may require manual handling or reel conversion for production environments. This option is suitable only when DC voltage specification is not critical to circuit operation.

Compliance Considerations: All substitute parts maintain ROHS3 compliance and MSL 1 rating. Approval agencies vary: original part includes CE and CSA certifications; substitutes feature cURus or AEC-Q200 certifications. Design validation should confirm certification alignment with end-application requirements.

Frequently Asked Questions (FAQ)

Q: Can 0679L4000-05 be used as a direct replacement for SSQ 4/5K? A: Yes. Both parts share identical current rating (4 A), voltage specifications (125 V AC/DC), response time (Fast Blow), and package type (2-SMD, Square End Block). Height dimension increases from 2.54 mm to 2.79 mm; verify PCB clearance before implementation.

Q: What is the difference between 0476004.MR and 0476004.MRSN? A: These are electrically and mechanically identical parts from Littelfuse NANO²®476 series. The difference is packaging: 0476004.MR and 0476004.MRSN are both supplied in Tape & Reel format. Both are suitable substitutes for applications requiring 250 V AC rating.

Q: Why does 0451004.MRSN list "DC voltage rating not specified"? A: The provided documentation for 0451004.MRSN does not include a DC voltage specification, only 125 V AC. This part should be used only in applications where DC voltage rating is not a design constraint or where 125 V DC operation is confirmed through alternative documentation.

Q: Are there dimensional compatibility issues with substitute parts? A: Yes. The original SSQ 4/5K measures 6.10 x 2.54 x 2.54 mm. Substitute parts vary: 0679L4000-05 is 6.10 x 2.54 x 2.79 mm; 0476004 series parts are 6.10 x 2.69 x 2.69 mm. PCB footprint and clearance verification is required before design release.

Q: What does the melting I²t difference mean for circuit protection? A: Melting I²t indicates thermal energy required to melt the fuse element. SSQ 4/5K has I²t of 5.1; substitutes range from 2.82 to 4.6. Lower I²t values result in faster response to overcurrent. Circuit protection characteristics may differ; thermal simulation is recommended for critical applications.

Q: Which substitute has the best supply availability? A: 0679L4000-05 shows 97,494 units in stock, significantly exceeding other options. This ensures production continuity and reduces lead time risk for ongoing designs.

Q: Can these parts be used interchangeably in existing PCB designs? A: Not without verification. Dimensional variations require footprint compatibility assessment. Melting I²t and resistance differences may affect circuit protection timing. Electrical validation is necessary before production implementation.