The most common causes of SPD failure are repeated surge stress, temporary overvoltage, incorrect Uc or Ucpv selection, wiring errors, unsuitable backup protection, excessive temperature, moisture, and the use of the wrong surge protective device for the system.
Lightning is only one possible cause. A surge protective device may also fail prematurely because the real operating voltage is higher than expected, a neutral fault creates abnormal voltage, a PV array exceeds the selected Ucpv, or the SPD is installed under unsuitable electrical or environmental conditions.
For maintenance teams, the important question is not only “Has the SPD failed?” It is also “Why did it fail, and will the replacement SPD fail again?”
Scope of this guide: This page explains why SPDs fail and why replacement modules sometimes fail again. For step-by-step inspection procedures, see How to Check SPD Failure . For plug-in cartridge replacement, use the related replacement guide.
Índice
What causes an SPD to fail?
An SPD can fail after accumulated surge exposure, one event above its discharge capability, a temporary overvoltage or another sustained abnormal voltage condition, Uc or Ucpv selected below the real system voltage, incorrect wiring, unsuitable fault-current backup protection, high enclosure temperature, moisture, or an incorrect AC or PV DC application.
If a new replacement SPD fails again, do not continue changing cartridges without investigating the system. Measure the actual voltage, verify the SPD label and application, check the neutral and wiring, inspect the backup fuse or MCB, and examine the base, terminals and enclosure conditions.
| Possible cause | What may happen | What maintenance teams should verify |
|---|---|---|
| Repeated surge exposure | MOV components gradually age after accumulated electrical and thermal stress. | Lightning exposure, switching events, surge history and replacement records. |
| Surge above SPD capability | A severe event can cause immediate degradation or operate the internal disconnector. | SPD type, In, Imax or Iimp, installation position and lightning protection concept. |
| Temporary or abnormal overvoltage | The SPD may conduct for too long, generate heat and disconnect. | L-N, L-PE, N-PE or DC voltage, neutral condition, transformer, generator and grid abnormalities. |
| Uc or Ucpv too low | Normal operating voltage or expected variation can accelerate varistor ageing. | Maximum continuous voltage, earthing arrangement, PV Voc and low-temperature correction. |
| Incorrect wiring or application | One protection mode may carry abnormal voltage or the intended protection path may not operate correctly. | Terminal assignment, polarity, pole configuration, N-PE arrangement and AC versus PV DC application. |
| Incorrect backup protection | A power-frequency fault may not be cleared correctly, or the upstream device may trip unnecessarily. | Fuse or MCB rating, breaking capacity, short-circuit current and manufacturer requirements. |
| Long leads or poor bonding | Protection performance may be reduced and the equipment may see a higher effective residual voltage. | PE continuity, equipotential bonding, conductor length, routing and terminal tightness. |
| High temperature or moisture | Thermal ageing, corrosion, insulation deterioration and terminal heating may be accelerated. | Enclosure temperature, ventilation, sunlight, condensation, IP protection and loose terminals. |
| Product or batch problem | Components, connections or disconnection mechanisms may not perform as declared. | Whether similar failures appear in one production batch or under verified normal conditions in different installations. |
Cause, failure mechanism and visible result are not the same
A useful SPD root-cause investigation separates four different levels. The visible red indicator is usually the final result, not the original cause.
| Root cause | Failure mechanism | Protective response | Visible result |
|---|---|---|---|
| Disconnected or unstable neutral | The applied voltage exceeds the normal operating range and the MOV conducts for too long. | Internal temperature rises and the thermal disconnector opens. | The status window changes, the remote contact changes state, or the SPD becomes disconnected. |
| Repeated surge exposure | MOV characteristics gradually change after accumulated electrical and thermal stress. | Leakage and heating increase until the protection element is isolated. | End-of-life indication appears after a later event or during operation. |
| One event above the SPD capability | The protection component experiences excessive impulse energy. | The disconnector operates or upstream fault protection clears the circuit. | Immediate indication, tripped protection or visible damage may occur. |
A red indicator identifies the SPD condition, not the root cause
Maintenance personnel still need to determine whether the original cause was normal accumulated surge ageing, abnormal system voltage, incorrect selection, installation conditions or a possible product problem.
1. Repeated surges gradually age the SPD
Many voltage-limiting SPDs use metal oxide varistors. During a surge, the varistor changes from a high-impedance state to a conductive state and limits the transient voltage across the protected circuit.
