AS 1768 air terminal placement, step by step
Four sphere radii, three placement steps and three spacing equations decide where every rod goes on an AS 1768:2021 design. Here is each rule with its clause, the standard's own worked example, and the mistakes reviewers find most often.
AS 1768 air terminal placement follows one method in the 2021 edition, the rolling sphere of Appendix D: roll a sphere of 20, 30, 45 or 60 m radius, set by the lightning protection level, over the structure and put an air terminal wherever it would touch. Large flat roofs are checked with a sphere of twice that radius.
AS 1768:2021, published by Standards Australia on 3 December 2021, sets the air terminal rules in Clause 3.2 and the placement method in Appendix D. Appendix D is normative, so it is a requirement of the standard, not guidance. Its three equations turn a rod height into a covered distance and a maximum spacing.
Every value on this page comes from the 2021 edition and carries its clause. The standard is copyright, and you need your own copy from Standards Australia to design or certify an installation. Lumex is independent of Standards Australia.
How does AS 1768:2021 place air terminals?
By the rolling sphere method alone. Clause 3.2.3 names it, Clause 3.2.2.2(b) makes the check mandatory for dedicated terminals, and Appendix D sets out how to do it.
The level comes first
The protection level is an output of the Section 2 risk assessment. Appendix D notes that it often lands on LPL I for hazardous, sensitive or critical sites and on LPL III for most ordinary buildings that need an LPS.
The sphere stands for the strike
The sphere radius is a striking distance, taken from Equation D.3.1 and the smallest stroke current the level must intercept. Any part of the building the sphere can touch is exposed and needs a terminal.
The check is not optional
Wherever dedicated air terminals are used, the rolling sphere check against Appendix D is a requirement. Natural components used as terminals are checked for sufficiency the same way (Clause 3.2.1).
The design passes when the sphere, rolled over the structure from every direction, rests only on air terminals and never on the building itself (Clause D.3.1). Weaker strokes than the level's minimum current can still slip through, which is why a higher level uses a smaller sphere.
What rolling sphere radius does AS 1768 use?
Four standard radii from 20 m to 60 m, each paired with a doubled radius for large flat surfaces. Every value below is AS 1768:2021.
| Protection level | Standard radius a (Table D.1) | Increased radius ai for flat surfaces (Table D.1) | Interception efficiency (Table 3.1) |
|---|---|---|---|
| LPL I | 20 m | 40 m | 99 % |
| LPL II | 30 m | 60 m | 97 % |
| LPL III | 45 m | 90 m | 91 % |
| LPL IV | 60 m | 120 m | 84 % |
The increased radius is always twice the standard one (Clause D.3.3). Table D.1 also gives the smallest stroke current each level is designed to intercept, from 2.9 kA at LPL I to 15.7 kA at LPL IV. The body text of Clause D.3.1 rounds the LPL III current to 10 kA while Table D.1 prints 10.1 kA, and the radius is 45 m either way. The efficiencies in Table 3.1 describe how much of the stroke current population each level is designed to intercept. Appendix B prints a separate set of LPS efficiencies for the risk calculation, so do not mix the two columns up.
What counts as an air terminal
Under AS 1768:2021 Clause 3.2.1 an air terminal can take three forms: an upright rod, a horizontal conductor raised above the roof, or a conductive part of the building itself. The standard calls the last kind a natural component, and gives metal roofs, parapet railings and poles as examples.
Dedicated terminals go on the parts of the building most likely to be struck, which are usually its points and corners (Clause 3.2.2.2(a)). A rod must be at least 500 mm long, stand on or above the part it protects, and sit back from the edge by no more than half its own height. A raised horizontal conductor must run directly over the part it protects. Conductors at roof level must link every terminal together so the current has several paths to earth, and they must be sized to carry the full lightning current.
