IEC 61643-31 Explained: The Standard Behind PV DC Surge Protection

Surge protective devices intended for the direct-current side of photovoltaic systems are evaluated against a distinct set of requirements. In many markets, the reference document for this application is IEC 61643-31, which addresses low-voltage surge protective devices for photovoltaic installations. For buyers and specifiers, an understanding of what this DC SPD standard requires can support more consistent product comparison and help avoid configurations that may not align with the intended application. This article reviews the principal test parameters defined in IEC 61643-31, compares them with the AC-oriented IEC 61643-11, and outlines practical checks for assessing a photovoltaic surge protective device.

1. What IEC 61643-31 Covers

IEC 61643-31 specifies performance and testing requirements for SPDs connected to the DC side of PV power supply systems. It forms part of the IEC 61643 series, which is structured so that Part 11 covers SPDs connected to low-voltage AC power systems, while Part 31 covers SPDs for PV DC circuits. The separation reflects the different electrical behaviour of DC circuits — in particular, the absence of a natural current zero-crossing, which influences how a DC SPD interrupts follow current after a surge event.

Because of this, a PV DC SPD is not simply an AC device relabelled for solar use; its construction and test basis are addressed by the DC SPD standard. The standard applies to devices intended to limit transient overvoltages and divert surge currents on the PV string and array side, typically between the modules, combiner boxes, and the inverter DC input. Many manufacturers, including Cresin, publish a dedicated Solar PV SPD product range that is declared to IEC 61643-31 for the DC side and IEC 61643-11 for the AC side.

2. Key Test Parameters: Uc, In, Imax, Up, TOV

2.1 Uc — Maximum Continuous Operating Voltage

The maximum continuous operating voltage, Uc, is the highest DC voltage the PV DC SPD can withstand continuously without conducting. IEC 61643-31 requires the declared Uc to be marked and verified. For PV applications, Uc selection is influenced by the array's open-circuit voltage, which can rise at low temperatures owing to the negative temperature coefficient of crystalline silicon modules (commonly near −0.3%/°C). Selecting Uc with margin for this effect is a recurring step in PV DC SPD specification.

A device rated for a 1500V system is therefore typically declared with a Uc above the highest expected operating voltage — for example, DC 1800V Uc is commonly applied where the array Voc may approach or exceed 1500V under cold conditions. Selecting Uc with adequate margin is a recurring theme in PV DC SPD specification, and it is one of the first values a buyer should confirm against the project's worst-case array voltage.

2.2 In — Nominal Discharge Current

The nominal discharge current, In, is the peak value of an 8/20µs impulse current that the SPD can withstand a defined number of times (commonly 15 positive and 15 negative impulses) without failure. It represents the rated discharge capability for repeated surge exposure. IEC 61643-31 requires In to be declared and tested. For a PV DC SPD such as the CSMS-B40, an In of 20kA (8/20µs) is a typical declared value.

In is frequently used as a basis for comparing devices of similar application, though it is reviewed together with Imax and Up rather than in isolation.

2.3 Imax — Maximum Discharge Current

The maximum discharge current, Imax, is the peak 8/20µs impulse current the SPD can withstand once without destruction. It is generally higher than In and indicates the upper surge level the device can survive for a single event. For DC PV SPDs, Imax values such as 40kA are commonly declared. The ratio and gap between In and Imax, together with the residual voltage at each level, provide information about the device's margin under severe but non-recurring events.

2.4 Up — Voltage Protection Level

The voltage protection level, Up, is the maximum voltage appearing across the SPD terminals under a specified discharge current. IEC 61643-31 requires Up to be declared; lower values generally indicate less overvoltage transmitted to protected equipment. For effective coordination, Up should be considered relative to the impulse withstand capability of the downstream components.

A PV DC SPD's Up is typically evaluated at In, and manufacturers may also provide values at Imax. When reviewing a photovoltaic surge protective device, comparing Up at the same reference current gives a more consistent picture than comparing headline numbers alone. This is particularly relevant where the DC side feeds sensitive inverter electronics.

