Air terminal lightning protection: the air-termination methods
IEC 62305-3 gives three ways to position the air terminations that catch a lightning strike: the rolling sphere method, the mesh method and the protection angle method. This guide explains how each one works, when it is used, how they combine and how they tighten at a more demanding protection level, then how NFPA 780 in the US and AS 1768 in Australia place air terminals instead.
IEC 62305-3 gives three methods for positioning the air terminations that catch a lightning strike: the rolling sphere method, the mesh method and the protection angle method.
Air terminal lightning protection stands or falls on placement. Each method decides where rods, wires or mesh conductors must go so that no part of a structure is left exposed. They are not competing choices; a designer uses them together on a single building, and all three tighten as the lightning protection level becomes more demanding.
The air termination is the part of a lightning protection system that a strike attaches to. Its job is to give lightning a preferred, controlled point to hit, so the current is caught by the system and led safely to ground rather than reaching an unprotected corner of the building. The three methods in this guide are the recognised ways to work out where those terminations go. This page explains how each method works, when each is used, how they combine on one structure, and how the geometry of all three follows directly from the lightning protection level the risk assessment arrived at.
Three ways to leave no surface exposed
Each method answers the same question from a different angle: given this structure and this protection level, where must the air terminations go so a strike cannot reach an unprotected surface? The three are summarised here, then taken one at a time below.
The general method
An imaginary sphere of a fixed radius is rolled over the structure. Anywhere it touches is a place a strike could land, so terminations are placed to keep it off every vulnerable surface. It works on any shape and is the fallback where the other two do not fit cleanly.
For roofs
A grid of conductors laid across a roof, so any strike is caught by the nearest conductor and shared between the surrounding ones. It suits flat and gently pitched roofs, where a regular grid sits naturally over the protected area.
For rods and small items
A rod or mast protects a cone of space beneath it, bounded by an angle from the vertical. Anything inside the cone is shielded. It suits simple structures and individual rooftop items such as a vent, an antenna or a small plant unit.
How does the rolling sphere method work?
The rolling sphere method is the most general of the three, and the easiest to picture. Take a solid sphere of a fixed radius and roll it over the structure from every direction: across the roof, against the walls, around the corners, up and over any mast or parapet. Wherever the surface of the sphere touches the building, a lightning strike could attach at that point. Wherever the sphere cannot reach, because an air termination holds it off, the surface beneath is protected.
Designing with the method means placing rods, masts and conductors so that the rolling sphere rests only on the air terminations and never on a part of the structure you need to protect. On a tall building the sphere touches the top edges and corners first, which is why terminations cluster there; lower down, a wall can be shielded by the roof edge above it if the sphere cannot curve in to reach it. For a single mast on flat ground you can size the protected zone with our rolling sphere calculator. The method makes no assumption about the shape of the building, which is what makes it the universal one: it works on a cube, a tower, a curved roof or a cluttered industrial structure equally well.
How does the mesh method work?
The mesh method protects a roof by covering it with a grid of conductors, the mesh, bonded into a closed pattern across the area to be protected and tied into the down conductors at the edges. The idea is that a strike to anywhere on the roof attaches to the nearest mesh conductor, and the current then divides between the conductors around it rather than concentrating in one. A roof covered by a sound mesh has no large gap where a strike could reach the surface between conductors.
This is the natural method for flat and gently pitched roofs, where a regular grid sits cleanly over the area and follows the edges and any rooftop features. It is less suited to steeply pitched or complex roofs, where a grid is awkward to lay and the rolling sphere or protection angle does the job better. In practice the mesh conductors are routed along ridges, edges and around plant, and any metal part on the roof that is large enough to take a strike is bonded into the mesh so it becomes part of the protected system rather than an unprotected target.
How does the protection angle method work?
The protection angle method places a vertical air termination, a rod or a mast, and treats the cone of space beneath its tip as protected. The cone is bounded by an angle measured from the vertical axis of the rod: anything that falls inside the cone is considered shielded from a direct strike, and anything outside it is not. A taller rod casts a wider cone in absolute terms, so a single mast can protect a surprisingly large area at its base, while a short rod protects only a small zone close around it.
This method suits simple shapes and individual items: a small building with a clean outline, or a single object on a larger roof such as a vent, an antenna, a chimney or a compact plant unit. For those isolated items the protection angle is often the most direct method, a rod beside the object sized so the object sits inside its cone. It becomes unwieldy on large or irregular structures, where the rolling sphere or mesh is cleaner, which is why it tends to be used for parts of a building rather than the whole of a complex one.
