Direct Answer

In design reviews you often hear "shielding failure" and "back-flashover". Where exactly does the lightning strike and why does it damage equipment? The answer: a shielding failure means the lightning bypasses the air-termination system and strikes the protected object directly (the protection zone is insufficient); a back-flashover means the lightning current, after entering the ground via the air-termination and down-conductor, discharges from the earthing system to nearby metallic objects not bonded to it (caused by a rise in earth potential or down-conductor potential). For the former you expand the protection zone; for the latter you add equipotential bonding and lower the earth potential rise.

An industrial building had an intact air-termination tape, yet a rooftop fan was damaged after a thunderstorm. Inspection showed the fan stood outside the air-termination protection zone (beyond the rolling-sphere radius), so the lightning bypassed the tape and struck the fan directly — a shielding failure. In another plant, a water-pump motor failed; inspection found the motor sat close to the down-conductor without equipotential bonding — when the lightning current entered the ground, the down-conductor potential rose and discharged to the motor — a back-flashover. The key distinction: a shielding failure is "protection zone not enough" (air-termination problem), while a back-flashover is "potential-difference discharge" (earthing/bonding problem) — mixing them up sends the rectification in the wrong direction.

Why:

  • Shielding failure is about position: the strike point is on the protected object (outside the air-termination zone);
  • Back-flashover is about path: the lightning "jumps" from the down-conductor/earth grid to a neighbouring metallic part;
  • Shielding failure → expand the zone: add air-termination devices / re-adjust the protection range;
  • Back-flashover → lower the potential: add equipotential bonding + reduce the earth potential rise.

Key insight: "A shielding failure is a missed strike; a back-flashover is a bounce-over" — one expands the zone, the other balances the potential. Misjudge it and the rectification goes off course.

A station once treated a shielding-failure incident as a back-flashover (adding only bonding, no air-termination) and the same location failed again the following year. After re-calculating the protection zone and installing an additional air-termination rod, no recurrence was observed. Lesson: a shielding failure needs a wider protection zone, not better earthing — misclassification makes the fix ineffective.

1. Shielding Failure: Insufficient Air-Termination Zone, Lightning Strikes the Protected Object Directly

Step one: how does a shielding failure happen.

Definition:

  • Lightning bypasses the air-termination device and hits the target;
  • The target lies outside the air-termination protection zone;
  • It is not that the air-termination failed (it is that the zone is insufficient);
  • A typical "missed strike".

Causes:

  • Protection zone under-calculated (rolling-sphere radius too small);
  • Target protrudes (higher than the air-termination device);
  • Air-termination devices spaced too far apart;
  • Complex target geometry (corners/protruding parts).

Typical positions:

  • Rooftop protruding equipment (fans/antennas/water tanks);
  • Building edges (air-termination tape coverage insufficient);
  • Parts higher than the air-termination device;
  • Equipment near the boundary of the protection zone.

Key insight: "Shielding failure = zone not enough" — the equipment sits outside the protected circle and is hit directly. Calculate the protection zone first, then talk about anything else.

2. Back-Flashover: High Potential During Ingress/Down-Conduction Discharges to Nearby Conductors

Step two: how does a back-flashover happen.

Definition:

  • Lightning current enters the ground via the air-termination/down-conductor;
  • The potential of the down-conductor/earth grid rises instantaneously;
  • It discharges to nearby metallic parts not equipotentially bonded;
  • A typical "bounce-over".

Causes:

  • Down-conductor too close to the protected object;
  • No equipotential bonding (potential difference not equalized);
  • Earth resistance too high (larger potential rise);
  • Protected object "floating" (not earthed/not connected to the air-termination system).

Discharge targets:

  • Nearby metal pipes/equipment enclosures;
  • Signal/power cables without equipotential bonding;
  • People (touching metal near the down-conductor);
  • Weak insulation points (breakdown).

Key insight: "Back-flashover = potential rise + potential difference" — the lightning does not strike you from the sky; it jumps from the ground or the conductor. Equipotential bonding is the core measure.

3. How the Two Differ in On-Site Damage Phenomena

Step three: how to tell them apart on site.

Strike-point position:

  • Shielding failure: strike point on the protected object itself (burn marks);
  • Back-flashover: strike point not on the equipment (the equipment is "discharged to");
  • Shielding failure: direct hit (damage trace on the target);
  • Back-flashover: nearby discharge (trace near the down-conductor/earth grid).

