Direct Answer

Are the down-conductor sizes and numbers on the drawing picked by feel, or calculated from the lightning-current thermal effect and mechanical forces? The answer: the down-conductor cross-section is determined by the thermal effect of the lightning current (temperature rise not exceeding the limit) and by electromechanical forces; the number and spacing determine how the current disperses and how much the potential rises on each down-conductor — multiple evenly arranged conductors split the large current and lower the potential gradient of each one. That is why the standard requires down-conductors to be evenly arranged around the building perimeter, in no less than a certain number.

A project configured down-conductors at the "drawing minimum" — only two, with spacing far larger than the standard, and without using the building reinforcement as natural down-conductors. After a strike, equipment near the conductors was damaged. Review: after adding more down-conductors arranged evenly around the perimeter, the current dispersed, the potential gradient fell, and the problem did not recur. The key distinction: the down-conductor size and number are not "picked by feel" — the cross-section handles the thermal effect and mechanical force, and the number handles current dispersion and the potential gradient; each has its own basis.

Why:

  • Cross-section: determined by the lightning-current thermal effect (temperature rise) and electromechanical force;
  • Number: determines the current dispersion and the potential gradient;
  • Even arrangement: multiple conductors split the current and lower each one's gradient;
  • Safe distance: the clearance between down-conductors and protected objects has requirements.

Key insight: "The cross-section handles 'surviving the heat and force'; the number handles 'dispersing the current and flattening the potential' — both sizes and numbers have calculation bases, not guesses."

At a substation, building reinforcement was used as natural down-conductors, but the electrical continuity of the steel was not verified (poor laps), and a strike caused local sparking. After rectification — checking/welding the lap joints per the standard and forming a continuous electrical path — the problem disappeared. Lesson: natural down-conductors must meet electrical-continuity conditions — it is not "steel exists, so it works", but "continuous, capable of shunting, and with a flat potential".

1. How the Cross-Section Is Determined: Thermal Effect and Electromechanical Force

Step one: why numbers like φ10 round steel.

The thermal effect:

  • The lightning current produces Joule heat in the down-conductor;
  • The temperature rise relates to I²t (current squared × time);
  • An insufficient cross-section → temperature rise over the limit → damage/melting;
  • The cross-section must keep the temperature rise within the limit.

The electromechanical force:

  • The lightning current produces electromagnetic force between conductors;
  • At large currents the mechanical force is considerable;
  • Insufficient cross-section/fixing → deformation/breakage;
  • The electromechanical force determines the mechanical-strength requirement.

Materials and sizes:

  • Round/flat steel per the standard minimum cross-section;
  • Differences in conductivity/corrosion resistance;
  • The size numbers come from the thermal + force double check;
  • Stricter for high-rise/large-current scenarios.

Key insight: "The cross-section is set jointly by the thermal effect (temperature rise) and the electromechanical force — the size numbers are calculated, not copied from a drawing."

2. Number and Spacing: Current Dispersion and Potential Gradient

Step two: why "at least two, evenly arranged".

The role of the number:

  • Multiple down-conductors shunt in parallel;
  • The current in each one falls → less heating/lower potential;
  • The current disperses into the ground at multiple points;
  • More conductors → a smaller burden on each one.

The role of spacing:

  • Even arrangement → even current distribution;
  • Spacing too close → mutual shielding/uneven distribution;
  • Spacing too large → local current concentration;
  • Even distribution around the perimeter is key.

The potential gradient:

  • A large current in a conductor → its own potential rises high;
  • A large gradient → risk to nearby equipment/people;
  • Multiple shunts → a smaller current each → lower gradient;
  • That is why "many and even" beats "few and thick".

Key insight: "The number and spacing decide the current dispersion and the potential gradient — many conductors arranged evenly shunt the current and lower the potential, which is why the standard requires a certain number."

3. Why Many and Even Beats Few and Thick

Step three: the merits of the configuration.

The problems of few and thick:

  • A large current in one conductor → a high potential gradient;
  • Concentrated local heating;
  • One break → the protection fails;
  • Low redundancy.

