SST 4 Surface Mount Board Mount Fuse - Equivalent & Substitute Parts

Part Overview

The SST 4 is a 4 A, 125 V AC/DC surface mount board mount fuse manufactured by Bel Fuse Inc. in a 2-SMD, Square End Block package. This component is classified as obsolete, making equivalent and substitute parts necessary for ongoing design support and production continuity. The SST 4 operates across a temperature range of -55°C to 125°C with slow blow response characteristics and is RoHS3 compliant with unlimited moisture sensitivity level (MSL 1).

Key Parameters

Parameter	Value
Manufacturer	Bel Fuse Inc.
Part Number	SST 4
Current Rating	4 A
Voltage Rating (AC)	125 V
Voltage Rating (DC)	125 V
Response Time	Slow Blow
Mounting Type	Surface Mount
Package / Case	2-SMD, Square End Block
Size / Dimension	0.240" L x 0.100" W x 0.100" H (6.10mm x 2.54mm x 2.54mm)
Operating Temperature	-55°C ~ 125°C
RoHS Status	RoHS3 Compliant
Product Status	Obsolete

Substitute Part Grouping Explanation

Substitute parts for the SST 4 are grouped into two categories based on electrical and mechanical compatibility:

Direct Manufacturer Substitutes (Bel Fuse Inc. 0680L Series): Parts 0680L4000-01, 0680L4000-11, 0680L4000-15, 0680L4000-21, and 0680L4000-25 share identical electrical ratings (4 A, 125 V AC/DC), package geometry (2-SMD, Square End Block), and physical dimensions (6.10mm x 2.54mm x 2.54mm). These parts are active products with enhanced breaking capacity specifications and are available in Tape & Reel packaging.

Parametric Equivalents: The 0680L4000-05 (Bel Fuse Inc.) and 0680H series parts (0680H4000-01, 0680H4000-05) maintain identical current and voltage ratings with the same package and dimensions. The 0680H series offers higher breaking capacity (100A AC, 500A DC) compared to the SST 4 (50A), making them suitable for applications requiring enhanced fault protection.

Cross-Manufacturer Alternatives (Littelfuse Inc.): Parts 0449004.MR and 0452004.MRL from Littelfuse Inc. NANO²® series provide equivalent electrical performance (4 A, 125 V AC/DC, Slow Blow) with identical mounting type and package classification. These parts feature slightly different physical dimensions (0.106" W x 0.106" H versus 0.100" W x 0.100" H) and different melting I²t characteristics.

Substitution is determined by matching: current rating, voltage rating (AC and DC), response time (Slow Blow), mounting type (Surface Mount), package type (2-SMD, Square End Block), and operating temperature range.

Parameter Comparison

Part Number	Manufacturer	Current Rating (A)	Voltage AC (V)	Voltage DC (V)	Response Time	Package / Case	Size (mm)	Breaking Capacity @ Rated Voltage	Melting I²t	DC Cold Resistance (Ω)	Product Status	Packaging
SST 4	Bel Fuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A	23	0.024	Obsolete	Cut Tape (CT) & Digi-Reel®
0680L4000-01	Bel Fuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680L4000-11	Bel Fuse	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680L4000-15	Bel Fuse	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680L4000-21	Bel Fuse	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680L4000-25	Bel Fuse	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680L4000-05	Bel Fuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	50A AC, 100A DC	24	0.026	Active	Tape & Reel (TR)
0680H4000-01	Bel Fuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	100A AC, 500A DC	15	0.032	Active	Tape & Reel (TR)
0680H4000-05	Bel Fuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.54 x 2.54	100A AC, 500A DC	15	0.032	Active	Tape & Reel (TR)
0449004.MR	Littelfuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.69 x 2.69	50A	40.8	0.0227	Active	Tape & Reel (TR)
0452004.MRL	Littelfuse Inc.	4	125	125	Slow Blow	2-SMD, Square End Block	6.10 x 2.69 x 2.69	50A	34.4	0.0186	Active	Cut Tape (CT) & Digi-Reel®

Engineering Selection Recommendations

Primary Recommendation: The 0680L4000-05 (Bel Fuse Inc.) is the preferred direct substitute for the obsolete SST 4. This part maintains identical electrical specifications, package geometry, and physical dimensions while offering active product status. It carries AEC-Q200, CE, and cURus certifications, matching the compliance profile of the original SST 4. Inventory availability (86,210 pcs) ensures supply continuity.

