Why Every PV String Needs Surge Protection

The $47,000 Lightning Strike That Could Have Been Prevented

It was a Tuesday morning in July when the maintenance team at a 500kW commercial solar installation in Arizona received the call they dreaded. A severe thunderstorm had passed through overnight, and the inverters were offline. When the technicians arrived on site, they discovered that a lightning strike had traveled through the unprotected PV strings, destroying three string inverters, damaging 24 solar modules, and corrupting the monitoring system. The total repair cost? $47,000. The system downtime? Three weeks. The cost of proper pv string surge protection they had skipped during installation to save budget? Less than $2,000.

This isn’t an isolated incident. According to industry data, lightning and surge-related damage account for up to 30% of all solar system warranty claims. Yet many installers and system owners still view surge protection devices (SPDs) as optional accessories rather than essential safety equipment. If you’re responsible for designing, installing, or maintaining solar arrays, this mindset could be costing you—or your clients—tens of thousands of dollars.

The Hidden Vulnerability of PV Strings

Solar arrays are essentially lightning magnets by design. Here’s why your PV strings are particularly vulnerable to surge events:

Elevated Exposure: Solar panels are intentionally installed in open, elevated locations with maximum sun exposure—the exact same characteristics that make structures attractive to lightning strikes. Rooftop installations can be the highest point on a building, while ground-mounted arrays in open fields have minimal natural lightning protection.

Long DC Cable Runs as Antennas: The DC cables connecting your PV strings act as enormous antennas, picking up electromagnetic interference from nearby lightning strikes. Even indirect strikes (lightning hitting the ground or nearby structures within 2km) can induce voltage surges exceeding 6,000V on unprotected cables.

Multiple Entry Points: Unlike traditional electrical systems with a single utility connection point, solar arrays have dozens or hundreds of potential surge entry paths—every string represents a pathway for destructive energy to reach your expensive inverter equipment.

DC Arc Persistence: When surges cause arcing in DC systems, the arc doesn’t self-extinguish at zero-crossing like AC systems. DC arcs can persist and escalate, creating fire hazards and catastrophic equipment damage.

Think of your solar array like a field of lightning rods connected directly to precision electronic equipment—without proper protection, it’s not a question of if you’ll experience surge damage, but when.

What Happens When Lightning Strikes Your Solar Array

The consequences of inadequate pv string surge protection extend far beyond immediate equipment damage:

Immediate Equipment Destruction

When a surge travels through unprotected PV strings, the first casualties are typically:

• Inverter input stages: IGBT modules, DC-link capacitors, and control boards (repair cost: $5,000-$15,000 per inverter)
• Bypass diodes in solar modules: Causes hot spots and permanent power loss (replacement cost: $400-$800 per module)
• Monitoring and communication equipment: Data loggers, sensors, and control systems ($2,000-$8,000)

Hidden Module Degradation

Even surges that don’t cause immediate failure can create micro-cracks in solar cells, accelerating long-term degradation. Studies show that modules exposed to repeated surge events without adequate protection can lose 15-25% more efficiency over their lifetime compared to protected systems.

System Downtime Costs

Pro-Tip: Many insurance policies won’t cover surge damage if you can’t prove that code-required surge protection was properly installed and maintained—always document your SPD installations with dated photos and commissioning reports.

Warranty Voidance Risk

Here’s the clause that many miss in manufacturer warranties: Most inverter and module warranties explicitly require “properly installed surge protection in accordance with local electrical codes and IEC 61643-31.” If you can’t demonstrate that appropriate SPDs were installed, you could void warranties worth tens of thousands of dollars.

Why String-Level Protection is Non-Negotiable

Understanding the surge path through your PV system reveals why protection at multiple levels is essential:

The Protection Cascade Concept

Effective pv string surge protection follows a coordinated protection cascade—think of it as a series of defensive barriers, each designed to handle specific threat levels:

First Line of Defense (String Level): Type 2 SPDs installed at or near the PV array handle the initial surge energy. These devices clamp high-voltage transients before they propagate through long cable runs where energy can accumulate.

