Wondering how to know if an SPD is bad? The status indicator is usually the first place to check, but it cannot tell you everything about the condition of a surge protective device.

A surge protective device (SPD) can continue to look completely normal from the outside even after part of its protection function has been lost.

This creates a common problem for installers and system owners:

How do you know if an SPD is bad?

The quickest place to start is the status indicator. On many modular SPDs, a normal green indication means the monitored protection module remains in its normal available state, while a red or failure indication means that the affected module should be checked or replaced.

However, the indicator is only one part of the diagnosis.

An SPD can also require attention because of overheating, visible damage, a remote alarm, an operated backup protective device, or exposure to an unusually severe electrical event.

And importantly:

A green indicator does not prove that the entire surge protection system is perfect.

The correct way to judge SPD condition is to combine the product indicator, visual inspection, installation condition, event history, and manufacturer instructions.

How Do I Know If My SPD Is Bad?

An SPD may be bad if its status indicator shows a failure condition, one or more modules turn red, the housing shows burn marks or deformation, the unit overheats, a remote alarm appears, or the SPD has experienced an abnormal electrical event. Always confirm the indicator meaning with the manufacturer’s instructions.

This guide explains 7 practical signs that an SPD may be bad, what each sign actually means, and when the module or complete SPD should be replaced.

Principaux enseignements

• A red or failed status indication is one of the clearest signs that an SPD module may no longer provide its intended protection.
• A green indicator usually confirms only the monitored condition of the module, not its complete remaining surge capacity.
• Burn marks, deformation, unusual odor, or abnormal heating require investigation.
• Equipment can often continue operating even when an SPD has failed because many power SPDs are connected en parallèle with the circuit.
• A normal digital multimeter cannot verify Up, In, Imax, Iimp, or remaining SPD life.
• Do not automatically replace every cartridge after every lightning event. Inspect the SPD and follow the manufacturer’s post-event instructions.
• For plug-in SPDs, a failed cartridge may sometimes be replaced without replacing the base, provided the base and surrounding installation are undamaged and the manufacturer permits it.

What Does It Mean When an SPD “Goes Bad”?

An SPD is designed to limit transient overvoltages and divert surge current away from sensitive electrical equipment.

Modern low-voltage SPDs typically contain one or more nonlinear protective components. MOVs, or metal oxide varistors, are commonly used in many Type 2 and combined SPD designs.

IEC standards describe SPDs as devices containing at least one nonlinear component that limits surge voltage and diverts surge current. Current IEC requirements distinguish between AC, general DC, and photovoltaic DC applications.

During normal system voltage, the surge protection element remains largely non-conductive.

When a transient overvoltage occurs, the SPD provides a low-impedance path for the surge current and helps limit the voltage reaching downstream equipment.

But this process creates electrical and thermal stress.

After repeated surge events, a severe surge, abnormal system voltage, or other stressful conditions, a protection component may deteriorate.

Many MOV-based modular SPDs therefore include a thermal disconnector.

If the protective component reaches an unsafe condition, the disconnector can separate it from the circuit. Many products mechanically link this condition to a red/green status window.

For example, Phoenix Contact documents surge protection plugs combining a varistor, thermal disconnect device and local green/red status indicator.

Phoenix Contact, for example, documents plug-in varistor modules incorporating a thermal disconnect device and local green/red indication.

This helps the SPD fail more safely.

But once the affected protection path has been disconnected, the system may no longer have the intended level of surge protection.

7 Signs an SPD May Be Bad

1. The SPD Status Indicator Shows Red or Failure

This is usually the first thing to check.

Many plug-in SPDs have a small mechanical status window on the front.

Depending on the product, the normal state may be shown as:

• Green
• OK
• Normal
• Protected

A failed or replacement state may be shown as:

• Red
• Replace
• Fault
• Failed

For many modular MOV-based products:

Green → monitored protection module remains available

Red → internal disconnection has occurred and the affected module normally requires replacement

However, this is not a universal color rule for every SPD.

Different manufacturers use different indicators. Some SPDs use LEDs instead of mechanical windows. Others use displays, alarm contacts, symbols, audible alarms, or combinations of these functions.

Schneider Electric, for example, documents SPD families where LED conditions are used to indicate whether surge suppression remains available on each phase.

Therefore, always check the marking and instructions for the exact SPD model.

Does red mean the whole electrical system stops working?

Usually not.

