3+1 vs 4+0 SPD: Differences, Grounding Systems, and Selection Criteria

A 3+1 SPD typically protects three-phase conductors through L-N paths, with one additional N-PE path. A 4+0 SPD uses L-PE paths for the three phases, plus an N-PE path.

Both configurations can have four protection elements, but they are not interchangeable simply because their voltage or surge-current ratings match.

The choice depends on the earthing system, the conductors available at the installation point, and the protection paths required by the electrical installation.

For an engineer comparing 3+1 and 4+0 SPDs, the useful starting point is the protection circuit—not the number of modules on the DIN rail.

What Is the Electrical Difference Between 3+1 and 4+0 SPDs?

The numbers describe how the surge protection paths are arranged. They do not describe the SPD's Type 1, Type 2, or Type 3 classification.

3+1 Protection Paths: L1/L2/L3–N and N–PE

A typical 3+1 SPD has three L-N protection paths and one N-PE path:

• L1 → N

• L2 → N

• L3 → N

• N → PE

Consider a surge between L1 and neutral. The L1-N element provides a direct protection path.

For a surge between L1 and PE, the L1-N and N-PE elements form part of the protection network. Their actual behavior depends on the device design and tested protection characteristics.

Some 3+1 designs use MOVs for L-N protection and a spark-gap or gas-discharge element for N-PE protection. The 3+1 designation alone, however, does not identify the internal components of every product.

4+0 Protection Paths: L1/L2/L3–PE and N–PE

A typical 4+0 SPD takes a different approach:

• L1 → PE

• L2 → PE

• L3 → PE

• N → PE

The phase conductors have direct protection paths to PE rather than separate L-N protection elements.

This difference matters when evaluating protection between phase and neutral. A 4+0 SPD may influence L-N surge voltage through its protection network, but its behavior cannot be assumed to match a 3+1 SPD.

A useful detail when reading product labels: 4P and 3P+N do not necessarily describe the internal protection topology. The connection diagram gives a clearer answer than the module count.

How Do TT, TN-S, TN-C-S, and IT Systems Affect SPD Selection?

The same four-module SPD can be evaluated differently depending on where it will be installed.

A distribution board with separate N and PE conductors presents different connection conditions from a point where neutral and protective earth still share a PEN conductor.

TT and TN-S: Check the Required Protection Modes

In a TT system, the installation's exposed conductive parts use an earthing arrangement independent of the source earth.

A suitable 3+1 SPD is commonly considered for three-phase TT systems with a distributed neutral. The L-N and N-PE arrangement must still meet the applicable installation requirements, including the relevant fault-protection conditions.

A 4+0 configuration should not be treated as a direct TT substitute without specific technical support for that arrangement.

TN-S systems have separate neutral and protective earth conductors. Both 3+1 and 4+0 products are available for TN-S applications.

The practical difference is the protection path. A project requiring particular L-N protection characteristics may lead to a different review from a project using direct conductor-to-PE protection paths.

Neither choice can be approved from the earthing-system name alone.

TN-C and TN-C-S: Check the PEN Separation Point

TN-C systems use a combined PEN conductor. A standard four-path diagram requiring separate N and PE conductors does not directly describe that installation point.

TN-C-S systems add another detail: the PEN conductor is separated into N and PE at a defined location.

Imagine a project drawing that identifies the supply as TN-C-S but does not show where the SPD will be installed.

If the SPD is before PEN separation, the available conductors differ from those after separation. A four-path arrangement intended for separate N and PE cannot simply be copied to both positions.

One detail in the electrical drawing—the PEN separation point—can therefore change which SPD configurations can be evaluated.

IT Systems: Why a Separate Compatibility Review Is Needed

IT systems deserve a separate review because the supply is not directly earthed in the same way as a typical TT or TN installation.

During a first earth fault, conductor-to-earth voltages can differ from normal operating conditions. Whether the neutral is distributed also affects the possible SPD connection arrangement.

A matching nominal voltage does not settle these questions.

The review needs the system voltage, conductor arrangement, earth-fault conditions, and the manufacturer's documented SPD suitability.

YQNT's SPD catalog describes IT systems and lists 3+1 and 4+0 protection configurations. The catalog does not, by those statements alone, confirm that every listed model is suitable for an IT installation.

Grounding-System Decision Reference

This reference identifies the relevant design conditions. It is not a model approval list.

For the broader choice between SPD types and installation positions, see the YQNT SPD Selection Guide.

Which Technical Ratings Must Be Checked Before Choosing 3+1 or 4+0?

For this comparison, the important point is which protection mode a published rating describes.