Second Line (Combiner Box): Additional Type 2 SPDs provide backup protection and handle any residual surges that passed through the string-level devices or entered through other paths.

Final Line (Inverter Input): Type 2 or fine-protection SPDs installed at the inverter’s DC input provide the last defense, ensuring that only clean power reaches sensitive electronics.

Key Principle: Each protection stage must be properly coordinated. The Voltage Protection Level (Up) of each successive stage should be progressively lower, and devices must be separated by at least 10 meters of cable or connected through decoupling inductors to prevent SPD interaction.

Compliance and Code Requirements

The National Electrical Code (NEC) Article 690.35(A) explicitly requires surge protection for PV systems. More specifically:

• All PV systems with exposed wiring on or in buildings must have SPDs
• SPDs must be listed and labeled for DC PV applications
• Protection is required on both DC and AC sides

IEC 61643-31 provides the international standard for SPD selection and installation in photovoltaic systems, specifying test procedures and minimum performance requirements.

Pro-Tip: During permit reviews and inspections, having properly rated and installed string-level SPDs demonstrates engineering due diligence and can expedite approval processes—inspectors look for this as a sign of quality installation.

The Four-Step Selection Method for PV String SPDs

Selecting appropriate pv string surge protection isn’t guesswork—follow this systematic approach to specify the right devices every time:

Step 1: Calculate Maximum System Voltage (Voc Consideration)

Your SPD’s maximum continuous operating voltage (Uc) must exceed the maximum open-circuit voltage (Voc) your system can produce under any conditions.

Calculation Formula:

Uc(min) = Voc(STC) × Temperature Correction Factor × Safety Margin

Temperature Correction Factor: For every 10°C below 25°C (STC), Voc increases by approximately 0.35-0.40% per °C for typical crystalline silicon modules.

Example Calculation:

• Module Voc (STC): 49.5V
• String length: 20 modules
• Voc at STC: 49.5V × 20 = 990V
• Lowest expected temperature: -20°C
• Temperature difference from STC: 45°C
• Voltage increase: 990V × (45°C × 0.0035) = 156V
• Maximum Voc: 990V + 156V = 1,146V
• Required Uc with 15% safety margin: 1,146V × 1.15 = 1,318V

Selection: Choose an SPD with Uc ≥ 1,500V DC for this 1000V nominal system.

Key Takeaway: Never select SPDs based on the nominal system voltage alone. Always calculate worst-case Voc including temperature effects and add a 15-20% safety margin to prevent SPD degradation during cold, high-irradiance conditions.

Step 2: Determine Required Voltage Protection Level (Up)

The Voltage Protection Level (Up) is the maximum voltage that will appear at the protected equipment during an SPD operation. This must be lower than the withstand voltage of your equipment.

Selection Criteria:

Up(SPD) < 0.8 × Equipment Withstand Voltage

For typical string inverters:

• 1000V system inverters: Withstand voltage typically 6-8 kV
• 1500V system inverters: Withstand voltage typically 10-12 kV

Recommended Up values for string-level SPDs:

• 1000V systems: Up ≤ 4 kV
• 1500V systems: Up ≤ 6 kV

Pro-Tip: Lower Up values provide better protection but may have shorter lifespans due to more frequent activation. Balance protection level with expected surge frequency in your location—high-lightning areas may need more robust specifications.

Step 3: Select Appropriate Discharge Current Rating (Iimp, Imax)

PV string SPDs must handle both direct and indirect lightning surges. The key ratings to understand:

Iimp (Impulse Current): The device’s ability to handle the high-energy surge from direct or nearby lightning strikes. Measured with a 10/350 μs waveform (Type 1 test).

Imax (Maximum Discharge Current): The device’s ability to handle multiple surges from indirect strikes. Measured with an 8/20 μs waveform (Type 2 test).

