直流ヒューズ直流システムにおける過電流保護のテクニカルガイド

Introduction: The Critical Difference Between AC and DCヒューズ

DC fuses represent a fundamentally different technology than their AC counterparts due to the unique challenge of interrupting direct current arcs. While AC current naturally crosses zero 120 times per second (at 60Hz), providing natural arc extinction points, DC current maintains constant voltage and must be forcibly interrupted through specialized fuse design.

This technical guide explores DC fuse construction, I²t characteristics, voltage ratings, and application-specific selection for solar photovoltaic systems, battery storage, electric vehicles, telecommunications, and industrial DC equipment.

Why DC Requires Specialized Fuses

The Arc Extinction Challenge:

When a fuse opens under load, an electrical arc forms between the separating conductors. This arc is essentially a plasma channel conducting current through ionized air.

AC Arc Behavior:

AC voltage/current waveform crosses zero 120 times/second
At zero crossing: No voltage = no energy to sustain arc
Arc naturally extinguishes every 8.3ms
Fuse element cools, preventing re-ignition

DC Arc Behavior:

DC voltage/current remains constant
No natural zero crossing
Arc sustained indefinitely by constant energy supply
Temperatures reach 3000-5000°C
Arc plasma maintains conductivity
Only mechanical/chemical arc suppression works

DC Fuse Design Requirements:

To interrupt DC arcs, fuses employ:

1. Arc chutes: Ceramic plates that divide arc into smaller segments
2. Silica sand filling: Absorbs arc energy, increases arc voltage
3. Longer fuse bodies: Greater separation distance for arc extinction
4. Multiple constriction points: Create multiple arcs in series (higher voltage drop)
5. Ceramic bodies: Withstand extreme temperatures without melting

Consequence of Using AC Fuse on DC:

AC fuse on DC circuit (DON'T DO THIS):
1. Overload condition occurs
2. Fuse element melts (correct operation)
3. Arc forms between molten ends
4. AC fuse expects natural zero crossing to extinguish arc
5. DC has no zero crossing
6. Arc continues indefinitely
7. Fuse body overheats and ruptures
8. Molten material ejected → FIRE HAZARD
9. Arc may weld fuse terminals together → NO PROTECTION

Result: Catastrophic failure, potential fire, equipment damage

DC Fuse Construction and Technology

Fuse Element Design

Single Element vs. Multi-Element:

Single Element (Fast-Acting):

Construction:
- Single wire or ribbon
- Uniform cross-section
- No mass concentration points
- Direct current path

Characteristics: – Very fast response (<10ms at high overcurrent) – Minimal time delay at low overcurrent – Precise I²t rating – Used for semiconductor protection

Applications: – Solar PV string protection (gPV fuses) – Battery disconnect (when fast trip required) – DC-DC converter protection

Multi-Element (Time-Delay):

Construction:
- Multiple parallel elements
- Mass concentrations at specific points
- Heat sinks attached to elements
- Solder bonds or spring-loaded mechanisms

Characteristics: – Slow response to moderate overload (minutes) – Fast response to severe overcurrent (milliseconds) – Tolerates inrush currents – Used for motor and capacitor loads

Applications: – DC motor protection (high inrush tolerance) – Capacitor charging circuits – Battery systems with surge current

Arc Quenching Technology

Silica Sand Filling (Most Common):

Material: High-purity quartz sand (SiO₂)
Particle size: 40-100 mesh
Filling ratio: 80-90% of fuse body volume

Arc Quenching Mechanism: 1. Fuse element melts, arc initiates 2. Arc heat melts surrounding sand into glass (fulgurite) 3. Glass formation absorbs energy (endothermic reaction) 4. Arc voltage increases (resistance of glass > air plasma) 5. Arc current decreases 6. When arc current < sustaining current → extinction

Arc Voltage: 20-100V per inch of arc length Total arc voltage: Can exceed system voltage (current-limiting effect)

Ceramic Fiber Filling:

Material: Alumina or zirconia ceramic fibers
Application: High-voltage DC fuses (>1000V)

