A surge protective device protects electrical equipment from transient overvoltage, but what protects the SPD if an internal fault or short circuit occurs?

This is where the SPD backup fuse or circuit breaker becomes important.

However, there is no universal rule such as:

“Every Type 2 SPD needs a 125 A fuse.”

or:

“A 40 kA SPD should use a 40 A breaker.”

Both statements can be wrong.

Richtig SPD backup fuse selection depends on the exact SPD model, upstream overcurrent protection, prospective short-circuit current, conductor size, system configuration and manufacturer instructions.

Some SPDs can use the existing upstream fuse or circuit breaker. Others require separate backup protection. Some products even integrate backup protection inside the SPD itself.

The most important rule is therefore simple:

Do not select an SPD backup fuse only from the SPD’s In, Imax or Type classification. Always check the manufacturer’s declared backup protection and the actual installation conditions.

Quick Answer: Does an SPD Need a Backup Fuse?

An SPD may need a backup fuse or circuit breaker, but not every installation requires a separate one.

If the existing upstream overcurrent protective device already satisfies the SPD manufacturer’s permitted protection arrangement, an additional backup fuse may not be required.

If the upstream protective device exceeds the manufacturer’s permitted value, or if the SPD instructions specify dedicated branch protection, an additional SPD backup fuse or circuit breaker may be necessary.

This means the correct question is not simply:

“What size fuse should I install?”

Instead, ask:

“Does the existing upstream protection already satisfy the requirements of this exact SPD?”

Wichtigste Erkenntnisse

• An SPD and a fuse or circuit breaker perform different protection functions.
• An SPD’s internal thermal disconnector does not automatically replace external short-circuit or overcurrent protection.
• Not every SPD requires a separate backup fuse.
• The upstream protective device must be compared with the SPD manufacturer’s declared limits.
• A larger fuse is not automatically better.
• A smaller fuse is not automatically safer because inappropriate sizing can result in unwanted disconnection.
• Fuse and circuit-breaker ampere ratings should not be treated as directly interchangeable.
• AC SPD and PV DC SPD requirements must be evaluated separately.
• Prospective short-circuit current at the installation point matters.
• Conductor size and SPD lead length must also be considered.

What Does an SPD Backup Fuse Actually Do?

An SPD and an overcurrent protective device solve different electrical problems.

A typical power SPD is connected in parallel with the circuit.

During normal operation, it carries little or no normal load current. When a transient overvoltage occurs, the SPD becomes conductive and diverts surge current through its designed protection path.

A fuse or circuit breaker has a different purpose.

Its job is primarily to interrupt sustained overcurrent or fault current.

Daher:

SPD ≠ fuse

and:

Fuse ≠ SPD

They provide complementary protection.

Why Can an SPD Need Backup Protection?

Many voltage-limiting SPDs use nonlinear components such as metal oxide varistors.

During normal system voltage, the protection element remains at high impedance.

During a surge, its electrical behavior changes rapidly and it conducts surge current.

However, surge-protection components can eventually reach end of life or become damaged because of:

• repeated surge stress,
• severe surge events,
• temporary overvoltage,
• incorrect system voltage,
• abnormal fault conditions,
• or internal degradation.

If a protection element develops a low-impedance fault, the installation must be able to disconnect the resulting power-frequency or DC fault current safely.

This is why SPD datasheets can include parameters such as:

• maximum backup fuse,
• maximum mains-side overcurrent protection,
• backup protection,
• short-circuit current rating,
• Isccr,
• SCCR,
• or recommended protective device.

For AC low-voltage SPDs, the current international product standard is IEC 61643-11:2025, which covers requirements and test methods for SPDs connected to AC circuits up to 1,000 V RMS.
IEC 61643-11:2025 — official IEC page

Internal SPD Disconnector vs External Backup Fuse

A common question is:

“If my SPD already has an internal disconnector, why would it need another fuse?”

Because they do not necessarily perform the same function.

Internal SPD Disconnector

The internal disconnector is generally associated with the surge-protection component inside the SPD.

For example, when an MOV reaches an unsafe thermal condition, the internal mechanism can disconnect the failed element.

On many modular SPDs, this condition is mechanically linked to a status indicator.

The window may change from green to red, depending on the manufacturer’s design.

External Backup Protection

The external fuse or circuit breaker is part of the installation’s fault-current protection and coordination.

Depending on the SPD, it can help:

• interrupt short-circuit current in the SPD branch,
• protect associated conductors,
• allow safe isolation,
• and keep the installation within the SPD manufacturer’s tested fault conditions.

