Selection Surge Protection Devices Selection for PV Systems - SPDs Installation
1. Factors Affecting SPD Installation
Overvoltages in rooftop photovoltaic (PV) systems are primarily caused by direct lightning strikes (on or near the building or PV equipment with LPS), induced voltage from lightning-related electric field changes, or switching operations in AC/DC power distribution networks. To assess the need for surge protection and determine the appropriate protective strategy, the rated impulse withstand voltage (Uw) of the equipment should be used as a reference. Unless specified otherwise by a risk assessment, SPDs should be installed on both the DC and AC sides of the PV system.
Devices within a PV system that may require protection include:
Inverters, including interfaces to both the low-voltage AC system and the DC system, PV arrays, internal PV system wiring, components installed between the inverter and PV arrays, such as combiner boxes; equipment used for monitoring and control of the PV system.
According to IEC 61643-12 and relevant sections of GB/T 21714, the selection and installation of SPDs for PV systems depend on multiple factors. These include ground flash density (NG) [1/km²·year] or the average number of thunderstorm days per year (T), characteristics of the low-voltage power supply system (e.g., overhead lines or underground cables) and the nature of the equipment being protected, whether an external Lightning Protection System (LPS) is installed to protect the PV system from direct lightning strikes.
If an external LPS is present, the SPD requirements are influenced by the protection level (Class) of the LPS and whether the required separation distance (S) between the LPS and the PV system is maintained (isolated vs. non-isolated LPS).
2. Influence of Different External LPS Configurations on Lightning Protection for PV Systems
SPD installation for rooftop PV systems generally falls into three categories, based on the presence and type of external LPS:
(1) PV Systems Without External LPS
As shown in the diagram (not included here), if the building is not equipped with an external lightning protection system, SPDs should be installed based on the outcome of a lightning risk assessment. It is recommended to install surge protection devices on both the AC and DC sides to protect critical PV components.
For DC side, a Type 2 DC SPD (designated as DC1) should be installed near the PV combiner box or the PV array (if no combiner box is used). This SPD protects the building from external surges transmitted through the PV cables and is typically installed just after the PV cables enter the building. Another Type 2 DC SPD (designated as DC2) should be installed near the DC input of the inverter. If the cable length between DC1 and the inverter is less than 10 meters, DC1 may be omitted.
For AC side, a Type 2 AC SPD (designated as AC1) is recommended at the AC mains input, usually located at the main distribution panel. This device protects against surges coming from the utility grid. Whether AC1 is needed in a system without external lightning protection depends on the results of a lightning risk assessment. To protect the expensive inverter, a Type 2 AC SPD (designated as AC2) is typically installed near the AC output of the inverter. If the cable length between AC1 and AC2 is less than 10 meters, AC2 may be omitted. Please refer to the configuration table for detailed guidance.
(2) PV Systems with External LPS That Meet the Required Separation Distance (Isolated LPS)
As shown in the diagram, when a building has an external lightning protection system (such as air-termination rods or tapes) and the rooftop PV array is protected by this system with proper separation distance maintained, the PV system is considered to be equipped with an isolated LPS. Even in this case, installing SPDs is still necessary.
For systems with an isolated LPS, the SPD setup is similar to systems without any external lightning protection. A Type 2 DC SPD should be installed near the combiner box, or near the PV array if no combiner box is used. Another Type 2 DC SPD should be placed near the DC side of the inverter, and a Type 2 AC SPD near the AC side of the inverter. However, because the PV system connects in and out of the building, a Type 1 AC SPD is also required at the main AC distribution board where the utility power enters the building. Refer to the configuration table for detailed layout guidance.
(3) PV Systems with External LPS That Don't Meet the Required Separation Distance (Non-Isolated LPS)
In rooftop PV systems where the panels are too close to the external lightning protection system (LPS) and can't meet the required safety distance, all metal parts of the PV system need to be connected to the LPS to ensure proper lightning equipotential bonding. This is the common approach used for rooftop lightning protection. In this setup, both AC and DC cables are treated like parallel conductors in the bonding system, which means they can be directly affected by lightning current. That's why the IEC recommends using Type 1 surge protective devices (SPDs)—usually together with Type 2 SPDs—at key points in the system: where the PV cables enter the building, and onboth the AC and DC sides of important devices like the inverter.
As shown in the diagram, DC1 should be installed as close as possible to the combiner box, or near the PV array if there's no combiner box. DC2 and AC2 should be placed near the inverter to give it better protection. In most cases, AC1 (at the main panel) and AC2 (near the inverter) are both needed. But if the inverter is installed inside the main panel and they share the same ground, with cable lengths less thanor equal to 0.5 meters, then AC2 might not be necessary. You can check the table for detailed installation guidance.
1：DC1 may be omitted under the following conditions: If the cable length between DC1 and the inverter is less than 10 meters, and the voltage protection level (Up) of AC2 is less than or equal to 0.8 times the PV array's rated impulse withstand voltage (Uw); or if the Up of AC2 is less than or equal to 0.5 times Uw and the PE (protective earth) conductor is routed close to the DC cables.
2：AC2 may be omitted under the following conditions: If the cable length between AC1 and AC2 is less than 10 meters; or if the inverter and the main distribution board share the same grounding electrode, and the SPD connection cables are each no longer than 0.5 meters—for example, if the inverter is installed inside the main panel.
3：Minimum cross-sectional area for SPD grounding conductors: For Type 2 SPDs, the copper grounding conductor should be no less than 6 mm²; for Type 1 SPDs, it should be no less than16 mm².
The choice of DC-side SPDs in rooftop PV systems mainly depends on how the building is protected from lightning and how the system is wired. According to the configuration table and IEC standards, there should always be a surge protector on the DC side of the inverter. But extra SPDs near the combiner box—or near the PV panels if there’s no combiner box—are only needed if the cable between the panels and the inverter is longer than 10 meters.
That said, this isn't how it's done everywhere. In some countries—Germany, for example—the rules are basically the opposite. There, DC surge protection is always installed right next to the PV panels, and you only need to add one near the inverter if the cable run is over 10 meters.