Direct Answer

Of the leakage current measured online on an SPD, how much is normal and how much is the real signal of MOV ageing? The answer: the SPD leakage consists of a capacitive component roughly in phase with the voltage and a resistive component tied to the resistance; the capacitive part drifts normally with grid voltage and temperature, while the resistive part rises significantly only as the MOV ages (grain-boundary breakdown, developing leakage paths). Separating the resistive component and watching its trend detects degradation earlier and more accurately than watching total leakage, and it also reduces false alarms.

At a distribution room, a total-leakage threshold alarm on one SPD kept tripping during the summer as the total drifted past the threshold; bench-testing with a resistive-current tester showed the resistive component had only risen slightly and the MOV was not degraded. After switching the monitoring to resistive-component trend judgment, the false alarms stopped, and another SPD that was truly ageing was caught early. The key distinction: total leakage = capacitive + resistive — the capacitive part drifts normally with the environment, and a rising resistive part is the real ageing signal; looking at the total lets temperature and voltage mislead you, while separating the resistive component gives a true reading.

Why:

  • Two components: the capacitive part is roughly in phase with the voltage (normal); the resistive part is tied to the resistance (ageing signal);
  • Why the capacitive part drifts: the MOV junction capacitance changes with voltage and temperature — normal fluctuation;
  • How ageing shows up: grain-boundary breakdown and developing leakage paths → a significant rise in the resistive component;
  • μA-level precision: the two components often differ by only tens of μA; without enough precision you cannot separate them.

Key insight: "The capacitive component is the shadow of the environment; the resistive component is the microphone of ageing — separate it and watch the trend to catch degradation earlier and more accurately."

At a data centre, total leakage was recorded monthly and stayed flat for a year, triggering no alarm; a routine check with a resistive-current analyser found the resistive component had been rising for three consecutive months, near the knee point. After replacing the SPD, the resistive component fell back. Lesson: a flat total can mask a quietly rising resistive component — only separation leaves ageing nowhere to hide.

1. The Two Components of Total Leakage: Capacitive vs Resistive

Step one: leakage is not "one number".

The capacitive component (the larger share):

  • The MOV behaves capacitively (junction capacitance);
  • The current leads the voltage by about 90°;
  • Its magnitude varies with voltage and temperature;
  • It is always present in normal operation.

The resistive component (the smaller share):

  • The MOV grain boundaries conduct resistively;
  • The current is roughly in phase with the voltage;
  • Its magnitude rises significantly with ageing;
  • It is the "sensitive indicator" of degradation.

Why separate them:

  • Total leakage = capacitive + resistive (vector sum);
  • The capacitive component masks the resistive change (larger share);
  • Looking at the total alone, a small resistive rise is invisible;
  • After separation, the resistive trend becomes clear.

Key insight: "Leakage is the sum of two vectors — the capacitive part is large, the resistive part small; without separation, the small part's deterioration is hidden by the large part."

2. Why the Capacitive Component Drifts with Voltage and Temperature

Step two: why environmental changes make the total "lie".

The effect of voltage:

  • Grid voltage fluctuates (±10% is normal);
  • The MOV junction capacitance changes with voltage;
  • The capacitive leakage follows the voltage;
  • Higher voltage → higher leakage (not ageing).

The effect of temperature:

  • Higher temperature → higher leakage;
  • The junction capacitance has a temperature coefficient;
  • Summer leakage is generally higher;
  • The temperature effect is easily confused with ageing.

Effect on the total:

  • The total drifts "normally" with the environment;
  • Threshold alarms mis-operate (in summer);
  • Real ageing is masked by the drift;
  • The total alone cannot distinguish.

Key insight: "The capacitive component follows voltage and temperature — a summer 'leakage exceedance' may just be heat, not damage."

3. How MOV Ageing Shows Up in the Rising Resistive Component

Step three: the physical process of ageing.

MOV ageing mechanism:

  • Grain boundaries degrade under long-term voltage stress;
  • Boundaries break down, forming leakage paths;
  • The resistive leakage increases;
  • Path expansion → accelerated ageing (thermal-runaway risk).

