Natural components of lightning protection: roof, frame, rebar

What can be used and under what conditions

DSTU EN 62305-3:2021 calls a natural component a conductive element that was not installed for lightning protection but can supplement it (cl. 3.15). The main condition from cl. 5.1.3: the part has its designated place, is reliably connected to the rest of the system and does not change over time. The standard gives two examples: interconnected reinforcement and the metal frame of a structure. Other natural components may only be taken as a supplement to an ordinary system.

A short overview of what works as what, and where the standard says so.

DSTU EN 62305-3:2021, cl. 5.2.5 and 5.3.5, abridged. The conditions for each row are explained below.

The roof as an air terminal

Metal covering counts as an air terminal on three conditions (cl. 5.2.5 a). The parts are joined reliably: hard soldering, welding, seaming, stitching, self-tapping screws or bolts. The thickness is not less than table 3 requires. There is no insulation on top.

Table 3 has two values. The larger, t, applies where puncture of the covering or local overheating must be prevented. The smaller, t′, is enough when puncture and ignition of materials under the sheet do not matter. For steel (in the table: stainless, galvanised) these are 4 mm and 0.5 mm, and the full table for different metals is in the article Steel mast as an air terminal and down-conductor. Where there is a risk of overheating or ignition under the covering, you must make sure that heating of the inner surface at the strike point is not dangerous (cl. 5.2.5 a, note 1).

A thin layer of paint or asphalt (1 mm) and PVC (0.5 mm) do not count as insulation (cl. 5.2.5 a, note 2). A covering under insulating material is not treated as an air terminal.

Under a non-metal roof, metal trusses and connected reinforcing steel can serve as air terminals, but only where damage to the non-metal roofing itself is acceptable (cl. 5.2.5 b). Decoration, handrails, pipes and parapets are taken if their cross-section is not smaller than that of standard air terminals (cl. 5.2.5 c). Pipes and tanks on the roof qualify if material and cross-section meet table 6 (cl. 5.2.5 d).

Frame and rebar as down-conductors

Metal structures become a down-conductor if the electrical continuity between their parts is reliable (cl. 5.5.3) and the dimensions are not smaller than table 6 gives for ordinary down-conductors (cl. 5.3.5 a).

The reinforcement of a reinforced-concrete structure qualifies too, if the steel frame is electrically continuous (cl. 5.3.5 b). Precast concrete needs interconnection points between reinforcement parts and a conductive link between them, and separate parts are joined on site.

If a steel frame or connected rebar serves as the down-conductors, ring conductors are not needed (cl. 5.3.5 c). Facade elements and profile rails qualify when their dimensions meet the down-conductor requirements (cl. 5.6.2), the sheet or tube is at least 0.5 mm thick and the vertical conductivity meets cl. 5.5.3 (cl. 5.3.5 d).

In a steel frame of bolted trusses, bolts tightened for mechanical strength make the parts electrically connected, and thin paint is punched through by the lightning current. As a reference, the standard suggests cleaning the seating surfaces and adding a weld of about 50 mm once the joint is complete (E.4.3.7).

A test joint is fitted on every down-conductor at the point where the air terminal is connected. The exception is natural down-conductors connected to foundation electrodes (cl. 5.3.6). The joint is opened only with a tool.

Rebar: joints and measurements

The standard treats rebar as electrically continuous when most of the joints between vertical and horizontal bars are welded or otherwise reliably connected (cl. 4.3). The criterion is simple: the total resistance between the highest part and ground level is not above 0.2 ohm. If that cannot be achieved or checked, the rebar should not be used as a natural down-conductor, and the standard recommends an external one (cl. 4.3).

As a reference, it is measured with a four-wire method between the air-termination system and the earthing plate near the ground, with a measuring current of about 10 A (E.4.3.1).

Joining methods recommended by informative annex E.4.3.6:

• welding: a weld of at least 50 mm, crossing bars bent before welding so they run parallel for at least 70 mm;
• clamps: special ones tested to the EN 62561 series; for a link to an equipotential bonding conductor, two bonding conductors are used, because the joint cannot be checked once the concrete has set;
• wire tying: only for additional equipotential bonding conductors and electromagnetic compatibility needs.

Tying is unsuitable for lightning current because the wire may explode and damage the concrete. A short weld where bars cross does not help either: when the concrete is poured, such joints often break (E.4.3.6). Welding to rebar is done only with the consent of the structural designer (E.4.3.3).

After construction the layout of the rebar is almost impossible to recover, so it is documented: drawings, descriptions and photos taken during construction (E.4.3.1). Welding and measurements at height and on electrical connections are done by qualified specialists, not by the customer alone.

What cannot be a natural component

A pipeline carrying a flammable or explosive mixture cannot serve as an air terminal or a down-conductor if the gaskets in flange joints are non-metallic or the flanges are not electrically bonded (cl. 5.2.5, note; cl. 5.3.5 a). Pipes and tanks with such a mixture on the roof may be used only if the wall thickness is not less than t from table 3 and heating of the inner surface at the strike point is not dangerous (cl. 5.2.5 e). If the thickness is not enough, they are connected to the structure being protected.

Covering with thick insulation on top is not treated as an air terminal (cl. 5.2.5 a). A natural component needs a designated place and permanence: the part must not change (cl. 5.1.3).

A steel mast: the same, but separately

When the shaft or mast itself serves as the air terminal and down-conductor, the wall thickness, section joints and earthing are covered in the article Steel mast as an air terminal and down-conductor. How to work out the protection zone of such a rod is in Lightning rod protection zone calculation.

The POLYGONAL company manufactures free-standing lightning rods on multi-sided bent shafts and masts of 5–40 m, and makes roof air terminals to the customer’s drawings or technical specification; see the page Lightning rods. Installation is carried out by a licensed construction partner company.

What to agree before work starts

Natural components must be in the lightning protection design, not appear on site. Agree with the designer:

• which elements count as air terminals and down-conductors, and whether they meet the thickness and cross-section of tables 3 and 6;
• where and how the rebar and frame are connected, and who consents to welding;
• who measures continuity, when and with what, and where it is recorded;
• where the test joints are and whether they are accessible for periodic checks: Lightning protection inspection.

If you need a lightning rod on a shaft, send the height of the structure, the size of the site and the intended location. For the terms used here see Lightning rod, air terminal: what is the difference, and for choosing a lightning rod see Lightning protection of masts and towers.