Selecting the appropriate Surge Protective Device (SPD) is a critical decision for ensuring the longevity and reliability of your solar power system. This comprehensive guide will walk you through the essential factors to consider when choosing SPDs for your photovoltaic installation, helping you protect your valuable investment from damaging electrical surges.
Why Your Solar System Needs SPD Protection
Solar power systems are particularly vulnerable to surge damage for several reasons:
- Exposed location: Solar arrays are typically installed in elevated, exposed positions
- Extended cable runs: DC power cables can act as antennas for induced surges
- Sensitive electronics: Inverters, monitoring systems, and control equipment contain vulnerable components
- Lightning attraction: Solar panel arrays can be attractive paths for lightning strikes
Without adequate surge protection, a single lightning strike or grid-switching event can cause thousands of dollars in damage to your inverters, charge controllers, panels, and monitoring systems. Even smaller, repeated surges can degrade system components over time, reducing efficiency and shortening operational lifespan.
Key Factors for Selecting the Right Solar SPD
1. Understand SPD Types and Their Applications
SPDs are categorized into three main types, each serving different protection needs:
Type 1 SPDs:
- Used for protection against direct lightning strikes
- Installed at service entrance points or utility connections
- Tested with 10/350μs waveform to handle high-energy impulses
- Required in buildings with external lightning protection systems
- Typically use spark gap technology
Type 2 SPDs:
- Provide protection against induced surges and switching events
- Installed at distribution boards or sub-panels
- Tested with 8/20μs waveform
- Use Metal Oxide Varistor (MOV) technology
- Most common type used in standard solar installations
Type 3 SPDs:
- Provide fine protection for sensitive terminal equipment
- Installed near specific electronic devices
- Have lower discharge capacity than Types 1 and 2
- Often used for monitoring systems and communication interfaces
For most residential and commercial solar systems, you’ll need a combination of SPD types for comprehensive protection. Type 1+2 combined SPDs are also available, which integrate the protective characteristics of both types into a single unit.
2. Evaluate Your System Voltage Requirements
One of the most critical selection criteria is matching the SPD’s voltage rating to your solar system’s requirements:
Maximum Continuous Operating Voltage (MCOV or Uc):
- DC side SPDs must have MCOV ratings higher than your system’s maximum open-circuit voltage (Voc)
- Remember that cold temperatures increase PV array voltage
- A good rule of thumb: Select DC SPDs with MCOV at least 10% above maximum system Voc
- For example, a 600V DC system requires SPDs with MCOV of at least 660V DC
- AC side SPDs should exceed nominal system voltage by at least 25%
Using an SPD with insufficient MCOV will result in premature failure and potentially create fire hazards, as the device will see normal system voltage as a surge condition.
3. Check the Voltage Protection Level (Up)
The voltage protection level or clamping voltage indicates the maximum voltage that will reach your equipment during a surge:
- Lower Up values provide better protection for sensitive components
- The Up should be below the impulse withstand rating of your equipment
- For best protection, choose an SPD with Up at least 20% lower than equipment tolerance
- Typical PV inverters require Up values below 2.5-4kV
When comparing SPDs, a lower voltage protection level generally indicates superior protection, but must be balanced with other parameters.
4. Assess Required Discharge Current Capacity
Two important ratings determine an SPD’s ability to handle surge currents:
Nominal Discharge Current (In):
- Indicates how much surge current the SPD can handle repeatedly
- Higher values mean better durability for frequent surges
- For Type 2 SPDs in solar applications, look for In ratings of 10-20kA or higher
Maximum Discharge Current (Imax):
- The highest single surge current the SPD can safely divert
- Type 2 devices typically range from 40-80kA
- Systems in high-lightning regions should use higher ratings
- Type 1 SPDs use Impulse Discharge Current (Iimp) rating instead
Balance these ratings based on your location’s lightning risk and system importance. Areas with frequent thunderstorms require higher ratings than locations with minimal lightning activity.
5. Consider Short Circuit Current Rating (SCCR)
The SCCR specifies the maximum prospective short-circuit current the SPD can safely handle if it fails:
- The SPD’s SCCR must be equal to or greater than the available fault current at its installation point
- This is a mandatory safety requirement in many electrical codes
- DC SPDs in high-voltage PV systems face significant challenges with fault current handling
- Some SPDs require external overcurrent protection devices to achieve their marked SCCR
6. Determine Optimal SPD Placement
Strategic placement of SPDs is crucial for effective solar system protection:
DC Side Placement Guidelines:
The “<10 Meter Rule” is widely adopted in the industry:
- If DC cable length is less than 10 meters: A single set of SPDs at the inverter DC input is generally sufficient
- If DC cable length exceeds 10 meters: Install two sets of SPDs – one near the PV array (in combiner boxes) and another at the inverter input
For larger systems, consider protection at these key points:
- Array level: Install SPDs at combiner boxes for distributed arrays
- Inverter DC input: Install SPDs immediately before inverter DC inputs
- String level: For systems with multiple strings, consider string-level protection
AC Side Placement:
- Grid interconnection point: Primary protection at main service panel
- Inverter AC output: Secondary protection near the inverter
- Distribution panels: Additional protection at sub-panels for larger systems
Also consider protection for communication and monitoring systems, which are often highly sensitive to surges.