Selection Guidelines by Application:

Calculation Example for String-Level Protection:
For a typical commercial rooftop array in a moderate lightning region:

• Exposure: Primarily indirect strikes
• Recommendation: Type 2 SPD
• Minimum Imax per pole: 40 kA (8/20 μs)
• For critical installations: Consider Type 1+2 hybrid with Iimp = 12.5 kA

Step 4: Choose Technology (MOV vs GDT)

The debate between Metal Oxide Varistor (MOV) and Gas Discharge Tube (GDT) technology for pv string surge protection often confuses engineers. Here’s the definitive comparison:

Hybrid Solution – The Professional Choice:

Modern high-performance PV SPDs combine both technologies in a staged protection approach:

• Primary Stage (GDT): Handles high-energy surges with excellent discharge capacity
• Secondary Stage (MOV): Provides fast response and low voltage clamping
• Arc Quenching Circuit: Prevents GDT follow current issues

Key Takeaway: For commercial and utility-scale installations where long-term reliability is critical, specify hybrid MOV+GDT technology SPDs. The slightly higher initial cost is offset by longer lifespan and superior protection performance.

Selection Decision Tree:

• Budget-conscious residential (< 20 kW): MOV-only Type 2 SPD
• Commercial rooftop (20-500 kW): Hybrid MOV+GDT Type 2 SPD
• Ground-mount or high-lightning areas: Hybrid Type 1+2 SPD with arc quenching
• Utility-scale (> 1 MW): Hybrid Type 1 SPD with remote monitoring

Critical Technical Parameters Explained

Understanding the datasheet specifications helps you make informed decisions about pv string surge protection:

Comprehensive Technology Comparison

Type 1 vs Type 2 SPDs for Solar Applications

Understanding when to specify Type 1 versus Type 2 devices is critical for proper pv string surge protection:

Pro-Tip: For optimal protection, use a Type 1+2 hybrid device at the array combiner point and Type 2 devices at the inverter input. This provides both high-energy handling and fine voltage clamping in a coordinated cascade.

Essential Ratings Decoded

Uc (Maximum Continuous Operating Voltage): The highest voltage the SPD can withstand continuously without degradation. Must exceed your system’s maximum Voc under all conditions.

Up (Voltage Protection Level): The voltage that appears at the protected equipment when the SPD operates. Lower is better, but must be balanced with energy handling capacity.

In (Nominal Discharge Current): The current used for classification and aging tests (typically 5 or 10 kA for Type 2 devices).

Imax (Maximum Discharge Current): The maximum surge current the device can handle in a single operation without damage.

Iimp (Impulse Current): For Type 1 devices, the high-energy surge current capability tested with 10/350 μs waveform.

TOV (Temporary Overvoltage) Capability: The device’s ability to withstand temporary voltage increases due to system faults or switching operations without permanent damage.

Installation Best Practices

Even the highest-quality pv string surge protection devices will fail to protect your system if improperly installed. Follow this proven installation sequence:

Critical Installation Requirements

1. Cable Length and Routing (The 0.5-Meter Rule)

The connection between your SPD and the protected equipment is critical. Every meter of cable adds inductance, which creates additional voltage during fast-rising surges:

Voltage Drop Calculation:

V_additional = L × (di/dt)
Where: L ≈ 1 μH per meter of cable
       di/dt for lightning ≈ 10-100 kA/μs

Example: Just 2 meters of connection cable can add 200V of additional voltage rise during a surge, partially negating your SPD’s protection!

Installation Rules:

• Keep total cable length from SPD to protected equipment < 0.5 meters (ideal: < 0.3 meters)
• Use shortest possible straight run—avoid loops or coils
• If longer runs are unavoidable, use larger conductors (min. 6 AWG / 10 mm²)
• Never bundle SPD cables with signal or communication wires

Pro-Tip: Pre-measure and cut your connection cables to exact length before installation. Mark the 0.5-meter limit on your installation template to ensure compliance during field installation.