Behaviour of the resistive component:

  • Early ageing: small resistive rise (below threshold);
  • Mid-stage: continuous rise (clear trend);
  • Late stage: significant increase (near end of life);
  • The resistive rise precedes any "perceptible" change in the total.

Why "earlier":

  • The resistive share is small; early changes do not affect the total's appearance;
  • After separation, tiny rises can be identified as a trend;
  • Early detection → early replacement;
  • Avoid developing into thermal runaway.

Key insight: "Ageing starts at the grain boundaries — the resistive component is a direct reading of the boundary state; a rising trend is ageing speaking."

4. What Separating the Resistive Component Means for Fewer False Alarms

Step four: fewer false alarms, earlier detection.

Sources of false alarms:

  • Total-leakage threshold + environmental drift → summer false alarms;
  • False alarms make operations "cry wolf";
  • Widening the threshold → real ageing is missed;
  • A dilemma.

The resistive-component approach:

  • Watch only the resistive trend, not the capacitive fluctuation;
  • Environmental drift does not trigger (normal fluctuation filtered);
  • Real ageing must trigger (rising trend);
  • Fewer false alarms and fewer missed detections.

Engineering effect:

  • More accurate alarms (less manual review);
  • Earlier ageing detection (a wider replacement window);
  • More credible data (the trend is explainable);
  • Operations focus on real problems.

Key insight: "Separating the resistive component filters out environmental noise and amplifies the ageing signal — fewer false alarms, yet earlier detection."

5. Why Online Monitoring Needs μA-Level Precision to Separate the Two

Step five: measurement precision is the precondition for separation.

Why it is hard:

  • The capacitive component is the main quantity (tens to hundreds of μA);
  • The resistive component is small (a few to tens of μA);
  • Both are summed in the total leakage;
  • Without enough precision, separation is impossible.

μA-level precision:

  • Requires synchronously sampling the voltage and current phase;
  • Computes the in-phase component (resistive) and the quadrature component (capacitive);
  • μA-level resolution is needed to recognise early changes;
  • This is the technical precondition for "separation".

Engineering requirements:

  • Sensor range and resolution matched;
  • Phase synchronisation (voltage reference);
  • Temperature compensation (distinguish environment from ageing);
  • Trend algorithms (baseline + knee point).

Key insight: "You can only separate what you can measure — without μA-level precision and phase synchronisation, the resistive component is invisible."

6. Product Implementation: Intelligent Lightning-Protection Online Monitoring

WeiWuLian products put resistive separation into practice:

  • FS/ESM SPD Monitor: synchronously samples voltage/current phase and separates capacitive/resistive components;
  • FSS Intelligent SPD: built-in monitoring with self-diagnosis;
  • Platform trend analysis: resistive-component baseline + knee-point alarms;
  • Temperature compensation: filters environmental drift and reduces false alarms.

The design principle in one sentence: "Separate the resistive component, watch the trend, set the knee point — ageing detected earlier, false alarms greatly reduced."

7. Common Misjudgments

  1. "Total-leakage exceedance means ageing": summer capacitive drift also exceeds. Result: frequent false alarms. Correct: judge by the separated resistive component.
  2. "A flat total means safe": the resistive part rises quietly and is masked. Result: ageing missed. Correct: track the resistive trend.
  3. A fixed threshold on the total: not corrected for the environment. Result: a dilemma (false/missed alarms). Correct: resistive trend + knee point.
  4. "Leakage measurement" without phase synchronisation: the two parts cannot be separated. Result: the measurement is useless. Correct: μA-level + phase separation.
  5. No temperature compensation: environmental drift treated as ageing. Result: unreliable alarms. Correct: compensate, then watch the trend.

Scope and Limitations

This article explains the capacitive/resistive composition of SPD leakage, why the capacitive part drifts with the environment, the principle of the resistive component as an MOV-ageing signal, and the value of separation for fewer false alarms, applicable to SPD online-monitoring selection and data interpretation. The quantitative threshold and knee point of the resistive component depend on the SPD type, manufacturer data and on-site voltage/temperature; follow GB/T 18802.11 and the product manual, determined by professional operations. All cases are anonymized (a distribution room, a data centre) and do not refer to any specific customer.