A DC surge protective device may look simple from the outside, but the markings printed on its label contain important information about where the SPD can be used and how it performs.

Consider this example from a 1000V DC solar SPD:

Ucpv: 1000V DC
Class II
In: 20kA
Imax: 40kA
Up: 4.0kV
Iscpv: 10kA
-40°C to +85°C
Indoor, IP20

For an experienced electrical engineer, these abbreviations are familiar.

For a buyer, distributor, solar installer or project purchaser, however, they can easily become confusing.

Does Class II mean a second-grade SPD?

Is 40kA Imax automatically better than 20kA?

Why does the same SPD show both 1000V and 4.0kV?

Does Iscpv 10kA mean that the SPD can discharge a 10kA lightning surge?

The short answer is: no single number describes the whole SPD.

A DC SPD should be read as a complete set of electrical characteristics. If you are new to surge protection, first understanding how a DC SPD works in a solar PV system can make these specifications easier to follow.

This guide explains how to read those markings correctly, using a Class II 1000V DC SPD as a practical example.

Quick Answer: What Do the Main DC SPD Specifications Mean?

Before going into the technical details, the table below gives a simple overview.

The most important point is:

Do not compare DC SPDs only by the largest kA number printed on the label.

For example, Imax 40kA, In 20kA and Iscpv 10kA all use the unit kA, but they describe completely different characteristics.

A Better Order for Reading a DC SPD Label

When buyers first see an SPD datasheet, they often start with the largest number.

That is usually the wrong approach.

A more useful reading order is:

1. Confirm that it is designed for PV DC
2. Check Ucpv
3. Check the SPD Type / Test Class
4. Check Up
5. Check In and Imax
6. Check Iscpv
7. Check the wiring and protection configuration
8. Check environmental ratings
9. Confirm the relevant standard and documentation

This order allows unsuitable products to be rejected before comparing headline surge-current figures.

VIOX uses a similar “reject incompatible specifications first” approach in its datasheet guide; for PV applications, the logic becomes even more important because Ucpv and PV short-circuit behavior need specific attention.

What Does Class II Mean on a DC SPD?

One of the first markings on the example label is:

Class II

This does not mean second-class quality.

Class I, Class II and Class III refer to standardized SPD test classifications, not product quality grades.

For PV surge protective devices, a Type 2 SPD is associated with Class II testing.

This is why different manufacturers or datasheets may use markings such as:

• Type 2
• T2
• Class II

They refer to closely related classification and testing concepts, although technically “Type” and “test class” are not simply two different spellings of the same term.

For most buyers, the practical relationship can be understood as follows:

For the example in this article:

Class II → Type 2 application direction

and therefore the most important current parameters shown are:

In = 20kA
Imax = 40kA

Is Class II Worse Than Class I?

No.

This is one of the most common misunderstandings when comparing SPDs.

The words Class I and Class II can sound like a ranking:

Class I = better
Class II = cheaper or weaker

That is not how SPD classification works.

They are intended for different surge conditions.

A Type 1 / Class I SPD is evaluated for lightning impulse-current capability, where partial lightning current may need to be handled at the installation point.

A Type 2 / Class II SPD is commonly used to limit transient overvoltages caused by induced lightning effects and switching events.

Therefore, the correct question is not:

Which class is better?

It is:

Which SPD type is required at this installation point?

A Type 1 SPD is not automatically the correct choice for every PV installation, and a Type 2 SPD is not automatically inadequate simply because Type 1 exists.

If you need a detailed explanation of this selection question, see our guide:

Type 1 vs Type 2 vs Type 1+2 DC SPD for Solar PV

Your existing article goes deeper into lightning exposure, installation location and when Type 1 capability should be evaluated.

Class I vs Class II DC SPD: What Is the Real Difference?

The biggest technical difference becomes clearer when we look at the test current.

Class I / Type 1

A Type 1 SPD is associated with an impulse-current rating:

Iimp

and a 10/350 μs current waveform is commonly associated with Class I testing.

The 10/350 μs waveform represents a much different current-energy duty than the 8/20 μs waveform used for Type 2 evaluation.

This means:

Iimp cannot be directly compared with Imax just because both are shown in kA.

For example:

Iimp = 12.5kA

and

Imax = 40kA

do not mean that the 40kA Type 2 product is “stronger.”