The benefits of many and even:

  • The current disperses; each conductor bears less;
  • The potential gradient is gentle;
  • A local break can still be shunted;
  • High redundancy and reliability.

The engineering comparison:

  • Two thick lines vs many even thin lines;
  • The latter has a flatter potential and higher reliability;
  • The standard takes "evenly around the perimeter, no less than a number";
  • Balancing safety and cost.

Key insight: "Few and thick means a high gradient and low redundancy; many and even means a flat potential and high reliability — the standard choosing 'many and even' is no accident."

4. The Safe Distance between Down-Conductors and Protected Objects

Step four: why the distance matters.

Where the safe distance comes from:

  • A down-conductor carrying current creates a strong surrounding field;
  • Too close to a protected object → air/gap breakdown;
  • Breakdown → side flash/counterattack (connecting to KD-0166);
  • Insufficient distance is a counterattack channel.

How the distance is set:

  • By the conductor current and potential level;
  • By the breakdown characteristics in air/soil;
  • The standard gives minimum clearance requirements;
  • When it cannot be met, use equipotential measures.

Engineering practice:

  • Route the conductors along exterior walls/independent columns;
  • Keep clearances from protected equipment;
  • Add protection/warnings in occupied areas;
  • Use an equipotential network to remove potential differences.

Key insight: "A down-conductor carrying current has a high surrounding potential — keep the safe distance from protected objects, or air breakdown forms a counterattack; where the distance cannot be met, equipotential bonding makes up for it."

5. The Conditions for Using Building Reinforcement as Natural Down-Conductors

Step five: what using the reinforcement requires.

The basic conditions:

  • Electrical continuity of the reinforcement (reliable laps);
  • A continuous electrical path;
  • A cross-section meeting the thermal/force requirements;
  • Equivalence with artificial down-conductors.

Continuity verification:

  • Are the laps/welds standard-compliant;
  • Are the crossing points reliably connected;
  • Electrical connection between ties and main bars;
  • Verify by continuity testing (connecting to the FR product).

Notes:

  • It is not "steel exists, so it works";
  • The reinforcement in concrete must be continuous;
  • Corrosion/breakage affects continuity;
  • Recheck the continuity regularly.

Key insight: "The premise of a natural down-conductor is electrical continuity — reliable laps, a continuous path, capable of shunting, and rechecked by continuity tests regularly."

6. Product Implementation: Earthing and Continuity Monitoring

WeiWuLian products manage the down-conductor system with data:

  • FR Earth-Resistance Monitor: online monitoring of the earthing-system state;
  • Continuity recheck: continuity tests of down-conductors/equipotential bonds;
  • Potential-risk assessment: flags abnormal potential gradients;
  • Data loop: recheck the down-conductor state after lightning events.

The design principle in one sentence: "Size by thermal and force, number by shunting, continuity by testing — manage the down-conductor system with data."

7. Common Misjudgments

  1. "Copy the sizes from the drawing": not understanding the basis. Result: misconfigured for the scenario. Correct: recheck by the thermal/shunting logic.
  2. "Two thick lines are enough": ignoring the potential gradient. Result: high local potential. Correct: many, evenly arranged.
  3. "Spacing is arbitrary": ignoring the breakdown risk. Result: counterattack damage. Correct: keep the safe distance + equipotential bonding.
  4. "Steel exists, so it is a natural conductor": ignoring electrical continuity. Result: local sparking. Correct: verify continuity + test.
  5. "Done once installed": no continuity monitoring. Result: a break goes unnoticed. Correct: recheck continuity regularly.

Scope and Limitations

This article explains the parameter basis of down-conductors: the cross-section (thermal effect and electromechanical force), the number and spacing (shunting and the potential gradient), the safe distance, and the conditions for natural down-conductors, applicable to the design and understanding of down-conductor systems. The exact cross-section/number/spacing values follow GB 50057-2010 and its revisions; this article explains only the physics and engineering logic behind the parameters. All cases are anonymized (a project, a substation) and do not refer to any specific customer.