Solar PV SPD: AC Side vs DC Side Protection Strategy

• Why PV Surge Protection Spans Two Standards
• PV System Architecture: Where SPDs Are Installed
• DC Side Protection: IEC 61643-31 Requirements
• AC Side Protection: IEC 61643-11 Requirements
• AC/DC PV SPD Parameter Comparison Table
• Voltage Levels: The Evolution from 600V to 1500V DC
• Configuration Principles for PV Systems
• Selection Considerations to Review
• Frequently Asked Questions
• Summary

Photovoltaic systems face surge threats on both the DC side (between solar panels and inverter) and the AC side (between inverter and grid). These two sides are governed by different IEC standards, operate at different voltage levels, and require different SPD configurations. This guide explains the distinction and helps engineers approach PV SPD selection systematically.

Why PV Surge Protection Spans Two Standards

A photovoltaic installation is not a single electrical environment. The DC side—carrying power from solar panels to the inverter—operates with direct current at voltages that have risen to 1500V DC in utility-scale plants. The AC side—carrying inverter output to the grid or local loads—operates with alternating current at standard low-voltage levels. Surge protective devices applied on these two sides face different electrical stresses and are evaluated under different IEC test standards, which is why AC DC surge protection for PV systems is addressed as a two-part strategy.

This is why solar surge protection is generally addressed as a two-part strategy: DC-side SPDs tested per IEC 61643-31 (the standard for SPDs in photovoltaic installations), and AC-side SPDs tested per IEC 61643-11 (the standard for low-voltage AC power SPDs). Understanding how these two standards differ in test methodology, voltage classification, and configuration principles helps in specifying PV SPDs that are suited to each side of the system. The Solar PV SPD product range from Cresin covers both DC-side and AC-side devices designed to the respective standards.

PV System Architecture: Where SPDs Are Installed

In a typical grid-connected PV system, surge protection is applied at several key points. On the DC side, SPDs are installed close to the inverter DC input—between the PV string or array and the inverter—to protect against surges entering via the DC cabling, which is often routed over long distances across rooftops or open ground. On the AC side, SPDs are installed at the inverter AC output or at the AC distribution panel, where they protect against surges entering from the grid or from switching operations within the AC network.

The distinction matters because DC and AC circuits behave differently under surge conditions. DC circuits have no natural current zero-crossing, which makes arc interruption more challenging for DC SPDs. AC circuits benefit from periodic zero-crossings that assist in extinguishing follow currents. These characteristics influence the internal component design and test requirements defined in each standard, and they are the reason AC DC PV SPD selection is approached as two separate decisions rather than one.

DC Side Protection: IEC 61643-31 Requirements

IEC 61643-31 is the international standard specifically addressing surge protective devices connected to photovoltaic DC circuits. It defines test classes, voltage ratings, and performance requirements that account for the sustained nature of DC current and the particular voltage characteristics of PV arrays.

One key consideration on the DC side is the selection of the maximum continuous operating voltage (Uc). The Uc of a DC PV SPD should be higher than the maximum open-circuit voltage (Voc) of the PV array at the lowest expected operating temperature, because PV array voltage rises as temperature drops. For example, a PV string with a rated Voc of 1000V at standard test conditions (25°C) may reach a significantly higher voltage at −10°C. Selecting a Uc that does not account for this temperature coefficient may lead to nuisance conduction or accelerated aging of the SPD.

The DC PV SPDs in Cresin's range cover Uc values from DC670V to DC1800V, corresponding to PV system nominal voltages of 600V, 1000V, 1200V, and 1500V DC. These devices are tested per IEC 61643-31 and carry an In (nominal discharge current) of 20kA and an Imax (maximum discharge current) of 40kA. The connection topology—whether the SPD is connected between positive and negative poles (PV+ to PV−), or between each pole and earth (PV+ to PE, PV− to PE)—depends on the system's earthing arrangement and the inverter's isolation design.

AC Side Protection: IEC 61643-11 Requirements

On the AC side of a PV system, surge protection is addressed using AC power SPDs tested per IEC 61643-11, the same standard applied to conventional low-voltage AC distribution. The AC side typically includes the inverter's grid-tie output, the AC distribution panel, and the connection to the utility grid or local loads.

For most residential and commercial PV systems connected to a 230/400V grid, a Type 2 AC SPD with a Uc of 275V or 385V is commonly selected. For larger commercial or utility-scale installations with higher AC voltage levels—such as systems feeding into a 690V or higher AC network—SPDs with correspondingly higher Uc values are used. Cresin's PV AC SPD range covers Uc values from AC385V to AC1500V, with In ratings from 20kA to 60kA and Imax from 40kA to 100kA, addressing a range of inverter output configurations.

The selection of Type 1 or Type 2 on the AC side follows the same principles as in any low-voltage AC installation: Type 1 (tested with 10/350μs impulse current) is generally considered when the building has an external lightning protection system, while Type 2 (tested with 8/20μs current) addresses indirect lightning and switching surges. More detail on AC-side Type 1 vs Type 2 selection can be found in the AC Power SPD Type 1 vs Type 2.