Daher:

An internal thermal disconnector should not automatically be treated as a replacement for an external SPD backup fuse.

The product documentation must still be checked.

Does Every SPD Need a Separate Backup Fuse?

Nein.

There are three common situations.

This is one of the most important points in SPD backup protection.

You cannot make the decision using only:

• Type 1,
• Type 2,
• Type 1+2,
• In,
• Imax,
• Uc,
• Ucpv.

Two Type 2 SPDs with similar voltage and Imax ratings may have different backup-protection requirements.

F1 and F2: Understanding SPD Backup Protection

SPD installation diagrams often use two designations:

F1 = upstream system protective device

F2 = dedicated SPD backup protective device

A simplified arrangement may look like:

Supply → F1 → Distribution System

with the SPD connected in parallel through F2 where dedicated F2 protection is required.

When selecting an SPD backup fuse, the first question should therefore not be:

“What size fuse should I buy?”

Instead ask:

“Can the existing F1 already provide the protection required by this SPD?”

If F1 meets the manufacturer’s conditions, F2 may not be necessary.

If F1 is larger than the permitted value, a dedicated F2 may be needed.

This principle is demonstrated clearly in several manufacturer’s published installation guides, which we will examine later.

7 Rules for Correct SPD Backup Fuse Selection

Rule 1: Start With the Exact SPD Datasheet

There is no universal SPD backup fuse size.

Always start with the exact manufacturer’s datasheet or installation instructions.

Look for wording such as:

• Maximum backup fuse
• Max. mains-side overcurrent protection
• Backup fuse
• Backup protection
• Recommended protective device
• Maximum upstream fuse
• Short-circuit current rating
• Isccr
• SCCR

Do not take a fuse value from another SPD simply because both devices are Type 2 or both have Imax = 40 kA.

Product construction and tested coordination can be different.

Rule 2: Check the Existing Upstream Protection First

Suppose an SPD manufacturer allows:

Maximum upstream protection = 125 A gG

and the system already has:

F1 = 63 A gG

If all other manufacturer requirements are satisfied, the existing upstream fuse may already provide the necessary protection.

Installing another 125 A fuse does not automatically make the installation safer.

Now consider:

F1 = 250 A gG

If the SPD is only approved with a maximum 125 A gG backup arrangement, the existing 250 A protection may no longer satisfy the SPD requirements.

A dedicated SPD backup fuse may then be necessary.

This is why F1 must be checked before adding F2.

Rule 3: Check the Prospective Short-Circuit Current

Fuse ampere rating alone is not enough.

You must also consider the available fault current at the installation point.

The same SPD can face very different fault conditions depending on where it is installed.

Zum Beispiel:

A small downstream distribution board may have relatively low prospective short-circuit current.

A main industrial panel close to a transformer can have significantly higher available fault current.

Therefore, always evaluate the combination of:

SPD + backup protection + available short-circuit current

Dies ist besonders wichtig bei:

• industrial distribution boards,
• large commercial buildings,
• transformer secondary panels,
• large PV installations,
• energy storage systems,
• and high-current electrical systems.

Rule 4: Never Size an SPD Backup Fuse From Imax

Consider a Type 2 SPD marked:

In = 20 kA

Imax = 40 kA

A very common mistake is:

“The SPD is 40 kA, so it should use a 40 A fuse.”

That is incorrect.

Imax = 40 kA describes a surge-current characteristic associated with the relevant impulse waveform.

A 40 A fuse is an overcurrent protective device rating.

These values describe different electrical phenomena.

Daher:

The SPD backup fuse rating cannot be calculated directly from the SPD’s Imax value.

If you are unsure about the difference between In, Imax, Up and Iscpv, see our Leitfaden für DC-SPD-Spezifikationen.

Understanding these parameters prevents one of the most common SPD selection errors.

Rule 5: Check the Conductor Size

The manufacturer’s maximum backup-fuse value does not mean that the largest permitted fuse should always be installed.

The protective device must also coordinate with:

• cable cross-sectional area,
• conductor insulation,
• terminal capacity,
• wiring method,
• installation temperature,
• enclosure conditions,
• and local electrical requirements.

For example, if an SPD permits a maximum backup fuse of 125 A, that does not automatically mean every installation should use a 125 A fuse.

Maximum is a limit, not always a recommendation.

Rule 6: Do Not Assume a Fuse and Circuit Breaker Are Equivalent

A fuse and a circuit breaker can both provide overcurrent protection, but their operating characteristics are different.