Choosing a method, and combining all three
The three methods are not a menu to pick one from. A real building usually uses all three at once, each applied to the part of the structure it suits best, all worked to the same protection level.
Rolling sphere for the whole and the awkward
Use it on any shape, and reach for it wherever the building is tall, complex or irregular and the other two do not fit cleanly. It is the method that checks exposed corners, edges and tall features the grid does not cover.
Mesh for the flat roof
Use it across the main roof area, where a regular grid follows the edges and ties cleanly into the down conductors. It is the efficient way to protect a large, gently pitched expanse that would take many rods to cover otherwise.
Protection angle for the small and simple
Use it to size masts protecting individual rooftop items, and on small, clean-outlined structures. It is the most direct method for a single object that needs its own rod.
A typical design on a flat-roofed building meshes the main roof, uses the protection angle to size the masts that shield rooftop plant and antennas, and then applies the rolling sphere to confirm that the exposed corners, parapet edges and any tall features are all covered. The methods overlap deliberately. What matters is the result they share: every surface a strike could reach is protected by at least one method, and all of them are worked to the same lightning protection level so the building meets one consistent class rather than a patchwork of stronger and weaker zones.
How the methods follow the lightning protection level
The single most useful thing to carry away is that all three methods are driven by one input, the lightning protection level, and all three tighten together as it becomes more demanding.
A lightning protection system is built to one of four classes, I to IV, and the class is decided by the lightning protection level (LPL I to IV) that the Part 3 design inherits from the Part 2 risk assessment. LPL I is the most demanding and LPL IV the least. The level is not chosen on taste; it is the output of the risk work, and it then sets the geometry of every air-termination method at once.
Smaller sphere
A more demanding level uses a smaller rolling sphere radius, which reaches into more places and forces more or better-placed air terminations across the structure.
Finer mesh
The same level uses a tighter grid, with less roof area between conductors, so a strike has a shorter path to the nearest mesh wire.
Narrower angle
The protection angle tightens, so each rod or mast shields a smaller cone and the structure needs more of them, or taller ones, to stay covered.
They all move the same way for the same reason: a more demanding level has to intercept a wider range of strikes, including the weaker, lower-current ones that a larger sphere, a coarser mesh or a wider angle would let through to the structure. The exact protection angle, mesh size and sphere radius for each level are set out in tables and a chart in the standard itself; the design has to be worked to those precise values once the level is known.
Air terminal lightning protection under NFPA 780 and AS 1768
Everything above is IEC 62305-3. The US and Australian standards place their air terminals by rules of their own, and none of the IEC methods or values carries across. The table sets the three side by side, each value cited to its own edition.
| Rule | IEC 62305-3:2024 | NFPA 780-2026 | AS 1768:2021 |
|---|---|---|---|
| What the standard calls the parts | Air-termination system: rods, catenary wires and mesh conductors | Strike termination devices: air terminals, masts, overhead ground wires and metal parts whose thickness reaches 3/16 in (4.8 mm) (3.3.42, 4.5.1.1, 4.5.1.4) | Air terminals: vertical rods, raised horizontal conductors or natural components such as metal roofs (Clause 3.2.1) |
| Methods allowed | Rolling sphere, mesh and protection angle (clause 5.2.2) | Roof placement rules, the angle method and the rolling sphere (4.7.1) | Rolling sphere only (Clause 3.2.3) |
| What sets the sphere radius | The lightning protection level (Table 2) | The type of structure: 150 ft (45 m) at most, 100 ft (30 m) for flammable vapors, explosives and watercraft (4.7.3.1.4, 7.3.2, 8.2.1, 10.3.1.1) | The protection level, with a sphere twice that radius on large flat surfaces (Table D.1) |
| Angle method | By height and class (Table 2) | Multiple-level roofs only: a one-to-two zone from a structure up to 25 ft (7.6 m), one-to-one up to 50 ft (15 m) (4.7.2.1, 4.7.2.3, 4.7.2.4) | Not offered for placement; Clause 3.2.3 specifies the rolling sphere |
| Flat roofs | Mesh size by class (Table 2) | No mesh method. Roofs over 50 ft (15 m) wide or long get interior devices no more than 50 ft (15 m) apart (4.6.5.1) | Corners first, the standard sphere on edges, the doubled sphere on flat areas; a surface is flat when nothing on it stands more than 300 mm proud (Clause D.3.2) |