Advantages over sand: – Lower weight (important for vibration environments) – Better high-temperature performance – Faster arc extinction at high voltages

Disadvantages: – Higher cost – More complex manufacturing

Vacuum Fuses (Specialty Applications):

Construction: Fuse element in evacuated glass tube
Pressure: <10⁻⁴ torr

Arc Extinction: – No air = no arc plasma medium – Arc extinguishes immediately when element parts – No arc voltage generated

Applications: – High-voltage DC transmission (HVDC) – Railway electrification (1500-3000V DC) – Not common in residential solar (<600V)

Limitations: – Very expensive ($200-1000 per fuse) – Fragile glass construction – Must maintain vacuum seal (limited lifespan)

I²t Rating and Fuse Coordination

Understanding I²t (Ampere-Squared Seconds)

Definition:
I²t represents the thermal energy that passes through a fuse before it clears a fault.

Formula:

I²t = ∫ i²(t) dt

Where: i(t) = instantaneous current as function of time Integration period = from fault initiation to final arc extinction

Physical meaning: – Energy dissipated in fuse element – Proportional to temperature rise – Determines fuse damage and let-through energy

Melting I²t vs. Clearing I²t:

Melting I²t (I²t_m):
- Energy required to melt fuse element
- Does NOT include arcing time
- Element physically melted, but circuit not yet opened

Clearing I²t (I²t_c): – Total energy from fault start to final arc extinction – Includes melting time + arcing time – Circuit fully interrupted, safe state achieved

Typical relationship: I²t_c = 1.2 to 2.0 × I²t_m (Arcing time adds 20-100% more energy)

Why I²t Matters for Coordination:

Fuse-Fuse Coordination Example:

Upstream fuse (main): 100A, I²t_c = 50,000 A²s Downstream fuse (branch): 30A, I²t_c = 5,000 A²s

Fault on branch circuit: – Downstream fuse should clear BEFORE upstream fuse melts – Required: Downstream I²t_c < Upstream I²t_m – Ratio: 5,000 < (50,000 / 1.5) = 33,333 A²s ✓ COORDINATED

If reversed (downstream 100A, upstream 30A): – Both fuses would melt simultaneously – Non-selective operation (entire system trips)

Fuse Selectivity (Discrimination)

Definition: Only the fuse closest to the fault opens, leaving rest of system energized.

Selectivity Ratio Method:

For two fuses in series to be selective:

Ratio = (Upstream fuse rating) / (Downstream fuse rating) ≥ 2:1

Example: Main battery fuse: 200A Branch inverter fuse: 80A Ratio: 200 / 80 = 2.5:1 (SELECTIVE)

Branch load fuse: 30A Sub-branch fuse: 20A Ratio: 30 / 20 = 1.5:1 (MARGINAL – verify I²t curves)

Time-Current Curve Method (Precise):

Procedure:
1. Obtain time-current curves for both fuses
2. Plot on log-log graph (current vs. time)
3. Verify vertical separation ≥ factor of 2 at all current levels
4. If curves cross: Non-selective at that current range

Solar PV Example: String fuse: 15A gPV (downstream) Combiner fuse: 60A gPV (upstream)

At 100A fault: – String fuse clears in 0.1 seconds – Combiner fuse clears in 5 seconds – Separation: 50× (highly selective)

Voltage Ratings and DC Interrupt Capacity

DC Voltage Rating vs. AC Voltage Rating

Why DC Voltage Ratings Are Lower:

Same fuse model:
- AC rating: 250V AC
- DC rating: 125V DC

Reason: DC arc more difficult to interrupt

AC has natural zero crossings → easier interruption DC requires forced interruption → needs more arc voltage

Rule of thumb: DC rating ≈ 50% of AC rating for same physical fuse

DC Voltage Rating Selection:

System voltage: 48V nominal (LiFePO4 battery)
Maximum charging voltage: 58.4V
Transient voltage: 65V (inverter startup spike)

Required fuse voltage rating: >65V minimum Select: 80V DC or 125V DC rated fuse

Undersizing consequence: If 32V DC fuse used on 48V system: – Arc voltage insufficient to interrupt – Arc sustains after element melts – Fuse body ruptures – Potential fire and equipment damage

Interrupt Rating (Breaking Capacity)

Definition: Maximum fault current the fuse can safely interrupt.

