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.
