Equivalent & Substitute Parts for BK/MDL-1-1/4 Glass Cartridge Fuse

Part Overview

The BK/MDL-1-1/4 is a 1.25 A, 250 V AC/DC slow-blow glass cartridge fuse manufactured by Eaton Electronics Division. This fuse is designed for 3AB and 3AG holder applications with dimensions of 0.250" diameter by 1.250" length (6.35mm x 31.75mm). The part is classified as obsolete, necessitating identification of active equivalent and substitute components that maintain electrical and mechanical compatibility while meeting current regulatory requirements.

Key Parameters

Parameter	Value
Current Rating	1.25 A
Voltage Rating (AC)	250 V
Fuse Type	Cartridge, Glass
Response Time	Slow Blow
Package / Case	3AB, 3AG, 1/4" x 1-1/4"
Size / Dimension	0.250" Dia x 1.250" L (6.35mm x 31.75mm)
Mounting Type	Requires Holder
Breaking Capacity @ Rated Voltage	100A

Substitute Part Grouping Explanation

Substitution eligibility for the BK/MDL-1-1/4 is determined by strict equivalence across the following parameters:

Primary Equivalence Criteria:

Current Rating: 1.25 A (exact match required)
Voltage Rating (AC): 250 V (exact match required)
Fuse Type: Cartridge, Glass (exact match required)
Response Time: Slow Blow (exact match required)
Package / Case: 3AB, 3AG, 1/4" x 1-1/4" (exact match required)
Size / Dimension: 0.250" Dia x 1.250" L (exact match required)
Mounting Type: Requires Holder (exact match required)

Secondary Considerations:

Breaking Capacity @ Rated Voltage: 100A (preferred for full compatibility)
Approval Agencies: CSA, UL (baseline certifications for the original part)
Product Status: Active status preferred for long-term availability

All substitute parts listed below meet the primary equivalence criteria. Variations in secondary parameters (DC Cold Resistance, Melting I²t, RoHS compliance status) do not affect functional interchangeability within the specified electrical and mechanical constraints.

Parameter Comparison

Part Number	Manufacturer	Current (A)	Voltage (V AC)	Response Time	Breaking Capacity (A)	Product Status	Approvals	RoHS Status
BK/MDL-1-1/4	Eaton	1.25	250	Slow Blow	100	Obsolete	CSA, UL	Non-compliant
BK/MDL-1-1/4-R	Eaton	1.25	250	Slow Blow	100	Active	CE, CSA, UL	ROHS3 Compliant
MDL-1-1/4-R	Eaton	1.25	250	Slow Blow	100	Active	CE, CSA, UL	RoHS Compliant
BK/MDL-1-1/4LX-R	Eaton	1.25	250	Slow Blow	100	Active	CSA, UL	RoHS Compliant
BK/MDQ-1-1/4	Eaton	1.25	250	Slow Blow	100	Active	CSA, UL	Non-compliant
MDQ-1-1/4	Eaton	1.25	250	Slow Blow	35	Active	CSA, UL	Not specified
0034.5227	SCHURTER	1.25	250	Slow Blow	100	Obsolete	CSA, UL	ROHS3 Compliant
0034.3418	SCHURTER	1.25	250	Slow Blow	35	Obsolete	CE	ROHS3 Compliant
03131.25HXP	Littelfuse	1.25	250	Slow Blow	100	Active	CE, CSA, PSE, UL	ROHS3 Compliant
03131.25MXP	Littelfuse	1.25	250	Slow Blow	100	Active	CE, CSA, PSE, UL	ROHS3 Compliant
03131.25VXP	Littelfuse	1.25	250	Slow Blow	100	Active	CE, CSA, PSE, UL	ROHS3 Compliant

Engineering Selection Recommendations

Tier 1 - Direct Active Replacements (Eaton MDL Series):

BK/MDL-1-1/4-R and MDL-1-1/4-R are the primary recommended substitutes. Both are active products from Eaton Electronics Division with identical electrical and mechanical specifications to the original BK/MDL-1-1/4. BK/MDL-1-1/4-R carries ROHS3 compliance and CE certification in addition to CSA and UL approvals. MDL-1-1/4-R maintains RoHS compliance with equivalent certifications. These parts ensure continuity within the Eaton MDL series platform.

