Direct Answer

O&M staff see SPD leakage currents of tens to hundreds of μA on the monitoring screen — is this value large, and at what point should they be alerted? The answer: SPD leakage current is a microamp-level current superimposed on the power-frequency voltage; you must first build a baseline for the same-model SPDs at this station, then look at the trend — a slow climb (e.g., from tens of μA toward the hundreds-of-μA level) that remains rising after decoupling from seasonal temperature/voltage fluctuations is the MOV-degradation signal; looking at the absolute value at a single moment has limited meaning.

At one power distribution room, 4 same-model SPDs were monitored on-line: Group A 45μA, Group B 52μA, Group C 38μA, Group D 180μA — by absolute value alone, D looked worst. After building a baseline, it was found that Group D sat on an outdoor high-temperature phase (ambient 15°C hotter than indoors) with higher phase voltage; after temperature/voltage correction, Group D's equivalent leakage was about 55μA — normal range. Three months later, Group D's leakage climbed to 140μA under equivalent conditions (others stable) — degradation confirmed and replaced early. Reading core: build the baseline first, then look at the trend — absolute values deceive (temperature/voltage effects); a sustained climb after decoupling is the degradation signal.

Because:

  • Leakage current naturally fluctuates: with voltage and temperature (not all degradation);
  • Absolute values have limited meaning: readings at different installation locations/conditions cannot be compared directly;
  • Trend is the signal: sustained climb under the same conditions = degradation (fluctuation does not count);
  • Joint reading: leakage + trip/remote signal + backup protector confirm.

Key insight: "baseline + trend + decoupling" is the three-step method for μA-level leakage reading. Build the baseline, watch the trend, exclude temperature/voltage interference — then the degradation inflection point is seen accurately.

At one plant, a "replace at 200μA" rule-of-thumb wrongly replaced 3 SPD groups (actually normal behavior under heat and high voltage), wasting tens of thousands of yuan. Rectification: build the station baseline first (with temperature/voltage correction), judge by trend. Lesson: concluding without a baseline causes misjudgment — "absolute value + single point" is unreliable; "baseline + trend" is reliable.

1. What the Leakage Current Consists Of

Step one: what is inside the leakage number.

Two components:

  • Capacitive component: determined by MOV varistor capacitance (varies with voltage);
  • Resistive component: determined by MOV grain-boundary state (varies with degradation);
  • Normal reading = capacitive dominant (tens of μA);
  • Degradation signal = resistive growth (total leakage climbs).

Sources of natural fluctuation:

  • Voltage fluctuation: system voltage up → capacitive component up;
  • Temperature change: temperature up → leakage up (seasonal);
  • Harmonic effects: waveform distortion → reading fluctuation;
  • Measurement error: instrument precision and wiring.

Reading prerequisites:

  • Distinguish components: capacitive fluctuation is "normal breathing";
  • Distinguish sources: resistive climbing is "true degradation";
  • Correct conditions: compare after temperature/voltage compensation;
  • Long-term view: a single point is meaningless; the trend is meaningful.

Key insight: "capacitance breathes with voltage, resistance climbs with degradation" is the key to reading. Understand the components and you will not mistake natural fluctuation for degradation.

2. How to Build a Normal Baseline

Step two: the baseline is the "ruler" of judgment.

Baseline definition:

  • Same model: same manufacturer/spec (consistent leakage level);
  • Same voltage: same system voltage class (consistent capacitance);
  • Same location: same installation environment (consistent temperature/humidity);
  • Same condition: same load/commissioning time.

Baseline-building methods:

  • Record at go-live: readings in the first 1–3 months after commissioning (establish the normal range);
  • Same-group comparison: compare same-model units within the station (lateral reference);
  • Seasonal records: cover winter and summer (temperature influence range);
  • Record conditions: log temperature/voltage alongside readings (for correction).

Baseline content:

  • Normal range: mean ± fluctuation band (e.g., 40–60μA);
  • Seasonal characteristics: higher in summer, lower in winter (normal);
  • Voltage correlation: each 1% voltage rise increases capacitance correspondingly;
  • Update mechanism: rebuild the baseline after replacing an SPD.

Key insight: "same model + same conditions + seasonal coverage" builds a credible baseline. Without a baseline, any absolute value is "an orphan number without reference."

3. Typical Magnitude and Trend Patterns of the Degradation Inflection

Step three: what pattern means degradation.

Trend-pattern comparison:

  • Linear climb: slow rise over weeks to months (typical degradation);
  • Step jump: sudden increase then plateau (strike/overvoltage impact);
  • Fluctuating type: rising and falling with temperature/voltage (normal breathing);
  • Sustained rise + threshold exceeded: degradation confirmed.

Typical magnitude references:

  • Normal: tens of μA (within the baseline band);
  • Watch: sustained climb to more than 2× the baseline;
  • Degraded: clear trend + still rising after decoupling;
  • Limit: hundreds-of-μA level (near failure).

