SST 4/5K Surface Mount Board Mount Fuse Equivalent & Substitute Parts

Part Overview

The SST 4/5K 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 part is classified as Obsolete, which necessitates identification of active equivalent and substitute components for ongoing design support and procurement. The SST 4/5K maintains electrical and mechanical compatibility with several active alternatives from both Bel Fuse Inc. and Littelfuse Inc. product lines.

Key Parameters

Parameter	Value
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
Physical Dimensions	0.240" L x 0.100" W x 0.100" H (6.10mm x 2.54mm x 2.54mm)
Operating Temperature Range	-55°C ~ 125°C
RoHS Status	ROHS3 Compliant
Moisture Sensitivity Level	1 (Unlimited)

Substitute Part Grouping Explanation

Substitution eligibility is determined by strict equivalence across the following parameters: current rating (4 A), AC voltage rating (125 V), DC voltage rating (125 V), response time (Slow Blow), mounting type (Surface Mount), package type (2-SMD, Square End Block), and operating temperature range (-55°C ~ 125°C). All substitute parts listed meet these core electrical and mechanical requirements.

Substitute parts are grouped into two categories:

Direct Manufacturer Substitutes (Bel Fuse Inc. 0680L Series): Parts 0680L4000-01, 0680L4000-05, 0680L4000-11, 0680L4000-15, 0680L4000-21, and 0680L4000-25 are active alternatives from the same manufacturer with identical electrical ratings and package specifications. These parts differ in breaking capacity and melting I²t characteristics but maintain full compatibility with the SST 4/5K electrical and mechanical interface.

Parametric Equivalents (Bel Fuse Inc. 0680H Series): Parts 0680H4000-01 and 0680H4000-05 are parametrically equivalent with identical current, voltage, and package specifications. These parts feature enhanced breaking capacity (100A AC, 500A DC) and lower melting I²t (15) compared to the SST 4/5K, making them suitable for applications requiring higher fault current handling.

Cross-Manufacturer Alternatives (Littelfuse Inc.): Parts 0449004.MR (NANO²® 449 Series) and 0452004.MRL (NANO²® 452 Series) are functionally equivalent components from Littelfuse Inc. with identical electrical ratings and 2-SMD package configuration. Minor dimensional variations exist in width and height (0.106" vs. 0.100"), but these remain within standard PCB footprint tolerances for this package class.

Parameter Comparison

Part Number	Manufacturer	Series	Current (A)	Voltage AC (V)	Voltage DC (V)	Response Time	Breaking Capacity AC (A)	Breaking Capacity DC (A)	Melting I²t	Product Status	Packaging
SST 4/5K	Bel Fuse Inc.	SST	4	125	125	Slow Blow	50	—	23	Obsolete	Cut Tape (CT) & Digi-Reel®
0680L4000-01	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680L4000-05	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680L4000-11	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680L4000-15	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680L4000-21	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680L4000-25	Bel Fuse Inc.	0680L	4	125	125	Slow Blow	50	100	24	Active	Tape & Reel (TR)
0680H4000-01	Bel Fuse Inc.	0680H	4	125	125	Slow Blow	100	500	15	Active	Tape & Reel (TR)
0680H4000-05	Bel Fuse Inc.	0680H	4	125	125	Slow Blow	100	500	15	Active	Tape & Reel (TR)
0449004.MR	Littelfuse Inc.	NANO²® 449	4	125	125	Slow Blow	50	—	40.8	Active	Tape & Reel (TR)
0452004.MRL	Littelfuse Inc.	NANO²® 452	4	125	125	Slow Blow	50	—	34.4	Active	Cut Tape (CT) & Digi-Reel®

Engineering Selection Recommendations

For Direct Replacement (Bel Fuse Inc. 0680L Series): The 0680L4000 variants are the primary recommended substitutes. All variants maintain identical electrical specifications and are Active products with full AEC-Q200 and CE certifications. The 0680L4000-05 offers the highest inventory availability (86,210 units) and is suitable for volume production requirements. Selection among 0680L4000-01, -11, -15, -21, and -25 variants should be based on packaging format requirements (Tape & Reel) and internal lead configuration specifications.

For Enhanced Fault Current Handling (Bel Fuse Inc. 0680H Series): The 0680H4000 variants provide superior breaking capacity (100A AC, 500A DC) and are recommended for applications subject to higher fault current conditions. These parts maintain identical current and voltage ratings but feature lower melting I²t (15 vs. 23), resulting in faster response characteristics. Certification is limited to cURus; applications requiring AEC-Q200 or CE compliance should utilize the 0680L series.

For Cross-Manufacturer Compatibility (Littelfuse Inc.): The 0449004.MR and 0452004.MRL are functionally equivalent alternatives when multi-source procurement is required. Both maintain 4 A, 125 V AC/DC ratings and Slow Blow response. The 0452004.MRL offers higher inventory (42,923 units) and is available in Cut Tape & Digi-Reel® packaging, matching the original SST 4/5K format. Minor dimensional variations (0.106" width and height vs. 0.100") are within standard PCB footprint tolerances for 2-SMD packages.

Frequently Asked Questions (FAQ)

Q: Can the 0680L4000 series directly replace the SST 4/5K without PCB modification?

A: Yes. The 0680L4000 series maintains identical electrical ratings (4 A, 125 V AC/DC, Slow Blow), package type (2-SMD, Square End Block), and physical dimensions (0.240" L x 0.100" W x 0.100" H). No PCB layout changes are required.

Q: What is the difference between the 0680L and 0680H series?

A: Both series share identical current (4 A) and voltage ratings (125 V AC/DC). The 0680H series provides enhanced breaking capacity (100A AC, 500A DC vs. 50A AC, 100A DC) and lower melting I²t (15 vs. 24), enabling faster response in high fault current environments. The 0680L series carries broader certifications (AEC-Q200, CE, cURus) compared to 0680H (cURus only).

Q: Are Littelfuse 0449004.MR and 0452004.MRL compatible with the SST 4/5K footprint?

A: Both Littelfuse parts are electrically equivalent (4 A, 125 V AC/DC, Slow Blow, 2-SMD package). Minor dimensional differences exist: Littelfuse parts measure 0.106" width and height versus the SST 4/5K's 0.100". These variations remain within standard PCB footprint tolerances for this package class and do not require layout modification.

Q: Which substitute offers the best inventory availability?

A: The 0680L4000-05 (Bel Fuse Inc.) provides the highest stock level at 86,210 units. For Littelfuse alternatives, the 0452004.MRL offers 42,923 units.

Q: Do all substitutes maintain RoHS3 compliance?

A: Yes. All listed substitute parts are ROHS3 Compliant and REACH Unaffected, matching the SST 4/5K environmental compliance status.

Q: What certifications should I verify for my application?

A: The 0680L series carries AEC-Q200, CE, and cURus certifications. The 0680H series is limited to cURus. Littelfuse 0449004.MR holds cULus and PSE certifications; 0452004.MRL holds CSA, PSE, and UR certifications. Verify that selected substitute certifications align with your application requirements.

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