Various devices in solar energy systems are naturally exposed to outdoor environments, with components installed on rooftops or open ground and DC cables extensively laid out, facing the constant assault of natural conditions. This makes photovoltaic systems the most direct target for lightning surges.
Therefore, it is essential to install surge protection devices (SPDs) to provide surge protection for photovoltaic systems. SPDs are the fundamental prerequisite for ensuring equipment safety, meeting IEC compliance requirements, and maintaining the installer’s own responsibilities. Correct selection of SPDs is crucial.
Why Solar PV Systems Need Both DC and AC SPD
Photovoltaic systems require surge protectors to be installed on both the DC side and the AC side, as surges can invade through two pathways: first, lightning induction, internal array transients, and long-distance cable coupling on the DC side directly threaten the inverter’s DC input and combiner box; second, grid switching, lightning entering the grid, and load switching surges on the AC side affect the inverter’s AC output, distribution board, and grid connection circuit.
For compliance purposes, different protection requirements should correspond to both the photovoltaic DC side and AC side respectively. Only by properly configuring SPDs (Surge Protective Devices) on both sides can a more comprehensive system protection strategy be established.
Understanading SPD Types: Type 1 SPD, Type 2 SPD, Type 3 SPD
IEC 61643 classifies SPDs into three types, each designed for different installation locations, surge sources, and protection objectives. In photovoltaic systems, this classification system applies to both the DC side and the AC side.
Type 1 SPD
Type 1 SPD is installed at the forefront of surge entry into buildings or systems and is specifically designed to withstand high currents. Its core components use metal oxide varistors (MOV) or gas discharge tubes (GDT), capable of diverting large currents to the ground in an extremely short time.
The test standard for Type 1 adopts a 10/350μs waveform, which simulates the characteristics of real direct lightning currents and has significantly more energy than the 8/20μs waveform. Therefore, Type 1 is larger in size and higher in cost but serves as an irreplaceable first level of protection in high-risk scenarios.
Type 2 SPD
Type 2 SPD is the most widely used protection type in photovoltaic systems. It is installed on the equipment side to protect against surge currents conducted or induced through cables, with its core component being a metal oxide varistor (MOV).
Type 2 uses an 8/20μs test waveform, with a discharge current range of 8kA to 65kA. The voltage protection level (Up) is usually lower than that of Type 1, providing more precise voltage clamping protection for precision equipment such as inverters.
Type 3 SPD
Type 3 SPDs are installed at the terminals of sensitive equipment to provide final clamping protection against residual surges. They have a smaller discharge current capacity (1.5kA–10kA) but an extremely fast response time (<25ns), specifically designed to protect precision electronic devices that cannot withstand any surge residual voltage.
Type 3 SPD does not have independent protection capability and must be installed downstream of Type 1 or Type 2, working in coordination with the upstream SPD. Using Type 3 alone will result in immediate damage due to energy overload when a large surge occurs.
| Item | Type 1 | Type 2 | Type 3 |
| Protection Target | Large surge current | Induced / conducted surges | Residual surges at sensitive end devices |
| Test Waveform | 10/350 μs | 8/20 μs | 1.2/50 μs |
| Discharge Current Level | 25–100 kA (Iimp) | 8–65 kA (In) | 1.5–10 kA |
| Typical Installation Location | Combiner box / Service entrance | Inverter side / Distribution board | Communication lines / Monitoring equipment |
| Applicable for DC / AC Systems | Yes | Yes | Yes |
| Standalone Use | Yes (high-risk environments) | Yes (standard applications) | No – must be used with Type 1 or Type 2 SPD |
DC SPD Selection: Protecting the Array Side
The installation position of the DC SPD directly determines the protection effect. In photovoltaic systems, there are two key installation points on the DC side, each corresponding to a different type of SPD and protection objective.
Install Node 1: Photovoltaic Array to PV Combiner Box
The combiner box is the first convergence point for DC-side surges entering the system. The cables of the photovoltaic array are concentrated here, and the extensive cable layout makes this location highly sensitive to lightning-induced voltages.
