Direct Answer

How do O&M personnel distinguish a normal, degraded, and failed SPD, and decide when to replace it? The answer: SPD degradation can be judged by combining "leakage-current trend (μA-level precision), trip/remote-signal state, and backup-protector state"; the standards (IEC 61643-11 / GB/T 18802.11) require SPDs to have status indication and failure protection — on-line monitoring makes the "degradation trend" visible, which is more reliable than single-point measurement.

At one power distribution room, 6 SPD groups were managed by "annual inspection + visual check"; after one lightning strike, 2 groups were found already failed (no visual anomaly, remote signals not connected). Rectification: added SPD monitors (FS/ESM, μA-level leakage current + remote signals), established a baseline per group and tracked continuously; six months later one group's leakage rose slowly from 35μA to 120μA (others stable), replaced early, avoiding failure exposure at the next strike. Judgment core: leakage-current trend + trip/remote signal + backup protector — three signals judged jointly, degradation discoverable before the next strike.

Because:

  • SPD degradation has no visual sign: MOV aging is internal; the eye cannot see it;
  • Leakage current is the "probe": μA-level trend changes reflect MOV degradation;
  • Standards require status indication: IEC 61643-11 / GB/T 18802.11 mandate status indication and failure protection;
  • Single-point measurement is unreliable: absolute values are affected by temperature/voltage; only trends are reliable.

Key insight: "three-signal joint judgment (leakage trend + remote signal + backup protection)" is the correct way to judge SPD state. Standard requirements + on-line monitoring — degradation trend visible, replacement justified.

At one plant, management relied only on "visual SPD checks + annual multimeter leakage measurement"; after one lightning strike a failed SPD went unnoticed (MOV cracked but housing intact), and equipment was damaged. Rectification: connected remote signals + added monitoring, leakage trends entered the ledger. Lesson: visual and single-point checks miss failures — trend monitoring + remote signals are the reliable judgment.

1. Why SPDs Degrade

Step one: what is the degradation mechanism.

MOV aging mechanism:

  • MOV (zinc-oxide varistor) repeatedly discharges lightning/operating overvoltages;
  • Each discharge causes micro-damage (grain-boundary degradation);
  • Long-term accumulation: leakage current grows, varistor voltage drifts;
  • Extreme case: thermal runaway (short-circuit failure).

Failure modes:

  • Degradation: leakage grows but protection still works (warning period);
  • Open failure: MOV burns open (protection lost);
  • Short failure: MOV breaks down (backup protection must disconnect);
  • Hidden failure: no visual anomaly (most dangerous).

Degradation triggers:

  • Repeated lightning impulses (count and amplitude);
  • Switching overvoltages (frequent operations);
  • Environmental factors (heat/humidity accelerate aging);
  • Grid harmonics/voltage anomalies (long-term stress).

Key insight: "MOV aging is gradual" is the theoretical basis of monitoring. Each discharge is micro-damage and the trend is measurable — that is why the leakage trend can find degradation early.

2. Standard Requirements for Status Indication and Safety Protection

Step two: what the standards say.

IEC 61643-11 points:

  • SPDs must have failure indication (visual status indicator);
  • Must have failure protection (disconnector/thermal trip);
  • Status-indication effectiveness verified by testing;
  • Classification tests (Class I/II/III) correspond to different applications.

GB/T 18802.11 corresponding requirements:

  • Status indication: normal/degraded/failed distinguishable;
  • Failure protection: thermal trip/backup protector;
  • Remote-signal output: state remotely visible (optional but recommended);
  • Replacement guidance: the manual defines replacement criteria.

Logic behind the standards:

  • Why status indication: SPD failure is hidden (no visual anomaly);
  • Why failure protection: short-circuit failure needs fast disconnection (fire prevention);
  • Why remote signals recommended: remotely visible (inspection coverage is limited);
  • On-line monitoring fits the standard spirit: makes the degradation trend visible.

Key insight: the standards require "state visible + failure protectable." Status indication is the baseline, remote signals are the upgrade — on-line monitoring fulfills the standard requirements properly.

3. The Role of Leakage Current in Degradation Judgment

Step three: how to read leakage.

Leakage-current composition:

  • Capacitive component: determined by varistor capacitance (varies with voltage);
  • Resistive component: determined by MOV grain-boundary state (varies with degradation);
  • Normal range: μA level (tens to hundreds of μA);
  • Degradation signal: resistive component grows (total leakage climbs).

Reading points:

  • Baseline first: reference value of the same model and voltage;
  • Look at trend: slow climbing is degradation (fluctuation is not);
  • Look at relative value: compare with own group/same model (absolute value has limited meaning);
  • Look at superposition: decouple seasonal temperature/voltage fluctuations (remove interference).

