SPD Backup Fuse vs Circuit Breaker: How to Select Backup Protection for an SPD Branch

An SPD does not always need a separate backup fuse or circuit breaker. The correct answer depends on the exact SPD design, its internal protection, the existing upstream protective device, the available fault current, and the manufacturer’s installation instructions.

Start with the SPD datasheet. Check whether it specifies a maximum backup fuse, an approved backup device, or integrated protection. Then check the upstream device F1. If F1 already meets the manufacturer’s stated conditions, a separate F2 may not be required. If it does not, install a dedicated F2 and verify the complete protection arrangement.

The key rule is simple: do not choose an SPD backup fuse from amperage alone.

Does an SPD Need a Separate Backup Fuse or Circuit Breaker?

Not every SPD requires an external OCPD. Some SPDs use internal overcurrent or thermal protection. Others rely on an external protective device.

The protection requirement also depends on the SPD failure mode. Under current UK guidance, SPD overcurrent protection may be internal or external, but it must follow the manufacturer’s instructions. The IET Wiring Matters guidance also notes that an upstream DNO fuse can sometimes provide short-circuit protection, but only when the stated installation and product conditions are met.

For a practical check, ask three questions:

1. Does the SPD have integrated protection?

2. Does the manufacturer allow the existing upstream device to protect it?

3. If not, what F2 type, rating, and characteristics does the manufacturer specify?

Do not assume that every Type 1 or Type 2 SPD follows the same rule.

What Does SPD Backup Protection Actually Protect?

An SPD is connected in parallel with the supply. During a surge, it conducts surge current away from the protected circuit. During normal operation, it should not carry the load current.

A fault at the end of the SPD’s service life is different. Depending on the SPD construction, the protective element may develop a low-impedance fault or another abnormal condition. The installation must then clear the fault without creating a hazardous condition.

That is the job of the overcurrent protection. It is not a replacement for the SPD. It protects the SPD branch against sustained fault current or overload conditions when required by the product design.

TOV, excessive surge energy, and MOV aging can contribute to SPD failure. But they do not by themselves determine the backup fuse size.

What Are F1 and F2 in an SPD Installation?

F1 is the upstream protective device. It may be a service fuse, main fuse, or another overcurrent protective device.

F2 is the dedicated protective device in the SPD branch.

A typical branch arrangement is:

Supply → F1 → Distribution Board

Distribution Board → F2 → SPD → PE

F2 is not automatically required. It can be omitted when the existing upstream protection is permitted to serve the SPD under the manufacturer’s documented conditions, and the installation meets the other applicable requirements.

F1 and F2 are part of the protection arrangement. For physical placement, wiring, protection modes, and installation checks, see our guide to Type 2 SPD installation and wiring.

This is why the rule “F1 ≤ maximum backup fuse, therefore F2 is not needed” is too broad. F1 rating is only one part of the decision. Short-circuit conditions, device characteristics, wiring, and the manufacturer’s tested arrangement can also matter.

For example, a Schneider Electric installation instruction states that its 920RM/1 SPD needs a backup fuse only when one is not already provided in the mains. It also gives different F2 arrangements based on the upstream fuse and short-circuit level. This is a product-specific rule, not a universal SPD rule.

Before selecting backup protection, confirm that the SPD itself matches the electrical system and installation point. You can first select the right SPD for the electrical system, then check its backup protection requirements.

How Do You Select an SPD Backup Fuse?

Use this order.

1. Check the Exact SPD Model

Start with the full model number. Do not select the fuse from “Type 2 SPD” alone.

The same SPD family can have different discharge ratings and backup protection limits.

2. Check the Maximum Backup Protection

Look for terms such as:

• Maximum backup fuse

• Maximum backup OCPD

• Backup protection

• Max. mains-side overcurrent protection

The manufacturer’s value is a limit for the specified arrangement. It is not automatically the fuse you must buy.