Common DC Fuse Interrupt Ratings:

Available Fault Current Calculation:

Battery Bank Example:
4× 200Ah LiFePO4 cells in parallel = 800Ah
Internal resistance: 0.005Ω per cell
Parallel resistance: 0.005Ω / 4 = 0.00125Ω
Wire resistance: 0.0005Ω (very short, large gauge)
Total circuit resistance: 0.00175Ω

Battery voltage: 51.2V (nominal) Fault current: 51.2V / 0.00175Ω = 29,257A

Required interrupt rating: >30,000A Select: Class T fuse (200kA interrupt) or gPV fuse (30kA) Inadequate: ANL fuse (10kA) – may rupture

Consequence of Insufficient Interrupt Rating:

Scenario: 10kA interrupt fuse on 30kA fault current circuit

Fault occurs: 1. Fuse element melts (correct operation) 2. Arc current = 30kA (exceeds fuse design) 3. Fuse body cannot contain arc pressure 4. Fuse ruptures violently 5. Molten material and plasma ejected 6. Secondary arc to ground or adjacent conductors 7. Fire, equipment damage, shock hazard

Prevention: Calculate available fault current, select fuse with adequate interrupt rating

Fuse Types for Specific DC Applications

Solar PV Fuses (gPV Rating)

What “gPV” Means:

g = Full-range breaking capacity (German: ganzbereichsschutz)
PV = Photovoltaic application

gPV rating indicates: – Tested for DC photovoltaic systems – Can interrupt reverse current (backfeed from battery) – Rated for high ambient temperature (70°C typical) – UV-resistant for outdoor mounting – Complies with IEC 60269-6 standard

Why Solar Requires Specialized Fuses:

Solar Array Characteristics:
1. High short-circuit current (Isc)
   - Modern panels: 10-12A Isc per panel
   - 10 panels in parallel: 120A short-circuit current

2. Reverse current capability – Battery can backfeed into faulted string – Fuse must interrupt reverse DC current

3. High ambient temperature – Rooftop installations: 70°C+ ambient – Standard fuses derate 20-30% at high temp – gPV fuses rated for 70°C ambient

4. Long service life required – 25-year system life expectancy – UV exposure degrades plastics – gPV fuses designed for longevity

gPV Fuse Sizing for Solar Strings:

NEC 690.9(B) Requirement:
Fuse rating ≥ 1.56 × String Isc

Example: Solar String Panels: 8× 400W, Isc = 10.5A each String Isc: 10.5A (series connection) Required fuse: 10.5A × 1.56 = 16.4A Select: 20A gPV fuse (next standard size)

Why 1.56× factor: – 1.25× for irradiance variation – 1.25× for temperature effects – Combined: 1.25 × 1.25 = 1.56×

Battery System Fuses

Main Disconnect Fuse:

Application: Between battery and busbar/inverter

Requirements: – Very high interrupt rating (battery = massive fault current) – Current-limiting preferred (protects downstream equipment) – Fast-acting to protect battery from internal faults

Recommended Types: 1. Class T fuses (best – 200kA interrupt, current-limiting) 2. MEGA fuses (good – 10kA interrupt, marine-grade) 3. ANL fuses (adequate for small systems – 10kA interrupt)

Sizing Example: Battery: 48V, 200Ah LiFePO4 Inverter: 5000W continuous Max current: 5000W / 42V (low voltage cutoff) = 119A Fuse: 119A × 1.25 = 149A → Select 150A or 175A Class T

Battery Management System (BMS) Integration:

Some BMS systems control fuse operation:

Active BMS with Contactor: – Mechanical contactor opens under fault – Fuse is backup protection only – Fuse sized for worst-case if contactor fails – Typical: Fuse = 2× normal operating current