A 3+1 SPD uses L-N and N-PE paths. A 4+0 SPD uses L-PE and N-PE paths. As a result, two products with the same Uc or headline discharge-current rating may still provide different protection characteristics.

IEC 61643-11:2025 specifies requirements, tests, and ratings for SPDs connected to AC low-voltage power systems.

Before comparing products, look at three things:

There is little value in comparing a total discharge-current figure from one topology with a single-path figure from another.

A detailed explanation of Uc, Up, TOV, In, Imax, and Iimp belongs in the existing YQNT SPD Datasheet Guide. Here, the focus stays on what those ratings mean for different protection connections.

YQNT 3+1 vs 4+0 SPD Models: What Does the Catalog Confirm?

A practical product example helps show why the topology matters even when two SPD model numbers look similar.

Compare 3+1 and 4+0 Variants Within the LDSP1-40 Series

The YQNT LDSP1-40 Type 2 AC series includes both configurations.

Two variants from the catalog illustrate the difference:

Source: YQNT SPD Product Catalog, LDSP1-40 Type 2 AC series, printed page 44.

Both catalog variants have the same Uc, but their protection paths differ. That difference is enough to require a separate electrical suitability review.

The catalog also contains surge-current and voltage-protection data. Any comparison of those figures must retain the original per-path or total-value basis. The topology comparison above is not a claim that the two products have identical performance.

What the Catalog Confirms—and What Still Requires Verification

The PDF confirms that YQNT lists both 3+1 and 4+0 arrangements in the LDSP1-40 series.

The PDF does not, by itself, approve either variant for every TT, TN-S, TN-C-S, or IT installation.

For a specific project, the remaining question is whether the documented protection arrangement meets the electrical system's requirements.

The YQNT LDSP1-40 Type 2 AC SPD product provides the related product reference. The catalog remains the technical parameter source for this comparison.

Can a 3+1 SPD Replace a 4+0 SPD?

Sometimes a different configuration can be considered, but a matching pole count or kA rating does not establish interchangeability.

Consider a hypothetical procurement situation.

An electrical panel BOM lists a four-module Type 2 SPD with a Uc of 275 Vac. The purchasing team finds another four-module SPD with the same Uc and a similar surge-current rating.

At first glance, the substitution seems straightforward.

Then the connection diagrams reveal that one device uses 3+1 protection and the other uses 4+0.

The procurement question has changed. It is no longer about finding a matching headline rating. It is about whether the alternative protection paths meet the original design requirements.

Conditions That Must Be Verified Before Replacement

Without enough information to resolve these differences, the substitution remains unapproved.

This is a hypothetical BOM review used to explain the decision. It is not a YQNT customer case.

What Should Engineers and Buyers Verify Before Ordering?

For a 3+1 or 4+0 SPD inquiry, a short electrical system description is often more useful than a long product keyword list.

Electrical System and Model Information Required

A buyer can start with the following information:

For example, “Type 2 SPD, 275 V, four modules” is not enough to distinguish the two protection arrangements described in this article.

Adding the earthing system and required protection paths makes the specification more useful.

Technical Evidence to Request Before BOM Approval

Some projects also require material documentation, topology and grid-compatibility evidence, stress and thermal-protection data, or laboratory and compliance records.

Those requirements depend on the application and purchasing specification. Availability must be established for the exact YQNT model rather than assumed from a product-family description.

The aim is simple: connect the ordered model to the required electrical configuration before procurement approval.

FAQs

Does a four-pole SPD always use a 4+0 configuration?

No. Four modules can also form a 3+1 arrangement. The connection diagram identifies the protection paths more reliably than the module count.

Does 3+1 mean Type 3 plus Type 1?

No. 3+1 describes a protection arrangement. Type 1, Type 2, and Type 3 describe SPD test classifications. They refer to different product characteristics.

Can a 3+1 SPD be used without a neutral conductor?

The typical 3+1 arrangement in this article uses L-N protection paths and therefore assumes a distributed neutral. A system without a neutral needs a different connection assessment.

Is the N-PE element always a GDT in a 3+1 SPD?

No. The internal N-PE technology depends on the product design. The manufacturer's technical documentation is needed to identify the actual element.

Can a higher Imax make up for an unsuitable SPD configuration?

No. A higher surge-current rating cannot make an incompatible protection arrangement suitable for the electrical system.

Confirm the Protection Configuration Before Ordering

The difference between 3+1 and 4+0 SPD configurations is more than a product label. The two arrangements protect different conductor paths, so a replacement or new selection needs to match the actual electrical installation.

For a YQNT model review, share the system voltage, earthing arrangement, installation position, required protection paths, and existing model or BOM.

YQNT can review the available SPD catalog information against those requirements and identify where additional technical evidence is needed.