Natural components are covered by Clause 3.2.2.3. Where the top of a structure is conductive and large enough, it must be bonded to the LPS and becomes part of the air terminal network. Sheet metal cladding, ornamental metalwork, handrails, pipes, tanks and parapets can all do the job, provided they are electrically continuous (Clause 3.1) and meet Table 3.3. For air terminals, Table 3.3 sets a minimum section of 35 mm², with a 25 mm by 3 mm strip or an 8 mm solid rod as typical examples. A reinforced concrete roof does not qualify on its own merits: it needs metalwork of that kind or dedicated terminals.
On tall buildings, the upper runs of dedicated downconductors are treated as part of the air terminal network, so they must sit as close as practical to the outer vertical corners (Clause 3.2.2.2).
The three placement steps
A sphere of constant radius treats a flat roof as if it were as exposed as a corner. Appendix D corrects for that with a mandatory three step procedure.
Points and corners
Start by putting air terminals on every point and corner of the structure, the places field data show are struck most.
Edges, standard radius
Roll the standard sphere, radius a, along the roof edges to find where extra terminals are needed between the corners.
Flat areas, doubled radius
Roll the larger sphere, radius ai, over big flat areas to find whether the middle of the roof needs terminals of its own.
Clause D.3.2 defines a flat surface as one with no projection taller than 300 mm. Clause D.3.2 explains that large flat roofs are struck less often than corners and edges, and the notes to Clause 3.2.3 give the field data: more than nine in ten recorded strike damage cases were on upper parts such as apexes, ridge ends and outer roof corners, while flat surfaces account for a small share. The doubled radius in step (c) is how AS 1768:2021 turns that field experience into fewer terminals in the middle of a roof.
How far apart air terminals can be
Clause D.3.3 gives three equations, all in metres. Equation D.3.3(1) gives the horizontal distance r along an edge that a terminal of height h protects: the square root of 2ah minus h squared, using the standard radius a. Equation D.3.3(2) is the same geometry with the doubled radius ai, and gives the distance ri a terminal protects across a large plane surface.
Equation D.3.3(3) turns those distances into array spacing. In a regular grid, vertical rods must be no further apart than ri times the square root of 2, and raised horizontal conductors no further apart than 2ri. Along an edge, the spacing between rods is twice r, which is how the standard's worked example uses it.
Table D.2 works these equations for rods and conductors 0.5 m, 1 m and 2 m high at each level. Two of its LPL III figures show the pattern: a 1 m terminal protects 9.4 m of edge with rods up to 18.9 m apart, and covers 13.4 m of flat roof with a rod grid up to 18.9 m apart. Doubling a rod's height does not double its cover: for short rods, the covered distance grows roughly with the square root of the height.
Read Table D.2 with care. Note 2 to Clause D.3.3 presents it as information, its footnote sends you to the equations for any other height, and the equations carry the requirement. On printed page 80 of AS 1768:2021, the LPL I rows of the three flat roof columns (ri, dvr and dhc) repeat the LPL II figures exactly. For a 1 m rod, for example, both levels show 10.9 m of flat roof cover. LPL I uses a 40 m increased radius against 60 m for LPL II, so Equation D.3.3(2) must give LPL I the shorter distance. For LPL I flat roof values, work the equation for your rod height instead of reading the table.
To sketch one rod quickly, our rolling sphere calculator works the same geometry as Equation D.3.3(1) and has AS 1768 presets for each Table D.1 radius and its doubled radius for ri. It does not apply the three steps, the setback rule or side strike protection, so it is a sketching aid only.
Worked example: a flat roof at LPL III
Appendix D walks one flat roofed building, 70 m by 50 m in plan, through every step at LPL III. These are the standard's own figures, summarised.
Corners and edges
Rods 1 m high go on the four corners. Equation D.3.3(1) with a 45 m sphere gives 9.4 m of protected edge, so rods may be up to 18.8 m apart along an edge (Table D.2 prints 18.9 m for the same rod). On the 70 m side that takes three more rods, evenly spaced at 17.5 m, and a final roll of the 45 m sphere confirms it.