Daher:

Do not assume that a 125 A gG fuse can automatically be replaced by a 125 A circuit breaker.

Manufacturers can specify different protective-device combinations.

For example, Schneider Electric publishes coordination tables pairing particular SPD models with specific protective-device ratings rather than applying one universal breaker value.

Rule 7: Separate AC and PV DC Requirements

Do not copy an AC SPD backup-protection rule directly into a photovoltaic DC system.

PV systems have different:

• voltage characteristics,
• fault-current behavior,
• source characteristics,
• protection devices,
• and disconnection requirements.

SPDs designed specifically for the DC side of photovoltaic installations are covered by IEC 61643-31:2018, which applies to PV DC SPDs up to 1,500 V DC.
IEC 61643-31:2018 — official IEC page

This is why an AC SPD backup-fuse example should never automatically be applied to a 1,000 V or 1,500 V PV system.

Real Manufacturer SPD Backup Fuse Examples

The following SPD backup fuse examples come from published manufacturer technical information.

They are useful because they demonstrate that backup-protection requirements vary significantly between products.

Do not copy these values to another SPD without checking its own datasheet.

Let’s look at these examples more closely.

Example 1: Schneider Electric — Existing F1 Can Sometimes Replace F2

Schneider Electric’s Resi MAX installation instructions provide an especially useful example.

For the documented SPD arrangement, Schneider states that the backup fuse is:

required only if suitable protection is not already provided in the mains.

The document lists ≤125 A gL/gG backup protection and shows a case where:

F1 ≤125 A

and:

Isc <50 kA

can result in:

No need for F2

under the manufacturer’s stated conditions.

Schneider Electric Resi MAX SPD installation instructions

This is an excellent example of why installing an additional fuse automatically is not always correct.

Example 2: Phoenix Contact — Too Small Is Not Automatically Better

Phoenix Contact provides a useful Type 2 SPD backup-fuse example for its VAL-SEC range.

Its published technical guidance explains that the device can be installed in the branch line without an additional F2 under specified upstream protection conditions.

Where a dedicated F2 is nevertheless required or desired, the document gives:

125 A gG

as an appropriate value for the example.

More importantly, Phoenix Contact also warns against selecting certain smaller backup fuses below 80 A gG in that specific arrangement because their impulse withstand may result in unwanted operation during surge events.

Phoenix Contact — The Right Backup Fuse for Surge Protective Devices

Dies bedeutet nicht mean that every Type 2 SPD needs an 80 A or 125 A fuse.

It demonstrates something more important:

An SPD backup fuse must be coordinated with the specific SPD. Choosing the smallest possible fuse is not automatically the safest option.

Example 3: DEHN — Integrated Backup Protection

Not every SPD requires an external backup fuse.

DEHN’s DEHNguard SE CI Type 2 SPD includes an integrated backup fuse.

DEHN states that no additional backup fuse is required for installations within the product’s specified maximum short-circuit current of 25 kArms.

DEHNguard SE CI with integrated backup fuse

This example shows why the statement:

“Every SPD must have a separate external fuse.”

is incorrect.

The correct arrangement depends on the tested product design.

Can an SPD Backup Fuse Be Too Small?

Ja.

A smaller fuse is not automatically safer.

The protective device must be capable of remaining intact during the surge conditions that the SPD is intended to handle while still interrupting a sustained fault when required.

If an undersized fuse operates unnecessarily during a surge, the SPD can become disconnected.

The electrical installation may then continue operating normally while surge protection has been lost.

This can be particularly difficult to notice in a parallel-connected SPD.

Phoenix Contact’s published example illustrates this issue by warning against certain undersized gG backup fuses in the specific Type 2 SPD arrangement described above.

Again:

This is a product-specific example, not a universal minimum fuse value.

Can an SPD Backup Fuse Be Too Large?

Ja.

If the external protective device exceeds the maximum value permitted by the SPD manufacturer, the SPD may no longer be operating within the tested fault-protection arrangement.

For example, if a product states:

Maximum backup fuse: 125 A gG

this should not be interpreted as:

“125 A must always be installed.”

Instead it means that the protection arrangement should not exceed the manufacturer’s stated limit unless another approved configuration is provided.

The actual selection must still consider:

• upstream protection,
• conductor size,
• short-circuit current,
• SPD instructions,
• and installation design.

Example: Small Commercial Distribution Board

Consider a Type 2 SPD in a small commercial distribution board.