YQNT data shows why this matters. Its AC SPD products use different maximum backup fuse values. The LDSP1s-D20 lists 32A gG. The LDSP1s-C40 lists 63A gG. The LDSP1s-B60 lists 100A gG. The LDSP1s7 lists 125 Å gG. The LDSP1s12.5 lists 160–200 Å gG. The LDSP1-A25 lists 315 Å gG.

These values are product-specific. They should not be converted into a universal “SPD fuse size” rule.

Check the exact value stated by the manufacturer. If you are not sure where to find maximum backup protection or Isccr, read an SPD datasheet before choosing the protective device.

3. Check the Existing F1

Compare the actual upstream protective device with the SPD manufacturer’s conditions.

If F1 is within the permitted range and the other conditions are satisfied, F2 may not be needed. If F1 exceeds the permitted protection, a separate F2 is normally required unless the manufacturer provides another approved arrangement.

YQNT’s catalog gives one useful example: the MY/MYC AC series can be used with an F1 up to 315A gG without an additional F2 under its stated installation conditions.

That does not mean every YQNT SPD can use a 315A upstream fuse without F2.

4. Check Available Fault Current and Isccr

This step is often missed.

The installation has a prospective short-circuit current. The SPD also has a declared short-circuit current rating, often shown as Isccr or a similar conditional rating.

Do not treat Isccr as the same value as the breaker’s Icn or Icu. They describe different aspects of short-circuit performance.

Also, do not assume that replacing a tested fuse with an arbitrary MCB leaves the SPD’s declared Isccr unchanged. The new arrangement must satisfy the manufacturer’s conditions and the applicable installation requirements.

5. Check Fuse Class and Installation Conditions

The fuse type matters. YQNT AC products specify gG backup fuses in the examples above. Its PV DC products use gPV protection, so you cannot mix AC and PV DC backup devices based on amperage alone.

The conductor size and connection method also matter. A fuse cannot be selected independently of the branch wiring.

Can a Circuit Breaker Replace an SPD Backup Fuse?

Sometimes, but not automatically.

A circuit breaker has a time-current characteristic. Its instantaneous or magnetic trip can respond differently from a fuse during a high-current surge.

The manufacturer therefore needs to specify or validate the breaker arrangement. YQNT’s technical catalog states that an equivalent MCB can be used in place of the fuse, but its tripping characteristics must be considered. It also notes that normative testing is performed for the SPD and fuse combination.

Phoenix Contact provides a useful example. For one SPD arrangement with a maximum 315A gG backup fuse, it shows a 125A C-characteristic MCB as a possible upstream protective device. It also states that higher nominal currents or other characteristics require individual consideration. 

So there is no safe universal rule such as “always use C-curve” or “always use D-curve.”

The correct question is: has the selected breaker characteristic been shown to be suitable for this SPD and installation?

What Does “Maximum Backup Fuse” Mean on an SPD Datasheet?

“Maximum backup fuse” means the maximum permitted backup fuse under the stated product conditions.

It does not mean the fuse is always the best choice.

A fuse that is too small can disconnect during a surge. Phoenix Contact warns that backup fuses below 80A gG should not be used for one of its typical Type 2 SPD arrangements because their impulse durability can be insufficient. 

A fuse that is too large can exceed the protection conditions specified by the SPD manufacturer.

The selected protective device must remain within the manufacturer’s permitted range and provide the required surge withstand and fault protection for the installation. This is also consistent with the IET’s current guidance, which says external OCPD ratings and characteristics should provide high surge current capability without exceeding the manufacturer’s requirements. 

How Does Isccr Affect SPD Backup Protection?

Think of Isccr as a short-circuit condition for the protection arrangement, not simply another SPD current rating.

YQNT AC products such as the LDSP1-40 list Isccr at 25kA together with a specified maximum backup fuse. The product data therefore gives two pieces of information that must be read together. 

