LPS Air-Termination: Rolling Sphere, Mesh, Protection Angle

Short answer

IEC 62305-3 accepts three methods for placing the air-termination of a building: protection angle, rolling sphere and mesh. The sphere rolls with a radius of 20, 30, 45 or 60 m depending on class I to IV, and the mesh is 5×5, 10×10, 15×15 or 20×20 m. The sphere works in every case; the angle and the mesh have limits.

Short answer

IEC 62305-3 allows three methods for deciding where the air-termination of a building goes: the protection angle, the rolling sphere and the mesh. All three use values that depend on the LPS class (I to IV): a sphere of 20, 30, 45 and 60 m radius, and a mesh of 5×5, 10×10, 15×15 and 20×20 m. The rolling sphere is suitable in all cases; the protection angle is for buildings of simple shape and has height limits; the mesh is suitable for flat surfaces. Air-termination is placed at corners, exposed points and edges, especially on the upper level of the facades.

What an air-termination is

Air-terminations are the metal elements of the external system that intercept the strike: rods (including free-standing masts), catenary wires or meshed conductors, in any combination. Two rules apply to all types:

Only the real physical dimensions of the metal count when working out the protected volume.
Individual rods must be bonded together at roof level so that the current is shared.

The standard does not allow radioactive air-terminations.

The values by class

LPS class	Rolling sphere radius	Mesh size
I	20 m	5 × 5 m
II	30 m	10 × 10 m
III	45 m	15 × 15 m
IV	60 m	20 × 20 m

The protection angle also changes with the class, and depends on the height of the air-termination above the reference plane to be protected. The standard gives it as curves (Figure 1): class I allows the narrowest angles. The curves stop being valid at the heights marked in the figure; beyond that point only the rolling sphere or the mesh can be used. For air-termination heights below 2 m the angle does not change.

Rolling sphere method

Imagine a sphere of the class radius rolling over the whole structure, on top of it and around it, in all directions. The air-termination layout is adequate if no point of the structure touches the sphere and the sphere touches only the air-termination system.

The points where the sphere touches the structure are possible points of strike. It is the method used for complex shapes, with projections, level changes or roof equipment, because it assumes no geometry.

Protection angle method

The volume protected by a vertical rod is a right cone with its apex on the rod axis; the half-angle of the cone is the protection angle of the class, and it depends on height. For a wire, the volume is built from that of virtual rods placed along the wire. For meshed wires, from the combination of the volumes of each conductor.

The structure must lie completely within the protected volume. One detail that causes mistakes: the height that enters the curve is measured above the reference plane of the area to be protected. If the roof is being protected, it is the height above the roof; if the ground is being protected, the height above the ground. The standard illustrates this with two different angles for the same rod (α1 with respect to the roof, α2 with respect to the ground).

Mesh method

The mesh is considered to protect the whole surface if all of the following are met:

There are conductors at the roof edges, at the eaves and on the ridges where the slope exceeds 1/10.
The mesh dimensions are no larger than those in the table for the class.
The network is built so that the current always finds at least two different metal paths to the earth-termination.
No metal installation projects beyond the volume protected by the air-terminations.
The conductors follow, as far as possible, the shortest and most direct route.

The mesh is suitable for horizontal and sloping roofs without curvature, and also for flat side surfaces. Where the slope exceeds 1/10, parallel conductors may be used instead of a mesh, provided the distance between them does not exceed the mesh width.

How to choose between the methods

Flat or low-slope roof, regular building: the mesh is simple to install and helps share the current.
Compact building with few projections: the protection angle with rods may be enough, within its height limits.
Complex shapes, roof equipment, large heights: rolling sphere, to check and decide where air-termination is missing.

In practice the methods are combined: a mesh is laid and the sphere is used to check that no point projects. The standard allows this: positions are determined with one or several methods.

Tall structures

The method changes for structures of 60 m or more, because they can receive side strikes; the guide on air-termination on tall structures and metal roofs explains it. For a substation yard (overhead shield wires and masts), see the guide on lightning shielding of substations.

Common mistakes

Using the protection angle above its maximum height. The curve does not apply beyond the points marked in the figure of the standard.
Counting as protected something that is not real metal. Only the physical dimensions of the air-termination count.
Leaving equipment projecting from the protected zone. In the mesh method, no metal installation may stick out of the protected volume.
Not bonding the rods together. Without bonding, the current is not shared and the separation distance grows.
Using the sphere radius of another class. The radius comes from the class given by the risk assessment.

Guides in the same series

The sphere radii by class come from the protection level: protection levels. The complete system is in LPS, classes I to IV, and the conductors that receive the current from the air-termination, in LPS down-conductors.

What we do

We place air-termination with the method that suits your structure and check with the rolling sphere that no exposed points remain. If you want to review an existing building or design a new one, we will quote the study.

References

IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 5.2.1 and 5.2.2 (air-termination and placement), Table 2 and Figure 1 (sphere, mesh and angle by class) and Annex A (A.1 protection angle, A.2 rolling sphere, A.3 mesh).

Frequently asked questions

Which air-termination method works for any structure?

The rolling sphere. IEC 62305-3 says it is suitable in all cases. The protection angle is reserved for simple shapes and has height limits; the mesh suits flat surfaces.

Why does the protection angle not apply to very tall structures?

Because the angle-versus-height curve in the standard only extends to certain heights, which depend on the class, marked in Figure 1. Beyond those points only the rolling sphere and the mesh can be used.

Must an air-termination mesh also cover the roof edges?

Yes. For the mesh to be considered to protect the whole surface, conductors must be placed at the roof edges, at the eaves and on the ridges where the slope exceeds 1/10, in addition to respecting the mesh size and giving the current at least two metal paths to earth.