Damage mode:

  • Shielding failure: the target body is burned/ruptured (energy enters directly);
  • Back-flashover: insulation breakdown/port damage (potential-difference discharge);
  • Shielding failure: single-point damage (the struck target);
  • Back-flashover: multi-point damage (related to potential distribution).

On-site judgment:

  • Check the position of the lightning trace (on the body vs nearby);
  • Check the positional relationship between equipment and air-termination;
  • Check whether equipotential bonding is complete;
  • Combine lightning-current monitoring records (FL monitor).

Key insight: "Shielding failure shows on the body, back-flashover shows nearby" — the damage location is the key to classification. Where the strike landed tells you which type it is.

4. Rectification Directions Compared

Step four: how to fix each type properly.

Shielding-failure rectification:

  • Expand the protection zone (add air-termination devices);
  • Re-arrange air-termination devices (reduce spacing);
  • Raise the air-termination (cover protruding equipment);
  • Re-calculate the rolling-sphere radius (per code).

Back-flashover rectification:

  • Add equipotential bonding (equipment to the air-termination system);
  • Lower the earth resistance (reduce the potential rise);
  • Increase the separation distance (down-conductor away from equipment);
  • Shielding/isolation (protect sensitive equipment).

Correspondence:

  • Shielding failure → air-termination problem → expand the zone;
  • Back-flashover → earthing problem → lower the potential;
  • Shielding failure → add air-termination rods/tapes;
  • Back-flashover → add bonding/reduce earth resistance.

Key insight: "Shielding failure expands the zone, back-flashover balances the potential" — the rectification direction is decided by the failure type. Get the direction right and the investment pays off.

5. On-Site Quick Judgment Based on Damage Location

Step five: a fast judgment flow.

Judgment flow:

  • Step 1: inspect the lightning trace (on the body vs nearby);
  • Step 2: check the protection zone (inside the air-termination circle or not);
  • Step 3: check the bonding (is the equipment connected);
  • Step 4: comprehensive judgment (shielding failure / back-flashover);
  • Step 5: targeted rectification + verification.

Judgment points:

  • Body burn marks → shielding failure (zone insufficient);
  • Nearby discharge → back-flashover (potential difference);
  • Both combined → handle separately;
  • Use monitoring records as supporting evidence (FL current records).

Verification methods:

  • Lightning-protection testing after rectification (zone / earth resistance);
  • FR earth-resistance monitoring (tracking back-flashover risk);
  • Lightning-event review (does it recur);
  • Archive of protection-zone calculations.

Key insight: "Classify first, rectify second, verify last" — a closed loop cures the root cause. Accurate classification means the fix works in one pass.

6. Product Implementation: Intelligent Lightning-Protection Online Monitoring

WeiWuLian products turn shielding-failure/back-flashover protection into practice:

  • FR Earth-Resistance Monitor: tracks earth resistance (the back-flashover risk indicator);
  • FL Lightning-Current Monitor: records lightning events (distinguishing the strike path);
  • Platform: event records + trend analysis + protection status;
  • Combined with protection-zone calculations and bonding rectification.

The design principle in one sentence: "Shielding failure expands the zone, back-flashover balances the potential, and monitoring verifies." Classify accurately and fix accordingly — lightning incidents stop recurring.

7. Common Misjudgments

  1. Treating a shielding failure as a back-flashover: only adding bonding. Result: the same position fails again next year. Correct: expand the air-termination protection zone.
  2. Treating a back-flashover as a shielding failure: only adding an air-termination rod. Result: the potential difference still discharges. Correct: add bonding + reduce earth resistance.
  3. "Intact air-termination means safe": ignoring the protection zone. Result: protruding equipment is struck. Correct: re-calculate the rolling-sphere radius.
  4. "Low earth resistance means safe": ignoring equipotential bonding. Result: back-flashover between equipment due to potential difference. Correct: bond the equipment.
  5. Not recording lightning events: no evidence to classify. Result: classification by guesswork. Correct: FL monitoring records + review.

Scope and Limitations

This article distinguishes the two easily-confused direct-strike concepts — shielding failure and back-flashover — and gives the rectification directions for each, applicable to lightning-protection design review and lightning-damage troubleshooting. Protection-zone calculations (rolling-sphere method) and equipotential design must be performed professionally per GB 50057 and similar codes; the target earth-resistance value must be determined by the lightning-protection class; on-site classification is best supported by lightning-current monitoring records. All cases are anonymized (an industrial building, a station) and do not refer to any specific customer.