- \n
- Status indicator window shows red or blank \u2014 [CRITICAL] Replace immediately; thermal disconnect has activated. <\/li>\n
- Visible burn marks or discoloration on housing \u2014 [CRITICAL] Isolate circuit and replace; internal MOV has likely failed short. <\/li>\n
- Enclosure cracking or deformation \u2014 [HIGH] Replace before next storm season; mechanical integrity is compromised. <\/li>\n
- Leakage current above 1 mA at rated voltage \u2014 [HIGH] Schedule replacement within 7 days; MOV degradation is measurable. See surge protection fundamentals<\/a> for baseline values. <\/li>\n
- Clamping voltage drifting above rated Up, such as exceeding 2.5 kV on a 1000 VDC system \u2014 [HIGH] Test with a calibrated surge tester; replace if Up is out of spec. <\/li>\n
- Insulation resistance below 1 M\u03a9 between L+ and PE \u2014 [HIGH] Replace; moisture ingress or MOV breakdown is indicated. <\/li>\n
- Repeated nuisance tripping of upstream DC circuit breaker<\/a> \u2014 [MEDIUM] Inspect the SPD for a partial short-circuit condition before resetting. <\/li>\n
- SPD installed beyond its service life, typically 10\u201315 years in high-surge environments \u2014 [MEDIUM] Schedule proactive replacement during the next planned outage. <\/li>\n
- System located in a high-lightning-density zone with no replacement since commissioning \u2014 [MEDIUM] Cross-reference local keraunic level data; replace per the manufacturer\u2019s surge-count rating. See regional lightning information from the World Meteorological Organization<\/a>. <\/li>\n
- No documentation of last inspection \u2014 [LOW] Treat as unknown condition; perform a full functional test or replace as a precaution. Review your SPD selection criteria to confirm the replacement unit is correctly rated for your system voltage and Imax.<\/li>\n<\/ol>\n
![\"DC](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/dc-spd-replacement-warning-signs-checklist-01.webp\")
Figure 1. Visual field checklist for identifying ten common DC SPD replacement indicators in service. – Suggested aspect ratio: 16:9<\/figcaption><\/figure>\n Why DC SPDs Fail: MOV Degradation Explained<\/h2>\n
To make good replacement decisions in the field, you need to understand what is failing inside the device long before the indicator turns red.<\/p>\n
DC SPD failure is usually a MOV (metal oxide varistor) problem. Under repeated surge events, MOV grain boundaries absorb energy and gradually lose clamping precision through a cumulative, irreversible process. IEC 61643-31 governs DC SPDs for photovoltaic applications and sets the test framework for surge withstand, voltage protection level (Up), and end-of-life behavior.<\/p>\n
How MOV Degradation Progresses<\/h3>\n
MOVs are zinc oxide ceramic discs that clamp transient overvoltages by shifting from a high-resistance to a low-resistance state in microseconds. Each clamping event deposits thermal energy into the varistor body. Over time, typically after cumulative surge energy exceeds the device\u2019s rated capacity, leakage current at continuous operating voltage (Uc) rises measurably.<\/p>\n
Once leakage current climbs above ~1 mA at rated Uc<\/sub>, thermal runaway becomes a real risk: increased leakage \u2192 more self-heating \u2192 further resistance drop \u2192 accelerated degradation. In 1000 VDC<\/sub> string circuits, this cycle can compress what should be years of service life into months under high-lightning-density environments.<\/p>\n
A 2023 field review of rooftop PV installations in Guangdong Province covering roughly 35 MW of installed capacity found that SPDs in combiner boxes without remote monitoring showed leakage current drift exceeding 2 mA in 18% of units after two monsoon seasons, well before any visible indicator had triggered.<\/p>\n
What IEC 61643-31 Actually Governs<\/h3>\n
IEC 61643-31 is the primary standard for low-voltage DC SPDs used in PV systems. It defines:<\/p>\n
- \n
- Impulse current (Iimp) and nominal discharge current (In) ratings under 8\/20 \u00b5s and 10\/350 \u00b5s waveforms<\/li>\n
- Voltage protection level Up, commonly \u2264 2.5 kV for Type 2 DC SPDs in 1000 VDC systems<\/li>\n
- Degradation test sequences that simulate cumulative surge stress to validate end-of-life signaling<\/li>\n<\/ul>\n
Understanding this degradation path is why pairing a DC SPD<\/a> with properly rated overcurrent protection matters. A degraded MOV drawing excess leakage current needs a reliable backup disconnect. For broader context on how SPDs fit into PV string protection, proper coordination with upstream DC fuses<\/a> and breakers is critical.<\/p>\n
\n
[Expert Insight]<\/strong>
\n– Leakage-current drift often appears before any mechanical indicator changes, so trend data is more useful than a one-time pass\/fail reading.
\n– If one SPD in a combiner box shows measurable degradation, test the neighboring units in the same exposure zone during the same visit.
