If the local mechanical status window on a conventional pluggable SPD has changed to red, treat that protection module as end-of-life and arrange replacement without delay. The distribution circuit may still be energized, but surge protection on that path is no longer assured. Before removing anything, record which module changed state and distinguish a red cartridge window from a red LED, remote alarm or panel-controller indication. Those signals do not always prove the same fault.

Replace a plug-in cartridge only with the exact compatible type specified for its base. Replace the complete assembly when the base, terminals, coding, remote-contact mechanism or insulation shows damage, or when the manufacturer does not permit cartridge-only replacement. If a new cartridge turns red again, do not keep fitting higher-voltage modules: investigate the continuous voltage, temporary overvoltage exposure, wiring, backup protection and installation conditions first.

A Red Window Is a State Signal, Not a Cause Code

Most modular SPDs contain a thermal disconnector associated with the voltage-limiting component. When ageing, abnormal power-frequency voltage or surge duty causes unsafe heating, the disconnector separates that component from the circuit. The mechanical window then changes state—commonly from green or white to red—to show that the module can no longer be relied upon for surge protection.

The indicator reports the condition of the protective module, not the event that produced it. A red window does not prove that lightning struck the building. It also does not reveal, by itself, whether the cause was accumulated surge stress, a temporary overvoltage, excessive continuous voltage, an incorrect cartridge, poor thermal conditions or another abnormal condition.

Another easy mistake is assuming that a failed SPD must disconnect the load. Many shunt-connected SPDs can reach end of life while normal supply continuity remains unchanged. Machines, lighting and controls may continue operating, but they are now exposed to the next transient. That is why a red window is a maintenance action, even when the panel appears to work normally.

For the wider context—Type 1, Type 2 and Type 3 positions, U_c, U_p, I_n and I_max—see the JUTRION surge protective device guide. This article stays with the narrower field decision: what the red indication proves, what to replace and what to check before returning the panel to service.

`Comparison of a local red SPD window, remote alarm contact and panel controller alarm.`

First Identify Which Indicator Changed

“The SPD is red” is not yet a complete fault report. A maintenance technician may be looking at the cartridge window, an electronic LED, a remote annunciator or an HMI alarm. Identify the signal source before ordering parts.

Observed indication	What it normally proves	What it does not prove	Immediate next step
Local mechanical window is red	The monitored protective module has operated its end-of-life disconnection mechanism	The original cause or the condition of every other pole	Record the affected pole; isolate safely; replace the compatible module or assembly
Remote alarm operates and a local window is red	The remote circuit is likely reporting an actual module status change	Whether the remote contact and alarm logic are correctly identified	Confirm the local indication, contact state and alarm label before replacement
Panel or controller is red while all local module windows remain normal	An alarm circuit has changed state	That the cartridge itself has failed	Check the product manual, auxiliary contact, wiring continuity, supply and controller logic
One pole is red	That protection path is unavailable	That the other modules experienced no stress	Record the phase or N–PE position and inspect the coordinated assembly
Several poles change together	A shared event or condition is plausible	That all modules failed for exactly the same reason	Preserve the pattern and examine system voltage, neutral condition and event records
Replacement turns red again	The installation or operating condition may still be stressing the SPD	That the new cartridge was defective	Stop repeat substitution and diagnose voltage, configuration, wiring and backup protection

Product-specific conventions take priority over color assumptions. Some monitored systems use more than two states, and some electronic indicators require an auxiliary supply. Check the label and current instruction sheet for the installed model rather than treating every red lamp as a universal cartridge-failure code.

Capture the Failure Pattern Before Removing the Cartridge

A failed cartridge contains useful diagnostic information until it is discarded. A short record can separate a one-time replacement from a recurring site problem.

Photograph the complete SPD assembly. Include every status window, pole marking, backup protective device and any visible damage.
Record the failed position. Note L1, L2, L3, N–PE or the protection mode printed on the product.
Copy the full markings. Manufacturer, series, part number, Type, U_c, U_p, I_n, I_max or I_imp are more useful than “red SPD.”
Save the time and operating context. Note recent thunderstorms, switching events, generator operation, utility abnormalities, neutral faults or maintenance work.
Check the remote alarm record. A timestamp from a PLC, BMS or monitoring relay can show whether several panels changed together.

The pattern guides the next measurement. One repeatedly failed phase suggests a local path, simultaneous changes across several boards suggest a shared event, and an N–PE failure calls for closer examination of the assembly and neutral/earth conditions. None proves the cause by itself.

Do not energize a replacement merely to see whether it survives. SPD work belongs inside an enclosure that may contain hazardous energy. Isolation, absence-of-voltage verification, replacement and testing should be performed by qualified personnel under the applicable site procedure and the product instructions.

Measurements That Help—and Tests That Can Mislead

The useful measurements are taken around the SPD and its network, not improvised across a removed MOV cartridge. A handheld resistance reading cannot certify the remaining life of a varistor, and a generic leakage-current limit cannot be applied across different SPD technologies and voltage classes. If the manufacturer provides a dedicated tester or a declared leakage-current test method, follow that method and its product-specific limits.

