UL 248-13 is the definitive North American safety standard for semiconductor fuses, also known as aR class fuses. These specialized fuses provide ultra-fast short-circuit protection for sensitive power semiconductor devices such as IGBTs, diodes, thyristors, and MOSFETs.
For design engineers, procurement managers, and quality assurance professionals working with power electronics, understanding UL 248-13 is essential for selecting the right protective components and ensuring product compliance.
In this comprehensive guide, we cover everything you need to know about UL 248-13, including its scope, key requirements, testing procedures, rating systems, and practical selection guidelines.
What is UL 248-13?
Overview
UL 248-13 is Part 13 of the UL 248 series of standards for Low-Voltage Fuses. It specifically addresses semiconductor fuses — high-speed fuses designed to protect solid-state devices from damage due to overcurrent conditions.
- Standard number: UL 248-13
- Full title: Low-Voltage Fuses — Part 13: Semiconductor Fuses (aR Class)
- Scope: Fuses with rated voltages up to 1500V DC / 1000V AC
- Fuse class: aR (short-circuit protection only)
- Region: North America (United States and Canada)
Relationship to Other Standards
UL 248-13 is part of a larger family of fuse standards:
| Standard | Part | Covers |
| UL 248-1 | Part 1 | General requirements |
| UL 248-2 | Part 2 | Class H and K fuses |
| UL 248-3 | Part 3 | Class R fuses |
| UL 248-4 | Part 4 | Class J fuses |
| UL 248-5 | Part 5 | Class T fuses |
| UL 248-8 | Part 8 | Class G fuses |
| UL 248-13 | Part 13 | Semiconductor fuses (aR) |
| UL 248-14 | Part 14 | Fuse holders |
| UL 248-19 | Part 19 | Photovoltaic fuses (gPV) |
International Equivalent
The international counterpart to UL 248-13 is IEC 60269-4, which covers semiconductor fuses under the IEC system. While similar in scope, there are important differences in testing methods and rating conventions.
Scope and Application
What UL 248-13 Covers
UL 248-13 applies to fuses that:
- Are rated for voltages up to 1500V DC or 1000V AC
- Provide short-circuit protection only (aR class)
- Are designed to protect semiconductor devices
- Have rated currents typically ranging from 10A to 2000A
- Feature extremely fast-acting characteristics
Typical Applications
Semiconductor fuses certified to UL 248-13 are used in a wide range of power electronics applications:
- Variable frequency drives (VFDs)
- Uninterruptible power supplies (UPS)
- Solar inverters (PV inverters)
- Battery energy storage systems (BESS)
- EV charging stations
- Industrial power supplies
- Motor drives and soft starters
- Rectifiers and converters
- Welding equipment
What UL 248-13 Does NOT Cover
- General-purpose fuses (covered by other UL 248 parts)
- Full-range protection fuses (gG, gL classes)
- High-voltage fuses above 1500V
- Automotive fuses (covered by SAE standards)
- Photovoltaic fuses (covered by UL 248-19)
Key Concepts and Definitions
aR Class Explained
The “aR” designation stands for “a” = partial range and “R” = semiconductor protection.
This means:
- a (partial range): The fuse provides protection only against short-circuit conditions, not against overloads
- R (rectifier/semiconductor): Designed specifically for protecting rectifiers and semiconductor devices
Important: aR fuses are NOT intended for overload protection. They protect only against short-circuit faults. For full-range protection, you need a gG or gR class fuse.
I²t Rating (Let-Through Energy)
The I²t rating (pronounced “eye-squared-tee”) is one of the most important parameters for semiconductor fuses. It represents the total energy the fuse lets through before clearing a fault.
Formula: I²t = I² × t
Where:
- I = fault current (in amperes)
- t = time to clear the fault (in seconds)
Why it matters: Semiconductor devices have their own I²t withstand rating. The fuse’s I²t must be LOWER than the device’s I²t withstand rating to provide effective protection.
Breaking Capacity
The breaking capacity (or interrupting rating) is the maximum short-circuit current the fuse can safely interrupt without causing damage or hazard.
Typical breaking capacities for UL 248-13 fuses:
- 50 kA
- 100 kA
- 200 kA
Voltage Rating
UL 248-13 fuses are rated for both AC and DC voltages. Key points:
- AC voltage rating: Typically up to 1000V AC
- DC voltage rating: Typically up to 1500V DC
- DC voltage is more challenging because DC arcs are harder to extinguish than AC arcs
- Always select a fuse with a voltage rating at least equal to the system voltage
Current Rating
The rated current (In) is the maximum current the fuse can carry continuously without exceeding temperature rise limits.