Assume:

Main upstream fuse F1 = 63 A gG

and the selected SPD permits:

Maximum upstream protection = 125 A gG

If:

• the available short-circuit current is within the SPD’s declared capability,
• the wiring is suitable,
• and the manufacturer permits the arrangement,

then the existing 63 A upstream device may already provide the required backup protection.

Adding a second 125 A fuse would not automatically improve the system.

Example: Industrial Main Distribution Board

Now consider a larger industrial panel.

Assume:

F1 = 400 A

while the SPD permits:

Maximum backup protection = 125 A gG

The upstream protection now exceeds the manufacturer’s stated limit.

A dedicated F2 may therefore be required.

The simplified arrangement may become:

400 A upstream protection → SPD branch → approved F2 → SPD

However, the designer must still verify:

• prospective short-circuit current,
• breaking capacity,
• conductor cross-section,
• SPD short-circuit rating,
• fuse class,
• and manufacturer instructions.

This is why a backup-protection arrangement that works in a small distribution board cannot simply be copied into an industrial main panel.

How Does SPD Backup Protection Work in a PV DC System?

Photovoltaic DC systems require additional care.

A PV DC SPD must be suitable for:

• the maximum PV system voltage,
• Ucpv,
• PV fault-current conditions,
• the relevant system topology,
• and the manufacturer’s specified disconnection arrangement.

The exact SPD backup fuse arrangement can depend on:

• maximum PV short-circuit current,
• number of parallel strings,
• existing string protection,
• array protection,
• SPD Iscpv,
• gPV fuse characteristics,
• and system voltage.

Before choosing backup protection, the SPD voltage itself must also be correct.

For more detail, see our DC SPD voltage selection guide for solar PV systems.

Should a PV SPD Always Use a gPV Fuse?

Nein.

A gPV fuse is specifically designed for photovoltaic applications, but this does nicht mean every PV SPD requires a separate gPV backup fuse.

The manufacturer’s instructions must be checked first.

Depending on the system design, existing PV overcurrent protection may already form part of the approved protection arrangement.

If a gPV fuse is required, it must also be suitable for:

• DC voltage,
• current,
• breaking capacity,
• PV application,
• conductor size,
• and environmental conditions.

For examples of PV-rated fuse links and holders, see KUANGYA’s DC gPV fuse range.

The product page is useful for understanding available PV fuse formats, but final SPD coordination must still follow the SPD manufacturer’s technical data.

Where Should the SPD Backup Fuse Be Installed?

Where dedicated backup protection is required, it is normally positioned in the SPD branch so that fault current flowing to the SPD can be interrupted.

A simplified concept is:

Busbar / Supply → F2 → SPD → required protection path

However, the exact wiring depends on:

• TN-S,
• TN-C,
• TN-C-S,
• TT,
• IT,
• single-phase,
• three-phase,
• PV DC configuration,
• SPD protection modes.

Always follow the wiring diagram for the exact SPD.

Do Not Forget SPD Lead Length

Adding a fuse or breaker should not result in unnecessarily long SPD connections.

SPD connection conductors have inductance.

During a fast-rising surge current, additional lead length can increase the voltage appearing across the protected equipment.

This means backup protection should be coordinated while keeping the SPD connection path short and direct.

For PV systems, there is also another distance issue: the distance between the SPD location and the inverter.

These two distances should not be confused.

For a detailed explanation, see our guide to SPD distance from inverter and SPD connection lead length.

Can One Breaker Protect Both the Load and the SPD?

Sometimes.

The existing protective device may be acceptable if:

• the SPD manufacturer allows it,
• its rating is within the required limit,
• the short-circuit conditions are acceptable,
• conductor protection is maintained,
• and the installation arrangement satisfies applicable requirements.

However, a dedicated SPD breaker can also provide practical advantages such as easier isolation and maintenance.

Der wichtige Punkt ist:

Dedicated protection does not automatically mean correctly coordinated protection.

The device must still match the SPD manufacturer’s requirements.

What Happens If the SPD Backup Fuse Opens?

Because many power SPDs are connected in parallel, the main electrical load can continue operating even after the SPD branch is disconnected.

That means:

Power ON does not necessarily mean surge protection ON.

If the backup fuse opens, the cause should be investigated rather than simply replacing the fuse and re-energizing the system.

Mögliche Ursachen sind:

• an SPD internal fault,
• severe surge stress,
• temporary overvoltage,
• incorrect SPD selection,
• inappropriate backup protection,
• wiring faults,
• or abnormal system conditions.

