Design Memo
CCC-DM-2026-041

Lightning Protection for Commercial Buildings

Section 1

What You Need to Know

Lightning strikes kill people and destroy equipment. AS 1768:2021 is the Australian Standard that tells you when a building needs a lightning protection system (LPS) and how to design one. The standard uses a risk assessment to decide if protection is needed. If your calculated risk hits or exceeds the tolerable limit, you must install an LPS. Most commercial buildings in areas with frequent storms will need one.

Section 2

The Rules

  • A risk assessment under AS 1768:2021 determines whether an LPS is required. If the risk ratio is 1.0 or greater, you must install protection (AS 1768:2021, Cl 3)
  • Ground flash density (Ng) is the starting point - it measures lightning strikes per km² per year for your location (AS 1768:2021, Cl 3.2 / BOM maps)
  • Level III protection (45 m rolling sphere, 15 m × 15 m mesh) is the standard choice for most commercial buildings, giving 90% interception efficiency (AS 1768:2021, Table 4.1)
  • Down conductors must be spaced no more than 20 m apart and take the most direct path to ground (AS 1768:2021, Cl 4.3)
  • Earth resistance must not exceed 10 ohm at each earth electrode (AS 1768:2021, Cl 4.4)
  • Surge protective devices (SPDs) are required at the point of entry for all conductive services - power, comms, data, CCTV, access control, and security (AS 1768:2021, Cl 5)
  • The lightning protection earth must be bonded to the mains protective earth for equipotential bonding (AS 1768:2021, Cl 4.5)
Section 3

What This Means in Practice

Take a three-storey commercial office in Brisbane (Ng around 4 flashes per km² per year). You run the risk assessment using the building's footprint, height, surrounding environment, and connected services. The formula compares your calculated risk against the tolerable level. In this case, the risk ratio will almost certainly exceed 1.0, so an LPS is mandatory.

You select Level III protection. That means a 45 m rolling sphere radius. Picture a ball with a 45 m radius rolling over the building. Any surface the ball touches is exposed and needs an air terminal. Flat roofs are easy - run a mesh of conductors at 15 m × 15 m spacing. Rooftop plant, antennas, and metal handrails all need bonding into the system.

Down conductors run from the roof mesh to the earthing system below. Space them no more than 20 m apart around the building perimeter. If the building has reinforced concrete columns, the rebar can serve as natural down conductors. But the rebar must overlap by at least 20 times its diameter at every joint, and the total resistance from the air terminal to the earth electrode must stay below 0.25 ohm.

At the switchboard, SPDs protect against surges entering through power cables. Every other conductive service - data, comms, CCTV, security, access control - also needs an SPD at the point of entry. Without these, a nearby strike can send a transient voltage straight into your electronics.

Section 4

Key Design Decisions

1

Does the Building Need an LPS?

Run the risk assessment first. Do not assume every building needs one. The assessment considers ground flash density, collection area, building contents, occupancy type, and connected services. A single-storey warehouse in a low-Ng area may not need protection. A multi-storey office in a storm-prone region almost certainly will.

Trade-off: The risk assessment costs $2,000–5,000 in engineering time. Skipping it means guessing - and guessing wrong means either wasted money or an unprotected building.
2

Which Protection Level?

Level III suits most commercial buildings. Level I or II is needed for buildings with sensitive contents (data centres, hospitals, heritage buildings). Level IV covers basic protection for low-risk structures.

Trade-off: Level I uses a 5 m × 5 m mesh and 20 m rolling sphere - roughly three times the conductor length of Level III. Specify what the risk assessment calls for, not the most expensive option.
3

Natural vs. Dedicated Down Conductors

Reinforced concrete columns can serve as natural down conductors, saving material and avoiding visible conductors on the facade. But the rebar must have tested electrical continuity - overlap joints of at least 20 times the bar diameter, and total resistance below 0.25 ohm.

Trade-off: Using rebar saves installation cost but requires testing during construction. If the rebar joints are not properly connected, you need dedicated copper or aluminium down conductors at $80–150 per metre installed.
4

SPD Coordination

Install SPDs at every service entry point. Use two-port SPDs with separate input and output connections and series impedance for best protection. Coordinate SPD ratings with the selected protection level.

Trade-off: A full SPD installation across power, data, comms, security, and CCTV costs $5,000–15,000 depending on the number of services. Without them, a single lightning event can destroy tens of thousands of dollars in equipment.
Section 5

Who Needs to Know What

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Section 6

References

  1. AS 1768:2021, Lightning protection
  2. IEC 62305 series, Protection against lightning
  3. AS/NZS 3000:2018, Electrical installations (Wiring Rules)
  4. National Construction Code 2022, references to AS 1768 for lightning protection provisions
  5. Bureau of Meteorology, Ground Flash Density Maps for Australia

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