AC/DC PV SPD Parameter Comparison Table

The table below compares the key parameters and standard requirements for DC-side and AC-side PV SPDs, providing a reference for selecting devices suited to each side of a photovoltaic system.

Voltage Levels: The Evolution from 600V to 1500V DC

The DC voltage of PV systems has increased over the past decade as a means of reducing conductor losses and improving system efficiency in larger installations. Understanding the voltage level of a given PV system is a starting point for selecting the appropriate DC PV SPD.

600V DC systems were common in earlier residential and small commercial installations. At this voltage level, a DC PV SPD with a Uc of approximately DC670V is typically selected, providing margin above the array's Voc at low temperature while remaining below the SPD's maximum continuous rating.

1000V DC systems are widely used in commercial and industrial PV installations. For these systems, a Uc of DC1200V or higher is commonly specified, depending on the temperature coefficient of the PV modules and the expected minimum operating temperature at the site.

1500V DC systems represent the current standard for utility-scale PV plants, where higher DC voltage reduces the number of parallel strings and lowers balance-of-system costs. At 1500V DC, the SPD's Uc is typically selected at DC1800V or higher to accommodate the temperature-induced rise in Voc. The 1500V PV SPD devices in Cresin's range are designed and tested to IEC 61643-31 for these higher-voltage applications.

Note: The Uc selection should account for the PV module temperature coefficient (typically around −0.3%/°C for crystalline silicon modules). At −10°C, a 1500V-rated array may produce a Voc approaching or exceeding 1800V, which is why DC1800V Uc devices are commonly applied in 1500V systems.

Configuration Principles for PV Systems

When configuring surge protection for a PV system, several principles are commonly followed to address the different surge environments on the AC DC sides of the installation:

DC-side configuration. The DC PV SPD is installed as close as practical to the inverter DC input, with short connecting conductors to minimize the additional voltage drop introduced during a surge event. The connection topology (PV+ to PV−, or PV+/PV− to PE) depends on the system earthing arrangement. In ungrounded (floating) systems common in transformerless inverters, SPDs are typically connected between each pole and a functional earth. In grounded systems, the topology follows the earthing point location.

AC-side configuration. The AC SPD is installed at the inverter AC output or at the AC distribution panel feeding the grid connection. The selection of Type 1 or Type 2 follows the same considerations as in conventional AC installations, based on whether the building has an external lightning protection system and the results of a risk assessment.

Coordination between DC and AC sides. DC-side and AC-side SPDs protect against different surge entry paths and do not typically require energy coordination with each other, because they are separated by the inverter's internal isolation. However, each side may benefit from multi-stage coordination within its own domain—for example, a DC SPD at the combiner box and another at the inverter DC input for longer DC cable runs.

For projects where the PV system configuration, earthing arrangement, or voltage level requires a tailored approach, the Cresin technical team can provide guidance. Readers are welcome to contact us for project-specific recommendations, or refer to the Solar PV System Surge Protection solutions page for reference configurations.

Selection Considerations to Review

When evaluating PV SPDs for a solar project, several considerations are worth reviewing:

• Uc vs. Voc at minimum temperature. The DC-side Uc should account for the PV array's open-circuit voltage at the lowest expected operating temperature, not just the rated Voc at standard test conditions. Overlooking this temperature coefficient may result in selecting an SPD with insufficient Uc.
• Standard compliance verification. Confirm that DC-side SPDs are tested to IEC 61643-31 (not merely IEC 61643-11), as the DC test methodology and requirements differ. A DC-rated SPD tested only to AC standards may not provide the intended protection in a PV DC circuit.
• Connection topology and earthing. The SPD connection topology (between poles, or pole to earth) should match the system's earthing arrangement. Mismatched topology may leave certain surge paths unprotected.
• Backup protection. DC PV SPDs require suitable upstream overcurrent protection that can interrupt DC fault current. Standard AC fuses or breakers may not be suitable for DC circuits. Reference the Specialized SPDs and Accessories page for dedicated backup disconnector options.

Summary

Solar surge protection involves two distinct strategies governed by two different standards: DC-side PV SPDs tested to IEC 61643-31 address the sustained DC current and temperature-dependent voltage characteristics of photovoltaic circuits, while AC-side PV SPDs tested to IEC 61643-11 address conventional AC power surge threats on the inverter output side. The selection of Uc, In, Imax, and connection topology should account for the system's voltage level (600V, 1000V, or 1500V DC), earthing arrangement, and the expected minimum operating temperature at the site.

By evaluating the DC and AC sides separately—each according to its applicable standard and electrical environment—engineers can configure PV surge protection that addresses the distinct threats present on each side of the system, in a manner consistent with IEC 61643-31 and IEC 61643-11.