The middle of the roof
The 90 m sphere is then rolled over the flat area. The solution is fourteen 1 m rods around the edges plus four 1.25 m rods in the field, which reach 15 m each under Equation D.3.3(2), and the whole roof is protected.
Fewer, taller rods
An alternative keeps the fourteen edge rods and uses just two field rods, each 1.6 m high, reaching 16.9 m. Taller rods trade quantity for height.
Railings as the edge terminals
With 1 m railings along all four edges, each covers 13.4 m of roof, so conductors may be 26.8 m apart. The railings are 70 m apart, so two more raised conductors 1 m high are added, or two 1.5 m rods reaching 16.4 m.
The note to Figure D.2 makes a practical point: in this example, metal railings along the roof edges, bonded to the downconductors, would protect every corner and edge without dedicated rods. Figure D.5 works that case through.
Does AS 1768:2021 allow the protective angle or mesh method?
Not for ordinary structures. Clause 3.2.3 specifies the rolling sphere method of Appendix D for placing air terminals, and AS 1768:2021 offers no protective angle table and no general mesh method beside it. If you have designed to IEC 62305-3, which offers all three methods, expect to redo the geometry with the sphere alone.
Angles and meshes do appear, but only in narrow places. Informative Appendix L, on high voltage power systems, describes the shielding angle used for overhead earth wires on power lines and lists fixed angle methods among those IEEE 998 uses for substations. Normative Appendix J allows a surface or buried mesh of 3 m to 8 m over explosives stores that are below ground or under more than 500 mm of soil (Clauses J.5.3 and J.5.4). Neither is a general alternative for a building.
Clause 3.2.2.2 also closes a common door. Air terminals that claim enhanced performance, such as early streamer emission devices, are outside the scope of the standard. To conform, any terminal must be placed by the same rolling sphere rules as a plain rod, whatever coverage its maker claims.
Side strike protection on tall structures
Above 60 m, the sides of a building need air terminals too. The band that needs them depends on the total height.
| Structure height | Faces needing an air terminal network | Source |
|---|---|---|
| 60 m or less | Clause 3.2.4 sets bands only for structures above 60 m; the rolling sphere check of Appendix D still applies | AS 1768:2021 3.2.4 |
| Between 60 m and 75 m | The part of the structure above 60 m | AS 1768:2021 3.2.4(a) |
| Over 75 m | The top 20 % of the structure's height | AS 1768:2021 3.2.4(b) |
Within that band, the design must take protruding parts such as corners, facade edges, balconies and viewing platforms into account. Note 1 to Clause 3.2.4 explains that this band may cover less of the facade than a strict rolling sphere would, and that metal cladding can provide it. Where there is doubt about whether external metal cladding is combustible, Note 2 recommends bonding it to the LPS at any height.
Downconductors and bonding, in brief
Air terminals only work if the current reaches earth. Under AS 1768:2021 Clause 3.3.2, downconductors run around the outside perimeter at no more than 20 m apart, measured the way a taut string follows the building, and every LPS has at least two. Lift shafts and service ducts are never acceptable routes for them. The standard recommends that they take the most direct route to earth, be evenly spaced and avoid places where people gather. Clause 3.3.3 lets structural steel, steel reinforcement, suitable metal cladding and curtain walls serve as natural downconductors.
Clause 3.6 then deals with side flashing from the LPS to nearby metal. Either bond that metal to the LPS (Clause 3.6.2), which as a minimum covers prominent roof plant, metal that could endanger people and metallic services at their point of entry, or keep it isolated by a separation distance worked from Equation 3.6.3 and the factors in Table 3.2. For the earthing that the downconductors feed, see the AS 1768 earthing guide.
Structures where the standard sets the level for you
For some structures AS 1768:2021 fixes or suggests the protection level, and therefore the sphere radius, or settles the air terminal question directly.
Explosives stores
Always LPL I, a 20 m sphere, by roof protection, masts or a suspended network, or one rod for small magazines. Masts and rods must stand at least 2 m clear of the structure, and the structure itself must not form part of the LPS unless that cannot ignite the contents.