They refer to different test waveforms and different performance characteristics.

Class II / Type 2

A Type 2 SPD is mainly characterized using:

• In
• Imax
• Up
• Ucpv for PV applications

The 8/20 μs current waveform is associated with Class II discharge-current testing.

Type 2 DC SPDs are commonly evaluated for use in locations such as:

• PV combiner boxes
• DC distribution equipment
• inverter DC inputs
• other coordinated PV protection points

However, the final SPD type must still depend on the actual lightning protection design and installation conditions.

What About Class III?

Class III exists within general SPD classification, but it should not become the main subject when discussing typical PV DC SPD selection.

Class III protection is generally associated with coordinated fine protection close to sensitive equipment.

For solar PV DC protection, buyers will much more commonly encounter:

• Type 2 / Class II
• Type 1
• Type 1+2

That is why this article focuses mainly on Class II and its relationship with Type 2 DC SPDs, rather than treating Class I, II and III as three equally common PV product categories.

What Does Ucpv 1000V DC Mean?

The example label shows:

Ucpv: 1000V DC

Ucpv means:

Maximum Continuous Operating Voltage for PV Application

In simple terms:

Ucpv tells you the maximum DC voltage that may be continuously applied to the SPD’s mode of protection.

IEC 61643-32 defines Ucpv in essentially this way and states that it must be equal to or higher than the maximum PV open-circuit voltage condition relevant to the installation.

This makes Ucpv one of the first specifications that should be checked when selecting a solar DC SPD.

Does “1000V System” Automatically Mean You Need a 1000V SPD?

Not necessarily.

This is important.

A PV system should not be evaluated using only a convenient nominal label such as:

1000V solar system

The actual maximum DC voltage of a PV string depends on factors including:

• module open-circuit voltage, Voc
• number of modules connected in series
• minimum design temperature
• module voltage temperature coefficient

PV module open-circuit voltage normally increases when temperature falls.

For a broader explanation of PV array voltage, string design and related system requirements, see our IEC 62548 PV array design guide.

Therefore, the selection process should be based on the maximum calculated PV open-circuit voltage, rather than simply copying the nominal inverter or system voltage.

A simplified workflow is:

Module Voc

→ Number of modules in series

→ Low-temperature correction

→ Maximum PV open-circuit voltage

→ Select suitable Ucpv

We have already covered this subject separately in our DC SPD Voltage Selection Guide, including 600V, 1000V and 1500V DC applications.

So in this article, the important point is simply:

Ucpv is a continuous-voltage compatibility rating, not a surge-current rating.

Ucpv Is Not the Same as Up

This is another common source of confusion.

Our example SPD shows:

Ucpv = 1000V DC

and

Up = 4.0kV

A buyer may reasonably ask:

Why does a 1000V SPD also have a 4kV voltage rating?

Because these two values describe different things.

So:

Ucpv is about surviving normal system operation.

Up is about limiting transient surge voltage.

They should never be treated as interchangeable voltage ratings.

What Does In 20kA Mean?

The label shows:

In: 20kA

In means:

Nominal Discharge Current

For a Type 2 / Class II SPD, In is associated with an 8/20 μs current waveform.

In technical terms, it represents the crest value of the current through the SPD having the specified 8/20 μs waveshape under the relevant standardized test conditions.

That definition is accurate, but for a buyer it can be simplified:

In is one of the main ratings used to describe the tested surge-discharge performance of a Type 2 SPD.

For this product:

In = 20kA

The important word here is nominal.

It should not be confused with Imax.

What Does Imax 40kA Mean?

The same SPD also shows:

Imax: 40kA

Imax means:

Maximum Discharge Current

For a Type 2 SPD, Imax is also normally associated with the 8/20 μs current waveform.

It represents the manufacturer’s declared maximum discharge-current capability under the applicable test conditions.

Therefore:

In 20kA and Imax 40kA are not duplicate specifications.

They represent different test-duty levels.

In 20kA vs Imax 40kA: What Is the Difference?

This is one of the most important parts of the entire datasheet.

Consider the example:

A simple way to understand them is:

In = nominal discharge-current rating.

Imax = maximum declared discharge-current rating.

One common purchasing mistake is to look only at Imax because it is the larger number.