Every significant operation creates electrical and thermal stress. A small event may have little measurable effect, while repeated or higher-energy events can gradually change the MOV characteristics, increase leakage current and reduce the remaining operating margin.
Eventually, the internal thermal disconnector may isolate the protection component and the visual indicator or remote contact may change state.
There is no universal number of lightning strikes for an SPD
SPD service life cannot be predicted by counting lightning events alone. A low-energy switching transient and a high-energy lightning current do not place the same stress on the device. Waveform, amplitude, duration, current distribution, SPD rating, installation position and previous degradation all affect the result.
A relatively new SPD can fail during one severe event. An older SPD may remain operational after many smaller events. A surge counter therefore cannot provide an exact percentage of remaining SPD life.
2. One surge can exceed the SPD’s discharge capability
An SPD is not an unlimited energy absorber. Its declared current ratings describe performance under specified test waveforms and conditions.
Type 2 SPDs commonly declare nominal discharge current Em and maximum discharge current Imax. Type 1 SPDs declare impulse current Iimp.
If an actual event is more severe than the device, installation position or protection concept can manage, the SPD may disconnect immediately, suffer internal degradation or lose protection without dramatic external damage.
A Type 2 SPD that fails near the service entrance is not automatically defective. The project may have required a Type 1 or Type 1+2 SPD, a different current rating, shorter conductors or better coordination with other protection stages.
Do not diagnose surge capacity from Imax alone
The correct SPD type and rating depend on installation position, lightning exposure, the presence of an external lightning protection system, incoming service type and coordination with downstream SPDs. See the Guia de Seleção Imax de SPD .
3. Temporary overvoltage is not the same as a lightning surge
Many premature or repeated SPD failures are blamed on lightning when the actual cause is a temporary overvoltage, or another abnormal voltage condition that remains present for much longer than a transient surge.
A transient surge may have a very high peak but last only microseconds. A temporary overvoltage usually has a lower peak than a lightning impulse, but it remains present for a much longer time.
A surge protective device is designed to limit short transient overvoltages. It is not intended to operate continuously as a voltage regulator.
If the applied voltage remains above the SPD’s continuous operating range, the voltage-limiting component may conduct for too long, produce heat and activate its thermal disconnector.
A compliant SPD should have a defined response to specified temporary overvoltage conditions. Depending on the declared design, it may withstand the condition or reach a controlled end-of-life state. This does not mean that it can safely clamp every sustained abnormal voltage indefinitely.
Possible sources of abnormal voltage
- Loose, damaged or disconnected neutral conductor.
- Incorrect transformer tap setting.
- Generator voltage-regulation problem.
- Phase imbalance or another network fault.
- Incorrect line, neutral or protective-conductor connection.
- PV earth fault, insulation fault or unexpected conductor-to-earth voltage.
- System voltage consistently higher than assumed during SPD selection.
Replacing the cartridge does not remove abnormal voltage
If the original failure was caused by a neutral problem, TOV or another sustained abnormal voltage condition, a replacement cartridge with the same rating may also overheat and disconnect. Measure and correct the system condition before repeatedly installing new cartridges.
4. Uc or Ucpv was selected below the real operating voltage
Uc is the maximum continuous operating voltage declared for an AC SPD. For photovoltaic SPDs, the corresponding DC parameter is commonly shown as Ucpv.
When Uc or Ucpv is too low, normal voltage variation or an expected operating condition may repeatedly bring the protection component closer to conduction.
The result can be higher leakage current, additional heating, accelerated MOV ageing and premature operation of the internal thermal disconnector.
Selecting the lowest available Uc does not automatically provide the best protection. The continuous voltage rating must first be high enough for the actual system voltage, expected variation and earthing arrangement.
Sistemas de distribuição AC
Do not choose Uc only from the nominal line-to-line voltage. Confirm which conductors each protection mode is connected between. TN, TT and IT systems can create different voltage conditions across the SPD protection modes.
For the complete AC selection process, see the SPD Uc Selection Guide .
Solar PV DC systems
The open-circuit voltage of a PV module rises at low temperature. String length, module data and the lowest expected site temperature must therefore be considered when calculating the maximum array voltage.
The SPD must also match the PV topology and maximum voltage from each active conductor to earth. For the separate PV calculation process, see the DC SPD Ucpv Selection Guide .
