A radial leaded fuse is a compact, non-resettable overcurrent protection device with two leads extending from the same side of its body. The leads are inserted into printed circuit board holes and soldered in place. Selecting the correct fuse requires both electrical matching and mechanical PCB compatibility.
What Is a Radial Leaded Fuse?
A radial leaded fuse has two wire leads that extend from one side of the fuse body. This arrangement allows the fuse to stand vertically or close to the PCB while both leads pass through nearby holes. It is commonly used where a compact, directly soldered protection device is preferred over a replaceable cartridge fuse and holder.
The term radial leaded fuse describes the lead arrangement and mounting style. Many radial fuses are also classified as subminiature fuses because of their small package size.
| Feature | Typical Description |
|---|---|
| Protection type | Non-resettable overcurrent protection |
| Lead arrangement | Two leads extending from the same side of the body |
| Mounting method | Through-hole PCB mounting and soldering |
| Common body styles | Square, rectangular or round subminiature packages |
| Operating characteristics | Fast-acting or time-lag |
| After operation | The fuse normally must be replaced after the element opens |
How a Radial Leaded Fuse Protects a PCB Circuit
Under normal operating conditions, current flows through the fuse element. When excessive current continues for long enough, the element heats and opens the circuit. This interrupts current flow before wiring, PCB traces, power components or connected loads are exposed to more damaging energy.
Current Passes Through
The fuse remains conductive while current stays within the conditions allowed by its rating and time-current characteristic.
The Element Heats
Higher current produces more heating. The opening time depends on current magnitude, duration and fuse construction.
The Element Opens
When the specified operating condition is reached, the fuse element melts or separates and interrupts the circuit.
The Fault Still Requires Diagnosis
A blown fuse indicates an overcurrent event, but it does not identify or remove the underlying circuit problem.
The fuse must be selected so it can tolerate normal startup and operating current while opening safely during abnormal overload or short-circuit conditions.
Radial Leaded Fuse vs Other Fuse Types
Radial leaded, axial leaded, SMD and cartridge fuses can all provide overcurrent protection, but their mounting methods and replacement characteristics are different.
| Fuse Type | Lead or Mounting Style | PCB Installation | Typical Advantage |
|---|---|---|---|
| Radial leaded fuse | Two leads on the same side | Through-hole soldering | Compact package and stable PCB mounting |
| Axial leaded fuse | One lead at each end | Horizontal or vertical through-hole mounting | Flexible lead forming and board layout |
| SMD fuse | Metalized end terminations | Surface mounting | Suitable for automated SMT assembly |
| Cartridge fuse | Cylindrical body with end caps | Holder, clips or optional leads | Broad size range and easier field replacement |
| Resettable PPTC | Radial or surface-mount package | Through-hole or surface mounting | Can recover after the fault and power are removed |
A radial leaded fuse is often chosen when the circuit needs a small one-time fuse that can be installed directly on the PCB without a separate holder.
Common Radial Leaded Fuse Body Styles
Radial lead arrangement can be used with several body shapes. The package affects board space, installation height, insulation and mechanical fit, but body appearance alone does not determine the electrical rating.
Square Miniature Fuse
A compact square or rectangular insulated body with two radial leads. This is the main style covered by the Blue Light 8ET, 8ED, 6EF and 6ET series.
Round Radial Fuse
A cylindrical or rounded body can also use two leads from the same side. Dimensions and PCB footprints vary by series.
Micro or PICO-Style Fuse
Very small leaded fuses may use round or encapsulated bodies. They require their own dimensional and electrical comparison.
Fast-Acting vs Time-Lag Radial Leaded Fuses
The response type determines how the fuse behaves under temporary overcurrent and sustained fault conditions. The correct choice depends on the circuit current profile, not only on the fuse amp rating.
| Response Type | Main Characteristic | Typical Circuit Condition | Selection Risk |
|---|---|---|---|
| Fast-acting | Opens relatively quickly when current exceeds its allowed time-current range | Circuits with limited normal startup or charging current | May nuisance-open if normal inrush is underestimated |
| Time-lag | Tolerates a specified short-duration inrush before opening | Power supplies, capacitive inputs, transformers or small motors | May provide unsuitable protection if selected only to prevent nuisance opening |
- Fast-acting does not mean the fuse opens instantly under every overload.
