SSQ 4 Surface Mount Fuse – Equivalent & Substitute Parts

Part Overview

The SSQ 4 is a 4A 125VAC/VDC surface mount board fuse manufactured by Bel Fuse Inc., designed for fast-blow protection in compact 2-SMD square end block packaging. This part is classified as obsolete, making equivalent substitutes necessary for ongoing production and maintenance applications. Substitute parts must maintain identical electrical ratings, mechanical compatibility, and thermal characteristics to ensure direct board-level replacement without circuit redesign.

Key Parameters

Parameter	SSQ 4 Specification
Current Rating	4 A
Voltage Rating – AC	125 V
Voltage Rating – DC	125 V
Response Time	Fast Blow
Mounting Type	Surface Mount
Package / Case	2-SMD, Square End Block
Breaking Capacity @ Rated Voltage	50A AC, 300A DC
Operating Temperature Range	-55°C ~ 125°C
RoHS Status	ROHS3 Compliant
Moisture Sensitivity Level	1 (Unlimited)

Substitute Part Grouping Explanation

Substitute parts for the SSQ 4 are categorized into two groups based on electrical and mechanical compatibility:

Direct Substitutes maintain identical voltage ratings (125VAC/125VDC), current rating (4A), response time (Fast Blow), and package form factor (2-SMD, Square End Block). These parts are interchangeable at the board level without functional compromise.

Similar Substitutes maintain the core electrical parameters (4A, 125VAC/125VDC, Fast Blow, Surface Mount) but may differ in package dimensions, melting I²t characteristics, or breaking capacity specifications. Selection requires verification of mechanical fit and thermal performance in the target application.

Key Parameters Determining Substitutability:

Current Rating: 4 A (mandatory)
Voltage Rating – AC: 125 V (mandatory for direct substitutes)
Voltage Rating – DC: 125 V (mandatory for direct substitutes)
Response Time: Fast Blow (mandatory)
Mounting Type: Surface Mount (mandatory)
Package / Case: 2-SMD configuration (mandatory for direct substitutes)
Operating Temperature: -55°C ~ 125°C (mandatory)
RoHS Compliance: ROHS3 (mandatory)

Parameter Comparison

Part Number	Manufacturer	Current (A)	Voltage AC (V)	Voltage DC (V)	Response Time	Package	Breaking Capacity AC	Breaking Capacity DC	Melting I²t	Size (mm)	DC Cold Resistance (Ω)	Product Status	Packaging Type
SSQ 4	Bel Fuse Inc.	4	125	125	Fast Blow	2-SMD, Square End Block	50A	300A	5.1	6.10 x 2.54 x 2.54	0.019	Obsolete	Cut Tape (CT) & Digi-Reel®
0679L4000-01	Bel Fuse Inc.	4	125	125	Fast Blow	2-SMD, Square End Block	50A	300A	4.6	6.10 x 2.54 x 2.79	0.02	Active	Tape & Reel (TR)
0476004.MR	Littelfuse Inc.	4	250	125	Fast Blow	2-SMD, Square End Block	100A	300A	2.82	6.10 x 2.69 x 2.69	0.01839	Active	Tape & Reel (TR)
0476004.MRSN	Littelfuse Inc.	4	250	125	Fast Blow	2-SMD, Square End Block	100A	300A	2.82	6.10 x 2.69 x 2.69	0.01839	Active	Tape & Reel (TR)
0451004.MRL	Littelfuse Inc.	4	125	125	Fast Blow	2-SMD, Square End Block	50A	N/A	3.152	6.10 x 2.69 x 2.69	0.016	Active	Cut Tape (CT) & Digi-Reel®
0451004.MRSN	Littelfuse Inc.	4	125	N/A	Fast Blow	2-SMD, Square End Block	50A	N/A	3.152	6.10 x 2.69 x 2.69	0.016	Active	Cut Tape (CT)
0453004.MR	Littelfuse Inc.	4	125	125	Fast Blow	2-SMD, Square End Block	50A	N/A	3.152	6.10 x 2.69 x 2.69	0.016	Active	Tape & Reel (TR)
3404.0016.11	SCHURTER Inc.	4	125	125	Fast Blow	2-SMD, J-Lead	100A	N/A	2.1	7.40 x 3.10 x 2.60	N/A	Active	Bulk
3404.0016.22	SCHURTER Inc.	4	125	125	Fast Blow	2-SMD, J-Lead	100A	N/A	2.1	7.40 x 3.10 x 2.60	N/A	Active	Tape & Reel (TR)