Many panel-mounted SPDs are installed en parallèle with the power circuit.

The electrical load therefore does not normally depend on current passing through the SPD.

If an internal SPD disconnector opens, the protected equipment may continue operating normally while the surge protection on that path has been lost.

This is why a failed SPD can easily remain unnoticed.

The lights stay on.

The inverter keeps running.

The machine continues operating.

But the next transient may no longer be limited in the same way.

2. One Module Is Red While the Others Are Green

A multi-pole SPD should not be judged by looking at only one cartridge.

For example, a multi-module installation may show:

Suppose a three-phase SPD has three or four plug-in modules.

If three remain green but one has changed to the manufacturer’s failure state, you should not describe the complete SPD as fully operational.

At least one intended protection path is no longer confirmed.

Should you replace all modules when only one is red?

Not automatically.

Some modular SPDs are specifically designed so individual cartridges can be replaced.

If:

• only one cartridge has failed,
• the base is undamaged,
• the terminals show no overheating,
• the correct replacement cartridge is available,
• and the manufacturer allows individual replacement,

then replacing the affected cartridge may be sufficient.

However, if there is:

• severe overheating,
• arcing,
• damage to the base,
• several failed modules,
• melted plastic,
• repeated unexplained failures,
• or uncertainty about the condition of the assembly,

the complete SPD and surrounding installation should be investigated.

3. There Are Burn Marks, Discoloration, Cracks, or Deformation

The status indicator is not the only thing that matters.

Always inspect the physical condition of the SPD.

Warning signs include:

• Blackening
• Brown discoloration
• Melted plastic
• Boîtier fissuré
• Bulging or deformation
• Damaged terminals
• Loose cartridge
• Corrosion
• Contamination par l'humidité
• Signes d'arc électrique

An SPD showing obvious thermal or mechanical damage should not simply be left in service because its indicator still appears green.

Physical damage can be caused by several different conditions, including:

• severe surge stress,
• abnormal overvoltage,
• poor terminal connection,
• overheating,
• contamination,
• incorrect installation,
• inappropriate system voltage,
• or an electrical fault.

The cause therefore matters.

Simply changing the cartridge without understanding why it was damaged can allow the same problem to happen again.

A damaged SPD should be treated differently from a normally disconnected module

There is an important difference between:

A clean module that has reached its manufacturer-defined replacement condition

et

An SPD showing burning, melting, arcing, or mechanical damage.

In the second situation, inspection should extend beyond the replaceable cartridge.

Check the:

• SPD base
• terminals
• conductor insulation
• backup protection
• earthing connection
• surrounding enclosure
• tension du système
• connection condition

Severe physical damage is a reason to investigate the entire installation rather than simply installing a new plug.

4. The SPD Is Abnormally Hot or Produces an Unusual Smell

Electrical equipment naturally operates at different temperatures.

But an SPD that becomes unusually hot, produces a burnt smell, or shows progressive discoloration deserves immediate attention.

MOV-based protective components can experience increasing leakage and thermal stress as they degrade.

Proper SPD designs therefore use internal disconnecting or protective mechanisms to reduce the risk of an unsafe end-of-life condition.

What should you do if the SPD feels hot?

Do not diagnose an energized SPD by repeatedly touching it.

If abnormal heating is suspected, inspection should be carried out by qualified personnel using appropriate procedures and instruments.

Les causes possibles sont les suivantes :

• Dégradation du parafoudre (SPD)
• sustained overvoltage
• incorrect Uc or Ucpv selection
• loose terminals
• excessive ambient temperature
• poor ventilation
• system faults
• unsuitable SPD application

A hot SPD does not automatically prove that the MOV itself has failed.

For example, a poorly tightened terminal can create local resistance and heating even if the surge component has not reached its end of life.

That is why the location of the heat and the overall installation condition should be investigated.

5. The SPD Remote Alarm Reports a Fault

Industrial SPDs frequently offer an optional remote signaling contact.

Instead of requiring someone to open the electrical cabinet and visually check the SPD, the remote contact can communicate the SPD’s model-defined condition to equipment such as:

• BMS (Système de gestion de batterie)
• PLC
• SCADA
• monitoring panel
• alarm lamp
• maintenance system

Phoenix Contact, for example, offers SPD assemblies combining local green/red indication with remote signaling.

This is useful in:

• solar PV plants,
• telecom systems,
• industrial plants,
• data centers,
• energy storage installations,
• remote substations,
• unattended electrical cabinets.

