Direct Answer

Once an earth electrode is buried, how does lightning current spread, and why does the potential drop far away? The answer: after lightning current is injected into the soil through the electrode, it spreads outward from around the electrode; the current density falls with distance, and the soil voltage drop concentrates in the region near the electrode — therefore the earth resistance is mainly determined by the current-spreading conditions of the soil near the electrode. That is why reducing resistance means enlarging the spreading area, backfilling low-resistivity soil or drilling deep wells, not endlessly lengthening a single electrode.

At a factory, a single vertical electrode measured a qualified earth resistance, yet nearby equipment still misbehaved after strikes; re-inspection found the soil around the electrode was compacted and dry, with poor spreading conditions. After backfilling low-resistivity soil around the electrode and enlarging the spreading area, the earth resistance dropped markedly and the equipment anomalies disappeared. The key distinction: earth resistance is not the resistance of the electrode itself but the resistance of the soil-spreading conditions near it — the drop concentrates near the electrode, and the spreading area sets the room for reduction.

Why:

  • Spreading process: current diffuses from the electrode in an approximate hemisphere; the current density falls with distance;
  • Concentrated drop: near the electrode the current density and the drop are greatest; far away the drop tends to zero;
  • Effective-length limit: an over-long electrode contributes little at the far end and gains nothing;
  • Basis for reduction: enlarging the area, backfilling soil, deep wells and resistance-reducing agents all improve the spreading conditions near the electrode.

Key insight: "Earth resistance is set mainly by the soil near the electrode — reduce resistance by enlarging the spreading area and improving the spreading conditions, not by lengthening one electrode."

At an airport, the aid-to-navigation lamp circuit earthing used multiple parallel vertical electrodes yet remained unsatisfactory. The design was changed to a combination of "vertical electrodes + a horizontal grid + low-resistivity backfill"; after the spreading area was enlarged the earth resistance met the code and the strike-related fault rate fell. Lesson: seeing resistance reduction as "driving a few more rods" is one-sided — the spreading area and soil conditions are what decide.

1. The Spreading Process: Current Diffuses from the Electrode in a Hemisphere

Step one: where does the current go once it enters the soil.

The geometry of spreading:

  • Current enters the soil from the electrode surface;
  • It diffuses outward with the electrode at the centre;
  • The equipotential surfaces are approximately hemispherical;
  • The current density falls with distance.

The current-density change:

  • The density is highest at the electrode surface;
  • It decays roughly with the inverse square of distance;
  • Far away the density tends to zero;
  • The soil participating in spreading is limited in extent.

The potential distribution:

  • Near the electrode the potential gradient is steep;
  • Far away the potential is flat and tends to zero;
  • "The drop concentrates in the near region" is the core idea;
  • This is the "spreading zone" concept.

Key insight: "Current spreads hemispherically from the electrode and the density decays with distance — spreading happens mainly near the electrode; the far field contributes little."

2. Why the Drop Concentrates near the Electrode

Step two: why "a small earth resistance" depends on the near region.

Drop = current × resistance:

  • Near the electrode the current density is high;
  • At the same soil resistivity the near-region drop is large;
  • Far away the density and the drop are small;
  • The total drop is dominated by the near region.

Composition of the earth resistance:

  • Earth resistance ≈ the near-region soil spreading resistance;
  • The electrode's own resistance is negligible;
  • The electrode–soil transition resistance also counts;
  • The far-region soil contributes little.

Engineering meaning:

  • High soil resistivity → high earth resistance;
  • Improving the soil near the electrode gives the largest effect;
  • Electrodes too close together → overlapping spreading zones;
  • Reduce resistance by improving the "near region" first.

Key insight: "The drop concentrates near the electrode — improving the spreading conditions of the near-region soil works better than lengthening the electrode."

3. The Effective Length of an Earth Electrode Has an Upper Limit

Step three: why "lengthening without end" does not help.

The physics of effective length:

  • Current spreads along/down the electrode;
  • At the far end the spreading current is already small;
  • Beyond the effective length the contribution tends to zero;
  • Adding length does not reduce the resistance.

A sense of magnitude:

  • A single vertical electrode has a finite effective length (related to soil resistivity);
  • Horizontal electrodes also have an effective length;
  • Over-long investment is wasted;
  • Better to use the same material for spreading area.