2.5 TOV — Temporary Overvoltage Withstand

Temporary overvoltage (TOV) is a power-frequency overvoltage that can appear for a longer duration than a surge — for instance, due to a fault on an adjacent circuit or a neutral displacement. IEC 61643-31 includes TOV withstand requirements, where the SPD is expected to either survive the TOV without damage or disconnect safely. For DC PV SPDs, TOV conditions are defined for both line-to-line and line-to-earth configurations.

A device that fails to manage TOV appropriately may enter a sustained conducting state, which is why the standard addresses this separately from the impulse current tests. Buyers reviewing a DC SPD standard declaration should confirm that TOV test conditions and outcomes are stated, not omitted.

2.6 Iimp — Where Combined Devices Are Assessed

Some PV DC SPDs are classified as combined Type 1 and 2, meaning they are tested for both the 10/350µs impulse current (Iimp, associated with direct lightning current) and the 8/20µs nominal current (In/Imax). IEC 61643-31 addresses such combined devices within the PV DC context. A PV SPD carries both an Iimp rating and an In/Imax rating, allowing a single device to address both partial lightning current at the origin and induced surges downstream. 

3. IEC 61643-31 vs IEC 61643-11: Comparison Table

The table below summarises the principal differences a buyer is likely to encounter when comparing a DC-side device evaluated to IEC 61643-31 with an AC-side device evaluated to IEC 61643-11. Note that the two standards address different circuit types, and a PV system typically uses both — one on the DC side and one on the inverter AC output.

4. Why the Two Standards Differ in Practice

Beyond the parameter list, the practical distinction lies in the circuit type each standard addresses. IEC 61643-11 uses AC test methods for low-voltage distribution, while IEC 61643-31 uses DC-specific arrangements that reflect the absence of zero-crossing and the higher voltages in PV strings. A PV DC SPD is tested for DC arc interruption; an AC SPD benefits from natural zero-crossings.

Consequently, a device certified only to IEC 61643-11 is not, by that certification alone, confirmed suitable for the PV DC side. A specification that simply requests "an IEC 61643 SPD" without stating the part number may leave this distinction unaddressed, which is why the part (IEC61643-31 vs IEC61643-11) is worth stating explicitly in a procurement document.

5. How Buyers Can Verify Compliance

Several checks are commonly applied before a specification is finalised:

• Confirm the certificate references IEC 61643-31 (not only IEC 61643-11) for DC-side models.
• Match the declared Uc to the array's worst-case Voc, accounting for low-temperature conditions.
• Verify that In, Imax, Up and TOV values are all declared and consistent with the test report.
• Check that the certified model number matches the product offered, including voltage class and pole configuration.
• Request the test report from an accredited laboratory where procurement volume or project risk justifies it.
• Review additional marks (such as TÜV or CB) that may be required by the target market.

For projects that need a coordinated DC-and-AC selection, the Cresin technical team can review the array voltage, inverter interface, and earthing arrangement. Readers are welcome to contact us to discuss a specific installation.

6. Common Compliance Gaps to Watch

Observations in the field sometimes include DC SPDs offered with only an IEC 61643-11 mark, Uc values selected without temperature margin, or Up figures quoted without a reference current. These gaps may not be apparent from a short datasheet, which is why a review of the full declaration and certificate is advisable before specifying. Pole configuration is a second recurring point: a device declared for a particular DC topology should match the plant's wiring, as confirmed against the single-line diagram.

7. Frequently Asked Questions

8. Summary

IEC 61643-31 provides the test framework for surge protective devices on the DC side of photovoltaic systems, with Uc, In, Imax, Up and TOV as the parameters most relevant to specification. Its DC-specific test basis distinguishes it from IEC 61643-11, which addresses the AC side. By confirming the correct standard part and reviewing the full declared parameters, buyers can select a PV DC SPD that is better aligned with the demands of solar surge protection in their installation.