Passive BMS (Monitoring Only): – Fuse is primary protection – BMS monitors but doesn’t interrupt – Fuse must handle all fault conditions – Typical: Fuse = 1.25× maximum current + margin

Industrial DC Equipment

DC Motor Fuses:

Motor Characteristics:
- High startup inrush (3-5× running current)
- Locked rotor current (6-8× running current)
- Requires time-delay fuse to avoid nuisance blowing

Motor Rating: 5HP at 250V DC Running current: 16A Locked rotor: 16A × 7 = 112A

Fuse Selection: – Standard fast-acting 20A: Will blow on startup – Time-delay 30A: Tolerates inrush, protects motor – Class CC 30A: Best choice (motor-rated, current-limiting)

Verification: – Check motor manufacturer’s recommendation – Test actual startup current with clamp meter – Confirm fuse doesn’t blow on 10 consecutive starts

DC-DC Converter Protection:

Converter Characteristics:
- Input current varies with output load
- Capacitor charging inrush (brief, high current)
- Electronic switching creates high-frequency noise

Protection Strategy: Input side: Fast-acting fuse (protect converter from supply faults) Output side: Fast-acting fuse (protect load from converter faults)

Example: 48V to 12V, 30A output converter Input current: 30A × 12V / 48V / 0.90 eff = 8.3A Input fuse: 8.3A × 1.5 (inrush margin) = 12.5A → 15A fast-acting Output fuse: 30A × 1.25 = 37.5A → 40A fast-acting

Testing and Verification

Pre-Installation Testing

Continuity Test:

Equipment: Digital multimeter (resistance mode)

Procedure: 1. Set meter to lowest resistance range (200Ω or less) 2. Touch probes to fuse terminals 3. Good fuse: <0.1Ω (essentially zero) 4. Bad fuse: OL (overload – infinite resistance)

Interpretation: – <0.1Ω: Fuse intact, safe to install – 0.1-1.0Ω: Possible corrosion, inspect visually – >1.0Ω or OL: Fuse blown or damaged, discard

Visual Inspection:

Class T / gPV Fuses (Opaque Body):
- Check for cracks in ceramic body
- Verify end caps tight (not loose)
- No discoloration or burn marks
- Manufacturer markings legible

ANL / MEGA Fuses (Transparent or Visible Element): – Element should be continuous (no breaks) – No discoloration of element – No sand leakage (if sand-filled) – Blade terminals not bent or corroded

In-Service Testing

Voltage Drop Test:

Purpose: Verify fuse not degraded, connections tight

Procedure: 1. Measure voltage at fuse input terminal 2. Measure voltage at fuse output terminal (under load) 3. Calculate drop: V_in – V_out

Acceptable: <0.1V at rated current Marginal: 0.1-0.3V (inspect connections) Failed: >0.3V (replace fuse or repair connections)

Example: 30A fuse, 25A load current Input: 51.2V Output: 51.1V Drop: 0.1V (acceptable) Resistance: 0.1V / 25A = 0.004Ω (good)

Thermal Imaging:

Equipment: Infrared camera or thermal gun

Target Temperature Rise: – <20°C above ambient: Excellent – 20-40°C above ambient: Acceptable – 40-60°C above ambient: Marginal (increased aging) – >60°C above ambient: Problem (corrosion, undersized, or near failure)

Hot Spots Indicate: – Corroded terminals – Loose connections – Undersized fuse (continuous overload) – Fuse nearing end of life

Procedure: 1. Operate system at 80% rated current for 30 minutes 2. Scan fuse holder and terminals with thermal camera 3. Compare fuse temperature to adjacent conductors 4. Fuse should be similar or slightly warmer than wire

メンテナンスとトラブルシューティング

Fuse Aging and Degradation

Causes of Fuse Aging:

1. Thermal Cycling:
- Operating near rated current generates heat
- Fuse element expands/contracts with temperature
- Repeated cycling weakens element microstructure
- Eventually fails prematurely (below rated current)