What does a remote alarm prove?

Usually, it tells you that the SPD’s monitored mechanism has changed state.

It does not automatically tell you:

• how large the surge was,
• why the SPD failed,
• how much lifetime remains in other modules,
• whether the earthing system is correct,
• whether the installation leads are too long,
• or whether the SPD rating is correct.

Remote signaling is therefore a maintenance tool, not a complete SPD diagnostic system.

If a remote alarm appears, confirm the local SPD status and inspect the device according to the manufacturer’s instructions.

6. The SPD or Its Backup Protective Device Operates Unexpectedly

SPDs can be installed with upstream or dedicated overcurrent protection depending on the SPD design and system requirements.

If the associated fuse or circuit breaker operates unexpectedly, do not immediately assume that the breaker itself is faulty.

Possible causes can include:

• internal SPD damage,
• abnormal power-frequency overvoltage,
• short-circuit conditions,
• incorrect SPD selection,
• unsuitable backup protection,
• wiring problems,
• or other system faults.

The backup protective device and the internal SPD disconnector do different jobs.

The internal thermal disconnector may operate because a surge protection element has reached an unsafe thermal condition without necessarily creating the type of sustained overcurrent required to trip a conventional breaker.

So this situation is possible:

SPD = failed

Breaker = still ON

This does not automatically mean the breaker has failed.

Likewise, repeatedly fitting a larger breaker is not a correct solution to repeated SPD disconnection.

The cause of the SPD failure must be investigated first.

7. The SPD Has Experienced a Major Lightning or Surge Event

A major lightning or switching event is an important reason to inspect an SPD.

But it is important to use careful wording here.

A lightning event does not automatically mean the SPD must be replaced.

SPDs are specifically designed to handle transient surge currents within their declared performance capabilities.

Par conséquent :

Lightning nearby ≠ SPD automatically destroyed

However, after a known severe event, especially at a critical facility, it is good maintenance practice to inspect:

• visual indicator status,
• remote alarm status,
• physical condition,
• cartridges,
• terminals,
• protective conductors,
• backup protection,
• and manufacturer-required diagnostics.

If a module has entered the manufacturer’s failed state, replace it.

If there is visible damage, investigate further.

If everything remains normal, follow the manufacturer’s maintenance and post-event instructions rather than replacing the SPD without evidence.

Does a Green SPD Indicator Mean the SPD Is Definitely Good?

Non.

This is one of the most important distinctions when inspecting an SPD.

If the manufacturer defines green as the normal state, a green indicator usually means something like:

The monitored disconnect mechanism has not indicated failure and the protection module remains in its normal available state.

That is useful information.

But it is not the same as proving every aspect of SPD performance.

A green indicator normally cannot prove:

• the SPD has its original remaining surge capacity,
• the voltage protection level Up is still exactly the declared value,
• the SPD is correctly sized for the system,
• Uc or Ucpv is correct,
• the earthing system is effective,
• all terminals are properly tightened,
• connection leads are sufficiently short,
• the required backup protection is correct,
• upstream and downstream SPDs are coordinated,
• every surge path is protected.

Think of the indicator as a condition signal, not a laboratory test.

If your SPD is still green, see our detailed guide to what an SPD green indicator can — and cannot — confirm about the protection system.

This is also why normal indication should be combined with periodic inspection.

Can an SPD Fail Without the Equipment Losing Power?

Oui.

This surprises many people.

A typical shunt-connected SPD is installed parallel to the circuit being protected.

A simplified arrangement looks like this:

Power Supply → Load

with the SPD connected from the live conductors toward the relevant protection path, such as PE.

The load does not normally receive its operating current through the SPD.

Therefore, when the SPD’s internal protective element is disconnected at end of life:

Load power may remain ON

while

surge protection may be reduced or lost.

This is exactly why local indicators and remote alarms are important.

Without inspection, a failed SPD can remain installed for months while everything downstream appears to operate normally.

Can You Test an SPD With a Multimeter?

When learning how to know if an SPD is bad, it is important to understand that a normal digital multimeter cannot fully verify surge protection performance.

This question needs a careful answer.

A normal multimeter cannot fully test SPD performance.

A digital multimeter cannot reproduce the standardized surge conditions used to determine major SPD ratings.