Important notes:
- The rated current is defined at a specific ambient temperature (usually 25°C or 30°C)
- At higher ambient temperatures, the current carrying capacity decreases (derating)
- UL 248-13 specifies maximum temperature rise limits for various components
UL 248-13 Key Requirements
1. Construction Requirements
UL 248-13 specifies detailed construction requirements for semiconductor fuses:
Fuse Elements:
- Must be made of materials with consistent and predictable melting characteristics
- Typically silver or copper with specialized designs
- Must provide consistent performance across the entire current range
Fuse Body:
- Must be made of insulating material with sufficient mechanical strength
- Must contain the arc during interruption
- Must withstand the pressures generated during fault clearing
- Common materials: ceramic, melamine glass fiber, high-temperature plastics
End Caps/Contacts:
- Must provide low-resistance electrical connections
- Must have sufficient mechanical strength
- Must resist corrosion
- Common materials: copper with tin or silver plating
2. Performance Requirements
UL 248-13 defines several critical performance tests:
Temperature Rise Test
- Fuse is operated at rated current in a specified ambient temperature
- Temperature rise of various components must not exceed specified limits
- Typical limits:
- Fuse element: varies by design
- End caps/contact surfaces: ~65-85°C rise above ambient
- Fuse body surface: varies by size and rating
Time-Current Characteristic Verification
- Fuse clearing time is verified at multiple current levels
- Must fall within specified tolerance bands
- Verifies consistent and predictable operation
Breaking Capacity Test
- Fuse is subjected to short-circuit currents at various levels up to the rated breaking capacity
- Must clear the fault safely without:
- Rupture of the fuse body
- External flashover
- Fire hazard
- Excessive peak let-through current
I²t Verification
- Total let-through energy (I²t) is measured during short-circuit tests
- Must not exceed published values
- Critical for coordination with semiconductor devices
Endurance Test
- Fuse is subjected to repeated cycles of current
- Verifies long-term reliability and stability
- Ensures characteristics don’t drift over time
3. Marking Requirements
UL 248-13 requires specific marking on each fuse:
- Manufacturer’s name or trademark
- Fuse catalog number or part number
- Rated current (amperes)
- Rated voltage (volts AC and/or DC)
- Fuse class (aR)
- UL recognition mark
- Interrupting rating
- Polarity markings (if polarity-sensitive)
- Date code or lot number
Testing Procedures
Type Testing vs. Production Testing
UL 248-13 distinguishes between:
Type Testing:
- Performed on representative samples to verify compliance with the standard
- Done once during product development and certification
- Includes all performance tests
- Expensive and time-consuming
Production Testing:
- Performed on every unit or on a sampling basis during manufacturing
- Verifies basic integrity and performance
- Typically includes visual inspection, continuity checks, and limited electrical tests
Key Test Procedures
1. Pre-conditioning
- Samples are stabilized at standard ambient conditions
- Initial resistance measurements are taken
- Visual and dimensional inspections are performed
2. Temperature Rise Test
- Fuse is mounted in a standardized test fixture
- Rated current is applied until thermal equilibrium is reached
- Temperatures are measured at multiple points
- Results are compared against standard limits
3. Low-Current Overload Tests
- Tests at currents just above rated current (e.g., 1.5× In, 2× In)
- Verifies the fuse does NOT operate at these levels (since aR is short-circuit only)
- Confirms the “a” (partial range) characteristic
4. Medium-Current Tests
- Tests at currents where the fuse begins to operate
- Verifies time-current characteristics in the transition region
- Typically 3× to 10× rated current
5. High-Current Short-Circuit Tests
- Tests at maximum rated breaking capacity
- Verifies safe interruption of high fault currents
- Measures peak let-through current and I²t
- Most critical test for semiconductor protection fuses
6. Endurance and Stability Tests
- Repeated cycling at rated current
- Verifies long-term stability of fuse characteristics
- Ensures the fuse doesn’t degrade over time
How to Read UL 248-13 Fuse Ratings
Typical Fuse Label Explained
Let’s look at a typical UL 248-13 semiconductor fuse label:
EssFuse Part No: GFEVT-74ES 630A 1000V AC / 1500V DC aR Semiconductor Fuse UL Recognized Interrupting Rating: 100kA I²t: 12,500 A²s (at 100kA)
Breaking it down:
| Marking | Meaning |
| EssFuse | Manufacturer |
| GFEVT-74ES | Part number / catalog number |
| 630A | Rated continuous current |
| 1000V AC | AC voltage rating |
| 1500V DC | DC voltage rating |
| aR | Fuse class (short-circuit protection only) |
| Semiconductor Fuse | Application type |
| UL Recognized | Certification mark |
| 100kA | Interrupting rating |
| 12,500 A²s | Total I²t at rated breaking capacity |
Understanding Time-Current Curves
Every UL 248-13 fuse comes with a time-current characteristic curve (TCC curve). This graph shows how long the fuse takes to operate at different current levels.