- Ground-mounted power station: The combiner box is fully exposed to an open environment, and a Type 1+2 SPD must be selected as the first level of protection to withstand possible high currents.
- Rooftop system (high lightning area): It is recommended to select a Type 1+2 combination SPD at the combiner box or array junction box.
- Rooftop system (low to medium lightning area): A Type 2 SPD can be selected at the combiner box as the starting point for array-side protection.
Installation Node 2: Combiner Box to Inverter DC Input
The DC input side of the inverter is the most critical protection point on the DC side, and a Type 2 DC SPD must be selected here regardless of the system size.
- Installation principle: The SPD should be as close as possible to the inverter’s DC terminal, with a wiring length controlled within 30cm.
- Protection objective: Clamp residual surges conducted through cables to a safe voltage level that the inverter can withstand (Up ≤ inverter DC-side withstand voltage).
- Coordination with combiner box SPD: If the combiner box is already equipped with Type 1, there must be >10m cable spacing between them to ensure proper cascading coordination.
5 Key Selection Parameters
Maximum Continuous Operating Voltage Uc
Uc is the most error-prone and critical parameter in DC SPD selection. It defines the maximum DC voltage that the SPD can withstand for a long time without conducting. Once the system voltage exceeds Uc, the SPD will continuously malfunction, causing rapid aging and failure of MOV components within weeks.
Selection formula:
Uc ≥ System maximum Uoc × 1.15
The maximum system Uoc must be calculated under low-temperature conditions. Low temperatures in winter can increase the open-circuit voltage of the modules, which is the most common reason for selecting a lower Uc rating.
Discharge Current Rating In / Imax
The discharge current parameter determines the magnitude of surge current that the SPD can safely divert. DC SPD has two related current specifications:
- In (Nominal Discharge Current): The test current that the SPD can repeatedly withstand, used for classification and model labeling.
- Imax (Maximum Discharge Current): The maximum single surge current that the SPD can withstand, typically 1.5–2 times of In.
The discharge current level should be selected based on the ground flash density (Ng value) at the project location.
Voltage Protection Level Up
Up is the maximum voltage peak that appears across both ends of the SPD during a surge—essentially, the highest voltage your inverter actually withstands during a surge event. The lower the Up, the more precise the protection for the equipment.
Selection principle: Up must be lower than the DC-side voltage resistance level of the inverter
The DC-side surge withstand voltage of most modern string inverters is 4kV to 6kV. When selecting, ensure:
Up < Inverter DC side withstand voltage × 0.8 (leave 20% safety margin)
The Up value is not necessarily better the lower it is—an excessively low Up means that the SPD will trigger during normal voltage fluctuations, accelerating aging. The Up should strike a balance between protection effectiveness and stability under normal operating conditions.
Short Circuit Current Rating SCCR
SCCR is the DC SPD parameter most easily overlooked by photovoltaic installers and also poses the greatest potential safety hazard. It defines the maximum short-circuit current that the SPD can safely withstand without causing an explosion or fire in case of internal failure.
Selection rules:
The SCCR of the SPD ≥ The expected short-circuit current (Isc) at the installation point
| System Size | Typical DC Side Isc | Minimum Required SCCR |
| Residential 3–15 kW | 500–2,000 A | ≥ 5 kA |
| Commercial 50–250 kW | 3,000–15,000 A | ≥ 25 kA |
| Utility-scale 1 MW+ | 10,000–30,000 A | ≥ 50 kA |
Ingress Protection Rating
DC SPDs are typically installed inside combiner boxes or next to inverters, and the environmental conditions at these locations directly determine the required IP protection level. Insufficient selection can lead to moisture and dust accumulation inside the SPD, accelerating failure or even causing short circuits.