If you suspect the SPD itself has failed, see our guide on wie man erkennt, ob ein SPD defekt ist.

Common SPD Backup Protection Mistakes

SPD Backup Fuse Selection Checklist

Before selecting an SPD backup fuse or circuit breaker, check the following:

If several of these answers are unknown, selecting a backup fuse by guesswork is not appropriate.

Relevant Standards to Check

IEC 61643-11:2025

IEC 61643-11 covers SPDs intended for connection to AC low-voltage power circuits and specifies performance and safety requirements, tests and ratings.

IEC 61643-11:2025

IEC 61643-31:2018

IEC 61643-31 applies specifically to SPDs intended for the DC side of photovoltaic installations up to 1.500 V DC.

IEC 61643-31:2018

IEC 60364-5-53

IEC 60364-5-53 covers the selection and erection of electrical equipment for protection, isolation, switching, control and monitoring.

The current IEC consolidated version includes the 2019 base publication plus amendments from 2020 and 2024.

IEC 60364-5-53:2019+A1:2020+A2:2024

Applicable national standards, installation rules and manufacturer instructions must also be followed.

Häufig gestellte Fragen

Does Every SPD Need a Backup Fuse?

Nein.

A separate SPD backup fuse is not always necessary.

If the existing upstream protective device already satisfies the manufacturer’s permitted backup-protection conditions, another fuse may not be required.

Some SPDs also include integrated backup protection.

What Size Fuse Should I Use for an SPD?

There is no universal fuse size for an SPD.

Use the exact SPD datasheet and check:

• maximum backup protection,
• existing F1,
• prospective short-circuit current,
• conductor size,
• and manufacturer instructions.

Do not calculate the fuse size from Imax alone.

Does a Type 2 SPD Always Need a 125 A Fuse?

Nein.

125 A appears in several manufacturer examples, but it is not a universal Type 2 SPD value.

Published examples show SPDs with different arrangements, including:

• existing upstream protection,
• dedicated backup fuse,
• integrated backup fuse,
• or other specified protection.

Can I Use a Circuit Breaker Instead of a Fuse?

Sometimes.

But the circuit breaker’s:

• current rating,
• trip characteristic,
• breaking capacity,
• surge behavior,
• and manufacturer-approved coordination

must be suitable.

Do not replace a specified fuse with a breaker simply because both devices have the same ampere rating.

Does an SPD’s Red Indicator Mean Its Backup Fuse Has Blown?

Nicht unbedingt.

A red status indicator commonly relates to the condition of the SPD’s internal protection module or disconnection mechanism.

An external backup fuse is a separate protective device.

Therefore, both the SPD status and the upstream or branch protection should be inspected.

Can the Electrical System Keep Working After the SPD Fuse Opens?

Ja.

Because many power SPDs are connected in parallel, the protected load may remain energized even if the SPD has been disconnected.

This means surge protection can be lost while the equipment appears to operate normally.

Does a PV DC SPD Always Need a gPV Backup Fuse?

Nein.

The requirement depends on:

• the exact SPD,
• its Iscpv or short-circuit data,
• manufacturer instructions,
• PV array design,
• available fault current,
• and existing overcurrent protection.

Never assume every PV SPD needs the same gPV fuse.

Is SPD Backup Fuse Rating Related to Imax?

Not directly.

Imax represents a surge-discharge current characteristic.

Fuse current rating relates to overcurrent protection.

A 40 kA Imax rating does not mean a 40 A backup fuse is required.

Schlussfolgerung

So, does an SPD need a backup fuse or circuit breaker?

Sometimes yes—but not always as a separate device.

Richtig SPD backup fuse selection starts with the manufacturer’s technical data rather than a universal ampere rating.

The correct decision depends on the relationship between:

SPD → F1 → F2 → available fault current → conductors → electrical system

Before installing additional backup protection, first check whether the existing upstream device already satisfies the SPD manufacturer’s requirements.

If dedicated protection is required, verify:

• fuse class or breaker characteristic,
• maximum permitted rating,
• short-circuit capability,
• conductor size,
• system voltage,
• and installation arrangement.

Most importantly:

Never select an SPD backup fuse only from Type 1 / Type 2, In or Imax. Use the exact manufacturer’s backup-protection requirements and the actual system conditions.

For photovoltaic applications, you can also review KUANGYA’s DC surge protective devices for solar PV systems when comparing Type 2 and Type 1+2 DC SPD options.