How to read a TCC curve:
- X-axis: Current (in amperes), usually logarithmic scale
- Y-axis: Time to clear (in seconds), usually logarithmic scale
- Curve: Shows the relationship between current and clearing time
Key points on the curve:
- Minimum melting time: The point where the fuse element just begins to melt
- Total clearing time: Melting time + arcing time
- Tolerance band: The range within which the fuse is guaranteed to operate
Selection Guide: How to Choose the Right UL 248-13 Fuse
Step 1: Determine the Application Requirements
Before selecting a fuse, gather the following information:
- System voltage (AC or DC, nominal value)
- Normal operating current
- Maximum available short-circuit current
- Type of semiconductor device being protected (IGBT, diode, thyristor, etc.)
- Device I²t withstand rating
- Ambient temperature range
- Mounting style (bolt-in, blade, PCB mount, etc.)
- Space constraints
- Certification requirements (UL, CSA, etc.)
Step 2: Select the Voltage Rating
Rule: The fuse voltage rating must be ≥ the system voltage.
- For AC systems: Use the AC voltage rating
- For DC systems: Use the DC voltage rating
- For systems with both: Both ratings must be sufficient
Important: Never use a fuse with a voltage rating lower than the system voltage. This can result in failure to clear the fault safely.
Step 3: Select the Current Rating
The current rating should be based on:
- Normal operating current: The fuse must carry this current continuously without overheating
- Ambient temperature: Higher temperatures require derating
- Pulse/Inrush currents: The fuse must withstand normal inrush currents without nuisance operation
- Coordination with upstream/downstream devices
General guideline:
- Fuse rated current = Normal operating current × Safety factor
- Typical safety factor: 1.25 to 1.5 (depending on conditions)
Temperature derating example:
- At 25°C: 100% of rated current
- At 40°C: ~90-95% of rated current
- At 60°C: ~80-85% of rated current
- At 80°C: ~70-75% of rated current
(Exact derating values vary by manufacturer — consult the datasheet)
Step 4: Verify I²t Coordination
This is the most critical step for semiconductor protection:
- Find the semiconductor device’s I²t withstand rating from its datasheet
- Find the fuse’s total I²t at the expected fault current level
- Ensure: Fuse I²t < Device I²t withstand
Safety margin: A good rule of thumb is to have at least a 20-30% margin between the fuse I²t and the device withstand rating.
Step 5: Verify Breaking Capacity
The fuse’s breaking capacity must be greater than or equal to the maximum available short-circuit current at the point of installation.
How to determine available fault current:
- Calculate based on system impedance
- Use arc flash study results
- Consult with a power systems engineer
- If unknown, select the highest breaking capacity available
Step 6: Consider Physical and Mechanical Factors
- Mounting style: Bolt-in, blade terminal, PCB mount, etc.
- Size: Ensure the fuse fits in the available space
- Fuse holder compatibility: Use a fuse holder rated for the same current and voltage
- Accessibility: For replacement and maintenance
- Vibration and shock resistance: Important for mobile or industrial applications
Common Mistakes to Avoid
1. Using aR Fuses for Overload Protection
Mistake: Using an aR fuse where overload protection is needed.
Why it’s wrong: aR fuses are designed for short-circuit protection ONLY. They will NOT protect against moderate overloads.
Solution: If you need both overload and short-circuit protection, use a gR class fuse (full-range semiconductor fuse) or combine an aR fuse with an overload protection device (like a circuit breaker).
2. Ignoring Temperature Derating
Mistake: Selecting a fuse based solely on rated current without considering ambient temperature.
Why it’s wrong: At higher temperatures, the fuse’s current carrying capacity decreases. This can lead to nuisance operation or accelerated aging.
Solution: Always apply appropriate temperature derating based on the actual operating environment.
3. Poor I²t Coordination
Mistake: Selecting a fuse without checking I²t coordination with the protected device.
Why it’s wrong: If the fuse’s I²t is too high, the semiconductor device may be damaged before the fuse clears the fault.
Solution: Always verify that the fuse’s total I²t is less than the device’s I²t withstand rating, with an appropriate safety margin.
4. Using the Wrong Voltage Rating
Mistake: Using an AC-rated fuse in a DC application, or using a fuse with insufficient voltage rating.
Why it’s wrong: DC arcs are much harder to extinguish than AC arcs. A fuse designed for AC may fail to interrupt a DC fault safely.