It cannot verify:

• In — Nominal Discharge Current
• Imax — Maximum Discharge Current
• Iimp — Impulse Discharge Current
• Up — Voltage Protection Level
• impulse durability
• complete remaining surge capacity

These characteristics require controlled test equipment and defined test procedures.

IEC SPD requirements use standardized testing to establish performance and safety characteristics; ordinary low-voltage resistance measurements are not substitutes for those tests.

What can a multimeter sometimes help check?

Depending on the SPD and the manufacturer’s instructions, a multimeter may help with:

• remote dry-contact continuity,
• basic wiring verification,
• investigation of an obvious short circuit,
• checking auxiliary circuits.

But there is no universal resistance reading that means “SPD good.”

For MOV-based products, the small test voltage produced by a normal multimeter may be far below the voltage at which the varistor significantly conducts.

A healthy MOV can therefore appear as very high resistance or open circuit.

A degraded MOV may also appear open under the same low test voltage.

Donc :

Open circuit on a multimeter does not prove the SPD is healthy.

Likewise:

A continuity beep is not a universal SPD pass/fail test.

Dedicated SPD or MOV test equipment can provide more useful diagnostic information for some products, but results must still be compared with manufacturer-defined values for that specific SPD.

Should You Use a Megohmmeter to Test an SPD?

Generally, do not apply an insulation resistance tester directly across an SPD unless the manufacturer specifically provides a procedure allowing it.

Megohmmeters use test voltages much higher than an ordinary multimeter.

Because an SPD is intentionally designed to conduct when voltage rises above certain conditions, insulation testing can:

• produce misleading readings,
• cause the SPD to conduct,
• interfere with the test,
• or potentially stress the device.

For system insulation testing, SPDs may need to be disconnected or removed according to the manufacturer’s instructions and applicable installation procedures.

Never assume an SPD should behave like ordinary insulation.

Why Do SPDs Fail?

Understanding the reason is important because simply installing another SPD may not solve the underlying problem.

1. Repeated Surge Exposure

Every surge does not necessarily destroy an SPD.

However, repeated surge activity can progressively stress protective components.

The actual service life depends on factors such as:

• surge magnitude,
• number of events,
• forme d'onde de choc,
• product design,
• system conditions,
• temperature,
• and protection coordination.

There is therefore no reliable rule such as:

“An SPD always lasts exactly five years.”

Calendar age alone does not determine SPD condition.

2. A Surge Exceeds the Device’s Capability

SPDs have declared surge-current capabilities.

A sufficiently severe electrical event can exceed the capability of a particular protection system.

This is one reason correct Type selection and coordinated surge protection are important.

Type 1, Type 2, and combined Type 1+2 SPDs are not simply different quality levels.

If you are unsure which classification applies, see our guide to Type 1 vs Type 2 vs Type 1+2 SPD for a more detailed comparison.

3. Temporary Overvoltage

A transient surge and a temporary overvoltage are not the same thing.

A surge is generally very short.

A temporary overvoltage, or TOV, can remain for much longer.

An SPD designed to conduct short transient energy should not be expected to clamp an abnormal power-frequency overvoltage indefinitely.

Improper TOV conditions can therefore create serious stress on an SPD.

Current IEC requirements specifically consider TOV and fault conditions as part of SPD application and testing.

4. Incorrect Uc or Ucpv Selection

Selecting an SPD with an unsuitable maximum continuous operating voltage can cause problems.

For AC systems, the relevant value is commonly expressed as Uc.

For photovoltaic applications, Ucpv is particularly important.

For photovoltaic DC applications, IEC 61643-31 specifies requirements and test methods for SPDs intended for the DC side of PV installations.

If you are not familiar with these ratings, our guide to DC SPD specifications explains Ucpv, In, Imax, Up and Iscpv separately.

If the SPD’s voltage rating is too low for the maximum voltage that can continuously appear in the system, the protection component can be stressed during normal or abnormal operating conditions.

For PV systems, remember that array voltage changes with temperature.

Maximum PV open-circuit voltage under expected low-temperature conditions therefore needs to be considered when selecting the DC SPD.

For the full selection process, see how to select the correct DC SPD voltage rating for a solar PV system.

5. Incorrect Installation

Even a good SPD can perform poorly if it is installed incorrectly.

Important installation factors include:

• wiring topology,
• polarity,
• grounding arrangement,
• la longueur des conducteurs,
• conductor routing,
• terminal torque,
• backup protection,
• SPD location.