2. Grounding Best Practices

Proper grounding is the foundation of effective surge protection:

• Ground Connection: Use minimum 6 AWG (10 mm²) copper conductor to the main PV system ground
• Low Impedance Path: Total ground resistance should be < 10 Ω (ideally < 5 Ω)
• Avoid Ground Loops: Connect SPD ground to the same ground bar as the protected equipment
• Equipotential Bonding: Ensure all metallic structures (array frame, equipment chassis, SPD housing) are bonded together

For PV Systems with Mid-Point Grounding:

• Connect both DC+ and DC- SPD poles
• Connect the PE terminal to the mid-point ground reference
• Verify that grounding complies with your local electrical code

3. Physical Installation Considerations

Location and mounting affect both protection effectiveness and maintenance:

• Mounting: Use DIN rail mounting for easy replacement; ensure secure mechanical connection
• Ventilation: Provide adequate airflow; SPDs can generate heat during operation
• Accessibility: Install where visual status indicators are easily visible for inspection
• Environmental Protection: Use appropriate IP-rated enclosures for outdoor installations (minimum IP65)
• Labeling: Clearly label SPD location, installation date, and next inspection due date

4. Connection Sequence

Always follow proper connection sequence to avoid ground faults or equipment damage:

1. Verify system is de-energized (check Voc = 0V)
2. Mount SPD in final location
3. Connect ground/PE terminal first
4. Connect DC- pole
5. Connect DC+ pole last
6. Verify all connections are tight (torque to manufacturer specification)
7. Check status indicator before energizing system

Pro-Tip: Install a disconnect switch between your PV strings and the SPD to allow safe maintenance and replacement without de-energizing the entire array. This is especially valuable for large commercial systems where downtime is costly.

Real-World Application Example: Sizing SPDs for a 10-String, 1000V System

Let’s work through a complete design example to demonstrate proper pv string surge protection selection for a typical commercial installation.

System Specifications

Array Configuration:

• 10 parallel strings
• 20 modules per string
• Module specifications:
  • Voc (STC): 49.5V
  • Isc (STC): 11.5A
  • Vmp: 41.8V
  • Imp: 11.0A
  • Temperature coefficient (Voc): -0.35%/°C

Environmental Conditions:

• Location: Arizona (high solar exposure, moderate lightning)
• Lowest expected temperature: -5°C
• Installation: Rooftop commercial building
• Exposure: Indirect lightning strikes expected

Equipment:

• String inverter: 100 kW, 1000V DC input rating
• Inverter withstand voltage: 6 kV
• Combiner box with 10 input strings

Step-by-Step SPD Selection

Step 1: Calculate Maximum System Voltage

Voc per string (STC) = 49.5V × 20 = 990V

Temperature correction:
ΔT = 25°C - (-5°C) = 30°C
Voltage increase = 990V × (30°C × 0.0035) = 104V
Voc (cold) = 990V + 104V = 1,094V

Required Uc with 20% safety margin:
Uc(min) = 1,094V × 1.20 = 1,313V

Selection: SPDs with Uc = 1,500V DC (standard rating)

Step 2: Determine Required Voltage Protection Level

Inverter withstand voltage = 6 kV
Maximum acceptable Up = 6 kV × 0.8 = 4.8 kV

Selection: SPDs with Up ≤ 4.0 kV (providing 33% safety margin)

Step 3: Select Discharge Current Rating

For rooftop installation in moderate lightning region:

• Primary threat: Indirect strikes
• Recommended: Type 2 SPD
• Minimum Imax: 40 kA (8/20 μs) per pole

For enhanced protection (optional but recommended):

• Consider Type 1+2 hybrid
• Iimp: 12.5 kA (10/350 μs)
• Imax: 60 kA (8/20 μs)

Selection: Type 2 SPD with Imax = 40 kA per pole (minimum), or Type 1+2 hybrid for critical loads

Step 4: Choose Technology

For this commercial application:

• Expected surge frequency: Moderate (10-20 events per year)
• System value: $150,000 (equipment + lost production risk)
• Maintenance access: Good

Selection: Hybrid MOV+GDT technology for optimal balance of performance and longevity

