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.
