Types of Air Terminals: Rods, Wires, Mesh and Natural Parts

Types of air terminals

Under DSTU EN 62305-3:2021 (cl. 5.2.1) an air-termination system may be made from any combination of rods (including free-standing masts), stretched wires and meshed conductors. Natural components, meaning metal parts of the structure that meet the standard’s conditions, are treated separately from these three elements (cl. 5.2.5).

Radioactive air terminals are not permitted by the standard.

DSTU EN 62305-3:2021, cl. 5.2.1, 5.2.2, 5.2.5, 5.3.2, Table 2, Figures E.13–E.16. The height limit of the protective angle method and the protection zone are covered in the calculation article.

Rod air terminals and free-standing masts

A rod air terminal is a vertical rod on a roof, on a pole or on a separate mast. The standard advises placing the capturing elements at corners, exposed points and edges, especially at the highest levels of every facade (cl. 5.2.2). Rods on a roof are bonded together so that the current is shared between them (cl. 5.2.1).

A separate mast stands apart from the structure and carries the rods of an isolated system (on one mast or on several). If the mast is metal or made of bonded reinforcing steel, no extra down-conductors are needed for it (cl. 5.3.2). A mast made of neither needs at least one down-conductor.

This is the role of our lightning rods: a multi-sided bent pole of 5–40 m with an air terminal on top. The mast can also carry lighting or antennas. How a steel pole works as a down-conductor and why section joints matter is explained in the article on a steel mast as an air terminal.

The protection zone of a rod is found by one of three methods. They, the system classes and a worked example are in the article on protection zone calculation; we do not repeat them here.

Stretched-wire air terminals

A stretched wire is a horizontal conductor fixed on supports. In an isolated system it is run between two masts, with a separation distance kept between the wire and the roof (Figure E.14 of DSTU EN 62305-3:2021). In a non-isolated system the wire may run along the roof ridge at a set distance from the roof (Figure E.16).

The down-conductor rule is simple: each supporting structure of a wire air terminal needs at least one (cl. 5.3.2). A single wire follows the same rule.

Mesh on a roof

A mesh is conductors laid over a surface. The standard calls this method suitable for flat surfaces (cl. 5.2.2). The largest cell size depends on the lightning protection system class (Table 2): 5×5 m for class I, 10×10 for II, 15×15 for III and 20×20 for IV.

If the mesh is made isolated, each supporting end of a wire needs at least one down-conductor (cl. 5.3.2).

On a flat roof where water can collect, the air terminals are fixed above the highest possible water level (cl. 5.2.4, note).

Natural air terminals

A metal part of the structure can serve as a natural air terminal when it is reliably bonded and meets the conditions of cl. 5.2.5. All other natural components the standard treats only as a supplement to the system (cl. 5.1.3).

• metal roof covering: reliable electrical contact between parts, thickness not less than the Table 3 value, no insulating layer; a thin coat of paint or asphalt (1 mm) or PVC (0.5 mm) is not counted as insulation;
• metal structures under a non-metallic roof (trusses, bonded reinforcing steel), if damage to the roofing material is acceptable;
• decoration, handrails, pipes, parapets: cross-section not less than that of standard air-termination elements;
• metal pipes and tanks on the roof: cross-section as in Table 6; for tanks with flammable or explosive contents the wall thickness must also be at least t from Table 3 and the heating of the inner surface at the strike point must be safe (Annex D). If this is not confirmed, the tank is not treated as an air terminal but is bonded to the structure.

A pipeline carrying a flammable mixture is not counted as a natural air terminal if the flange gaskets are non-metallic or the flanges are not electrically bonded (cl. 5.2.5, note).

Whether a particular roof qualifies is a matter for calculation and a site survey, not for this list.

Isolated and non-isolated systems

An isolated external lightning protection system is one where the air terminals and down-conductors are placed so that the lightning current path does not touch the protected building. In a non-isolated one the path may be in contact with the building (cl. 3.3, 3.4).

In most cases the external system can be attached to the building. An isolated one is worth considering if thermal and explosive effects at the strike point or on the conductors could endanger the building or its contents: typical examples are a combustible roof, combustible walls, and zones with a risk of explosion or fire (cl. 5.1.2). Another reason is expected changes to the building or its use that would require reworking the system.

Non-isolated air terminals are laid on the surface of a non-combustible roof. For a combustible one the standard asks for special attention to clearance: not less than 0.15 m is enough for thatched roofs without steel supports and not less than 0.1 m for other combustible materials (cl. 5.2.4).

The protective angle construction is shown in Figures E.13 and E.14 for an isolated system and in Figures E.15 and E.16 for a non-isolated one.

What to send for choosing a lightning rod

For a lightning rod on our pole, tell us what the site is, the size of the area to be protected, the height you think is needed and the wind region. Add whether the mast should carry lighting or antennas. Give the system class if the risk assessment is already done; leave blank what you do not know.

The air-terminal type, the system class and the building’s lightning protection design are set by the designer. We propose the pole design from your input data; erection is carried out by a licensed construction partner company. The products are in the Lightning rods and air terminals section; how to choose a lightning rod is in the article Lightning protection of masts and towers, and the article on terms separates the words “lightning rod” and “air terminal”.