IEC 62305 Lightning Protection Levels — When You Need What
Four protection levels (I-IV), rolling sphere radii from 20m to 60m, and the risk assessment that determines which level applies. Decoded in 5 minutes.
February 26, 2026
The Four Protection Levels
IEC 62305-1 defines four Lightning Protection Levels (LPL), each with increasing capture efficiency:
| Parameter | LPL I | LPL II | LPL III | LPL IV |
|---|---|---|---|---|
| Rolling sphere radius | 20m | 30m | 45m | 60m |
| Mesh size | 5m × 5m | 10m × 10m | 15m × 15m | 20m × 20m |
| Protection angle | 25° (at 20m height) | 35° | 45° | 55° |
| Min peak current captured | 3 kA | 5 kA | 10 kA | 16 kA |
| Capture efficiency | 99% | 97% | 91% | 84% |
| Conductor cross-section (Cu) | 50mm² | 25mm² | 16mm² | 16mm² |
LPL I captures 99% of all lightning strikes. LPL IV captures only 84%. The 16% of strikes that bypass LPL IV are low-current events (<16 kA) that are less likely to cause structural damage but can still destroy electronics.
How to Determine the Required Level
IEC 62305-2 prescribes a risk assessment based on four risk components:
- R1: Risk of loss of human life
- R2: Risk of loss of service to the public
- R3: Risk of loss of cultural heritage
- R4: Risk of loss of economic value
For each risk component:
R = N × P × L
Where:
- N = number of dangerous events per year (based on flash density, building dimensions, environment)
- P = probability of damage (depends on protection measures)
- L = consequential loss (ratio of expected loss)
If R exceeds the tolerable risk (R_T), lightning protection is required. The LPL is selected to reduce R below R_T.
Simplified Selection Guide
For projects without a full risk assessment:
| Building Type | Typical LPL |
|---|---|
| Hospitals, data centres, emergency services | LPL I |
| Schools, office towers (>30m), public assembly | LPL II |
| Commercial buildings, industrial facilities | LPL III |
| Residential, small structures | LPL IV |
| Explosive/flammable storage | LPL I (mandatory) |
Rolling Sphere Method
The rolling sphere is the primary design tool. Imagine a sphere of radius R (per LPL) rolling over the building:
- Where the sphere touches the building surface: that surface needs protection
- Where the sphere rests on air terminals or natural components: those points become the lightning capture zone
- Areas in the "shadow" under the sphere between air terminals: protected zone
For LPL I with R = 20m, air terminals must be closely spaced. For LPL IV with R = 60m, fewer air terminals are needed because the larger sphere "bridges" larger gaps.
Down Conductors
| LPL | Minimum Down Conductors | Maximum Spacing |
|---|---|---|
| I | As required by design | 10m |
| II | As required | 10m |
| III | As required | 15m |
| IV | As required | 20m |
Down conductors must be routed as directly as possible from the air termination to the earth termination, avoiding sharp bends (<90° preferred, never <60°).
Earth Termination
All LPLs require earth resistance ≤ 10Ω (lower values required for sensitive installations). Type A (vertical/horizontal electrodes) or Type B (ring earth electrode around the building) arrangements are specified per IEC 62305-3.
Design your system: Calculate rolling sphere coverage and protection zones 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.
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