Direct Answer

Why does one "lightning strike" appear as a string of pulses in the monitoring waveform instead of a single "bang"? The answer: the full course of one cloud-to-ground flash consists of the downward leader (building the channel), the first return stroke (largest amplitude, concentrated energy), multiple subsequent return strokes, and possibly a continuing current (small amplitude but long duration) — what the monitor records as "one strike" is actually a superposition of several discharge stages, which is why an SPD can be progressively degraded by repeated impacts.

At a substation, the FL Lightning-Current Monitor recorded one ground flash: first a large 28 kA pulse (first return stroke), then three 3–8 kA pulses within 200 ms (subsequent return strokes), and finally a low-amplitude current lasting tens of milliseconds (continuing current). If one only reads "strike count = 1", the cumulative stress on the SPD across this chain of impacts is missed. The key point: the first return stroke sets the "peak level"; the subsequent strokes and continuing current set the "cumulative energy" — SPD degradation is often driven not by the single big pulse but by the accumulated multiple impacts.

Why:

  • Downward leader: the stepped leader extends toward the ground, establishing the discharge channel (stage one);
  • First return stroke: the first large current surge once the channel is bridged — largest amplitude, concentrated energy (stage two);
  • Subsequent strokes: the channel can re-ionize, producing several smaller return strokes (stage three);
  • Continuing current: finally, a low-amplitude, long-duration current whose heat accumulation is not negligible (stage four).

Key insight: "One ground flash = leader + first return stroke + subsequent strokes + continuing current — 'strike count' is not 'one stress event'."

At a factory, an SPD failed after "1 strike" was counted in one thunderstorm. Reviewing the waveform: it was actually 1 first return stroke + 4 subsequent strokes + a continuing current — the SPD suffered repeated impacts before degrading to failure. Lesson: do not judge lightning by count alone; look at the staged waveform — cumulative stress is the real enemy of SPDs.

1. Downward Leader: Building the Lightning Channel

Step one: a strike begins with an "invisible channel".

The nature of the leader:

  • After charge accumulates in the cloud, the channel extends downward in stepped leaders;
  • The leader carries little current, "paving the way" for the large current to follow;
  • Leader direction is influenced by the electric field and terrain, and it may branch;
  • Channel establishment takes milliseconds (much longer than the return stroke).

Why it matters:

  • The leader decides "where the lightning hits" (the strike point);
  • Branching leaders create multiple strike points;
  • The leader current is small and often does not trigger monitoring;
  • But the leader channel is the "carrier" of the later return stroke.

Meaning for monitoring:

  • The small leader current is usually not recorded as a "strike";
  • But strike-point determination relies on leader-channel analysis;
  • The leader stage sets the "starting point" the down-conductor/air-termination receives;
  • Understanding the leader explains "why lightning hits here and not there".

Key insight: "The leader is the 'pathfinder' of the channel — small current, big influence; it decides where the strike lands."

2. First Return Stroke: The Largest Short Pulse

Step two: the "main event" of the strike.

The nature of the return stroke:

  • When the leader channel is bridged, ground charge surges upward along the channel;
  • The first return stroke has the largest amplitude (typically 10–30 kA, sometimes more);
  • Duration is short (tens of microseconds);
  • Energy is concentrated near the peak.

Why it is "most visible":

  • The largest pulse in the monitoring waveform is usually the first return stroke;
  • The peak drives the Iimp selection for SPDs;
  • The concentrated energy hits equipment hardest;
  • The steep waveform (large di/dt) gives strong induction effects.

Engineering significance:

  • Peak and waveform are used to assess SPD discharge requirements;
  • The energy of the first stroke directly affects SPD lifetime;
  • Strike counting often keys on the first return stroke;
  • But looking only at the first stroke underestimates cumulative damage.

Key insight: "The first return stroke is the 'peak event' — big and short, it drives selection; but it is not the whole stress picture."

3. Subsequent Strokes and Continuing Current: Small but Grinding

Step three: why the strike "is not over".