For example:

SPD A: In 20kA / Imax 40kA
SPD B: In 10kA / Imax 40kA

A buyer may see “40kA” on both products and assume that their surge performance is equivalent.

That conclusion is incomplete.

The full declared performance needs to be checked.

VIOX makes the same useful procurement distinction in its SPD datasheet guide: In and Imax should be read separately, and Imax should not be interpreted as the SPD’s normal repeated discharge capacity.

Does Higher Imax Always Mean a Better SPD?

No.

This is probably the most useful purchasing rule in this article:

The highest kA value does not automatically identify the best SPD.

Suppose two products are advertised as:

SPD A

• Ucpv: 1000V
• In: 20kA
• Imax: 40kA
• Up: 4.0kV

SPD B

• Ucpv: 1000V
• In: 20kA
• Imax: 60kA
• Up: different
• short-circuit rating: unknown

It would be incorrect to select SPD B immediately just because:

60kA > 40kA.

Other questions still matter:

• What is its Type / Class?
• What is In?
• What is Up?
• What is Ucpv?
• What is Iscpv?
• What protection mode is declared?
• What backup protection is required?
• Is it specifically tested for PV DC use?
• Does its documentation match the project requirement?

A larger number may be useful when the complete design calls for it.

But:

More kA is not a substitute for correct coordination.

Can a 40kA Type 2 SPD Replace a Type 1 SPD?

Not simply because it says 40kA.

This is another very common mistake.

A Type 2 product may show:

Imax = 40kA, 8/20 μs

while a Type 1 product may show:

Iimp = 12.5kA, 10/350 μs

Looking only at the peak current numbers:

40 > 12.5

may make the Type 2 SPD appear stronger.

But that comparison is technically incorrect.

The two values are based on different test waveforms and different protection duties.

Therefore:

Imax cannot be used as a substitute for a specified Iimp requirement.

If the project requires Type 1 lightning-current capability, a large Type 2 Imax value alone does not satisfy that requirement.

What Does Up 4.0kV Mean?

The example label shows:

Up: 4.0kV

Up means:

Voltage Protection Level

It describes the SPD’s declared voltage-limiting performance under specified standardized test conditions.

In practical terms, Up helps answer the question:

How effectively does the SPD limit the transient voltage during the relevant test?

Generally, a lower Up can provide a lower level of surge voltage at the SPD terminals.

However, this needs an important qualification.

The printed Up value should not be treated as the exact maximum voltage that every connected inverter or device will experience in a real installation.

Why?

Because the actual voltage seen by the protected equipment can also be affected by:

• SPD connection lead length
• conductor routing
• inductance
• installation arrangement
• earthing and bonding
• distance between the SPD and equipment
• coordination with other SPDs

VIOX correctly makes the same distinction: the declared Up is established at the SPD under standardized conditions, while installation conductors can increase the voltage presented to the protected equipment.

Is Lower Up Always Better?

Not by itself.

It is reasonable to prefer effective voltage limitation, but the selection must remain compatible with the complete electrical system.

A low Up does not compensate for:

• insufficient Ucpv
• wrong SPD Type
• incorrect wiring
• poor earth connection
• unsuitable short-circuit characteristics
• excessively long leads
• incorrect coordination

So instead of saying:

“Always buy the SPD with the lowest Up.”

A more technically correct rule is:

Select an SPD with a suitable Up while also verifying Ucpv, SPD Type, protected equipment withstand level and installation coordination.

What Does Iscpv 10kA Mean?

Now we come to one of the most misunderstood PV-specific parameters:

Iscpv: 10kA

This is particularly important because it also uses the unit kA.

It is easy to look at:

In 20kA
Imax 40kA
Iscpv 10kA

and assume they are three levels of surge current.

They are not.

Iscpv means:

Short-Circuit Current Rating of the SPD for the PV application.

IEC terminology defines Iscpv as the maximum prospective short-circuit current from the power system for which the SPD, together with the specified disconnector, is rated.

A simpler explanation is:

Iscpv tells you whether the SPD and its specified disconnection arrangement are suitable for the prospective PV short-circuit current at the installation point.

This is a fault-current / short-circuit characteristic.

It is not a lightning-surge rating.