Phoenix Contact also publishes SPD data where the declared short-circuit rating changes with the backup fuse arrangement. This shows why the protective device and the SPD cannot always be evaluated separately. 

The practical rule is:

Available fault current must not exceed the verified short-circuit capability of the complete SPD protection arrangement.

The exact verification method depends on the product, standard, and installation.

What Happens If the Backup Fuse Is Too Small or Too Large?

Too small can cause unwanted disconnection during a surge. The SPD may then be left disconnected when another surge occurs.

Too large can leave the SPD outside the manufacturer’s specified protection arrangement.

There is a third problem: choosing a breaker or fuse without checking its actual characteristics. Two devices with the same 63A rating can behave differently under a surge or fault.

This is why “63A is enough” is not a complete engineering answer.

What Do Real Manufacturer Examples Show?

Schneider Electric shows an SPD where the backup fuse is required only when suitable mains protection is not already present. Its instructions also vary the backup arrangement with upstream fuse rating and short-circuit level.

Phoenix Contact shows both sides of the problem. Some products can operate without a separate F2 under specified conditions. Other arrangements require a sufficiently large backup fuse to carry the surge current. Phoenix also documents breaker characteristics for specific cases.

DEHN takes another approach with products that integrate the backup fuse into the SPD. Its DEHNguard CI and DEHNvenCI families state that no additional backup fuse is required because the protection is integrated. 

These examples lead to one conclusion:

Backup protection is manufacturer-specific.

YQNT SPD Backup Protection Examples

The following values are useful when reviewing YQNT AC SPD selections:

Always check the exact model and current installation instructions before specifying the protective device.

SPD Backup Protection Checklist for Engineers and Buyers

Before approving an SPD branch, confirm:

• Exact SPD model

• AC or DC application

• System voltage and earthing arrangement

• SPD type

• Integrated protection, if any

• Maximum backup protection

• Fuse class or approved breaker characteristic

• Existing upstream F1

• Available prospective short-circuit current

• SPD Isccr or conditional short-circuit rating

• Conductor size and connection method

• Manufacturer coordination requirements

• Local code and project requirements

If one item is unknown, do not guess the backup device from the SPD’s Imax value.

FAQ

Does every SPD need a separate backup fuse?

No. Some SPDs have integrated protection. Others allow the existing upstream protective device to provide the required protection. Follow the exact manufacturer’s instructions.

Can the existing upstream fuse protect an SPD?

Yes, in some installations. It must satisfy the manufacturer’s conditions and the applicable short-circuit and installation requirements.

Can an MCB replace an SPD backup fuse?

Sometimes. The breaker’s trip characteristics and breaking capability must be suitable for the exact SPD arrangement. Do not assume that an MCB with the same ampere rating is equivalent to a fuse.

Is the maximum backup fuse the recommended fuse size?

No. It is the maximum permitted value for the stated conditions. The selected device must also withstand the expected surge and provide the required fault protection.

Does SPD Imax determine the backup fuse size?

No. Imax describes the SPD’s surge discharge capability. Backup protection depends on the manufacturer’s protection requirements, fault conditions, wiring, and protective-device characteristics.

Does Isccr equal the MCB’s Icu or Icn?

No. These ratings are not interchangeable. Evaluate the complete SPD and protective-device arrangement.

Conclusion

Choosing SPD backup protection is a coordination task, not a simple fuse-size calculation. Start with the exact SPD model and check the manufacturer’s maximum backup protection, the existing upstream F1, and the available fault current. If a circuit breaker is used, also verify its breaking capacity, operating characteristics, and compatibility with the specified SPD protection arrangement.

The main point is simple: do not select a fuse or circuit breaker from the SPD’s kA rating alone. The protective device must match the manufacturer’s requirements and the actual electrical installation.

If you are sourcing an SPD for a new project, review our YQNT AC and DC surge protective devices and confirm the required backup protection from the product documentation before ordering.