Reading points:

  • Exclude fluctuation: look after temperature/voltage correction;
  • Confirm multiple times: trend over consecutive readings (not a single point);
  • Dynamic threshold: updated with the baseline (not a fixed number);
  • Joint verification: remote signal/trip/backup protector confirm.

Key insight: "linear climb/step jump + sustained rise after decoupling" is the degradation-inflection pattern. Fluctuation is not degradation — confirm multiple times and verify jointly before concluding.

4. Why μA-Level Precision Has Judgment Value

Step four: why precision matters.

Precision determines resolution:

  • SPD normal leakage is at the tens-of-μA level;
  • Degradation trends change at the ±10–20μA level;
  • Insufficient precision (mA level) cannot resolve the trend;
  • A "yes/no" judgment is even worse (equivalent to no monitoring).

Precision requirements:

  • μA-level resolution (can see tens-of-μA changes);
  • Temperature/voltage compensation (readings comparable);
  • Stable measurement (no environmental drift);
  • Periodic calibration (stay accurate).

Consequences of insufficient precision:

  • Trend drowned in noise (cannot see the climb);
  • Many false alarms (fluctuation treated as degradation);
  • Missed warnings (tiny changes invisible);
  • Insufficient replacement basis (cannot trace).

Key insight: "μA-level precision + compensation" is the prerequisite for trend judgment. Without precision, the trend is untrustworthy — equivalent to no monitoring.

5. How to Jointly Read Leakage Readings with Trip/Remote-Signal State

Step five: how to combine the judgment.

The roles of the three signals:

  • Leakage current: degradation trend (earliest warning);
  • Trip/remote signal: state change (confirms failure);
  • Backup protector: safety bottom line (disconnects isolation).

Joint-reading rules:

  • Leakage climb + remote normal: degradation warning (arrange replacement);
  • Leakage abnormal + remote operated: failure confirmed (replace immediately);
  • Remote operated + leakage normal: mechanical/indication issue (verify on site);
  • Three signals normal: healthy (keep monitoring).

Reading flow:

  • Step 1: look at the leakage trend (baseline comparison);
  • Step 2: look at the remote-signal state (operated or not);
  • Step 3: look at the backup protector (disconnected or not);
  • Step 4: comprehensive judgment (warning/replacement/verification).

Reading practice examples:

  • Scenario 1: summer leakage 60μA (baseline 40–60μA), temperature 35°C — normal (within the seasonal band);
  • Scenario 2: leakage climbs from 45μA to 130μA over three months, temperature/voltage unchanged — degradation warning (arrange replacement);
  • Scenario 3: after a strike, leakage jumps from 40μA to 90μA then plateaus — impact degradation (add to focused observation);
  • Scenario 4: leakage normal but remote operated — indication/mechanical issue (verify on site);
  • Reading order: correct conditions first, compare baseline next, confirm jointly last — never conclude from a single reading.

Key insight: "leakage gives the trend, remote gives the state, backup gives the bottom line" — only the three-signal combination is complete. A single signal is unreliable; combined reading is accurate.

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

FEXLINK products land leakage reading:

  • FS/ESM SPD Monitor: μA-level leakage + temperature/voltage compensation;
  • Baseline management: same-model group baselines built automatically;
  • Trend analysis: climb/jump recognition + threshold alarms;
  • Joint reading: leakage + remote signal + backup protector summarized.

Design principle in one sentence: "build the baseline first, look at the trend, read jointly." μA-level precision + trend analysis — the degradation inflection is seen accurately and replaced correctly.

7. Common Reading Misconceptions

  1. Judging by absolute value: replace at 180μA, fine at 40μA. Consequence: temperature/voltage differences mislead. Correct: baseline + trend.
  2. Concluding from a single point: judging from one measurement. Consequence: fluctuation treated as degradation. Correct: multiple readings, watch the trend.
  3. Ignoring temperature/voltage: not recording conditions. Consequence: seasonal fluctuation misjudged. Correct: compare after condition correction.
  4. Reading with insufficient precision: watching trends with an mA-level instrument. Consequence: trend drowned in noise. Correct: μA-level precision + compensation.
  5. Looking at leakage only: ignoring remote/trip signals. Consequence: failure confirmation lags. Correct: three-signal joint reading.

Scope and Limitations

This article provides methods for reading SPD leakage-current μA-level readings (baseline building, trend patterns, precision requirements, joint reading), applicable to daily reading and replacement decisions of SPD monitoring data in low-voltage distribution systems. Specific magnitude references must follow the equipment model and system voltage; baseline-building periods relate to environmental conditions; monitoring-instrument precision must meet μA-level requirements with periodic calibration. All cases are anonymized (a certain power distribution room/a certain plant) and do not refer to any specific customer.