5. Incorrect wiring changes the intended protection path
An SPD can show a green indicator while still being installed incorrectly. The indicator normally reports the condition of a monitored internal protection component. It does not verify every external conductor, terminal and protection mode.
Typical wiring and application problems include:
- Line, neutral or PE connected to the wrong terminal.
- Incorrect 1P+N, 2P, 3P+N or 4P configuration for the network.
- Incorrect N-PE protection arrangement in a TT system.
- Loose terminals creating resistance heating.
- Connection conductors routed in a large loop.
- Very long line and PE conductors.
- DC positive, negative and PE protection paths connected incorrectly.
- An AC SPD installed on the PV DC side.
- A cartridge installed in an incompatible base.
Incorrect wiring does not always destroy the SPD immediately. In some installations, the more direct result is poor equipment protection. In other cases, one protection mode can be exposed to a voltage or fault condition outside its intended application.
Need a wiring reference?
Use the Guia de Diagrama de Fiação de SPD Trifásico or the Single-Phase SPD Wiring Diagram Guide . Installation and voltage measurement must be performed by qualified electrical personnel.
6. The backup fuse or MCB is missing or incorrectly coordinated
The upstream fuse or circuit breaker does not normally limit a microsecond surge. Its main role in the SPD branch is to clear power-frequency fault current if the SPD or its connection develops a fault.
Correct coordination should consider the SPD manufacturer’s declared maximum backup protection, the conductor, the upstream device, the prospective short-circuit current and the required breaking capacity.
If the protective device is unsuitable:
- An internal SPD fault may not be disconnected safely or quickly enough.
- An undersized fuse or unsuitable MCB may operate during expected surge events.
- The SPD branch may remain disconnected without being noticed.
- The interrupting capacity may be insufficient for the panel fault level.
- Replacing the SPD alone may leave the original coordination problem unchanged.
Do not copy a backup fuse rating from another SPD
Two SPDs with similar Uc, In or Imax values may have different backup protection requirements. Confirm the documentation for the exact model and the actual short-circuit conditions of the panel.
7. Long leads and poor bonding mainly reduce protection performance
During a fast surge, conductor inductance matters. A long connection conductor can develop additional voltage while surge current is flowing.
This voltage is added to the SPD’s own protection voltage and can increase the voltage experienced by downstream equipment.
Loose PE connections, poor equipotential bonding, corrosion and unnecessarily long conductors can therefore make the complete protection system less effective.
Long conductors do not automatically mean the SPD itself will fail
The most direct effect of excessive connection length or poor bonding is usually reduced protection performance and a higher effective voltage at the equipment. These conditions may contribute to abnormal current distribution or additional stress, but they should not be described as an automatic direct cause of SPD failure.
Check the complete connection path
- Line or neutral connection from the busbar to the SPD.
- Connection from the SPD to PE, PEN or the equipotential bonding bar.
- Total conductor length, not only one side of the connection.
- Loops and separation between incoming and protected conductors.
- Terminal torque, corrosion and conductor cross-section.
- Coordination with upstream and downstream SPDs.
8. High enclosure temperature accelerates SPD ageing
SPD components already experience thermal stress during surge operation and abnormal voltage conditions. High ambient temperature reduces the available thermal margin and can accelerate ageing of varistors, insulation, terminals and disconnection components.
Common high-temperature situations
- Outdoor solar combiner boxes exposed to direct sunlight.
- Sealed cabinets with limited ventilation.
- SPDs installed close to contactors, power supplies or other heat sources.
- Loose terminals producing local resistance heating.
- High-current conductors heating the same enclosure.
- Ambient temperature outside the product’s declared operating range.
Moisture and condensation can cause corrosion, contamination and insulation deterioration. For outdoor PV, telecom and industrial installations, inspect cable glands, enclosure sealing, drainage and condensation control—not only the plug-in module.
9. Solar and DC SPD failures need a separate review
A PV DC system is not simply an AC distribution board with a different voltage label. Long string cables, voltage rise at low temperature, conductor-to-earth voltage, insulation monitoring and DC fault interruption all affect SPD selection.