Protection Architecture Design

graph TB
    subgraph "PV Array - 10 Strings"
        S1[String 1: 20 Modules]
        S2[String 2: 20 Modules]
        S3[String 3: 20 Modules]
        S10[String 10: 20 Modules]
    end
    
    S1 --> SPD1[String-Level SPD<br>Type 2, Uc=1500V<br>Up=4kV, Imax=40kA]
    S2 --> SPD2[String-Level SPD]
    S3 --> SPD3[String-Level SPD]
    S10 --> SPD10[String-Level SPD]
    
    SPD1 --> CB[Combiner Box]
    SPD2 --> CB
    SPD3 --> CB
    SPD10 --> CB
    
    CB --> SPD_CB[Combiner SPD<br>Type 2, Uc=1500V<br>Up=3.5kV, Imax=60kA]
    
    SPD_CB --> |10m Cable| INV[String Inverter<br>100kW, 1000VDC]
    
    INV --> SPD_INV[Inverter Input SPD<br>Type 2, Uc=1500V<br>Up=3.0kV, Imax=40kA]
    
    SPD1 -.->|Ground| GND[System Ground<br>< 5Ω Resistance]
    SPD_CB -.->|Ground| GND
    SPD_INV -.->|Ground| GND
    
    style SPD1 fill:#90EE90
    style SPD2 fill:#90EE90
    style SPD3 fill:#90EE90
    style SPD10 fill:#90EE90
    style SPD_CB fill:#87CEEB
    style SPD_INV fill:#FFD700

Final Specification Summary

String-Level Protection (10 units):

• Technology: Hybrid MOV+GDT
• Configuration: 2-pole (DC+, DC-)
• Uc: 1,500V DC
• Up: ≤ 4.0 kV
• Imax: 40 kA (8/20 μs) per pole
• Mounting: DIN rail in junction boxes near array
• Estimated cost per unit: $180
• Total cost: $1,800

Combiner Box Protection (1 unit):

• Technology: Hybrid MOV+GDT Type 1+2
• Configuration: 2-pole (DC+, DC-)
• Uc: 1,500V DC
• Up: ≤ 3.5 kV
• Iimp: 12.5 kA (10/350 μs)
• Imax: 60 kA (8/20 μs)
• Remote monitoring: Contact output for status
• Estimated cost: $450

Inverter Input Protection (1 unit):

• Technology: Hybrid MOV+GDT
• Configuration: 2-pole (DC+, DC-)
• Uc: 1,500V DC
• Up: ≤ 3.0 kV
• Imax: 40 kA (8/20 μs)
• Estimated cost: $220

Total Protection System Cost: $2,470

Key Takeaway: This comprehensive three-stage protection cascade costs less than 1.5% of the total system value but protects against damage that could cost $47,000 or more. The ROI calculation is simple: one prevented surge event pays for the entire protection system 19 times over.

The Cost of NOT Having Protection

When evaluating whether to specify pv string surge protection, consider the true cost of going without:

Direct Cost Comparison

ROI Analysis

Break-Even Calculation:

Initial SPD Investment: $3,270
Average Surge Damage Cost: $22,500
Break-even point: 0.145 surge events

If your region experiences just 1 significant surge event every 7 years,
the SPD system pays for itself.

According to IEEE data, most commercial solar installations experience
2-4 damaging surge events over a 25-year lifespan without protection.

Expected ROI Over 25 Years:

• Initial investment: $3,270
• SPD replacement (year 10, year 20): $3,000
• Total investment: $6,270
• Prevented damage (3 events × $22,500): $67,500
• Net savings: $61,230
• ROI: 977%

Pro-Tip: When presenting surge protection to budget-conscious clients, frame it this way: ‘We can either invest $3,000 today for protection, or budget $20,000-50,000 for repairs later. The protection system is not an expense—it’s damage insurance with a 1000% ROI.’