Long connection leads are particularly important because the inductive voltage developed along the conductors during a rapidly changing surge current can add to the voltage seen by the protected equipment.

SPD location and lead length are two different installation issues. See why SPD distance from the inverter matters for a detailed explanation of the 10 m system-distance rule and the approximately 0.5 m connection-lead recommendation.

This means SPD performance depends not only on the number printed next to Haut de la page, but also on how the SPD is actually installed.

6. Environmental Conditions

Environmental stress can also shorten SPD service life.

En voici quelques exemples :

• excessive cabinet temperature,
• moisture,
• condensation,
• dust,
• corrosive atmosphere,
• vibration,
• poor ventilation.

These conditions can affect terminals, insulation, contacts, and surrounding installation components even before the internal surge element reaches normal end of life.

How to Check an SPD: A Practical Inspection Process

A simple field inspection can follow this order.

Step 1: Identify the Exact SPD

Check:

• manufacturer,
• model,
• AC or DC application,
• Type classification,
• voltage rating,
• replacement cartridge number.

Do not install a cartridge just because it physically fits the base.

Step 2: Read the Status Indicator

Confirm what the indicator means for that exact product.

Do not automatically assume every green window means normal and every red window means failure without checking the manufacturer’s markings.

Step 3: Inspect Every Module

For multi-pole SPDs, inspect all cartridges.

One failed module can mean the complete intended protection arrangement is no longer available.

Step 4: Look for Physical Damage

Vérifier pour :

• burning,
• melting,
• discoloration,
• cracks,
• moisture,
• loose modules,
• damaged terminals.

Step 5: Check Remote Signaling

If remote signaling is installed, compare the remote alarm state with the local status.

A mismatch should be investigated.

Step 6: Consider Recent Events

Ask whether the installation recently experienced:

• lightning,
• switching events,
• abnormal system voltage,
• equipment faults,
• fuse operation,
• circuit breaker operation.

Step 7: Decide Whether to Replace or Investigate Further

Replace the SPD Cartridge or the Whole SPD?

Many DIN-rail SPDs are modular.

They consist of:

Base + replaceable protection cartridge

For reference, you can also view our DC surge protective device range, including modular Type 2 and Type 1+2 models with replaceable cartridges.

This allows easier maintenance.

Replace only the cartridge when:

• the product is designed for plug-in replacement,
• only the cartridge has reached its replacement condition,
• the base is undamaged,
• terminals are sound,
• there are no signs of overheating or arcing,
• and the correct compatible cartridge is available.

Consider replacing the complete SPD when:

• the base is burnt,
• terminals are damaged,
• plastic has melted,
• there has been severe arcing,
• multiple components are damaged,
• the SPD is non-modular,
• the original replacement cartridge is unavailable,
• or the manufacturer requires complete replacement.

Never bypass the SPD’s internal thermal disconnector or attempt to reset a sealed failed cartridge.

A cartridge that has entered its defined end-of-life condition is normally intended to be replaced rather than repaired.

Does a Failed SPD Need to Be Replaced Immediately?

If the manufacturer-defined indication shows that the intended surge protection path is no longer available, replacement should not be unnecessarily delayed.

The electrical system may continue running without it.

But that does not mean continued operation without surge protection is desirable.

This is especially important for:

• solar inverters,
• industrial control systems,
• PLCs,
• communication equipment,
• data centers,
• EV charging infrastructure,
• energy storage systems,
• expensive electronic loads.

The longer the failed SPD remains in place, the longer the equipment may remain exposed to the next transient event.

How Often Should an SPD Be Inspected?

There is no single inspection interval suitable for every installation.

Inspection frequency should consider:

• manufacturer instructions,
• site maintenance procedures,
• lightning exposure,
• system importance,
• environmental conditions,
• past surge activity,
• criticality of protected equipment.

Inspection is particularly useful:

• during routine electrical maintenance,
• after a known severe surge or lightning event,
• after unexplained protective-device operation,
• after changes to the electrical system,
• when a remote alarm occurs.

For unattended or critical installations, remote signaling can make it easier to detect a failed SPD quickly.

SPD Failure Does Not Always Mean the SPD Was Poor Quality

Seeing a failed SPD sometimes leads to the assumption:

“The SPD failed, so it must be defective.”

That conclusion is not always correct.

An SPD is a protective device exposed directly to electrical stress.