Iscpv 10kA Is Not the Same as Imax 10kA

This distinction deserves to be stated clearly:

Iscpv ≠ Imax

Even though both can be expressed in kA.

For example:

Imax

Describes surge discharge-current performance.

Iscpv

Describes the SPD’s PV short-circuit-current rating in conjunction with the specified disconnection arrangement.

So on our example product:

Imax = 40kA

does not mean:

PV short-circuit rating = 40kA.

And:

Iscpv = 10kA

does not mean:

The SPD can only discharge a 10kA surge.

These numbers belong to different electrical events.

Why Is Iscpv Especially Important for PV DC Systems?

PV DC circuits behave differently from ordinary AC circuits.

One important difference is that DC current does not naturally pass through zero every half-cycle as AC current does.

For protection devices, switching equipment and disconnecting arrangements, this makes DC fault interruption an important design consideration.

That is one reason PV-specific SPDs have characteristics and test requirements beyond simply printing:

“DC 1000V”

on an AC-style product.

IEC 61643-31 is specifically intended for SPDs connected to the DC side of photovoltaic installations and establishes PV-specific requirements, ratings and test methods.

Therefore, when buying a PV SPD, do not ask only:

“Is this 1000V DC?”

Also ask:

What is the declared Iscpv and required disconnection arrangement?

In, Imax, Iimp and Iscpv: Do Not Mix Them Up

The easiest way to remember these parameters is with this table:

Notice something important:

Three or four specifications can all contain:

kA

but this does not mean they can be compared directly.

Before comparing any two kA ratings, first ask:

1. What parameter is this?
2. What waveform is used?
3. What SPD Type is involved?
4. Is this a surge test or a short-circuit rating?

This prevents a large number of purchasing errors.

What Does “-40°C to +85°C” Mean?

The label also declares:

-40°C to +85°C

This is the manufacturer’s declared temperature range for the product.

Environmental ratings should not be ignored simply because electrical parameters look correct.

PV equipment may be installed in:

• rooftop combiner boxes
• inverter cabinets
• outdoor electrical enclosures
• industrial PV rooms
• high-temperature regions
• cold-weather installations

The internal temperature of an enclosure can be very different from the normal outdoor air temperature.

Therefore, project engineers should confirm that the SPD’s declared environmental conditions are suitable for the actual enclosure and installation environment.

What Does 5%–95% Mean?

Where the datasheet declares:

5%–95%

this normally represents the manufacturer’s declared relative humidity operating range.

However, humidity specifications should always be read together with conditions such as:

• condensation requirements
• enclosure protection
• installation location
• temperature
• pollution conditions

The percentage alone does not mean that an exposed SPD can be installed directly in rain or other outdoor environments.

What Does Indoor Mean?

The label states:

Indoor

This does not necessarily mean that the SPD can never form part of an outdoor PV installation.

It means the SPD itself is intended for a protected environment according to its declared construction and installation conditions.

For example, an indoor-rated SPD may be installed inside:

• a suitable outdoor PV combiner box
• an IP-rated enclosure
• an inverter cabinet
• a protected electrical cabinet

The important distinction is:

The SPD can be inside outdoor equipment without the SPD itself being directly exposed to outdoor conditions.

What Does IP20 Mean?

IP20 is the enclosure protection rating shown for the SPD itself.

It should not be interpreted as a weatherproof rating.

A DIN-rail SPD with IP20 is normally installed inside another suitable enclosure or cabinet.

For an outdoor PV installation, the final environmental protection depends on the complete enclosure, not only the bare SPD module.

So:

IP20 SPD inside a suitable outdoor enclosure

can be a normal arrangement.

But:

IP20 SPD directly exposed to rain, dust and outdoor weather

should not be assumed to be acceptable.

How Should You Read This 1000V DC SPD Label as a Whole?

Now we can return to the original example.

Ucpv: 1000V DC
Class II
In: 20kA
Imax: 40kA
Up: 4.0kV
Iscpv: 10kA

Instead of reading each number independently, read it as one technical description.

Ucpv 1000V DC

This tells us the declared maximum continuous PV operating voltage of the SPD.

Class II

This indicates Class II testing / Type 2 application direction.

In 20kA

This is the nominal discharge-current rating associated with the relevant Type 2 surge test.

Imax 40kA

This is the declared maximum discharge-current rating.