| Edges and corners | Found by the chosen method | A device within 24 in (600 mm) of each corner and ridge end, then no more than 20 ft (6 m) apart, or 25 ft (7.6 m) for devices 24 in (600 mm) or taller (4.6.2.1 to 4.6.2.3) | Terminals on points and corners, then spacing from Equations D.3.3(1) to D.3.3(3) (Clauses 3.2.2.2, D.3.3) |
| Terminal height | Set by the geometry of the chosen method | Tip at least 10 in (254 mm) above what it protects; braced at half height or above once taller than 24 in (600 mm) (4.5.2.1, 4.5.2.2.2) | A rod at least 500 mm long, set back from the edge no more than half its height (Clause 3.2.2.2) |
| Size of a solid rod | Material and size tables (clause 5.6, Table 7) | Copper, with the rod size set by the class of materials, which follows the height (Tables 4.1.1.1.1, 4.1.1.1.2) | A minimum cross-section for the rod, read from Table 3.3 |
| Tall buildings | Flashes to the side (clause 5.2.3) | Above the striking distance the sphere reaches the walls (4.7.3.2) | Side strike protection above 60 m, or on the top 20 percent above 75 m (Clause 3.2.4) |
| Enhanced or ESE terminals | Not part of IEC 62305 | Not covered by the standard (1.1.3) | Outside its scope; placed as a plain rod to conform (Clause 3.2.2.2) |
A US air terminal follows the roof, not a level
NFPA 780 has no protection levels, so a lightning protection air terminal on a US job is placed by the Chapter 4 roof rules and checked with a sphere whose size follows the type of structure. Anything inside a zone of protection may carry devices but does not need them (4.5.1.6). Detail: NFPA 780 air terminal spacing and the NFPA 780 zone of protection.
An Australian air terminal follows the level
The Section 2 risk assessment gives the level and Table D.1 the sphere. Appendix D, which is normative, then works rod height into covered distance with three equations. An IEC mesh size or protection angle has no standing in an AS 1768:2021 design. Detail: AS 1768 air terminal placement.
Where air-termination design goes wrong
The methods are simple to state and easy to misapply. A few failures recur often enough to name, because each leaves a structure with a gap its paperwork does not show.
Forgetting the new rooftop item
A plant unit, antenna or solar array added after handover sits above the mesh and outside every protection angle, exposed. New rooftop equipment has to be brought back inside the protection, not left standing proud of it.
Stretching the protection angle too high
The angle method stops being valid above a certain height for a given level. Using it on a tall mast where only the rolling sphere applies leaves a zone that looks protected on paper but is not.
Mixing levels across one building
Working part of a structure to one level and part to another leaves a weakest link. Every method on a single building should be sized to the same lightning protection level so the whole meets one class.
Trusting the mesh on the wrong roof
A mesh is for flat and gently pitched roofs. On a steep or complex roof it leaves gaps a regular grid cannot follow, and the rolling sphere or protection angle should carry those areas instead.
Air termination in the wider standard
Positioning the air terminations is one step in a longer chain. The air termination catches the strike, the down conductors carry the current down the structure, and the earth termination dissipates it into the soil; the three together make up the external lightning protection system, described in IEC 62305-3. Getting the air termination right is what stops a strike reaching an unprotected surface in the first place, which is why the three methods on this page sit at the front of the design.
They also sit downstream of a decision made earlier. The lightning protection level that fixes the sphere radius, mesh size and protection angle is itself an output of the Part 2 risk assessment, which decides whether a structure needs protection at all and how strong it must be. And once the system is installed, the air terminations are checked at every periodic inspection, because a missing or displaced termination, or a new rooftop item left outside the protection, is one of the most common ways a sound design quietly stops being sound.
Questions answered
What are the air-termination methods in IEC 62305?
How does the rolling sphere method place air terminations?
What is the mesh method?
What is the protection angle method?
When is each air-termination method used?
How does the lightning protection level affect the air-termination methods?
Why does a higher lightning protection level use a smaller rolling sphere?
Are the exact sphere radius, mesh size and protection angle values in IEC 62305?
How is air terminal lightning protection placed under NFPA 780 and AS 1768?
The tolerable risk in IEC 62305-2 is not a fixed constant.
Clause 7.3 NOTE 1 gives RT = 1 × 10⁻⁵ per year as a
representative value of tolerable risk and adds that another value may be set once
the case has been investigated in detail. The standard's introduction then lets national or local regulations fix
RT, the tolerable frequency of damage FT
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