- Time-lag does not mean the fuse is less protective when correctly matched to the circuit.
- Two fuses with the same current rating can have very different opening times.
- Compare manufacturer time-current curves and inrush data before substitution.
- Verify performance under startup, steady-state and abnormal conditions.
Important Radial Fuse Ratings
Current rating is only one part of fuse selection. A radial fuse should also match circuit voltage, available fault current, operating characteristic, temperature and PCB requirements.
| Parameter | Why It Matters | Common Selection Error |
|---|---|---|
| Rated current | Defines the current class of the fuse under specified test conditions. | Choosing the same value as normal load current without derating or inrush review. |
| Rated voltage | Defines the maximum circuit voltage at which the fuse is intended to interrupt safely. | Assuming voltage rating is irrelevant because the current rating matches. |
| Breaking capacity | Defines the maximum prospective fault current the fuse can interrupt safely under stated conditions. | Selecting only by amperage and ignoring available short-circuit current. |
| Time-current characteristic | Shows how opening time changes with overcurrent magnitude. | Treating all fast-acting or all time-lag products as identical. |
| I²t | Describes energy let-through during pre-arcing and interruption. | Ignoring energy limits of downstream components. |
| Cold resistance | Affects voltage drop, power loss and low-voltage circuit performance. | Ignoring fuse resistance in low-voltage or precision circuits. |
| Operating temperature | Ambient heat can change current-carrying and opening behavior. | Using room-temperature assumptions next to hot components. |
| Safety approvals | May be required for equipment certification and target markets. | Replacing with an electrically similar part that lacks required approvals. |
Radial Fuse Dimensions and PCB Mounting
Electrical compatibility does not guarantee mechanical compatibility. The body must fit the available space, and the leads must align with the existing PCB footprint.
Length, Width and Height
Check the complete package dimensions and available clearance above and around the PCB.
Lead Spacing
Measure the center-to-center distance between leads. A small difference can prevent installation or stress the body.
Lead Diameter and Hole Size
The PCB hole must accept the lead while still supporting reliable soldering and mechanical retention.
Soldering Conditions
Excessive soldering temperature, duration or lead stress can damage the fuse or change its reliability.
Blue Light square miniature radial fuses currently include body sizes of approximately 8.5 × 5.0 × 4.0 mm and 8.5 × 8.0 × 4.0 mm. These dimensions alone are not enough for substitution. Always confirm lead spacing, lead diameter, recommended PCB hole size and the product drawing.
- Confirm the exact lead spacing before PCB production or replacement.
- Check lead diameter and recommended PCB hole diameter.
- Avoid sharp lead bending close to the fuse body.
- Follow specified soldering temperature and duration limits.
- Keep the fuse away from unnecessary external heat sources.
- Confirm whether the selected series permits the intended board-cleaning process.
How to Choose a Radial Leaded Fuse
Use the following sequence to narrow the selection. Final verification should be based on the manufacturer data sheet and testing in the actual circuit.
- Confirm whether the circuit is AC or DC and determine its maximum operating voltage.
- Measure normal steady-state current in the protected branch.
- Record startup, capacitor-charging, transformer, motor or lamp inrush current.
- Choose fast-acting or time-lag response according to the current profile.
- Check the expected short-circuit current and required breaking capacity.
- Compare the time-current curve and I²t with circuit protection requirements.
- Apply the manufacturer’s load and ambient-temperature derating guidance.
- Match body size, lead spacing, lead diameter and installation height.
- Confirm required safety approvals and environmental specifications.
- Test startup, continuous operation, overload and expected fault conditions.
Start with the Circuit, Not the Package Color
Body color and general shape are not reliable electrical identifiers. Read the original marking, product drawing and data sheet whenever possible.
Separate Normal Inrush from a Real Fault
A short, expected startup surge may require a time-lag fuse. A sustained overload or downstream short circuit must be corrected rather than masked with a larger or slower fuse.
Verify Both Electrical and Mechanical Fit
A replacement should match current, voltage, response type, breaking capacity and approvals while also fitting the existing PCB holes and available space.
Common Applications of Radial Leaded Fuses
Radial leaded fuses may be considered for many PCB-level protection tasks when the selected series meets the circuit voltage, current, fault interruption and environmental requirements.