Up 4.0kV

This is the declared voltage protection level.

Iscpv 10kA

This is the PV short-circuit-current rating and must be understood together with the applicable disconnector/protection arrangement.

Now the label makes much more sense.

It is not saying:

“1000V, 20kA, 40kA, 10kA — bigger numbers are better.”

It is describing different electrical functions and limits of the same SPD.

6 Common Mistakes When Reading a DC SPD Datasheet

1. Treating Class II as a Quality Grade

Incorrect:

Class I is premium and Class II is lower quality.

Correct:

Class I and Class II represent different test/protection classifications.

2. Selecting an SPD Only by Imax

Incorrect:

This one says 60kA, so it is automatically better than the 40kA model.

Correct:

Check:

• Type
• Ucpv
• In
• Imax
• Up
• Iscpv
• standard
• wiring and protection requirements

before making the comparison.

3. Comparing Iimp Directly with Imax

Incorrect:

Imax 40kA is higher than Iimp 12.5kA, so Type 2 is stronger.

Correct:

Iimp and Imax refer to different test waveforms and different surge duties.

4. Confusing Ucpv with Up

Incorrect:

The SPD is rated 1000V but Up is 4kV, so the specifications conflict.

Correct:

Ucpv and Up describe different voltage characteristics.

5. Treating Iscpv as a Surge Rating

Incorrect:

Iscpv 10kA means the SPD can discharge 10kA of lightning current.

Correct:

Iscpv is a PV short-circuit-current rating.

6. Ignoring Installation Conditions

Even a correctly selected SPD can provide poor protection if installation is poor.

Important details include:

• lead length
• conductor routing
• PE connection
• enclosure
• coordination
• backup protection
• distance from protected equipment

A datasheet tells you what the device is capable of under defined conditions.

Installation determines how effectively that capability is used in the real system.

The SPD should also be considered together with other solar PV electrical protection devices, including DC fuses, isolators, circuit breakers and the overall protection arrangement.

How to Check a DC SPD Before Buying

For buyers, distributors and EPC procurement teams, the following sequence is more useful than simply asking for:

“1000V 40kA SPD price.”

Step 1 — Confirm the Application

Is the product specifically intended for:

PV DC surge protection?

Do not assume an AC SPD with a similar voltage or current marking is interchangeable.

Step 2 — Check Maximum PV Voltage

Determine the maximum possible PV open-circuit voltage.

Then confirm that the selected Ucpv is suitable.

For the full calculation method, refer to the DC SPD Voltage Selection Guide mentioned earlier.

Step 3 — Confirm Type / Class

Determine whether the installation requires:

• Type 2
• Type 1
• Type 1+2

Do not make this decision simply from product price or kA rating.

Our detailed Type 1 vs Type 2 vs Type 1+2 DC SPD guide explains this separately.

Step 4 — Check In and Imax

For a Type 2 / Class II SPD, confirm both values.

Do not check only Imax.

Step 5 — Check Up

Verify the declared voltage protection level and whether it is appropriate for the protected equipment and protection concept.

Step 6 — Check Iscpv

Confirm that the SPD and specified disconnection/protection arrangement are compatible with the prospective short-circuit current of the PV system.

Step 7 — Check Installation Configuration

Confirm:

• pole configuration
• DC+/DC− arrangement
• PE connection
• protection mode
• backup protection
• wiring diagram

Step 8 — Check Environmental Conditions

Verify:

• temperature range
• humidity
• indoor/outdoor condition
• enclosure IP rating
• installation altitude where relevant

Step 9 — Verify the Standard and Documentation

For PV DC SPDs, IEC 61643-31 is an important product standard to check.

IEC 61643-31:2018 covers SPDs intended for the DC side of photovoltaic installations up to 1500V DC, while IEC 61643-32:2017 addresses their selection, installation and coordination.

Depending on the project and market, buyers may also need:

• certificate
• test report
• datasheet
• wiring diagram
• dimensional drawing
• declaration of conformity
• project-specific approvals

Do not assume that a logo printed on a product is sufficient evidence.

Match the documentation to the exact model being purchased.

DC SPD Specification Checklist

Before approving a PV SPD, use the following checklist.

Example: How a Buyer Should Request a 1000V DC SPD

A weak RFQ might say:

Please quote 1000V 40kA solar SPD.

