Challenge: Design Lightning Protection for a Flat-Roof Data Centre
A 60m × 40m flat-roof data centre with rooftop HVAC units. LPL I required. How many air terminals, what mesh size, and where do the down conductors go?
February 26, 2026
The Building
A single-storey data centre:
- Footprint: 60m × 40m, flat roof at 6m height
- Rooftop equipment: 4 large HVAC units (3m tall, located centrally), cable trays, satellite dishes
- Required protection level: LPL I (highest — the facility houses critical data infrastructure)
- Soil: Clay, measured earth resistance target: <1Ω
The Challenge
Design the external lightning protection system:
- Air termination layout (mesh and/or rods)
- Down conductor count and spacing
- Earth termination arrangement
- Protection of rooftop equipment
The Solution
Air Termination: Mesh Method
For LPL I, the mesh size is 5m × 5m per IEC 62305-3 Table 2.
Roof dimensions: 60m × 40m
Conductors required:
- Long axis: 60m / 5m = 12 spans → 13 parallel conductors (running 40m each)
- Short axis: 40m / 5m = 8 spans → 9 parallel conductors (running 60m each)
Total roof conductor: (13 × 40m) + (9 × 60m) = 520 + 540 = 1,060m of conductor
Material: 50mm² copper tape (LPL I requirement) or 8mm diameter copper rod.
Rolling Sphere Verification
LPL I rolling sphere radius: 20m
For a 6m high flat roof, the rolling sphere analysis:
- A 20m radius sphere resting on the roof has its centre at 20m above the surface
- The sphere touches the roof at the contact point
- Maximum height of unprotected equipment above the mesh: where the sphere would touch between mesh conductors
For a 5m × 5m mesh, the maximum "sag" of the sphere between conductors:
h_sag = R − √(R² − (d/2)²)
Where R = 20m and d = 5m (diagonal of mesh = √(5²+5²) = 7.07m):
h_sag = 20 − √(400 − 12.5) = 20 − 19.69 = 0.31m
Any object less than 0.31m above the mesh plane is protected. Objects taller than 0.31m (like HVAC units at 3m) need additional protection.
HVAC Unit Protection
Each HVAC unit protrudes 3m above the roof. Options:
Option A: Vertical air terminals on the HVAC units Place 1m tall air terminals at each corner of each HVAC unit. The rolling sphere analysis verifies coverage:
- Air terminal tip at 3+1 = 4m above roof
- Protection angle for LPL I at 4m above reference plane: per IEC 62305-3 Table 3, protection angle = 71° at 2m effective height (measured from the HVAC top)
- Each 1m rod protects a circle of radius 1m × tan(71°) ≈ 2.9m around its base
With air terminals at all four corners of each HVAC unit, the unit is fully protected.
Option B: Elevated catenary conductors String conductors above the HVAC units, elevated on masts. More effective for grouped equipment.
Down Conductors
LPL I requires maximum spacing of 10m between down conductors.
Building perimeter: 2×(60+40) = 200m
Minimum down conductors: 200m / 10m = 20 down conductors
Distribution: 7 per long side (60m ÷ 10m + 1), 5 per short side (40m ÷ 10m + 1), minus shared corners = 20 total.
Each down conductor: 50mm² copper or 8mm diameter copper rod, routed externally on the building facade, as direct a path as possible from roof to earth.
Earth Termination
For <1Ω target in clay soil (typical resistivity: 40–100 Ω·m):
Type B ring electrode around the building perimeter: 200m of 50mm² bare copper buried at 0.5m depth, 1m from the building foundation.
Supplementary vertical electrodes at each down conductor: 3m copper-clad steel rods driven at each of the 20 down conductor positions.
Estimated resistance: R_ring ≈ ρ/(2π × √A) where A = area = 2,400m² R_ring ≈ 80/(2π × 49) = 0.26Ω — meets <1Ω target.
Equipotential Bonding
Per IEC 62305-3 Clause 6.2, all metallic services entering the building must be bonded to the lightning protection system at the point of entry: water, gas, telecoms, power supply cable screens.
Design your system: Calculate rolling sphere coverage and earth resistance with the Lightning Protection Calculator.
Frequently Asked Questions
What is protection coordination?
Protection coordination ensures that the protective device closest to a fault operates first, minimizing the affected area. This requires analyzing time-current curves (TCC) for all devices in series per IEEE 242.
How do I select between MCB and fuse?
MCBs offer adjustable trip settings and reusability but cost more. Fuses are cheaper, faster at high fault currents, and better for motor starting (withstand inrush). Choice depends on application per IEC 60947-2.
Related Articles
- Lightning - Interactive calculator with standards compliance
Try It Yourself
Run the calculations from this article using our free calculators: