Air-Termination on Tall Structures and Metal Roofs

Short answer

IEC 62305-3 distinguishes two situations. On structures under 60 m, side strikes are not considered; the roof and horizontal projections are protected according to the LPS class. On structures of 60 m or more, side strikes can occur, and an air-termination system is installed on the upper part, typically the top 20 % of the height where that part is above 60 m, with requirements of at least class IV. In addition, parts of the structure (roof sheets, parapets, tanks and pipes) can be used as natural air-terminations if they meet continuity and minimum thickness requirements.

Structures under 60 m

According to the standard, studies indicate that the probability of low-amplitude strikes to the vertical face of a structure under 60 m is low enough to disregard. The roof and horizontal projections are then protected according to the class given by the risk assessment. Air-termination placement in those cases is explained in the guide on air-termination by rolling sphere, mesh and angle.

Structures of 60 m or more

On taller structures, side strikes can occur, especially at points, corners and edges. The standard clarifies that the risk is usually low, because only a few percent of strikes to tall structures are lateral and their parameters are considerably lower than those of strikes to the top. However, electrical and electronic equipment installed on the facades can be damaged even by strikes with a low peak current.

An air-termination system is therefore required to protect the upper part and the equipment it carries. The points set by the standard are:

• The zone to protect is typically the top 20 % of the height, provided that part is above 60 m. For the part below 60 m, protection may be omitted. On structures between 60 and 75 m, the protected area does not have to extend below 60 m.
• The placement rules must meet at least the requirements of protection level IV, with emphasis on corners, edges and significant projections such as balconies or observation platforms.
• If there is sensitive equipment on the outer wall of the upper part, it should be protected with special measures: horizontal air-terminals, meshed conductors or equivalent.
• With the rolling sphere method, the standard applies it only to the upper part (see the figure in Annex A).

The side air-termination requirement can be met by the metal cladding itself or by metal curtain walls, if they meet the dimensions of Table 3, or by external down-conductors at the vertical edges where there are no natural conductors. Air-terminations formed this way can use installed down-conductors or connect to natural down-conductors such as the steel frame or continuous reinforced concrete.

Roofs, tanks and parapets as air-terminations

The standard lists the parts of a structure that can be used as natural air-terminations, always in accordance with the general rule on natural components (they stay in place and are not modified):

1. Metal roof sheets, with durable electrical continuity between parts (brazing, soldering, welding, crimping, seaming, screwing or bolting) and without insulating coating.
2. Metal elements of the roof structure (trusses, interconnected reinforcing steel) under a non-metallic roof covering, where damage to that covering is acceptable.
3. Ornaments, railings, pipes and parapet covers, with cross-sections no smaller than those of a standard air-termination.
4. Pipes and tanks on the roof, with thicknesses and cross-sections in accordance with the standard.
5. Pipes and tanks carrying flammable or explosive mixtures, with thickness not less than the value t of Table 3 and without dangerous heating of the inside at the point of strike.

Pipes with flammable or explosive mixtures are not considered natural air-terminations if the flange joints are not metallic or if the flanges are not otherwise bonded. A thin layer of paint, about 1 mm of asphalt or 0.5 mm of PVC is not considered insulating.

Minimum thicknesses (Table 3)

Table 3 gives two thicknesses per material, valid for all classes:

• t, to prevent puncture of the sheet or where hot spots or ignition of combustible material underneath must be considered.
• t′, accepted when neither preventing puncture nor the ignition hazard matters.

If there is a risk of hot spots or ignition, you must verify that the temperature rise of the inner surface at the point of strike is not dangerous. That problem can be ignored when the sheet is inside a zone LPZ 0B or higher.

Combustible materials under the roof

If the roof is made of easily combustible material, the standard gives minimum distances between the air-termination conductors and the material: on thatch roofs without steel bars, at least 0.15 m; on other combustible materials, not less than 0.10 m. Easily combustible parts must not be in direct contact with the components of the external system, or under a metal membrane that lightning could puncture. On a flat roof where water can collect, air-terminations are installed above the maximum likely water level.

Equipment on the roof

Conductive parts on the roof, such as metal tanks, are often connected to equipment inside the structure. To prevent the whole lightning current from passing through the interior, the standard recommends a good connection between those natural components and the air-termination mesh. Where a direct strike on the conductive part is not acceptable, it must lie inside the space protected by an air-termination system.

Common mistakes

• Accepting a metal roof without checking thickness and continuity. A thickness below t can be punctured.
• Using a pipe carrying gas as an air-termination without checking the flanges. Without metallic joints or bonding between flanges, it does not count.
• Not protecting equipment mounted on the wall of the upper part. It is the equipment that gets damaged by side strikes.
• Covering the sheet with insulating material. It loses its status as a natural air-termination.

Guides in the same series

The complete system and its classes are in LPS, classes I to IV; the sphere radii by level, in protection levels. The conductors that receive the current from the roof are explained in LPS down-conductors.

What we do

We assess whether the roof, parapets and tanks of your building can work as air-termination, and what needs to be added. We will quote the study.

Related guides

• IEC 62305: lightning protection, what each part covers
• LPS air-termination: rolling sphere, mesh and protection angle
• Lightning protection system (LPS): classes I to IV
• LPS down-conductors: number, spacing and step voltage
• Lightning shielding of substations

References

• IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 5.1.3 (natural components), 5.2.3 (side strikes on tall structures), 5.2.4 (construction), 5.2.5 and Table 3 (natural components and thicknesses), Annex A.2 and Annex E.5.2.

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