Insurance and Warranty Implications

Warranty Coverage:
Most major manufacturers include surge protection requirements in their warranties:

• Without SPDs: Warranty claims denied if surge damage occurs and no protection was installed
• With SPDs: Full warranty coverage maintained, manufacturer may even cover SPD replacement costs

Insurance Premiums:
Commercial insurance providers increasingly require documentation of surge protection:

• Systems without adequate protection: 15-25% higher premiums
• Systems with documented, code-compliant protection: Standard rates
• Annual savings on $100,000 system: $300-500

Downtime Risk:
For critical facilities (hospitals, data centers, manufacturing) or systems under Power Purchase Agreements (PPAs):

• PPA performance penalties: $5,000-15,000 per week of downtime
• Critical load impact: Immeasurable risk to operations
• Reputation damage: Lost customer confidence

Key Takeaways

⚡ Lightning doesn’t have to strike your array directly to cause damage. Indirect strikes up to 2km away can induce surges exceeding 6,000V on unprotected PV strings. String-level protection is your first line of defense.

💰 The cost of protection is trivial compared to damage costs. A comprehensive three-stage SPD system costs $2,000-5,000 for typical commercial installations but protects against $20,000-100,000+ in potential damage. Break-even occurs after just 0.15 surge events.

🔧 SPD selection requires four critical calculations: Maximum system voltage (Voc × temperature × safety margin), required protection level (Up < 0.8 × equipment withstand voltage), discharge current rating (based on exposure level), and technology choice (hybrid MOV+GDT for best performance).

📐 Installation quality determines protection effectiveness. Keep connection cables under 0.5 meters, use minimum 6 AWG ground conductors, avoid cable loops, and ensure all connections are torqued to specification. Poor installation can reduce protection effectiveness by 50% or more.

🎯 Coordinated cascade protection is essential. Use Type 1+2 SPDs at the array combiner, Type 2 at string level, and final Type 2 protection at the inverter input. Each stage must have progressively lower Up values and be separated by adequate cable length for proper coordination.

✅ Code compliance is mandatory, not optional. NEC Article 690.35 and IEC 61643-31 require surge protection for PV systems. Proper SPD installation is necessary for permit approval, warranty validity, and insurance coverage. Document everything with photos and commissioning reports.

🔄 Plan for SPD lifecycle maintenance. Even the best SPDs have finite lifespans (typically 5-10 years depending on surge frequency). Specify devices with visual status indicators and remote monitoring capability, and schedule annual inspections to verify continued protection.

Frequently Asked Questions

Do I need SPD on every string or just at the combiner box?

Best practice is protection at both levels. While combiner-level protection is the minimum requirement, string-level SPDs provide the first defense against surges before they propagate through the system. For optimal protection:

• Critical installations (commercial, utility-scale): Install SPDs at both string and combiner levels
• Budget-conscious residential (< 20kW): Minimum protection at combiner or inverter input is acceptable
• High lightning regions: String-level protection is non-negotiable

String-level protection becomes especially important when strings are separated by significant distances (> 50 meters) or when array wiring is exposed. The additional cost is minimal (typically $150-200 per string) compared to the protection benefit.

What’s the difference between Type 1 and Type 2 SPDs for solar?

Type 1 SPDs handle direct lightning strikes; Type 2 SPDs handle indirect strikes and switching surges.

Type 1 devices are tested with a 10/350 μs impulse current waveform, representing the high energy from direct strikes. They can dissipate 40-50 times more energy than Type 2 devices but are larger and more expensive. Use Type 1 SPDs when:

• Arrays are in open fields (ground-mount installations)
• Installation is the highest point in the area
• Local lightning density exceeds 3 strikes/km²/year
• Regional code requires Type 1 protection

Type 2 devices are tested with an 8/20 μs waveform and handle indirect strikes (the most common threat). They provide better voltage clamping (lower Up) and are sufficient for most rooftop installations.

Modern hybrid “Type 1+2” devices provide both capabilities in a single unit—ideal for combiner box protection where both direct and indirect surge threats exist.

Can I use AC SPDs on the DC side?

Absolutely not—AC and DC SPDs are fundamentally different and not interchangeable.