2. Environmental Exposure: – UV radiation degrades plastic holders – Moisture causes corrosion of terminals – Salt air accelerates corrosion (marine environments) – High ambient temperature accelerates aging

3. Repeated Fault Clearing: – Each near-overload event stresses element – Element gradually thins at hot spots – I²t rating decreases over time – Nuisance blowing increases

4. Harmonic Currents: – High-frequency switching (inverters) generates harmonics – Harmonics increase RMS current above DC value – Additional heating accelerates aging – Fuse rated for DC may be inadequate for switching loads

Recommended Replacement Intervals:

Solar gPV Fuses:
- Inspect annually
- Replace every 10 years (preventive)
- Replace immediately if discolored or loose

Battery Fuses: – Inspect quarterly – Replace every 5 years (high cycle count) – Replace after any short-circuit event

Marine Fuses: – Inspect quarterly (corrosion risk) – Replace every 3-5 years – Replace if any corrosion visible

Industrial Fuses: – Inspect per manufacturer schedule – Replace based on fault counter logs – Replace if thermal imaging shows hot spots

Troubleshooting Nuisance Fuse Blowing

Problem: Fuse blows repeatedly at normal load

Diagnostic Steps:

Step 1: Measure Actual Load Current
- Use DC clamp meter
- Measure for 10 minutes (capture transients)
- Compare to fuse rating

If current < 80% of fuse rating: → Fuse problem (undersized, damaged, or wrong type)

If current > 100% of fuse rating: → Load problem (overload or short circuit)

Step 2: Check Voltage Drop – Measure voltage across fuse under load – >0.3V indicates high resistance – Causes: Corrosion, loose connection, damaged fuse

Step 3: Verify Correct Fuse Type – Fast-acting fuse on motor load → Use time-delay – AC fuse on DC system → Replace with DC-rated – Undersized voltage rating → Upsize voltage rating

Step 4: Check for Intermittent Faults – Insulation resistance test: Should be >1MΩ – Flex wires while measuring resistance – Low resistance indicates chafed insulation

Step 5: Temperature Effects – Check ambient temperature at fuse location – >40°C ambient → Fuse derates 10-20% – Improve ventilation or upsize fuse rating

よくある質問

1. Can I use an AC-rated fuse for DC applications?

No, absolutely never. AC fuses rely on the natural zero-crossing of alternating current (120 times/second at 60Hz) to extinguish arcs. DC has no zero-crossing, causing arcs to sustain indefinitely in AC fuses. When an AC fuse attempts to interrupt DC current, the arc continues burning, overheating the fuse body until it ruptures violently, ejecting molten material and creating fire hazard. Always use fuses specifically rated for DC voltage with appropriate arc-quenching technology (silica sand, ceramic plates).

2. What does the gPV rating mean on solar fuses?

gPV stands for “general purpose Photovoltaic” – a specialized rating for solar PV fuses per IEC 60269-6. These fuses are tested to interrupt reverse DC current (backfeed from batteries), operate reliably at high ambient temperatures (70°C), withstand UV exposure for outdoor mounting, and provide full-range breaking capacity. Standard DC fuses may not safely interrupt the unique fault conditions in PV systems. NEC-compliant solar installations require gPV-rated fuses for string and combiner protection.

3. How do I calculate the required interrupt rating for a DC fuse?

Calculate available fault current: I_fault = System Voltage / Total Circuit Resistance. Include battery internal resistance, wire resistance, and connection resistance. Example: 48V battery (0.01Ω internal) + 0.002Ω wiring = 0.012Ω total. Fault current = 48V / 0.012Ω = 4,000A. Select fuse with interrupt rating exceeding this value (5kA or 10kA minimum). Lithium batteries have very low internal resistance and can deliver massive fault currents exceeding 10,000A – Class T fuses (200kA interrupt) provide maximum safety margin.