Steel tanks under a non-metallic roof
These tanks are not self-protecting, so their air terminals must be placed under Section 3 and Appendix D at LPL I.
Houses and small buildings
A mostly metal roof can be the air terminal network. For a mostly non-metallic roof, informative Appendix I recommends a network and says LPL IV is generally enough.
Antennas on a protected building
An antenna inside the zone of protection needs nothing more. For one outside it, informative Appendix I recommends bonding it to the LPS, with extra bonding and SPDs where a tower carries it.
Common air terminal mistakes under AS 1768
Using the doubled radius on edges. The increased radius ai applies only to large flat areas in step (c). Edges and corners are always checked with the standard radius a (Clause D.3.2).
Calling a roof flat when it is not. Plant, parapets or skylights taller than 300 mm mean the surface is not flat for Appendix D, and each projection needs its own check.
Rods set too far back. A rod more than half its own height from the edge it protects fails Clause 3.2.2.2(a)(ii), even if a sphere drawing looks acceptable.
Choosing the level by habit. The sphere radius follows the protection level from the risk assessment (Clause D.2), or the level Appendix J fixes. Picking LPL III because most jobs use it, with no assessment behind it, leaves the design without a basis.
Crediting enhanced terminals. Coverage claimed for early streamer emission or similar devices counts for nothing under Clause 3.2.2.2. Place them as ordinary rods.
Unbonded roof metal. Prominent roof plant must be bonded to the LPS (Clause 3.6.2(a)), and conductive upper parts large enough to act as terminals must be bonded in (Clause 3.2.2.3).
Forgetting the tall building band. Above 60 m, a roof that passes the sphere check is not enough; the side strike band of Clause 3.2.4 still applies.
Carrying rules across standards. A protection angle or mesh size from IEC 62305-3, or a zone of protection rule from NFPA 780, has no standing in an AS 1768:2021 design. For how the US standard does it, see NFPA 780 air terminal spacing.
The level that sets the sphere
Every placement in Appendix D starts from a protection level, and that level comes from the risk assessment. Lumex runs the AS 1768:2021 assessment: it judges R1 to R4 each against its own tolerable value, then searches for the least protection that clears all four under Clause C.3.4.2, working up from LPL IV to LPL I and adding surge protection where needed. The result names the LPL, which gives you the sphere radius in Table D.1, and the report shows the working behind every figure. If you do not yet know your level, run the Section 2 assessment in Lumex first and size the sphere from its result.
Lumex does not design the air terminal layout, draw the sphere or certify the installation. That remains the designer's work against the published standard. It sets a review date 24 months out for an AS 1768 assessment, in line with the inspection interval of Clause 5.4.4, and sends a reminder when it falls due.
Read more: how Lumex runs the AS 1768:2021 risk assessment, which lightning standard applies to your project, AS 1768:2021 explained section by section, what changed from AS/NZS 1768:2007, earthing under AS 1768, AS 1768 surge protection and the lightning protection standards used in Australia.
Questions answered
What rolling sphere radius does AS 1768 use?
Does AS 1768:2021 allow the protective angle method?
How are air terminals placed on a flat roof under AS 1768?
What is the minimum height of an air terminal under AS 1768?
When does AS 1768 require side strike protection?
Can a metal roof or parapet rail act as the air terminal?
Are ESE or enhanced air terminals allowed under AS 1768:2021?
How far apart must downconductors be under AS 1768?
Lumex computes the method of the standard you choose, IEC 62305-2:2024, AS 1768:2021 or NFPA 780-2026, and shows the working. It does not certify a structure. You may not issue or submit a Lumex output until a competent person, qualified where the structure is located, has reviewed the inputs and the result and signed it.
AS 1768:2021 Table 2.1 sets a tolerable value for each type of loss, and the owner chooses the one for economic loss. Every AS 1768 assessment in Lumex states the values that applied.
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