| DC-specific issue | Por que isso importa | O que confirmar |
|---|---|---|
| Ucpv below maximum array voltage | The SPD can remain exposed to excessive DC voltage instead of seeing only short transient events. | Maximum corrected string Voc at the lowest expected site temperature. |
| AC SPD used on the DC side | The device may not have the required DC voltage, disconnection or arc-handling design. | PV DC declaration, product application and relevant IEC 61643-31 requirements. |
| Wrong PV topology assumption | Positive or negative conductor-to-earth voltage may differ from the value assumed during selection. | Floating, earthed or other PV system arrangement. |
| Incorrect protection mode | One internal protection mode may carry voltage or surge stress outside its intended design. | Positive-to-PE, negative-to-PE and positive-to-negative configuration. |
| Outdoor thermal stress | High combiner-box temperature reduces thermal margin and accelerates ageing. | Direct sunlight, internal temperature, ventilation and installation spacing. |
| Long DC cable routes | Surges may enter from different points and require coordinated protection near arrays, combiner boxes and inverter inputs. | Cable route, SPD locations and coordination between protection stages. |
Do not apply one polarity rule to every DC SPD
DC SPD internal protection circuits differ. Some products use symmetrical protection arrangements, while others have defined terminal or polarity requirements. Follow the diagram for the exact model rather than applying a general rule from another product.
10. Product quality or manufacturing defects are also possible
System voltage, selection and installation should be investigated carefully, especially when replacement SPDs repeatedly fail in the same location. However, product quality problems cannot be excluded automatically.
Possible product-related causes include inconsistent MOV characteristics, unsuitable thermal disconnection, weak internal connections, poor terminal contact, incorrect materials, inadequate sealing, incorrect labels or insufficient batch consistency.
A product or batch issue becomes more likely when:
- Similar failures concentrate in the same production batch.
- Units fail under verified normal voltage and temperature conditions.
- The same unusual damage pattern appears in unrelated installations.
- Wiring, protection coordination and system voltage have been independently verified.
- Failure occurs much earlier than comparable units under similar conditions.
- The label or supplied replacement cartridge does not match the approved specification.
A balanced root-cause investigation protects both buyer and supplier
Not every failed SPD is defective, but not every failure should be blamed on the installation. Recording product labels, batch information, operating conditions, wiring and damage patterns makes the conclusion more reliable.
11. Why does the replacement SPD fail again?
Repeated failure shortly after replacement is a strong sign that the original cause may still be present. Installing another cartridge restores protection only when the base, wiring and system conditions remain suitable.
Frequent causes of repeated SPD failure
- The replacement has the same unsuitable Uc or Ucpv as the original.
- A neutral fault or abnormal system voltage was not corrected.
- The cartridge was replaced but the base or terminal was heat-damaged.
- The backup fuse or MCB remains incorrectly selected.
- Loose wiring continues to produce local heating.
- The enclosure remains too hot or contains condensation.
- A Type 2 SPD is exposed to a duty requiring Type 1 protection.
- The PV array voltage exceeds the original design assumption.
- Only one pole was replaced although all modules experienced the same event.
- An incompatible replacement cartridge was installed in the existing base.
Do not continue replacing cartridges without measuring the system
A second failure should trigger a root-cause review. Continuing to install the same SPD specification without checking the operating voltage, neutral, PV Voc, wiring, base and enclosure can reproduce the same failure.
12. When should several SPD modules be checked or replaced?
One failed cartridge does not automatically mean that every SPD in the facility must be replaced. The decision should be based on the event, installation history, common exposure, system criticality and whether the remaining protection condition can be verified.
Correct the common cause before replacing a full batch
Batch replacement without correcting abnormal voltage, unsuitable Uc or Ucpv, incorrect wiring, heat or moisture can reproduce the same failure across the new modules.
13. Root-cause checklist before ordering replacement SPDs
The following information helps maintenance teams and suppliers distinguish normal end-of-life replacement from a system, selection, installation or possible batch problem.
Product information
- Full SPD model and clear label photo.
- Type 1, Type 2 or Type 1+2.
- Uc or Ucpv.
- In, Imax or Iimp.
- Up and pole configuration.
- Plug-in cartridge and base model.
- Production batch and installation date.
System information
- Nominal and measured operating voltage.
- AC earthing system or PV topology.
- Maximum corrected PV string Voc.
- Backup fuse or MCB model and rating.
- Prospective short-circuit current if available.
- Installation position in the protection system.
- Upstream and downstream SPD arrangement.
Failure evidence
- Indicator and remote-contact status.
- Cracking, discoloration or burning.
- Base or terminal heat damage.
- Number of failed units.
- Failure date and operating condition.
- Recent lightning, outage or grid event.
- Whether a replacement also failed.
Installation environment
- Panel and wiring photos.
- Approximate SPD conductor length.
