In a non-isolated LPS the standard requires at least two down-conductors, spread around the perimeter at typical spacings of 10 m (classes I and II), 15 m (III) or 20 m (IV), with one at each exposed corner. Near the down-conductors the step and touch voltage risk is considered tolerable if nobody is within 3 m, there are at least 10 down-conductors or the soil has a surface resistance of at least 100 kΩ.
Short answer
Down-conductors carry the lightning current from the air-termination to the earth-termination. In a non-isolated system, IEC 62305-3 requires at least two, spread around the perimeter, with typical spacings of 10 m for classes I and II, 15 m for III and 20 m for IV, and one at each exposed corner where possible. Near them there can be a hazard from step and touch voltages: the standard considers it tolerable when one of three conditions is met (nobody within 3 m, at least 10 down-conductors, or a high-resistance surface soil layer) and otherwise requires additional measures.
What down-conductors must achieve
The standard asks that, between the point of strike and earth:
- there are several parallel paths for the current;
- the length of the paths is a minimum;
- equipotential bonding is made to the conductive parts of the structure as applicable.
There is a physical reason. Sharing the current among several down-conductors reduces the voltage along each one and, with it, the separation distance needed and the risk of sparking toward the inside. In addition, down-conductors should be as far as possible from internal circuits and metal parts to avoid having to bond them to the LPS. Lateral connections between down-conductors (ring conductors) improve current sharing; the standard considers them good practice.
Number and position
Isolated LPS. If the air-terminations are rods on separate non-metallic masts, at least one down-conductor is needed per mast; metal masts or masts of interconnected reinforcing steel need no additional down-conductors. With catenary wires, at least one down-conductor at each supporting structure. With a network of conductors, at least one at each end of the supporting wire.
Non-isolated LPS. Not fewer than two down-conductors are installed, distributed around the perimeter within architectural and practical limits, and preferably equally spaced. One is installed at each exposed corner where possible.
| LPS class | Typical distance between down-conductors |
|---|---|
| I | 10 m |
| II | 10 m |
| III | 15 m |
| IV | 20 m |
The informative annex of the standard adds that an exposed corner can do without a down-conductor if it lies no more than half the table distance from the two neighboring down-conductors, or no more than a quarter of it from one of them. A variation of ±20 % in the spacing is acceptable if the average meets the table. Down-conductors that cannot be placed on one side are compensated on the other sides, at distances not less than one third of the table value. In closed inner courtyards with a perimeter over 30 m, down-conductors must be installed. Inner corners can be ignored.
How they are installed
- Straight and vertical, as a direct continuation of the air-terminations, by the shortest path. Loops are avoided; if that is not possible, the distance between two points of the loop and the conductor length between them must meet the separation distance (see the guide on distance s).
- Never in gutters or downpipes, even with insulation.
- Near doors and windows, the separation distance is recommended.
- Non-combustible wall: the down-conductor may run on the surface or inside the wall.
- Combustible wall: on the surface if the heating is not dangerous for the material; if it is, more than 0.1 m from the wall (the clamps may touch the wall). Where the distance to a combustible material cannot be assured, the cross-section of steel or thermal equivalent must not be less than 100 mm².
At the foot of each down-conductor, at the connection to the earth-termination, a test joint is installed that can be opened with a tool for measurement and stays closed in normal use. It is not required on natural down-conductors combined with foundation electrodes.
Natural components as down-conductors
The following can serve: metal installations with durable continuity and dimensions at least equal to those of a standard down-conductor, reinforced concrete with electrical continuity, interconnected steel frames, and facade elements, profiles and metal substructures (with sheets or tubes of at least 0.5 mm and vertical continuity). Pipes with flammable or explosive mixtures do not count if the flange joints are not metallic or not bonded. When the structure uses its steel frame or interconnected reinforcing steel as down-conductors, ring conductors are not needed. See the guide on reinforcing steel of the concrete.
Step and touch voltages near down-conductors
Even if the LPS is well designed, the area near a down-conductor can be dangerous for people. The standard considers the risk falls to a tolerable level if one of these conditions is met, for both touch and step:
- in normal operation there are no people within 3 m of the down-conductors;
- a system of at least 10 down-conductors conforming to the natural components is used;
- the contact resistance of the surface soil layer within 3 m of the down-conductor is at least 100 kΩ. A layer of 5 cm of asphalt or 15 cm of gravel generally achieves it.
If none is met:
- Against touch voltage: the exposed down-conductor is insulated with insulation that withstands 100 kV impulse 1.2/50 μs (for example, at least 3 mm of cross-linked polyethylene) and access is limited with physical restrictions or warning notices.
- Against step voltage: equipotentialization with a meshed earth-termination, plus physical restrictions or warning notices that limit access to the area within 3 m.
These conditions come from the lightning standard and are for impulse. Step and touch voltages from a 50/60 Hz fault in a substation follow other criteria; for that, see the guide on step and touch voltage.
Common mistakes
- A single down-conductor «because the building is small». The minimum in a non-isolated system is two.
- Down-conductors inside the gutter. They corrode, and the standard prohibits it.
- Applying the spacing of the wrong class. The typical distance comes from the LPS class.
- Forgetting the test joint. Without it, each down-conductor cannot be measured.
- Ignoring people. A down-conductor next to a public entrance requires one of the conditions of clause 8.
Guides in the same series
Down-conductors end at the earth-termination described in LPS earth-termination and are sized using LPS materials and dimensions. The air-terminations they connect to are in LPS air-termination.
What we do
We define the number and position of the down-conductors together with the air-termination and earth-termination, and review the areas where people have access. We will quote the study.
Related guides
- IEC 62305: lightning protection, what each part covers
- LPS earth-termination: type A and type B electrodes
- Reinforcing steel of the concrete as down-conductor and electrode
- Separation distance s against dangerous sparking
- Step and touch voltage
References
- IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 5.3 (down-conductors: 5.3.1 to 5.3.6), Table 4, Figure 2, Cl. 8 (measures against touch and step voltages) and Annex E.5.3 (number and position of down-conductors).
Frequently asked questions
What is the minimum number of down-conductors in an LPS?
In a non-isolated external system, not fewer than two, spread around the perimeter with equal spacing where possible. In an isolated system, at least one per mast, one at each supporting structure of a catenary wire and one at each end of the supporting wire of a conductor network.
Can a down-conductor run in a rainwater gutter or downpipe?
No. The standard prohibits installing down-conductors in gutters or downpipes, even if they are covered with insulation, because moisture causes severe corrosion.
How is step voltage hazard reduced near a down-conductor?
If none of the standard conditions is met (nobody within 3 m, at least 10 down-conductors or a surface soil resistance of at least 100 kΩ), use an equipotential earthing mesh together with physical restrictions or warning notices that limit access to the area within 3 m.