\n– In coastal or humid sites, inspect terminal seals and enclosure gaskets along with electrical values; moisture ingress can accelerate MOV aging.<\/p>\n<\/blockquote>\nHow to Test a DC SPD Yourself<\/h2>\n
Testing a DC SPD does not require a full lab setup. A digital multimeter, safe isolation procedure, and a clear pass\/fail threshold are enough to catch many failures before they damage other equipment.<\/p>\n
Step 1: Visual Pre-Check<\/h3>\n
Inspect the SPD housing for burn marks, cracked enclosures, or a tripped status indicator window showing red. A triggered remote signaling contact is a hard stop: do not proceed to electrical testing without isolating the circuit first.<\/p>\n
Step 2: Disconnect and Isolate<\/h3>\n
De-energize the string or combiner circuit. Verify zero voltage across the SPD terminals using your meter set to DC voltage. In a 1000 VDC string system, residual voltage above 5 V after isolation suggests a leakage path worth investigating before contact.<\/p>\n
Step 3: Measure MOV Resistance<\/h3>\n
Set your multimeter to the highest resistance range, typically 20 M\u03a9. A healthy MOV usually reads above 1 M\u03a9 between line and ground terminals. Readings below 100 k\u03a9 indicate degradation, meaning the varistor is conducting at rest and may no longer clamp correctly during a surge event. For context on clamping behavior, see this engineering breakdown of 600V DC SPD clamping behavior<\/a>.<\/p>\n
Step 4: Check Leakage Current (If Equipment Available)<\/h3>\n
Using a DC insulation tester at 500 V test voltage, leakage current through a serviceable SPD should remain below 1 mA. Values exceeding 5 mA at 500 V DC indicate the MOV is thermally degraded and the Up (protection level) can no longer be guaranteed within the rated \u00b110% tolerance.<\/p>\n
Step 5: Verify Backup Protection Continuity<\/h3>\n
Confirm the series-connected backup fuse or DC circuit breaker<\/a> is intact. An open backup fuse with an apparently healthy MOV reading is a common false negative: the SPD looks fine but has no fault-clearing path. In a 60 MW ground-mount installation in Hebei Province (2023), maintenance crews found that roughly 15% of flagged SPD failures were actually blown backup fuses, not MOV degradation.<\/p>\n
![\"DC](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/dc-spd-testing-sequence-resistance-leakage-check-02.webp\")
Figure 2. Field test workflow for DC SPD diagnosis using visual, electrical, and continuity checks. – Suggested aspect ratio: 16:9<\/figcaption><\/figure>\n What Happens If You Don’t Replace a Failed SPD<\/h2>\n
If an SPD fails inspection and stays in service anyway, the issue shifts from routine maintenance to preventable equipment risk.<\/p>\n
A failed DC SPD does not simply stop protecting; it can leave the circuit effectively open to the next surge event.<\/p>\n
MOV Bypass and Loss of Clamping<\/h3>\n
When the MOV inside the SPD degrades past its service limit, its clamping voltage rises sharply, often from a rated Up near 2.0 kV to values above 4.0 kV, or the MOV shorts internally and the thermal disconnect isolates it from the circuit. In either case, the next lightning-induced or switching transient passes through without proper attenuation. Once the MOV is bypassed, the SPD\u2019s protection guarantee is gone.<\/p>\n
Unprotected Equipment Exposure<\/h3>\n
With no functional SPD in the string, every connected component, including inverters, charge controllers, DC distribution boxes, and monitoring hardware, is exposed to full transient energy. Inverter IGBT modules are especially vulnerable; a single unclipped surge above 1500 V in a 1000 VDC string circuit can damage gate oxide layers and create latent failures that appear later under normal load cycling. In a 35 MW ground-mount installation in Hebei Province (2023), post-storm inspection found that three inverters failed within 10 days of a lightning event, and forensic analysis traced the damage to an SPD whose indicator had shown fault for more than two months.<\/p>\n
Downstream Damage Cascade<\/h3>\n
The operational and financial impact compounds quickly. A single failed surge protection device left unreplaced can trigger a cascade: inverter failure leads to string downtime, which may stress adjacent fuse links through unbalanced loading and, in worst cases, contribute to DC arc-fault conditions requiring full combiner box<\/a> replacement. Repair costs often run 15\u201340 times the cost of the SPD itself.<\/p>\n
The cost comparison is clear: an SPD replacement often costs less than $50 in string applications, while inverter replacement can run from $2,000 to $8,000. Skipping the inspection is not savings; it is deferred loss.<\/p>\n
![\"Failed](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/failed-dc-spd-consequence-chain-inverter-damage-03.webp\")
Figure 3. Failure progression after a degraded DC SPD remains in service without replacement. – Suggested aspect ratio: 16:9<\/figcaption><\/figure>\n \n
[Expert Insight]<\/strong>
\n– After any nearby lightning event, compare current SPD readings against the last maintenance record rather than relying only on the status window.
\n– When one inverter fails after a surge, inspect the associated combiner-box SPD and backup fuse before energizing a replacement inverter.
\n– If replacement parts are delayed, isolate the affected string or circuit instead of operating knowingly without surge protection.<\/p>\n<\/blockquote>\nReplacement Decision Matrix: Replace Now, Monitor, or Escalate<\/h2>\n
When a DC SPD shows warning signs, the main decision is whether to shut down immediately, monitor for a short interval, or involve a specialist.<\/p>\n
Condition \u2192 Action \u2192 Urgency Table<\/h3>\n
![\"DC](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/dc-protection-coordination-chain-spd-breaker-combiner-04.webp\")