Engineering meaning:

  • Design the electrode length by the effective length;
  • Mind the spacing in multi-electrode parallel connections (avoid shielding);
  • Too-close spacing → overlapping spreading zones → lower efficiency;
  • Use grids/loops instead of endless lengthening.

Key insight: "The electrode's effective length has an upper limit — lengthening gains nothing; the material is better spent on area and soil."

4. The Physical Basis of Resistance-Reduction Methods: Area, Soil, Deep Wells, Agents

Step four: every measure improves the "near-region spreading".

Enlarging the spreading area:

  • A horizontal grid/loop extends the spreading range;
  • More injection points, lower density;
  • Multi-electrode parallel (with enough spacing);
  • Directly lowers the near-region drop.

Backfilling low-resistivity soil:

  • Replace the soil around the electrode;
  • Improves the electrode–soil contact;
  • The near-region drop falls significantly;
  • Simple construction, stable effect.

Drilling deep wells:

  • Reach deep low-resistivity soil/groundwater layers;
  • Bring the spreading to a low-resistivity depth;
  • Suited to high-resistivity surface sites;
  • Deep-well earth electrodes.

Resistance-reducing agents / modules:

  • Lower the effective resistivity near the electrode;
  • Increase the effective electrode radius;
  • Improve the contact (lower transition resistance);
  • Mind long-term stability and environmental requirements.

Key insight: "Area lowers density, backfill lowers the near-region resistivity, deep wells find low-resistivity layers, agents enlarge the effective radius — all four improve the spreading conditions."

5. Why Grid Equalization Matters More than a Single Small Resistance

Step five: the engineering landing point of the spreading concept.

The limitation of a single small resistance:

  • However small a single point is, its spreading zone is limited;
  • The potential gradient can still be steep (a local high drop);
  • Potential differences between equipment still exist;
  • Equalization is not solved.

The advantage of grid equalization:

  • Multi-point injection, dispersed current;
  • Uniform surface-potential distribution;
  • Small potential differences between equipment earth points;
  • Combined with bonding it lowers backflashover risk (connecting to KD-0173).

Engineering combination:

  • A large grid + dense meshes (equalization);
  • Multi-point earthing (current dispersal);
  • A bonding network (removing potential differences);
  • FR Earth-Resistance Monitor tracking the state online.

Key insight: "Grid equalization makes the potential distribution flat — more than a single small resistance, it lowers the potential differences between equipment and the backflashover risk."

6. Product Implementation: Intelligent Lightning-Protection Online Monitoring

WeiWuLian products manage the spreading state:

  • FR Earth-Resistance Monitor: tracks the earth-resistance trend online (warnings when the spreading conditions degrade);
  • Monitoring platform: recognises changes in the electrode–soil contact state;
  • Retrofit verification: before/after resistance comparison for area/soil/deep-well retrofits;
  • Design support: unified planning of grid equalization + bonding.

The design principle in one sentence: "The spreading area sets the room for reduction, equalization sets the safety margin — FR monitoring makes changes in the spreading conditions visible."

7. Common Misjudgments

  1. "Lengthen one electrode without end to reduce resistance": beyond the effective length it gains nothing. Result: wasted investment. Correct: enlarge the area / backfill soil.
  2. "A few more rods are enough": too-close spacing overlaps the spreading zones. Result: low parallel efficiency. Correct: design by effective length and spacing.
  3. Looking only at the single-point resistance: ignoring equalization. Result: potential differences remain. Correct: grid equalization + bonding.
  4. Ignoring the near-region soil: compacted/high-resistivity soil. Result: the resistance stays high. Correct: backfill low-resistivity soil.
  5. No monitoring after reduction: soil changes are unknown. Result: the resistance rises again. Correct: FR tracks the trend online.

Scope and Limitations

This article explains the physical process of electrode current spreading (hemispherical diffusion, near-region concentrated drop), the effective-length limit, the physical basis of resistance-reduction methods (area/soil/deep wells/agents), and the value of grid equalization, applicable to earthing design and resistance-reduction retrofits. Quantitative calculation must follow GB 50057-2010 and GB/T 21714, combined with soil resistivity and electrode layout, completed by professional design. All cases are anonymized (a factory, an airport) and do not refer to any specific customer.