Direct Answer

The earthing resistance passes every year, so why does lightning still damage the low-voltage (LV) equipment? The answer: a passing steady-state earthing resistance only means good power-frequency dissipation — it does not mean the transient counterattack and equipotential bonding are sufficient. LV equipment damage usually comes from the potential difference caused by the earth-potential rise, missing equipotential bonding at zone boundaries, or no SPD on the signal/power paths. The diagnosis must expand from "the earthing resistance" to the equipotential continuity and the cross-zone SPD coordination, instead of only measuring the resistance.

At a park, the earthing resistance passed every year, but the LV equipment was still damaged by surges in the storm season. Measuring only the resistance could not explain it; after expanding the diagnosis, it was found that there was no equipotential bonding at the cabinet-tray zone boundary — when the earth potential rose during a strike, the potential difference between the two sides was large, and the surge entered through the signal lines. After adding cross-zone bonding and signal-line SPDs, there was no more damage. The key distinction: a passing resistance is not transient safety — the root causes of LV damage are mostly equipotential continuity and cross-zone coordination; the diagnosis must expand from the resistance to these two.

Why:

  • The resistance dimension: a passing steady state only means good power-frequency dissipation;
  • Equipotential bonding: is the continuity/cross-zone interface sufficient;
  • SPD coordination: are the power and signal paths protected;
  • The diagnosis: resistance → equipotential → SPD coordination.

Key insight: "A passing resistance is not transient safety — for LV damage, check the equipotential continuity and cross-zone SPD coordination; do not stare only at the resistance value."

At a plant, an LV box beside the down-conductor was damaged by a strike even though the resistance passed; the cause was insufficient clearance and no equipotential bonding with the down-conductor, so the current path counterattacked the LV equipment. Adding the equipotential connection fixed it. Lesson: a passing resistance is only the baseline — the potential difference from the earth-potential rise is the real enemy of LV equipment.

1. A Passing Steady-State Resistance Is Not Transient Safety

Step one: why it still breaks even when it passes.

Steady state vs transient:

  • The steady-state resistance measures the power-frequency dissipation;
  • The transient looks at the potential rise under the lightning current;
  • The two are not the same;
  • A passing resistance is not transient safety.

Why a potential difference appears:

  • The earth potential rises the moment the lightning current enters the ground;
  • The rise amplitude differs at different points;
  • A potential difference forms between the devices;
  • The difference drives a surge current into the equipment.

The misconception:

  • "A passing resistance means safe";
  • Measuring only the resistance, not the bonding;
  • Cannot explain why the equipment broke;
  • No direction for rectification.

Key insight: "A passing steady-state resistance only means good power-frequency dissipation — the potential difference from the earth-potential rise is the main cause of LV damage; do not treat a pass as an immunity certificate."

2. Is the Equipotential Continuity and the Cross-Zone Interface Complete

Step two: check the potential-difference channels.

Check the equipotential continuity:

  • Are the cabinets/trays/shielding all bonded;
  • Are the connection points reliable (connecting to KD-0187);
  • Are new metal bodies brought in;
  • The continuity decides whether the potential is one.

Check the cross-zone interfaces:

  • The lightning-protection-zone (LPZ) boundaries (connecting to KD-0165);
  • The entering/exiting pipes and cables across the zones;
  • Is the bonding done at the interface;
  • Missing bonding → the potential-difference entrance.

Common misses:

  • The tray and the cabinet not connected;
  • No bonding busbar at the cross-zone interface;
  • Signal lines/shielding not bonded;
  • New pipes not brought in.

Key insight: "Check the equipotential continuity and the cross-zone interface — all cabinets/trays bonded, bonding at the interfaces; one miss is a potential-difference entrance."

3. Are the Power and Signal Path SPDs in Place

Step three: check the surge entrances.

The power path:

  • An SPD at the distribution entry (connecting to KD-0161);
  • Protection of the LV equipment power;
  • A missing power SPD → the surge enters through the power;
  • Configure by LPZ stages.