Power Supplies and Adapters
Input or internal branch protection where compact through-hole installation is required.
Battery Chargers
Protection for charging circuits, subject to normal charging surge and fault-current analysis.
Household Appliance Boards
PCB protection in control modules, power sections and auxiliary circuits.
LED Lighting Equipment
Compact protection for drivers and control boards when inrush behavior is properly evaluated.
Industrial Control Boards
Branch and board-level protection in compact electronic control equipment.
Consumer Electronics
Space-limited devices requiring directly soldered, non-resettable overcurrent protection.
Application names alone are not enough for selection. Two power supplies or appliances can have very different startup current, fault current and certification requirements.
Radial Leaded Fuse Replacement and Troubleshooting
Before replacing a radial fuse, record all available information from the original part and circuit. Do not choose a replacement only because the new part fits the PCB holes.
| Check Before Replacement | Why It Matters |
|---|---|
| Rated current and voltage | The replacement must match the circuit’s normal current and interruption voltage requirements. |
| Fast-acting or time-lag response | A different response can cause nuisance opening or unsuitable fault protection. |
| Breaking capacity | The fuse must interrupt the available fault current safely. |
| Body size and lead geometry | The part must fit without forced lead bending or body stress. |
| Approvals and temperature range | Equipment compliance and environmental performance may depend on them. |
If the New Fuse Blows Immediately
Possible causes include a downstream short circuit, failed semiconductor, damaged power component, incorrect fuse response, excessive inrush or a replacement with an unsuitable rating.
If the Fuse Blows After Running for a While
Check sustained overload, rising ambient temperature, cooling problems, intermittent load faults and whether the selected current rating was properly derated.
Do Not Short-Circuit the Fuse Position
Bypassing a fuse removes an intended safety function and can expose PCB traces, wiring and components to damaging fault energy.
Explore Our Radial Leaded Fuse Series
Blue Light currently offers four square miniature radial leaded fuse series covering fast-acting and time-lag characteristics in two compact body sizes.
8ET Series
Compact time-lag square miniature fuse for through-hole PCB protection where specified short-duration inrush must be tolerated.
8ED Series
Compact time-lag PCB fuse series in the smaller square miniature body format.
6EF Series
Fast-acting square miniature fuse for circuits that require a quicker response to excessive current.
6ET Series
Time-lag square miniature fuse for circuits with approved startup or charging inrush conditions.
Radial Leaded Fuse Guides
These L4 guides will expand the main topics below. Keep them as plain text until each page is published.
Related Fuse Guides
Frequently Asked Questions
What is a radial leaded fuse?
It is a compact overcurrent protection device with two leads extending from the same side of the body. The leads are inserted into PCB holes and soldered in place.
Is a radial leaded fuse the same as a subminiature fuse?
Not exactly. Subminiature fuse describes a broad size category, while radial leaded describes the lead arrangement and through-hole mounting style. Many radial leaded fuses are also subminiature fuses.
Is a radial leaded fuse the same as a resettable fuse?
No. Some resettable PPTC devices have radial leads, but a conventional radial leaded fuse opens permanently and normally must be replaced.
What is the difference between radial and axial fuses?
A radial fuse has both leads on the same side of the body. An axial fuse has one lead extending from each end.
Can a fast-acting radial fuse replace a time-lag fuse?
Not based on current rating alone. The circuit inrush, time-current curves, breaking capacity and protection requirements must be compared first.
How do I choose the current rating?
Start with the actual continuous current in the protected branch, then consider inrush, ambient temperature, manufacturer derating guidance and the required opening behavior.
Are two radial fuses with the same amp rating interchangeable?
Not necessarily. They may differ in voltage rating, response type, breaking capacity, I²t, resistance, dimensions, lead spacing or approvals.
Why does a new radial fuse blow immediately?
The circuit may have a shorted component, excessive startup current, the wrong fuse response type or another unresolved fault. Do not repeatedly replace the fuse without investigating the cause.
Need Help Selecting a Radial Leaded Fuse?
Prepare the circuit voltage, normal current, startup current, expected fault current, required response type, body dimensions, lead spacing and approval requirements. Blue Light can then help narrow the suitable square miniature fuse series.
Contact Blue Light