Magnitude references:

  • Normal: tens of μA (within baseline);
  • Watch: continuous climb to the hundreds-of-μA level;
  • Degraded: obvious trend (deviation beyond threshold);
  • Combined: confirm with trip/remote signal (joint reading).

Why μA-level precision is the key:

  • SPD normal leakage is in the tens-to-hundreds-of-μA range; degradation trends often change at the ±10–20μA level;
  • Insufficient precision (e.g., mA level or "yes/no" judgment) cannot resolve the trend — equivalent to no monitoring;
  • The monitor must have μA-level resolution with temperature/voltage compensation so trend data is credible;
  • When reading, first exclude measurement error and seasonal fluctuation, then conclude degradation.

Key insight: "μA-level precision + trend judgment" is the core of leakage monitoring. Absolute values deceive (temperature/voltage effects); trends are reliable — without precision the trend is untrustworthy.

4. On-Line Monitoring vs Periodic Inspection

Step four: how to choose between the two.

Periodic inspection:

  • Approach: visual + multimeter single-point measurement;
  • Pros: low cost, simple equipment;
  • Cons: single-point absolute values unreliable; failures invisible between inspections;
  • Suitable for: low-importance/few-point sites.

On-line monitoring:

  • Approach: μA-level leakage + remote signals + trend records;
  • Pros: trend visible, failure reported immediately, unattended;
  • Cons: equipment investment (monitor) required;
  • Suitable for: critical circuits/many points/unattended rooms.

Complementary relationship:

  • On-line monitoring covers "trend + timeliness";
  • Periodic inspection covers "on-site confirmation + supplementary checks";
  • Recommended combination: on-line monitoring primary + inspection review;
  • Monitor alarm → on-site confirmation → replacement decision.

Key insight: "on-line monitoring + inspection review" is the best combination. Monitoring gives trend and timeliness; inspection gives on-site confirmation — complementary makes it complete.

5. Practical SPD Replacement Decision Flow

Step five: how to decide whether to replace.

Replacement criteria (any one met):

  • Leakage-current trend clearly degraded (continuous climb + threshold exceeded);
  • Status indicator shows failure;
  • Remote signal reports trip/failure;
  • Backup protector operated (disconnected);
  • Post-strike test fails (varistor-voltage drift beyond limit).

Decision flow:

  • Step 1: record the alarm/anomaly (leakage/remote/trip);
  • Step 2: on-site review (status indicator/visual/multimeter);
  • Step 3: compare baseline (trend confirms degradation);
  • Step 4: arrange replacement (de-energize + safe operation);
  • Step 5: post-replacement test (verify new SPD + update ledger).

Notes:

  • Replacement must be de-energized (shock prevention);
  • Replace with the same model and parameters (system matching);
  • Establish a new baseline immediately after replacement;
  • Archive records (replacement ledger and reasons).

Key insight: "alarm → review → confirm → replace → archive" is the replacement closed loop. Three-signal joint judgment, process-managed — replacement is evidence-based.

6. Product Landing: Intelligent Lightning-Protection On-Line Monitoring

FEXLINK products land SPD state judgment:

  • FS/ESM SPD Monitor: μA-level leakage monitoring + remote signals;
  • FSS Intelligent Surge Protective Device: built-in status indication + thermal trip;
  • Monitoring platform: trend records + threshold alarms + baseline management;
  • Joint reading: leakage + remote signal + backup-protector state summarized.

Design principle in one sentence: "three-signal joint, trend judgment, standard landing." Leakage trend + remote signal + backup protection — SPD degradation visible before the next strike.

7. Common Judgment Misconceptions

  1. Visual-housing judgment: intact housing = healthy. Consequence: internal MOV degradation invisible. Correct: leakage trend + status indicator.
  2. Single-point leakage reading: judging from one absolute measurement. Consequence: temperature/voltage effects mislead. Correct: build a baseline, watch the trend.
  3. Remote signals not connected: state not remotely visible. Consequence: failure unknowable between inspections. Correct: connect remote signals to the platform.
  4. Ignoring degradation: keep using after leakage grows. Consequence: failure exposed at the next strike. Correct: replace once the trend is confirmed.
  5. Replacement without records: no reason recorded. Consequence: ledger gaps, no traceability. Correct: closed-loop replacement with archiving.

Scope and Limitations

This article provides SPD degradation-judgment methods (leakage-current trend, trip/remote signal, backup-protector joint reading) and standard basis (IEC 61643-11 / GB/T 18802.11), applicable to SPD state monitoring and replacement decisions in low-voltage distribution systems. Specific leakage-magnitude references must follow the equipment model and system voltage; standard clauses follow the latest effective versions; replacement work must be de-energized and follow safety codes. All cases are anonymized (a certain power distribution room/a certain plant) and do not refer to any specific customer.