Disconnect or remove the SPD before an installation insulation-resistance test unless its instructions explicitly state otherwise. A megohmmeter can drive an SPD into conduction, distort the result and, at some test voltages, damage connected protection components. Do not use a 500 V or 1,000 V insulation test as a bench health check for an unknown cartridge.

`Field voltage measurements around an SPD and isolation before insulation-resistance testing.`

Field check	How to take it	What makes the result actionable
L–N and L–L RMS voltage	Measure every phase and, where recurrence is intermittent, use min/max or a power-quality logger through utility, generator and transfer states	Compare the highest sustained value with the voltage across the actual SPD mode, the declared U_c and the product’s TOV data
N–PE voltage	Measure at the affected board and compare it with upstream and nearby boards under representative load	A material, unstable or load-dependent rise calls for neutral and bonding investigation; one universal 5 V, 20 V or 50 V threshold does not fit every earthing system
Neutral integrity	Inspect and test the neutral path, terminations and phase balance under the site’s approved safe procedure	Phase-to-neutral voltages moving in opposite directions or changing strongly with load can indicate a loose or open neutral
Backup protective device	Record the exact fuse or breaker type, rating and curve together with conductor size and prospective short-circuit current	Pass only when the combination appears in the SPD manufacturer’s coordination data and also protects the connecting conductors
Remote contact	Check the local window first, then verify contact state at the SPD and at the PLC or BMS input	This separates a failed module from broken alarm wiring, loss of control power or inverted controller logic

These checks deliberately avoid universal numbers such as “less than 20 μA means healthy” or “use a 25–63 A fuse.” MOV leakage depends on applied voltage, temperature, waveform and construction, while backup protection differs by SPD Type, short-circuit capability, conductor arrangement and manufacturer. The current datasheet and coordination table are the acceptance criteria.

Replace the Cartridge or the Complete SPD Assembly?

A plug-in design is intended to simplify service, but “it fits the socket” is not a compatibility test. Mechanical coding, protection technology, voltage rating, protection mode, remote signaling and product generation can differ even when two cartridges look similar.

`Decision between replacing a compatible SPD cartridge and replacing a damaged complete assembly.`

Cartridge-Only Replacement Is Appropriate When

the manufacturer identifies the installed base as reusable;
the replacement is the exact compatible part or an explicitly approved successor;
the base, terminals, locking features and insulation show no heat, arc, contamination or mechanical damage;
the remote contact or status mechanism operates correctly after replacement; and
the original Type and rating set remain correct for the system.

Replace the Complete Assembly When

the SPD is a fixed, non-replaceable construction;
the base or terminal area is discolored, cracked, carbonized, loose or distorted;
there is evidence of arcing, moisture ingress or damage outside the cartridge;
the remote signaling mechanism does not reset or its condition cannot be verified;
the compatible cartridge is obsolete or cannot be positively identified; or
the installed configuration was wrong for the network and must be redesigned rather than restored.

For a multipole assembly, follow the manufacturer’s replacement policy. Replacing only the red pole may be acceptable for some systems; other maintenance policies replace a coordinated set after a major event or when all modules have similar service exposure. The product documentation and the recorded event should decide—not a universal rule based only on color.

The JUTRION AC and DC SPD range includes modular product families intended to make inspection and cartridge servicing practical. Match the replacement to the exact base and declared protection circuit rather than ordering by pole count or color alone.

Why a New SPD Cartridge Can Turn Red Again

A second red indication soon after replacement changes the job from routine maintenance to failure analysis. Repeated cartridge consumption is evidence that the installation, selection or operating environment may not match the SPD.

Evidence to collect	Condition it helps distinguish	Engineering response
Highest steady-state RMS voltage across each actual protection mode	Normal supply versus sustained voltage too close to or above U_c	Correct the supply or select the SPD from verified network and TOV requirements—not by simply choosing the next higher cartridge
Voltage events, neutral condition and generator/transfer records	Temporary overvoltage, neutral displacement or source-change disturbance	Correct the system cause and confirm SPD TOV behavior for the earthing arrangement
Installed cartridge code and base code	Correct replacement versus an incompatible or incorrectly rated insert	Use the approved module/base combination and protection mode
Terminal torque, conductor routing and enclosure temperature	Local heating or installation stress versus electrical overstress	Restore the installation to the specified mechanical and thermal conditions
Backup fuse or breaker and prospective short-circuit conditions	Correct disconnection coordination versus unsupported fault duty	Verify manufacturer-declared backup protection and short-circuit capability
Event timing across several panels	Localized failure versus a common incoming surge or supply event	Inspect upstream stages, bonding and coordination across the installation

The continuous operating voltage U_c deserves special attention. Moving from a 275 V cartridge to 320 V or 385 V may reduce stress from an elevated power-frequency voltage, but it may also change the declared voltage protection level U_p. A higher U_c is therefore not a heavier-duty cure. The replacement must still provide an effective protection level suitable for the downstream equipment. The separate guide to choosing between 275 V, 320 V and 385 V SPD U_c ratings explains that trade-off in detail.