IP Rating Selection Rules
| Installation Environment | Minimum IP Rating | Typical Application |
| Indoor inverter area | IP20 | Indoor equipment room, distribution room |
| Outdoor combiner box | IP65 | Standard rooftop / ground-mounted PV systems |
| Coastal high-humidity environment | IP66 | Coastal projects, high salt mist areas |
| Desert / dusty environment | IP66 / IP67 | Desert utility-scale solar plants |
| Direct submersion risk area | IP67 / IP68 | Flood-prone ground-mounted solar plants |
AC SPD Selection: Protecting the Grid Side
The surge threat on the AC side comes from the grid direction—lightning strikes on the grid, transformer switching, and capacitor bank operations generate transient overvoltages that reverse into the inverter’s AC output end and distribution system through AC lines. The installation position of the AC SPD determines the scope and precision of protection.
Installation Node 1: Inverter AC Output Terminal
The AC output of the inverter is the most critical protection point on the AC side and also the standard installation location for all photovoltaic systems. The Type 2 AC SPD selected here serves a dual purpose: intercepting incoming surges from the grid side and preventing internal transients generated by inverter switching from spreading to the grid.
- Installation principle: The SPD should be installed between the inverter AC terminal and the AC circuit breaker, with a wiring length of <50cm.
- Single-phase system: 1P+N or 2P configuration, depending on the type of grounding system.
- Three-phase system: 3P+N configuration, covering phase-to-ground and interphase protection.
- Applicable scale: Suitable for residential to large commercial systems; it is the minimum compliance requirement for AC side protection.
Installation Node 2: Main Distribution Board
The main distribution board is the service entrance where the power grid enters the building, and it is also the first convergence point for AC surges in the entire electrical system. In areas with high lightning activity or systems containing overhead lines, a Type 1 AC SPD must be selected at the service entrance as the first level of protection for buildings.
- Type 1 Application Scenarios: Service entrance with overhead incoming lines, high lightning areas (Ng>25), commercial/industrial power distribution systems
- Type 2 Application Scenarios: Residential distribution boards in low to medium lightning areas, as a supplementary protection layer for the service entrance
- Coordination with inverter-side SPD: Maintain >10m cable distance between main distribution board SPD (upstream) and inverter-side SPD (downstream) to ensure cascading coordination
4 Key Selection Parameters
Maximum Continuous Operating Voltage Uc
Uc is the highest AC operating voltage that an AC SPD can withstand for a long time without conducting, equivalent to the concept of Uc in DC SPDs. Selecting a low Uc value is the primary cause of premature failure in AC SPDs, as the grid voltage itself has a normal fluctuation range of ±10%, and Uc must cover this upper limit of fluctuation.
Selection formula:
Uc ≥ System rated voltage × 1.1 (minimum value), it is recommended to leave a margin of 15–20% for actual selection.
Uc Reference Table
| System Type | Nominal Voltage | Minimum Uc Calculation | Recommended Minimum Uc Rating | Common Standard Ratings |
| Single-phase | 120V (North America) | 132V | 150V | 150V / 175V |
| Single-phase | 230V (Europe / Australia) | 253V | 275V | 275V / 320V |
| Single-phase | 240V (Australia / UK) | 264V | 275V | 275V / 320V |
| Three-phase | 400V L-L (Europe) | 440V | 440V | 440V / 480V |
| Three-phase | 480V L-L (North America) | 528V | 550V | 550V / 600V |
| Three-phase | 690V L-L (Industrial) | 759V | 760V | 760V / 800V |
Discharge Current Rating In / Imax
The selection logic for AC-side discharge current is the same as that for the DC side, also referencing the ground flash density (Ng value) of the project location. However, the current level requirement on the AC side is usually lower than that on the DC side because the AC side benefits from natural shielding provided by building structures and distribution systems, and surge energy has already attenuated before reaching the AC side.
| Installation Point | System Scale | Recommended In | SPD Type | Notes |
| Inverter AC output | Residential single-phase | 20 kA | Type 2 | NEC 285 minimum requirement |
| Inverter AC output | Commercial three-phase | 40 kA | Type 2 | Standard commercial configuration |
| Main distribution board | Residential (low–moderate lightning risk) | 20–40 kA | Type 2 | Additional protection at service entrance |
| Main distribution board | Commercial (high lightning risk) | ≥ 25 kA Iimp | Type 1 | Standard for high-risk applications |
| Service entrance | Overhead line supply | ≥ 25 kA Iimp | Type 1 | Based on 10/350 μs lightning current waveform |
| Service entrance | Utility-scale PV plant | ≥ 25 kA Iimp | Type 1 | Mandatory for large-scale systems |
It is recommended to install an SPD at both the AC output of the inverter and the main distribution board in a commercial three-phase system, forming dual-level AC protection. The cable distance between the two is usually naturally over 10 meters, requiring no additional coordination measures.