Solution: Always use a fuse with the appropriate DC voltage rating for DC applications.
5. Ignoring Peak Let-Through Current
Mistake: Only looking at I²t and ignoring the peak let-through current.
Why it’s wrong: Even if the total energy (I²t) is acceptable, the peak current can still damage semiconductor devices due to di/dt effects and magnetic forces.
Solution: Check both peak let-through current AND I²t when coordinating with semiconductor devices.
UL 248-13 vs. IEC 60269-4: Key Differences
While both standards cover semiconductor fuses, there are important differences:
| Aspect | UL 248-13 | IEC 60269-4 |
| Region | North America | Europe / International |
| Voltage classes | Different categorization | Different categorization |
| Test methods | Some differences in procedure | Some differences in procedure |
| Marking requirements | UL specific | IEC specific |
| Size formats | Inch-based common | Metric-based common |
| Certification marks | UL, CSA | CE, VDE, etc. |
Important: Fuses certified to one standard are not automatically compliant with the other. If you need both UL and IEC certification, look for fuses that are dual-certified.
Frequently Asked Questions (FAQ)
Q1: What does “aR” mean on a fuse?
A: “aR” stands for “partial range semiconductor protection.” The “a” indicates the fuse provides only short-circuit protection (not overload protection), and “R” indicates it’s designed for protecting rectifiers and semiconductor devices.
Q2: Can I use an aR fuse for overload protection?
A: No. aR fuses are designed ONLY for short-circuit protection. They will not provide reliable protection against moderate overload conditions. For full-range protection, use a gR class fuse or combine an aR fuse with an overload protection device.
Q3: What is I²t and why is it important?
A: I²t (eye-squared-tee) is a measure of the thermal energy let through by the fuse during a fault. It’s critical for semiconductor protection because the fuse’s I²t must be lower than the protected device’s I²t withstand rating to prevent damage.
Q4: How do I know if a fuse is UL 248-13 certified?
A: Look for the UL recognition mark on the fuse or its packaging. The marking should include “aR” class designation and the UL mark. You can also verify certification in the UL Online Certifications Directory.
Q5: Can I replace a UL 248-13 fuse with a different brand?
A: Yes, as long as the replacement fuse has the same or better ratings: same current rating, same or higher voltage rating, same or higher breaking capacity, same or lower I²t, and appropriate certifications. Always verify compatibility before replacement.
Q6: What’s the difference between UL 248-13 and UL 248-19?
A: UL 248-13 covers semiconductor fuses (aR class) for general power electronics applications. UL 248-19 covers photovoltaic fuses (gPV class) specifically designed for solar PV systems. While both are DC-rated, they have different performance characteristics optimized for their respective applications.
Q7: How often should UL 248-13 fuses be replaced?
A: Under normal operating conditions, semiconductor fuses can last for many years. They should be replaced if:
- They have operated (blown) due to a fault
- They show signs of damage or discoloration
- They are past their expiration date (if specified)
- After a major fault event, even if they appear intact (some degradation may have occurred)
Q8: Are UL 248-13 fuses polarity-sensitive?
A: Some DC-rated semiconductor fuses are polarity-sensitive, meaning they are designed to interrupt current flowing in one direction more effectively. Always check the manufacturer’s datasheet and follow polarity markings if present. For bidirectional applications, use fuses specifically designed for that purpose.
Conclusion
UL 248-13 is the gold standard for semiconductor fuse safety in North America. Understanding its requirements and specifications is essential for selecting the right fuse for your power electronics application.
Key takeaways:
- aR class = short-circuit protection only, not overload protection
- I²t coordination is the most critical factor for semiconductor protection
- Always verify voltage rating, current rating, and breaking capacity
- Temperature derating is important for reliable operation
- Look for UL certification to ensure compliance and safety
About EssFuse Semiconductor Fuses
EssFuse manufactures a complete line of UL 248-13 certified semiconductor fuses for demanding power electronics applications. Our aR class fuses feature:
- UL 248-13 recognition (North American safety certification)
- Voltage ratings up to 1500V DC / 1000V AC
- Current ratings from 10A to 2000A
- Breaking capacities up to 200kA
- Low I²t design for optimal semiconductor protection
- High cycling capability for long service life
- Multiple form factors to fit various applications
For more information about our UL 248-13 certified semiconductor fuses, or to request a quote for your specific application, contact our engineering team today.
gpv fuse https://essfuse.com/product-category/photovoltaic-fuse/
semiconductor fuses https://essfuse.com/product-category/north-american-semiconductor-fuse/
automotive fuse https://essfuse.com/product-category/automobile-fuse/