Tier 2 - Active Eaton Alternatives (MDL and MDQ Series):

BK/MDL-1-1/4LX-R provides an active alternative within the Eaton MDL series with RoHS compliance and CSA/UL approvals. BK/MDQ-1-1/4 and MDQ-1-1/4 represent the MDQ series variant, both active products. MDQ-1-1/4 carries a reduced breaking capacity of 35A compared to the 100A specification of the original part; this reduction may impact performance in high-fault-current applications.

Tier 3 - Cross-Manufacturer Active Substitutes (Littelfuse 313 Series):

03131.25HXP, 03131.25MXP, and 03131.25VXP are active Littelfuse products from the 313 series. All three maintain full electrical and mechanical equivalence with 100A breaking capacity. These parts carry expanded certifications including CE, CSA, PSE, and UL, with ROHS3 compliance. Operating temperature range is specified as -55°C to 125°C.

Obsolete Parts (Not Recommended for New Designs):

0034.5227 and 0034.3418 (SCHURTER) are classified as obsolete. While 0034.5227 maintains 100A breaking capacity, both parts have limited availability and should not be selected for new applications or long-term inventory planning.

Frequently Asked Questions (FAQ)

Q: Can BK/MDL-1-1/4-R be used as a direct replacement for BK/MDL-1-1/4?

A: Yes. BK/MDL-1-1/4-R is an active product with identical electrical ratings (1.25 A, 250 V AC), fuse type (cartridge glass), response time (slow blow), and mechanical dimensions (0.250" x 1.250"). Both require the same 3AB/3AG holder. The primary difference is product status (active vs. obsolete) and enhanced compliance certifications on the replacement part.

Q: What is the difference between MDL-1-1/4-R and BK/MDL-1-1/4-R?

A: Both parts are electrically and mechanically identical with 1.25 A, 250 V ratings and 100A breaking capacity. The "BK/" prefix designation does not affect functional performance. MDL-1-1/4-R carries RoHS compliance, while BK/MDL-1-1/4-R carries ROHS3 compliance. Both are active Eaton products suitable for direct substitution.

Q: Can Littelfuse 03131.25HXP, 03131.25MXP, or 03131.25VXP replace the Eaton BK/MDL-1-1/4?

A: Yes. All three Littelfuse parts meet the primary equivalence criteria: 1.25 A, 250 V AC, slow-blow glass cartridge fuses with 0.250" x 1.250" dimensions for 3AB/3AG holders. All three are active products with 100A breaking capacity and carry CE, CSA, PSE, and UL certifications. Cross-manufacturer substitution is electrically and mechanically valid.

Q: Why is MDQ-1-1/4 listed as a substitute if it has only 35A breaking capacity versus 100A?

A: MDQ-1-1/4 meets all primary equivalence criteria for current rating, voltage, fuse type, response time, package, and dimensions. The reduced breaking capacity of 35A represents a secondary parameter variation. This part is suitable for applications where the fault current does not exceed 35A. For applications requiring 100A breaking capacity, Tier 1 or Tier 2 alternatives with 100A ratings are recommended.

Q: Are there any compatibility issues when switching from Eaton to Littelfuse fuses?

A: No compatibility issues exist. Both manufacturers produce glass cartridge fuses with identical 0.250" x 1.250" dimensions for 3AB/3AG holders. Electrical performance (1.25 A, 250 V, slow-blow response) is equivalent. Littelfuse parts carry additional certifications (PSE) and specify operating temperature range (-55°C to 125°C), which may provide advantages in temperature-sensitive applications.

Q: Should I select a part with ROHS3 compliance over RoHS compliance?

A: ROHS3 compliance represents the current regulatory standard and is preferred for new designs and long-term inventory. However, both ROHS3 and RoHS compliant parts are functionally equivalent for the fuse application. Selection should align with your organization's compliance requirements and supply chain strategy.

Q: What holder types are compatible with these fuses?

A: All listed substitute parts require 3AB or 3AG holder types. The fuses are not self-contained and must be installed in appropriate holders designed for 0.250" diameter x 1.250" length cartridges. Verify that your existing holder is rated for 3AB/3AG before installation.

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