Voltage Protection Level Up
The Up requirement on the AC side is stricter than that on the DC side. The electronic components at the AC output of the inverter are more sensitive to surge residual voltage, and the withstand voltage level of precision instruments, power metering devices, and communication modules in distribution systems is usually only 1.5kV to 2.5kV.
Two-level Up objectives:
- Basic protection: Up ≤ 2.5kV – Meets the withstand voltage requirements on the AC side of the inverter, preventing damage to the inverter output stage.
- Precision protection: Up ≤ 1.5kV – Suitable for systems with precision monitoring equipment, smart meters, or energy storage BMS.
Pole Configuration: 1P / 2P / 3P+N
The number of poles of the AC SPD must match the grounding method of the system—this is a structural parameter that affects protection integrity. Choosing the wrong number of poles may result in certain surge paths being completely unprotected.
Pole Configuration Quick Reference
| Grounding System Type | Typical Region | Recommended Poles | Protected Paths |
| Single-phase TT system | Europe, parts of Australia | 2P | L-PE + N-PE independent protection paths |
| Single-phase TN-S system | Europe, Asia | 1P+N | L-N + N-PE coordinated protection |
| Single-phase TN-C system | Legacy buildings | 1P | L-PEN single protection path |
| Single-phase 120/240V split-phase | North America | 2P | L1-N + L2-N dual-line protection |
| Three-phase TT / TN-S system | European commercial | 3P+N | Three-phase to earth + neutral protection |
| Three-phase TN-C-S system | Industrial distribution | 3P+N | Full path protection |
| Three-phase 480V delta system | North American industrial | 3P | Phase-to-phase protection (no neutral) |
When the grounding system type is uncertain, prioritize choosing 3P+N (three-phase) or 2P (single-phase), these two configurations provide the most comprehensive protection paths and are suitable for the two most mainstream grounding systems, TT and TN-S.
Common Mistakes Encountered When Choosing SPD for the Solar System
| Common Mistakes | Problem Description | Possible Consequences |
| Only focusing on price, not parameters | Selecting SPD based only on cost, ignoring key parameters such as Uc, In, Imax, and Up | Ineffective protection or premature SPD failure |
| Incorrect Uc selection (too low) | SPD continuous operating voltage is lower than actual system operating voltage | Overheating, nuisance tripping, or SPD burnout |
| Using Type 2 in direct lightning zones | Installing only Type 2 SPD in areas with LPS or high lightning exposure | Cannot withstand lightning current; high risk of equipment damage |
| Mixing AC SPD with DC applications | Using AC SPD on photovoltaic DC side | Insufficient arc extinguishing capability, safety hazard |
| Ignoring Up protection level | Not checking whether residual voltage is below equipment withstand level | Inverters or monitoring devices may still be damaged |
| Excessive SPD installation distance | SPD installed too far from protected equipment | Increased lead inductance, reduced protection effectiveness |
| Poor grounding system | High grounding resistance or excessively long grounding conductors | SPD cannot effectively discharge surge energy |
| Ignoring grounding system type | Not selecting poles/configuration based on TN-S, TT, IT systems | Improper operation or protection failure |
| No backup protection device | Missing fuse or circuit breaker coordination | Possible short circuit or fire during SPD failure |
| Protecting only AC side | SPD installed only on AC distribution, no DC-side protection | PV modules and inverter DC side remain exposed to surges |
| No remote monitoring function | Commercial systems without SPD remote signaling contacts | No alarm feedback, inefficient maintenance |
| Using non-certified SPD | Products without IEC certification | Compliance risks and unreliable performance |
| Not replacing aged SPD | SPD not replaced after end-of-life degradation | Loss of protection capability without visible signs |
| Lack of coordination between SPD stages | No energy coordination between Type 1 and Type 2 SPD | Overstress and failure of upstream or downstream SPD |
| Ignoring local lightning density (Ng) | No design based on regional lightning risk | Undersized or over-specified SPD selection |
LSP Surge Protection Solutions for Solar Systems
LSP DC and AC SPD Products
Since 2010, LSP has provided surge protective devices (SPDs) for both DC and AC solar systems. LSP offers:
- AC SPDs: Type 1, Type 2, and Type 3 for main panels, distribution panels, and sensitive loads.