AC SPDs rely on the natural current zero-crossing that occurs 100-120 times per second in AC systems to extinguish any follow current after surge protection. DC systems have no zero-crossing, meaning:

• GDT-based AC SPDs can latch into short-circuit mode on DC systems, creating a permanent fault
• Arc extinction mechanisms designed for AC won’t function properly in DC applications
• Voltage ratings differ significantly between AC and DC due to different stress characteristics

DC SPDs must be specifically designed and rated for photovoltaic applications with:

• Arc quenching or current limiting circuits for GDT technology
• Proper Uc ratings based on DC voltage stress
• Thermal disconnectors suitable for DC arcing
• Testing and certification to IEC 61643-31 (PV-specific standard)

Using AC SPDs on DC circuits is a code violation, warranty voidance, and serious safety hazard. Always specify DC-rated, PV-specific surge protection devices.

How do I know when my SPD needs replacement?

Most quality SPDs have visual status indicators—but don’t rely on visual inspection alone.

Modern pv string surge protection devices include multiple failure indication methods:

Visual Indicators:

• Green/Red indicator windows showing operational status
• “OK” vs “FAULT” markings visible without opening enclosure
• Some devices include pop-out mechanical indicators

Electrical Indicators:

• Remote contact outputs (normally closed contact opens on failure)
• Dry contact signals to monitoring systems
• Some advanced models support Modbus/SNMP remote monitoring

Inspection Schedule:

• Annual visual inspection: Check status indicators during routine maintenance
• Post-storm inspection: Inspect within 24 hours after severe weather events
• Quarterly remote monitoring check: If connected to SCADA/monitoring system

When to Replace:

• Status indicator shows “FAULT” or red condition
• Remote monitoring shows SPD failure
• After a known direct lightning strike (replace as precaution)
• After 5-10 years regardless of apparent condition (preventive replacement)
• When leakage current measurements exceed 10× the rated value

Pro-Tip: Document SPD installation dates on device labels and in maintenance logs. Set calendar reminders for preventive replacement based on manufacturer recommendations—don’t wait for failure in critical applications.

What voltage rating should I choose for a 1000V/1500V system?

Choose SPD voltage ratings based on worst-case Voc, not nominal system voltage.

For 1000V nominal systems:

Typical maximum Voc (cold): 1,100-1,200V
Recommended SPD Uc rating: 1,500V DC
Standard protection level (Up): 3.5-4.0 kV

For 1500V nominal systems:

Typical maximum Voc (cold): 1,650-1,800V
Recommended SPD Uc rating: 2,000V DC
Standard protection level (Up): 5.0-6.0 kV

Critical calculation steps:

1. Calculate string Voc at Standard Test Conditions (STC)
2. Apply temperature correction for lowest expected temperature
3. Add 15-20% safety margin
4. Select next higher standard SPD voltage rating

Example for 1500V system:

• Module Voc (STC): 52V
• String length: 28 modules
• Voc at STC: 1,456V
• Lowest temperature: -10°C (35°C below STC)
• Temperature increase: 1,456V × 35°C × 0.0035 = 178V
• Maximum Voc: 1,456V + 178V = 1,634V
• With 20% safety margin: 1,634V × 1.2 = 1,961V
• Select SPD with Uc = 2,000V DC (standard rating)

Never undersize SPD voltage ratings to save cost—undersized SPDs will degrade rapidly or fail prematurely when exposed to high Voc conditions.

MOV or GDT – which is better for solar applications?

Neither is universally “better”—the optimal choice depends on your specific application requirements.