7. Check Compliance with Relevant Standards
Verify that your selected SPDs comply with applicable standards:
- IEC 61643-31: Standard specifically for SPDs in photovoltaic applications
- IEC 61643-32: Selection and application principles for PV SPDs
- UL 1449: Safety standard for SPDs in North America
- IEC 62305 Series: Standards for lightning protection systems
- NEC Article 690.7(C): National Electrical Code requirements
Products meeting UL 1449 with Type 1 or Type 2 designation are generally accepted for PV applications in North America.
8. Evaluate Influence of External Lightning Protection
If your building has an external Lightning Protection System (LPS), you need to consider the “separation distance ‘s'” between it and your PV system:
- If separation distance can be maintained: Type 2 SPDs may be sufficient
- If separation distance cannot be maintained: Type 1 SPDs become mandatory
This is a fundamental design consideration that significantly impacts your SPD selection strategy.
9. Understand Your System’s Grounding Configuration
Different grounding configurations require specific SPD connection schemes:
DC Side Configurations:
- Functionally earthed: One DC pole connected to earth
- High-resistance earthed: DC pole connected to earth through resistance
- Unearthed/floating: Neither pole directly connected to earth
AC Side Configurations:
- TN-C, TN-S, TN-C-S systems
- TT systems
- IT systems
Each configuration requires a specific SPD connection scheme to ensure effective protection. For example, ungrounded (IT) PV systems often need SPDs with “Y-configurations” for comprehensive protection.
Installation Best Practices for Optimal SPD Performance
Minimize Connection Lead Lengths
The physical wiring of an SPD critically impacts its performance:
- Keep connecting leads as short as absolutely possible
- Ideal total lead lengths should be less than 0.5 meters
- Never exceed 1 meter for total connection length
- Avoid sharp bends in conductors as they increase inductance
During fast-rising surge currents, even short lengths of connecting wire develop substantial inductive voltage drop. This adds directly to the SPD’s clamping voltage, potentially compromising protection.
Ensure Proper Conductor Sizing
- For Type 2 SPDs, use minimum 6 mm² copper conductors for protective earth connections
- For Type 1 SPDs, use 16 mm² copper or larger for protective earth connections
- Live conductors should be at least equal to system wiring, if not larger
- Always follow manufacturer recommendations and relevant standards
Route Cables Appropriately
- Route AC, DC, and data cables together with their associated equipotential bonding conductors
- This reduces the area of loops formed by wiring, minimizing induced overvoltages
- Create designated cable paths that minimize exposure to electromagnetic interference
Maintenance Requirements for Long-Term Protection
Even the best SPDs have a finite lifespan:
- Most quality SPDs have a 10-15 year expected service life under normal conditions
- Visual indicators should be checked regularly for signs of SPD activation or failure
- For critical installations, select SPDs with remote monitoring capabilities
- Replace SPDs after major surge events even if no external damage is visible
- Establish regular inspection schedules, particularly before storm seasons
Common SPD Selection Mistakes to Avoid
Avoid these frequent errors when choosing surge protection for your solar system:
- Undersizing protection: Selecting SPDs with insufficient energy handling capacity
- Ignoring thermal performance: Failing to account for high temperatures in outdoor enclosures
- Overlooking coordination: Installing mismatched SPDs that don’t properly coordinate energy dissipation
- Incomplete protection: Protecting only the DC or AC side, leaving vulnerabilities
- Using AC SPDs for DC protection: AC and DC SPDs are NOT interchangeable due to their different arc-quenching capabilities
- Compromising on quality: Choosing the cheapest option rather than properly certified devices
- Improper grounding: Installing even the best SPDs with inadequate grounding systems
- Missing indicators: Selecting devices without status indicators, making maintenance difficult
Conclusion: Protecting Your Solar Investment
Selecting the right SPD for your solar system requires careful consideration of system characteristics, environmental factors, and protection requirements. By properly assessing your needs and implementing a coordinated protection strategy, you can significantly reduce the risk of surge-related damage.
Remember these key takeaways:
- Choose SPDs specifically designed and rated for photovoltaic applications
- Match voltage ratings to your system’s requirements
- Implement protection on both DC and AC sides
- Select appropriate protection levels based on geographical lightning risk
- Ensure proper installation according to manufacturer guidelines
- Maintain and replace SPDs according to recommended schedules
The relatively small investment in quality surge protection can prevent thousands of dollars in potential damage and system downtime. Don’t compromise on protecting your solar system—it’s an essential component of ensuring your renewable energy investment delivers returns for decades to come.