The signal path:

  • Signal/data/control lines;
  • Matching signal SPDs;
  • Surges often enter through the signal lines;
  • The signal path is the easiest to ignore.

The coordination principle:

  • Both the power and the signal need SPDs;
  • Select by the path characteristics;
  • The signal SPD coordinates with the power SPD;
  • Focus protection on the interfaces.

Key insight: "Check the power and signal path SPDs — LV surges often enter through the signal lines; protecting both the power and the signal is complete protection."

4. Is the Earthing Grid Equalization Insufficient

Step four: check the grid itself.

The equalization role of the grid:

  • The equalization net makes the potential rise consistent during a strike;
  • The grid area/mesh affects the equalization;
  • Insufficient equalization → a large local potential difference;
  • A grid retrofit improves the equalization.

How to check:

  • The grid mesh density;
  • Local grids in the equipment areas;
  • Whether independent earthing is merged (counterattack prevention);
  • The equalization net connected to the main grid.

Common problems:

  • Too large a grid mesh;
  • No local equalization in the equipment areas;
  • Independent earthing not merged;
  • A large local potential difference on a strike.

Key insight: "Check the grid equalization — a dense mesh, local equalization, merged independent earthing; the potential rise is consistent and the difference is small."

5. The Diagnosis Order: Resistance → Equipotential → SPD Coordination

Step five: check in order.

The diagnosis order:

  • ① The resistance: a pass is only the baseline (measured by the FR);
  • ② Equipotential: continuity + cross-zone interface;
  • ③ SPD coordination: the power + signal paths;
  • ④ Grid equalization: mesh/local/merging;
  • Check in order and record the data.

The treatments:

  • A large difference → add bonding;
  • A missing interface → do the cross-zone bonding;
  • A missing path → add the power + signal SPDs;
  • Poor equalization → denser mesh/local equalization.

Practice verification:

  • Review after the retrofit in the storm season;
  • The FR monitor + the FS SPD monitor;
  • Event records closed;
  • No more damage means it is good.

Key insight: "Diagnose in order: resistance (baseline) → equipotential (continuity/interface) → SPD coordination (power/signal) → grid equalization — treat the cause and verify by monitoring after the retrofit."

6. Product Implementation: Monitoring and Diagnosis as One

WeiWuLian products manage LV protection:

  • FR Earth-Resistance Monitor: online resistance and earthing state;
  • FS SPD Monitor: online SPD state/events;
  • Equipotential check: continuity/interface data;
  • Closed-loop diagnosis: resistance-bonding-SPD data.

The design principle in one sentence: "Resistance is the baseline, bonding is the main battlefield, SPD is the entrance — a three-dimensional data loop; LV protection no longer relies on guessing."

7. Common Misjudgments

  1. "A passing resistance means safe": ignoring the transient. Result: damage from the potential difference. Correct: expand the diagnosis to the bonding.
  2. "Measure only the resistance": ignoring the bonding. Result: no root cause found. Correct: resistance → bonding → SPD.
  3. "Only the power SPD": ignoring the signal path. Result: the surge enters through the signal lines. Correct: protect both.
  4. "No bonding across the zones": ignoring the interface. Result: a potential-difference entrance. Correct: bond the interface.
  5. "Independent earthing is fine": ignoring the equalization. Result: a large local difference. Correct: merge/local equalize.

Scope and Limitations

This article explains the diagnosis of the counter-intuitive "passing resistance but damaged LV equipment" failure (steady vs transient, equipotential continuity, cross-zone interfaces, power/signal SPD coordination, grid equalization and the diagnosis order), applicable to troubleshooting and understanding LV lightning damage. The exact parameters and requirements follow GB 50057-2010, GB 50343-2012 and the manufacturers' data; this article explains only the diagnostic logic and judgment basis. All cases are anonymized (a park, a plant) and do not refer to any specific customer.