- PE and equipotential-bonding arrangement.
- Ambient and internal enclosure temperature.
- Direct sunlight, moisture or condensation.
- Nearby heat-producing equipment.
- Enclosure type and ingress protection.
Not sure why several SPDs failed?
Send us the SPD label, measured system voltage, panel wiring photo, backup protection, failure quantity and base condition. LEEYEE can help compare the failed model with the real application and recommend a suitable replacement specification for maintenance, panel-building or OEM orders.
Perguntas mais frequentes
What is the most common cause of SPD failure?
There is no single cause for every installation. Repeated surge stress and abnormal system voltage are common electrical causes. Incorrect Uc or Ucpv, wiring errors, unsuitable backup protection, high temperature and incorrect application often explain premature or repeated SPD failure.
Is SPD failure always caused by lightning?
No. Lightning is only one possible cause. A disconnected neutral, temporary overvoltage, incorrect voltage rating, wiring error, overheating, moisture, unsuitable system configuration or a possible product defect can also cause an SPD to disconnect or fail prematurely.
Why does an SPD keep failing after replacement?
Repeated failure usually means the original cause remains. Possible causes include abnormal system voltage, Uc or Ucpv selected too low, incorrect wiring, unsuitable backup protection, a damaged plug-in base, high temperature or an incompatible replacement cartridge.
Can repeated lightning surges wear out an SPD?
Yes. Repeated surge current creates electrical and thermal stress in the protection components. The degradation rate depends on event energy, waveform, SPD rating, installation conditions and previous ageing. There is no fixed number of lightning events that applies to every SPD.
Can high mains voltage damage an SPD?
Yes. If the voltage remains above the SPD’s continuous operating range, the voltage-limiting component may conduct for too long, generate heat and activate its thermal disconnector.
What happens if SPD Uc is too low?
Normal operating voltage or expected voltage variation may repeatedly stress the SPD. This can increase leakage current, heating and MOV ageing and lead to premature disconnection.
Can poor grounding cause SPD failure?
Poor bonding, loose PE connections and long conductors mainly reduce overall protection performance and increase the effective voltage at the equipment. They may contribute to abnormal stress, but they should not be treated as an automatic direct cause of SPD failure.
Does an SPD need a backup fuse or MCB?
The requirement depends on the SPD, upstream protective device and installation. When required, backup protection clears power-frequency fault current in the SPD branch. Its rating and breaking capacity must follow the exact product documentation and the panel fault level.
Why does a solar DC SPD fail repeatedly?
Common causes include Ucpv below the corrected maximum array Voc, use of an AC SPD on the DC side, incorrect PV topology, unsuitable protection modes, high combiner-box temperature, moisture or unresolved insulation and earth faults.
Should all SPD modules be replaced when one module fails?
Not automatically. Inspect the remaining poles, base, terminals, event history and system condition. Broader replacement may be appropriate after a common severe event, repeated failures or when the remaining condition cannot be verified.
Can I replace only the plug-in SPD cartridge?
Yes, when the replacement cartridge is exactly compatible and the base, contacts, terminals and wiring are undamaged. Replace or further inspect the complete unit when there is melting, discoloration, cracking, loose contacts or repeated failure.
Does a green indicator prove that the SPD is correctly installed?
No. A green indicator normally reports the condition of a monitored internal component. It does not confirm correct wiring, suitable Uc or Ucpv, correct backup protection, short conductors or effective bonding.
Technical Basis and Application Notes
SPD failure analysis should be based on the exact product documentation, system measurements and the applicable installation requirements. The following standards are particularly relevant to the topics discussed in this guide:
- IEC 61643-11: requirements and test methods for SPDs connected to low-voltage AC power systems.
- IEC 61643-31: requirements and test methods for SPDs used in photovoltaic installations.
- IEC 60364-5-53: selection and erection requirements for isolation, switching, control and surge protective devices in low-voltage installations.
- The manufacturer’s declared Uc or Ucpv, TOV behaviour, backup protection, short-circuit withstand information and installation instructions for the exact SPD model.
Standards and local installation rules may be revised. Project engineers should confirm the edition and regional requirements applicable to the specific installation.
Safety note: SPD inspection, live-voltage measurement and replacement must be performed by qualified electrical personnel using appropriate isolation, verification and personal protective procedures. Product ratings and replacement decisions should be confirmed against the documentation of the exact SPD model and the applicable installation rules.