Subsequent strokes:

  • The channel can re-ionize, producing multiple return strokes;
  • Amplitude is smaller than the first (typically 1/3–1/2 of it);
  • Intervals are tens of milliseconds;
  • Multiple strokes add up to significant cumulative energy.

Continuing current:

  • Finally, a low-amplitude current may last tens to hundreds of milliseconds;
  • Small amplitude (hundreds of amperes to a few kiloamperes);
  • Long duration makes heat accumulation (Joule heating) significant;
  • It is an important source of heating in down-conductors and connections.

Meaning for SPDs:

  • Multiple impacts consume SPD lifetime with every discharge;
  • Continuing current accelerates degradation through heat;
  • Monitoring must record "staged waveforms", not just a count;
  • Degradation often appears as "sudden failure after many small impacts".

Key insight: "Subsequent strokes and continuing current are 'cumulative events' — small and long, the main force that grinds down SPDs."

4. What Multi-Stage Discharge Means for Cumulative SPD Damage

Step four: connecting the waveform to SPD lifetime.

Cumulative damage mechanism:

  • Every discharge imposes electrical and thermal stress on the SPD;
  • Multiple strokes mean multiple impacts (not one);
  • Continuing current means sustained heat input;
  • Once the accumulation reaches a critical point → thermal disconnection/failure.

What monitoring must provide:

  • Record the peak and timing of every stroke;
  • Distinguish the first stroke from subsequent strokes;
  • Identify the continuing-current segment (duration/charge);
  • Correlate SPD state (leakage/temperature) changes.

Why "count is not enough":

  • A count of 1 is not a stress of 1;
  • The same count can hide very different waveforms and damage;
  • Count-only monitoring cannot warn of progressive degradation;
  • Staged waveforms are needed for predictive maintenance.

Key insight: "The SPD's enemy is 'cumulative stress' — monitoring must record staged waveforms to support predictive maintenance."

5. Why "Strike Count" Is Not "One Damage Event"

Step five: the real relationship between count and stress.

Count vs stress:

  • One ground flash may contain 1 first stroke plus several subsequent strokes;
  • The counter records "1", but the SPD may suffer 4–5 impacts;
  • The long-term heat input of the continuing current is not in the count;
  • Stress = frequency × amplitude × duration (combined).

Engineering conclusions:

  • Strike statistics serve environmental assessment (related to N/Ng);
  • SPD lifetime assessment needs cumulative stress (waveform);
  • The monitoring platform should provide waveform playback;
  • Degradation warnings combine leakage and strike history.

Product implementation:

  • FL Lightning-Current Monitor records staged waveforms;
  • The platform splits statistics by "first stroke/subsequent strokes/continuing current";
  • Correlate with SPD state (FSS/leakage monitoring);
  • Achieve "assess SPD remaining life after a strike".

Key insight: "Count serves environmental assessment; waveform serves lifetime judgment — record both to understand lightning."

6. Common Misjudgments

  1. Treating a strike as a single "bang": looking only at the count. Result: cumulative stress underestimated. Correct: read the staged waveform.
  2. Watching only the first-stroke peak: ignoring subsequent strokes. Result: energy accumulation underestimated. Correct: count all strokes and the continuing current.
  3. Blaming "lightning too strong" when the SPD fails: not checking the waveform. Result: the real cause (multiple-impact accumulation) is hidden. Correct: replay the strike waveform and correlate with SPD failure.
  4. Dismissing the continuing current: thinking a small current is harmless. Result: heat-accumulation degradation ignored. Correct: watch long, low-amplitude current segments.
  5. Disconnecting count from lifetime: counting strikes without correlating equipment. Result: no predictive maintenance. Correct: correlate waveforms with device state.

Scope and Limitations

This article describes the four stages of one cloud-to-ground flash (downward leader, first return stroke, subsequent strokes, continuing current) and their meaning for cumulative SPD damage, applicable to lightning-current waveform interpretation and lightning-protection equipment lifetime assessment. The quantitative characteristics of ground flashes (typical amplitude, duration, charge) vary strike by strike; specific engineering parameters need site monitoring data and the GB/T 21714 series. All cases are anonymized (a substation, a factory) and do not refer to any specific customer.