A failure indication may mean that it experienced the kind of abnormal condition its protective and disconnecting mechanisms were designed to handle safely.

The more useful question is:

Why did the SPD reach this condition?

Investigate:

• event severity,
• system voltage,
• Uc or Ucpv selection,
• SPD Type,
• installation,
• grounding,
• lead length,
• backup protection,
• TOV exposure,
• environmental conditions.

If the same SPD position fails repeatedly, replacing cartridges without investigating the system is not enough.

How to Know If an SPD Is Bad: Final Inspection Checklist

The easiest sign of SPD failure is a manufacturer-defined failure or replacement indication, commonly red on many modular SPDs.

But that should not be the only check.

An SPD should also be investigated if you find:

1. A red or failed status indicator
2. One failed module among several modules
3. Burning, cracking, melting, or discoloration
4. Abnormal heating or burnt odor
5. A remote fault alarm
6. Unexpected SPD or backup-protection operation
7. Evidence of a significant lightning, surge, or abnormal electrical event

At the same time, do not assume that a green indicator proves the entire surge protection system is perfect.

It usually confirms only the condition monitored by that particular SPD.

And do not rely on a normal multimeter resistance test to prove surge performance.

Correct SPD assessment combines:

product status + visual inspection + installation condition + event history + manufacturer instructions.

If a modular SPD has reached its defined end-of-life state, replace the affected cartridge or assembly as specified.

More importantly, if failure is repeated or accompanied by physical damage, investigate the underlying electrical system before simply installing another SPD.

Ultimately, how to know if an SPD is bad comes down to combining the status indication, physical inspection, event history, installation condition and manufacturer guidance rather than relying on a single test.

Questions fréquemment posées

How to Know If an SPD Is Bad During a Basic Inspection?

Start with the manufacturer’s status indicator. If the product defines the displayed condition as normal and there is no physical damage or alarm, the monitored module remains in its normal indicated state. However, the status indicator does not prove complete surge performance or remaining lifetime.

Does green mean an SPD is good?

On many modular SPDs, green means that the monitored protection module has not entered its failure or disconnection state.

But always confirm the manufacturer’s markings because indicator systems vary between products.

Green should not be interpreted as proof that the complete installation is perfectly protected.

What does a red SPD indicator mean?

On many plug-in MOV-based SPDs, a red indicator means the protection module has reached a manufacturer-defined failed or replacement condition, often associated with an internal disconnector.

The affected cartridge normally needs to be replaced.

Check the exact product instructions before taking action.

Can an SPD still supply power when it has failed?

Oui.

Most panel-mounted SPDs are connected in parallel with the protected circuit.

The load can therefore remain energized even if the SPD protection element has been disconnected.

Can I test an SPD with a multimeter?

Only to a limited extent.

A multimeter may help check remote dry contacts or investigate certain basic electrical conditions when permitted by the manufacturer.

It cannot verify surge ratings such as Up, In, Imax, or Iimp, and there is no universal resistance value that proves an SPD is healthy.

Should I replace an SPD after every lightning strike?

Not automatically.

Inspect the SPD after a significant event and follow the manufacturer’s maintenance instructions.

Replace it if it shows a failure indication, physical damage, abnormal condition, or if the manufacturer specifically requires replacement after the event.

Can I replace only the red SPD cartridge?

Replaceable cartridges are common in modular SPDs. For example, Schneider Electric explains that when the SPD end-of-life indicator turns red, the cartridge should be replaced.

Often yes, if the SPD is designed with replaceable plug-in modules and the base is undamaged.

Use only the correct compatible replacement cartridge specified for that SPD.

If the base, terminals, or surrounding components show heat or arc damage, the complete assembly should be investigated.

Why does my SPD keep turning red?

Repeated failure can indicate more than ordinary surge aging.

Les causes possibles sont les suivantes :

• repeated severe surges,
• incorrect SPD voltage rating,
• temporary overvoltage,
• incorrect application,
• installation problems,
• grounding problems,
• inappropriate backup protection,
• excessive environmental stress.

If cartridges repeatedly fail at the same location, investigate the system instead of continuing to replace modules without identifying the cause.

Can an SPD last forever if the indicator stays green?

No reliable fixed lifetime can be determined from indicator color alone.

SPD life depends on the electrical stress it experiences, environmental conditions, product design, and installation.

The indicator should be treated as one part of a wider inspection and maintenance process.