Choose MOV-only SPDs when:

• Budget is the primary constraint (residential installations)
• Surge frequency is low (< 5 significant events per year expected)
• Fast response time is critical (< 25 nanoseconds)
• Lower voltage clamping (Up) is required
• System is in low-to-moderate lightning exposure area

Choose GDT-only SPDs when:

• High discharge current capacity is required (direct strike zones)
• Maximum lifespan is critical (minimal degradation over time)
• System operates in high-temperature environments
• Zero leakage current is essential
• Budget allows for higher initial investment

Choose Hybrid MOV+GDT SPDs when:

• Commercial or utility-scale installations (> 50kW)
• Long-term reliability is paramount
• System is in moderate-to-high lightning exposure
• Remote monitoring and status indication are available
• Total cost of ownership (not just initial cost) drives decisions

The industry trend is toward hybrid designs because they combine the best characteristics of both technologies:

• Fast MOV response with robust GDT energy handling
• Arc quenching circuits eliminate GDT follow-current concerns
• Superior long-term reliability justifies slightly higher cost

For professional installations where system uptime and long-term protection are priorities, specify hybrid technology—the 20-30% higher initial cost is recovered through extended lifespan and superior protection performance.

How close should the SPD be installed to the equipment?

Maximum 0.5 meters (50 cm) of total cable length between SPD and protected equipment—shorter is always better.

The critical principle: Every meter of connecting cable adds inductance (approximately 1 μH/meter), which creates additional voltage rise during fast surge events:

Voltage rise calculation:

V_additional = L × (di/dt)

Example with 2 meters of cable:
L = 2 meters × 1 μH/meter = 2 μH
di/dt = 50 kA/μs (typical lightning surge rate)
V_additional = 2 μH × 50,000 A/μs = 100V per meter

Total additional voltage = 200V

This additional voltage appears at the protected equipment on top of the SPD’s voltage protection level (Up), effectively reducing protection performance.

Installation best practices:

• Ideal distance: < 0.3 meters (30 cm)
• Maximum acceptable: 0.5 meters (50 cm)
• If longer runs unavoidable: Use larger conductors (min. 6 AWG / 10 mm²) and twisted-pair routing
• Cable routing: Avoid loops, coils, or parallel runs with signal cables
• Mounting location: Install SPD as close as physically possible to equipment terminals

Pro-Tip: Pre-cut SPD connection cables to exact required length before installation. Use short, direct cable runs even if it requires relocating the SPD mounting position—protection effectiveness is more important than neat cable management.

For large systems with multiple combiner boxes, place SPDs at each combiner box rather than using long runs to a central SPD location. Distributed protection is more effective than centralized protection with long cable runs.

Will SPDs affect my system performance or efficiency?

Properly selected and installed SPDs have zero impact on system performance during normal operation.

During Normal Operation:

• Voltage drop: Effectively zero (SPDs are open circuits in normal conditions)
• Power loss: Negligible (< 0.001% of system output)
• Efficiency impact: None measurable
• EMI/RFI effects: None (SPDs may actually reduce electrical noise)

Leakage Current Considerations:

• MOV-based SPDs: 10-100 μA leakage (aging increases this)
• GDT-based SPDs: < 1 μA leakage
• For a 100kW system operating at 1000V: 100 μA leakage = 0.1W power loss (0.0001% of output)
• Impact on performance: Immeasurable

During Surge Events:

• SPD activates in nanoseconds, clamping voltage to safe level
• After surge, SPD returns to high-impedance state
• No residual effect on system operation
• Modern SPDs self-test and indicate any degradation

Potential Issues Only if Incorrectly Applied:

• Undersized Uc rating: SPD may clamp during high Voc conditions, appearing as system fault
• Failed SPD not replaced: May show as short circuit, preventing system operation
• Incorrect polarity: Can cause ground faults (follow installation instructions carefully)

The Bottom Line: Quality SPDs are transparent to system operation. Any performance impact from properly installed surge protection is far outweighed by the protection benefit. The only “performance issue” you’ll experience is continued operation after surge events that would have otherwise destroyed your equipment.

Final Thought: In the photovoltaic industry, we often hear “every dollar saved in installation costs is profit.” But skipping pv string surge protection to save $2,000-3,000 upfront is like canceling your car insurance to save on premiums—it works great until you need it. The question isn’t whether you can afford surge protection; it’s whether you can afford to replace an entire inverter, dozens of modules, and absorb weeks of downtime when lightning strikes. Make surge protection a non-negotiable part of every PV system design—your clients (and your reputation) will thank you.