If the underlying problem is a sustained supply rise or sag rather than a microsecond transient, an SPD is not the device that regulates or disconnects that condition. The over- and under-voltage protector guide explains that protection boundary and the settings used for abnormal mains voltage.

A red module after a storm does not automatically justify a larger I_max. The event may instead have exposed poor bonding, long connecting conductors, missing upstream coordination or an abnormal mains condition.

The Same Red Window Can Mean a Different Investigation

Application	First evidence to collect	Replacement boundary	Recurrence question
Solar PV combiner box or inverter DC input	String configuration, maximum open-circuit voltage, minimum design temperature, polarity, U_cpv and DC short-circuit current	Use a PV DC SPD with the correct topology and DC disconnection behavior; never fit an AC cartridge as a temporary substitute	Does U_cpv remain above the calculated maximum array voltage? A common IEC design screen is U_cpv ≥ 1.2 × U_oc,max, subject to the applicable design and product instructions
Cold store or automated factory board with VFD loads	Bus RMS voltage, min/max record, source-transfer events, neutral condition, switching history and the affected phase	Do not blame VFD harmonics or fit a higher-U_c cartridge from the red window alone	Is the module seeing sustained voltage/TOV stress, repeated switching transients, poor connection geometry or inadequate upstream coordination?
Unattended telecom, water or remote utility cabinet	Local status window, remote-contact state, enclosure temperature, supply events and alarm timestamp	Restore protection with the exact compatible spare and test the complete alarm path	Can operations identify the affected panel and dispatch the right cartridge before the next surge event?

`Different SPD red indicator investigations for solar PV, VFD loads and remote cabinets.`

The solar and industrial cases should not be merged into one generic replacement rule. PV DC systems have no AC current zero and require dedicated DC-rated disconnection and voltage selection. An industrial AC panel, by contrast, may need event logging to distinguish sustained supply stress from switching activity and installation effects.

Close the Job Locally and Remotely

A green window after insertion proves only that the status mechanism has returned to its normal state. Before the maintenance ticket is closed, verify the rest of the protection path.

Confirm the installed identity. Record the part number, protection Type, U_c, U_p and discharge-current ratings.
Inspect the complete path. Check conductors, terminals, protective earth, backup protection and enclosure condition against the product instructions.
Verify every status point. Confirm the local windows and, where fitted, the remote contact, PLC, BMS or alarm relay.
Close the record. Note the date, affected pole, removed and replacement codes, measurements and suspected or confirmed cause.

This record makes a repeat failure comparable with the first. On unattended telecom, solar, cold-storage, water or industrial sites, a volt-free contact can also shorten the time between loss of protection and maintenance response. Commission the complete signal path to the PLC or BMS, including its behavior during loss of control power or a broken wire; the local mechanical window remains the device-level confirmation.

PV installations need a separate spare list because an AC cartridge must not be substituted on the DC side. Array voltage, U_cpv, DC disconnection behavior and protection configuration are covered in the DC SPD selection guide for solar PV systems.

*`Workflow for recording SPD labels, building a spare cartridge BOM and matching compatible replacements.`*

Build the Spare-Cartridge Kit from the Installed Base

An emergency SPD kit should be a controlled bill of materials, not a box of cartridges grouped only by voltage. For every critical board, record the complete SPD assembly code and the replaceable-module code, protection mode, Type, U_c or U_cpv, remote-contact option and compatible base. Separate AC and PV DC stock physically so that an incorrect emergency substitution is difficult.

BOM field	Why purchasing and maintenance need it
Site, board and protection stage	Identifies where the spare can be used and whether it belongs at the service entrance, sub-board or equipment level
Base and cartridge part numbers	Prevents a mechanically similar but electrically incompatible insert from being fitted
Protection mode and pole position	Distinguishes L–N, L–PE and N–PE duties and the modules used in 1+1, 3+1 or 4+0 assemblies
Type and voltage data	Preserves Type 1, Type 1+2 or Type 2 duty and the approved U_c/U_cpv selection
Remote-contact requirement	Keeps the replacement compatible with the existing PLC, BMS or alarm circuit
Installed quantity and criticality	Lets the operator set stock from installed population, replacement lead time and acceptable unprotected time rather than a generic percentage

Panel builders, EPC contractors and distributors can send this BOM together with product-label photographs and the required destination market to JUTRION technical sales. This gives the factory enough information to confirm replacement compatibility, prepare matched spare-cartridge sets and discuss OEM labeling or project quantities without guessing from “1P+N” or “40 kA” alone.

A confirmed red end-of-life window calls for replacement, but the color does not reveal the cause. Restore protection with the exact compatible part. If the base is damaged, replace the assembly; if the indication is remote only, verify the alarm circuit; and if a new cartridge fails again, stop substituting parts and investigate the installation.

Technical References