4. What is I²t rating and why does it matter?

I²t (ampere-squared seconds) represents thermal energy passing through a fuse during fault clearing. It determines the “let-through energy” that reaches protected equipment. Lower I²t means faster clearing and better protection for sensitive electronics like inverters. I²t is critical for fuse coordination: downstream fuse I²t must be significantly less than upstream fuse I²t to ensure selective operation (only fuse nearest to fault opens). Calculate by integrating current squared over clearing time – manufacturers provide I²t curves in datasheets.

5. Can I parallel DC fuses to increase current capacity?

No, never parallel fuses. Manufacturing tolerances cause slight resistance differences between fuses. Lower-resistance fuse carries more current and blows first, forcing remaining fuse(s) to carry full fault current, blowing immediately afterward. This defeats overcurrent protection entirely during fault conditions. For higher current capacity, use a single appropriately rated fuse. If no single fuse available for your current, use multiple parallel conductors with one large fuse protecting all conductors together.

6. Why did my DC fuses blow when the load current was below the fuse rating?

Several possible causes: (1) High ambient temperature causing fuse derating (20-30% capacity loss at 50-70°C), (2) Inrush current from motor or capacitor startup exceeding instantaneous fuse rating (use time-delay fuse), (3) Harmonic currents from switching inverters increasing RMS current above DC measurement, (4) Fuse aging from thermal cycling or previous near-overload events, (5) Wrong fuse type (fast-acting instead of slow-blow for motor loads), (6) Intermittent short circuit or ground fault. Measure actual current including transients and check ambient temperature.

7. How long do DC fuses last before requiring replacement?

Lifespan depends on application: Solar gPV fuses (outdoor) 10-15 years with annual inspection; Battery disconnect fuses 5-10 years or after any short-circuit event; Marine environment fuses 3-5 years due to corrosion; Industrial fuses per manufacturer schedule based on fault history. Replace immediately if visual damage, discoloration, corrosion, or thermal imaging shows >40°C temperature rise above ambient. Fuses age from thermal cycling, environmental exposure, and near-overload events – preventive replacement is cheaper than equipment damage from failed fuse.

Conclusion: Engineering Reliable DC Overcurrent Protection

DC fuses represent sophisticated overcurrent protection technology specifically engineered to safely interrupt direct current arcs through specialized construction and arc-quenching materials. Proper selection requires understanding of I²t characteristics, interrupt capacity, voltage ratings, and application-specific requirements.

Key Selection Criteria:

Voltage Rating:
– Must exceed maximum system voltage (including transients)
– DC rating typically 50% of equivalent AC rating
– Verify with manufacturer for series battery strings

Current Rating:
– Standard loads: 1.25× continuous current
– Solar PV: 1.56× string Isc (NEC 690.9)
– Motors: 1.5-2.0× running current (time-delay type)

Interrupt Rating:
– Calculate available fault current from battery/source
– Select fuse interrupt rating ≥ 2× fault current
– Lithium batteries: Class T (200kA) recommended
– Lead-acid batteries: 10kA often adequate

Fuse Type by Application:
– Solar PV strings: gPV-rated fuses (IEC 60269-6)
– Battery disconnect: Class T (current-limiting, high interrupt)
– Automotive/Marine <80A: ANL or MEGA fuses
– Industrial motors: Time-delay, motor-rated fuses

Critical Safety Rules:
– NEVER use AC fuses on DC circuits
– NEVER parallel fuses to increase capacity
– NEVER exceed fuse voltage rating
– ALWAYS verify interrupt rating adequate
– ALWAYS coordinate I²t for selective operation

Installation Best Practices:
– Install within 7″ of battery positive (NEC 690.71)
– Use proper fuse holders (matching fuse type)
– Torque connections per manufacturer spec
– Protect from environmental exposure
– Label with fuse type and rating

Maintenance Schedule:
– Annual inspection (visual + thermal imaging)
– Replace per application schedule (3-15 years)
– Replace after any short-circuit event
– Replace if corrosion, damage, or high temperature observed

Properly engineered DC fuses protection provides reliable, selective overcurrent interruption for decades of service in solar, battery, electric vehicle, and industrial DC applications.