Littelfuse 0242 Series: Specs, Stock Check & Alternatives
What the 0242 Series is for
The Littelfuse 242 Series is a barrier fuse designed for barrier circuits in intrinsically safe applications relating to hazardous locations. In plain terms: it provides overcurrent protection where equipment operates in potentially explosive environments, and the fuse sits in the barrier network that keeps energy to the hazardous area within safe limits. That application is why the details below matter more than usual — a fuse chosen on package size alone is not a safe substitute.
The series covers 40mA to 250mA, all at 250VAC/VDC, with a high interrupting rating and both axial and surface-mount versions available. The part used as the example here, 0242.050UR · Littelfuse, is the 50mA surface-mount version.
0242.050UR specifications
| Parameter | 0242.050UR value |
|---|---|
| Series | Littelfuse 242 Series Barrier Fuse |
| Current rating | 50mA (0.050A), fast-acting |
| Voltage rating | 250VAC / 250VDC |
| Mounting | Surface mount, cartridge |
| Size | 0.119 in dia x 0.331 in L (3.02mm x 8.40mm) |
| Cold resistance | 11.34 ohms |
| Nominal melting I²t | 0.000103 A²sec |
| Interrupting rating | 4000A at 250V |
| Operating temperature | -40°C to +125°C (consider rerating) |
| Approvals / compliance | UR, RoHS compliant, lead free |
Opening characteristics for the series, from the same datasheet: at 100% of rated current the fuse must not open for at least 4 hours; at 300% it opens within 10 seconds maximum; at 1000% it opens within 0.002 seconds maximum. These are series-level figures — always confirm the exact rating against the datasheet revision for your design before ordering.
Other ratings in the same series
The 242 Series spans eight current ratings at the same 250V. Changing the current rating changes the protection behaviour, so a different rating is a design decision, not a like-for-like swap:
| Part number | Current rating | Cold resistance | Nominal melting I²t |
|---|---|---|---|
| 0242.040UR | 40mA | 16.48 ohms | 0.000078 A²sec |
| 0242.050UR | 50mA | 11.34 ohms | 0.000103 A²sec |
| 0242.080UR | 80mA | 8.19 ohms | 0.000214 A²sec |
| 0242.100UR | 100mA | 3.60 ohms | 0.000977 A²sec |
| 0242.125UR | 125mA | 3.78 ohms | 0.001026 A²sec |
| 0242.160UR | 160mA | 3.00 ohms | 0.00157 A²sec |
| 0242.200UR | 200mA | 2.68 ohms | 0.0025 A²sec |
| 0242.250UR | 250mA | 1.60 ohms | 0.00579 A²sec |
Note the suffix as well as the rating: within this ordering system, the UR surface-mount version and the axial version of the same rating are different packages. They do not fit the same board footprint, so a package change needs a board-level check even when the current rating matches.
How to check stock without guessing
Catalogue pages on this site show sourcing scope, not live stock, and quotes are confirmed after the lot, date code and packaging are verified. That is the correct order for a barrier fuse, where the lot and date code are part of the verification, not an afterthought. To get a usable answer, send through the inquiry page:
- The full part number, including every suffix — 0242.050UR, not "0242" or "50mA fuse".
- The target quantity and any packaging requirement.
- Any date code or lot limits your application imposes.
- Whether a different rating or package is acceptable at all, and who on your side approves it.
Treat the answer as applying to that inquiry only. Barrier-fuse stock is lot-specific, and a previous quotation does not confirm that the same lot is still available.
Before accepting an alternative
Use this checklist before any substitute goes near a barrier circuit:
- Current rating: does the alternative open at the same overload behaviour your barrier design was certified with? A higher rating may fail to protect; a lower rating may nuisance-trip.
- Voltage and interrupting rating: both must meet or exceed the original at 250V.
- Package and footprint: surface-mount and axial versions are not interchangeable without a board change.
- Cold resistance and I²t: both affect normal-operation losses and let-through energy; compare them, do not assume they match because the current rating looks close.
- Approvals: confirm the alternative carries the same agency approvals your application requires.
- Certification impact: if the barrier network is part of a certified intrinsically safe design, a part change may need re-assessment. When in doubt, ask the design authority before substituting.
For the general discipline of checking a lot before payment — photos of the actual lot, date code plausibility, lot-code agreement between label and parts — see our earlier guide, How to Source Obsolete & Hard-to-Find ICs Without Getting Fakes.
FAQ
What is the 0242 Series used for?
Barrier circuits in intrinsically safe applications for hazardous locations. It provides overcurrent protection in the barrier network that limits energy reaching equipment in potentially explosive environments.
Is 0242.050UR the same as other 50mA fuses?
No. The current rating is only one parameter. Voltage rating, interrupting rating, opening time, cold resistance, I²t, package and agency approvals all have to match the design. A general-purpose 50mA fuse is not automatically a barrier-fuse replacement.
Can I replace a 50mA part with an 80mA part from the same series?
Only after engineering review. The higher rating changes when the fuse opens and how much energy it lets through, which can break the protection logic of the barrier circuit. Same series does not mean interchangeable.
How do I confirm availability of 0242.050UR?
By inquiry, with the full part number, quantity and any date code or packaging limits. Availability is confirmed lot by lot, not from a catalogue listing.
What does the UR suffix mean here?
In this ordering system it identifies the surface-mount version of the rating, as opposed to the axial version. Package matters: the two do not share a footprint, so check the board layout before accepting a package change.