This leaves many important questions unanswered.

A better RFQ is:

Please quote a Type 2 / Class II PV DC SPD for a 1000V DC application. Please confirm Ucpv, In, Imax, Up, Iscpv, pole configuration, required backup protection, IEC 61643-31 documentation and wiring diagram.

If known, also include:

• actual maximum string Voc
• quantity
• installation point
• system configuration
• minimum temperature
• enclosure/application
• OEM requirements

This allows the manufacturer to evaluate the complete requirement instead of matching only the words:

“1000V” and “40kA.”

Frequently Asked Questions

Is Class II the Same as Type 2 SPD?

They are closely related but technically describe classification/testing from slightly different perspectives.

For practical PV product selection, a Type 2 SPD is associated with Class II testing, which is why labels and datasheets may show Type 2, T2 or Class II terminology.

Is Class II SPD Lower Quality Than Class I?

No.

Class I and Class II are not quality levels.

They are intended for different surge-protection duties and test conditions.

What Is the Difference Between In and Imax?

In is the nominal discharge current.

Imax is the maximum discharge current.

For Type 2 SPDs, both are normally associated with the 8/20 μs waveform, but they represent different test-duty levels.

Is 40kA Imax Better Than 20kA?

Not enough information is available to answer that from Imax alone.

A proper comparison also requires Ucpv, Type, In, Up, Iscpv, standard, protection configuration and installation conditions.

Can a 40kA Type 2 SPD Replace a Type 1 SPD?

Not simply because its peak current number is larger.

Type 1 is characterized by lightning impulse-current capability such as Iimp and the associated Class I test conditions.

Imax and Iimp should not be directly compared.

What Is the Difference Between Ucpv and Up?

Ucpv is the maximum continuous operating voltage for the PV application.

Up is the declared voltage protection level under specified surge test conditions.

They perform completely different functions in the datasheet.

What Does Iscpv Mean?

Iscpv is the short-circuit current rating of the PV SPD.

More precisely, it describes the maximum prospective short-circuit current for which the SPD, together with its specified disconnector arrangement, is rated.

Is Iscpv the Same as Imax?

No.

Imax is a surge discharge-current rating.

Iscpv is a short-circuit-current rating.

The fact that both use kA does not make them interchangeable.

Is a 1000V DC SPD Suitable for Every 1000V Solar System?

Not automatically.

Maximum PV open-circuit voltage should be calculated under the project’s design conditions before selecting Ucpv.

For more detail, see our DC SPD Selection Guide: How to Choose the Right Voltage Rating.

Can an IP20 SPD Be Used in an Outdoor Solar Installation?

It may be installed inside a suitable outdoor enclosure, provided the complete installation satisfies the required environmental protection.

IP20 on the SPD itself should not be treated as a weatherproof outdoor rating.

What Standard Should Be Checked for a PV DC SPD?

IEC 61643-31 covers requirements and test methods for SPDs intended for the DC side of photovoltaic installations up to 1500V DC.

IEC 61643-32 provides selection and application principles for PV SPDs.

Conclusion

Reading a DC SPD datasheet becomes much easier once each parameter is treated according to its actual purpose.

For the example:

Class II
Ucpv 1000V DC
In 20kA
Imax 40kA
Up 4.0kV
Iscpv 10kA

the correct interpretation is not simply:

“This is a 1000V, 40kA SPD.”

A more complete interpretation is:

• Class II identifies its Class II / Type 2 protection direction.
• Ucpv 1000V DC describes continuous PV voltage compatibility.
• In 20kA describes nominal discharge-current performance.
• Imax 40kA describes maximum declared discharge-current performance.
• Up 4.0kV describes the declared voltage protection level.
• Iscpv 10kA describes the PV short-circuit-current rating.

Each specification answers a different engineering question.

That leads to one simple purchasing principle:

Never select a DC SPD from one voltage number, one kA number or one Type label alone.

The correct SPD is the one whose complete set of ratings matches the PV system voltage, surge environment, equipment protection requirements, short-circuit conditions and installation design.

For solar PV projects requiring 600V, 1000V or 1500V surge protection, explore KUANGYA DC surge protective devices to compare available Type 2 and Type 1+2 models by voltage rating, discharge-current capability and application.