- DC SPDs: Type 1+2 and Type 2 for photovoltaic arrays and inverters.
These devices clamp overvoltage and safely divert surge current to ground during lightning or switching events. LSP SPDs are compatible with solar panels, inverters, and energy storage systems, helping engineers select the right SPD for each part of a solar setup.
Key Features and Certifications
| Feature | Description |
| Status Window | Green = normal, Not green = problem |
| Fast Response | Stops overvoltage in nanoseconds |
| IEC Compliance | Meets IEC 61643-31 (DC) & IEC 61643-11 (AC) |
| Certifications | TUV, CB, CE |
| Warranty | 5 years standard, up to 10 years optional |
LSP SPDs are made with durable materials, fast reaction to surges, and clear visual indicators. The certifications ensure reliable protection and international compliance.
Why Choose LSP for Solar Systems
- Trusted by over 1,200 customers in 35 countries.
- Fast shipping: most orders within 15 days.
- Reliable protection for solar panels, inverters, and connected loads.
- Expert support for selecting the right SPD based on system voltage, current, type, wiring, and grounding.
Tip: Always install SPDs close to the equipment, use proper wiring and grounding, and check the status window regularly to maintain full protection. LSP provides certified SPDs and expert guidance to ensure solar systems remain safe from lightning surge and switching events.
Conclusion
To protect the solar energy system from surge damage, the correct AC SPD should be selected for the AC side, and the correct DC SPD should be selected for the DC side. The two cannot replace each other because the voltage characteristics, surge paths, and protection requirements of AC circuits and DC circuits are different.
FAQ
What is a Surge Protective Device (SPD) in a solar system?
A surge protective device (SPD) is a critical safety device protecting solar components from transient overvoltages caused by lightning or switching. Within nanoseconds, it shunts excess energy to ground and limits voltage, shielding expensive inverters and panels from destruction, ensuring long-term system reliability and operational safety in harsh environments.
What IEC standards apply to SPDs for solar systems?
IEC 61643-31 is the primary standard for DC protection in solar systems, while IEC 61643-11 applies to the AC side. Additionally, IEC 60364-7-712 and IEC 62305 govern installation and risk assessment, ensuring proper coordination, safety design, and system reliability in photovoltaic applications.
What SPD type should engineers use for solar panels?
Engineers must use DC SPDs certified to IEC 61643-31. Use Type 1+2 for high-risk or utility-scale sites to handle direct strikes, and Type 2 for standard rooftops to block induced surges, ensuring proper coordination with system voltage levels, grounding design, and installation environment conditions for optimal protection performance.
What happens if the SPD is not grounded properly?
Improper grounding renders an SPD useless by blocking the “floodway” for surge current. High-impedance connections increase residual voltage, allowing destructive spikes to reach sensitive electronics. This leads to catastrophic inverter failure, fire hazards, and voided warranties, and may also cause repeated system downtime and costly maintenance interventions over time.
What maintenance does an SPD require?
An SPD requires regular visual inspections of its status indicator window. Green signifies healthy operation, whereas Red indicates a failed module requiring immediate replacement. Additionally, check for terminal tightness and grounding integrity to ensure long-term stable performance and continuous system protection reliability.