Engineering Selection Recommendations

Direct Replacement (Preferred): The 0679L4000-01 (Bel Fuse Inc., Active status) is the primary direct substitute for the SSQ 4. It maintains identical electrical ratings (4A, 125VAC/125VDC), fast-blow response, and 2-SMD square end block packaging. The minor height difference (2.79mm vs. 2.54mm) and lower melting I²t (4.6 vs. 5.1) are within acceptable mechanical and thermal tolerances for board-level replacement. This part carries AEC-Q200 and cURus certifications, confirming automotive and safety compliance.

Alternative Substitutes (Voltage-Rated): The 0476004.MR and 0476004.MRSN (Littelfuse Inc., NANO²® 476 series, Active status) provide 250VAC rating while maintaining 125VDC and 4A current specifications. These parts are suitable for applications requiring higher AC voltage margin. Both are ROHS3 compliant with cURus certification. Dimensional variations (2.69mm height vs. 2.54mm) require PCB footprint verification.

Functional Equivalents (Same Electrical Ratings): The 0451004.MRL and 0453004.MR (Littelfuse Inc., NANO²® series, Active status) maintain identical 4A, 125VAC/125VDC ratings and fast-blow response. These parts feature updated packaging (Tape & Reel or Cut Tape) and are widely available. The 0451004.MRL includes PSE and UL certifications in addition to CSA compliance.

Mechanical Alternatives (Different Package Form): The 3404.0016.11 and 3404.0016.22 (SCHURTER Inc., OMF 125 series, Active status) maintain electrical equivalence but use J-Lead package configuration instead of square end block. These parts feature higher breaking capacity (100A AC) and are suitable only if PCB footprint accommodates J-Lead geometry. Bulk and Tape & Reel packaging options are available.

Frequently Asked Questions (FAQ)

Q: Can the 0679L4000-01 replace the SSQ 4 without PCB modification? A: Yes. The 0679L4000-01 maintains identical electrical ratings and 2-SMD square end block packaging. The height difference (2.79mm vs. 2.54mm) is within standard PCB tolerance for surface mount components. No circuit redesign is required.

Q: What is the difference between the 0476004.MR and 0476004.MRSN? A: Both parts are electrically and mechanically identical. The difference is packaging format: 0476004.MR is supplied in standard Tape & Reel, while 0476004.MRSN is also Tape & Reel. Both are Active status parts from Littelfuse Inc. with identical specifications.

Q: Why do the SCHURTER 3404.0016 series parts have different dimensions? A: The SCHURTER parts use J-Lead package configuration instead of square end block. This results in larger overall dimensions (7.40 x 3.10 x 2.60mm vs. 6.10 x 2.54 x 2.54mm for the SSQ 4). These parts are suitable only if the PCB footprint is designed to accommodate J-Lead geometry.

Q: Are all substitute parts ROHS3 compliant? A: Yes. All listed substitute parts carry ROHS3 Compliant status, matching the SSQ 4 specification.

Q: What does melting I²t indicate, and why does it vary among substitutes? A: Melting I²t is the thermal energy required to melt the fuse element. The SSQ 4 has a melting I²t of 5.1, while substitutes range from 2.1 to 3.152. Lower values indicate faster thermal response. Selection should be based on circuit protection requirements and thermal design margins.

Q: Can I use the 0451004.MRSN as a direct replacement? A: The 0451004.MRSN maintains 4A and 125VAC ratings but does not specify 125VDC voltage rating in the provided data. Verify DC voltage requirement in your application before selection. If 125VDC is required, use 0451004.MRL or 0453004.MR instead.

Q: What is the difference between Cut Tape and Tape & Reel packaging? A: Cut Tape (CT) packaging provides components in individual strips suitable for manual assembly or small-volume production. Tape & Reel (TR) packaging provides continuous reels for automated pick-and-place assembly. Both formats contain identical components; packaging choice depends on assembly process requirements.

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