Alternative within Bel Fuse 0680L Series: Parts 0680L4000-01, 0680L4000-11, 0680L4000-15, and 0680L4000-21 are functionally equivalent to 0680L4000-05 with identical electrical and mechanical parameters. Selection among these variants depends on packaging requirements and internal lead configuration specifications not detailed in this reference.

Enhanced Protection Option: The 0680H4000-01 and 0680H4000-05 (Bel Fuse Inc.) provide higher breaking capacity (100A AC, 500A DC) while maintaining the same current and voltage ratings. These parts are suitable for applications requiring enhanced fault protection margins. Note that cURus certification is the only approval agency listed for the 0680H series.

Cross-Manufacturer Alternative: The Littelfuse Inc. 0452004.MRL (NANO²® 452 series) offers equivalent electrical performance with active product status and CSA, PSE, and UR certifications. Physical dimensions differ slightly (2.69mm width and height versus 2.54mm), requiring PCB layout verification. This part is available in Cut Tape & Digi-Reel® packaging, matching the original SST 4 packaging format.

Littelfuse 0449004.MR: The NANO²® 449 series part provides equivalent electrical specifications with cULus and PSE certifications. Dimension differences and higher melting I²t (40.8 versus 23) require application-specific evaluation.

Frequently Asked Questions (FAQ)

Q: Can I directly replace SST 4 with 0680L4000-05? A: Yes. The 0680L4000-05 maintains identical current rating (4 A), voltage rating (125 V AC/DC), response time (Slow Blow), package type (2-SMD, Square End Block), and physical dimensions (6.10mm x 2.54mm x 2.54mm). Both are RoHS3 compliant with MSL 1. The primary difference is product status (obsolete versus active) and packaging format (Cut Tape & Digi-Reel® versus Tape & Reel).

Q: What is the difference between 0680L and 0680H series fuses? A: Both series maintain identical current and voltage ratings (4 A, 125 V AC/DC) and package dimensions. The 0680H series offers significantly higher breaking capacity (100A AC, 500A DC) compared to 0680L (50A AC, 100A DC). The 0680H series has lower melting I²t (15 versus 24), indicating faster response to overcurrent conditions. Selection depends on application fault protection requirements.

Q: Are Littelfuse NANO²® parts compatible with the SST 4 footprint? A: Littelfuse parts 0449004.MR and 0452004.MRL maintain the same electrical specifications and package classification (2-SMD, Square End Block). However, physical dimensions differ: Littelfuse parts measure 6.10mm x 2.69mm x 2.69mm versus SST 4 dimensions of 6.10mm x 2.54mm x 2.54mm. PCB layout and land pattern verification is required before substitution.

Q: What certifications should I verify for my application? A: The SST 4 carries AEC-Q200, CE, CSA, and cULus certifications. The 0680L4000-05 maintains AEC-Q200, CE, and cURus certifications. If your application requires specific certifications (such as CSA), verify that the substitute part carries equivalent or superior certifications. The 0452004.MRL carries CSA, PSE, and UR certifications.

Q: Does packaging format affect electrical performance? A: No. Cut Tape (CT) & Digi-Reel® and Tape & Reel (TR) packaging formats do not affect electrical performance. Selection depends on assembly equipment compatibility and procurement workflow. The SST 4 uses Cut Tape & Digi-Reel® packaging, while most active substitutes use Tape & Reel format.

Q: What is the significance of melting I²t values? A: Melting I²t indicates the thermal energy required to melt the fuse element. Lower values indicate faster response to overcurrent. The SST 4 has melting I²t of 23, while 0680L series parts have 24, and 0680H series have 15. For applications with sensitive circuits, lower melting I²t values provide faster protection but may increase nuisance trip risk.

Q: Can I use 0680H4000-05 as a direct replacement for SST 4? A: Electrically, yes. The 0680H4000-05 maintains identical current and voltage ratings with the same package and dimensions. However, the higher breaking capacity and lower melting I²t represent different protection characteristics. Application-specific testing is necessary to confirm compatibility with circuit protection requirements.

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