Calculate the protection radius (Rp) of an ESE lightning arrester with the NF C 17-102 formula. Enter the protection level, the arrester's ΔT and the mast height, then check whether a roof is covered, the height a single arrester would need, and how many arresters a larger roof needs.
ESE systems follow NF C 17-102 and similar national standards; they are not covered by IEC 62305 or NFPA 780. Check that the authority having jurisdiction accepts ESE before designing with it.
| Height h | Level I | Level II | Level III | Level IV |
|---|
Rp(h) = √( 2·r·h − h² + ΔL·(2·r + ΔL) ) for h ≥ 5 m
Rp(h) = h × Rp(5) ÷ 5 for 2 m ≤ h < 5 m
Example: Level I, ΔT = 60 µs, h = 5 m: Rp = √(2 × 20 × 5 − 25 + 60 × (40 + 60)) = √6175 = 78.6 m. Raising the mast above 5 m adds very little: at h = 10 m Rp is 79.4 m. The ESE efficiency and the protection level matter far more than extra height.
Rp in metres for common ΔT values:
| ΔT | Level | h = 2 m | h = 3 m | h = 4 m | h = 5 m | h = 6 m | h = 8 m | h = 10 m | h = 15 m | h = 20 m |
|---|---|---|---|---|---|---|---|---|---|---|
| 25 µs | Level I | 17 | 25 | 34 | 42 | 43 | 43 | 44 | 45 | 45 |
| 25 µs | Level II | 20 | 29 | 39 | 49 | 49 | 50 | 51 | 53 | 54 |
| 25 µs | Level III | 23 | 34 | 46 | 57 | 58 | 59 | 61 | 63 | 65 |
| 25 µs | Level IV | 26 | 39 | 52 | 65 | 66 | 67 | 69 | 72 | 75 |
| 45 µs | Level I | 25 | 38 | 51 | 63 | 63 | 64 | 64 | 65 | 65 |
| 45 µs | Level II | 28 | 42 | 57 | 71 | 71 | 72 | 72 | 73 | 74 |
| 45 µs | Level III | 32 | 48 | 64 | 81 | 81 | 82 | 83 | 85 | 86 |
| 45 µs | Level IV | 36 | 54 | 72 | 89 | 90 | 91 | 92 | 95 | 97 |
| 60 µs | Level I | 31 | 47 | 63 | 79 | 79 | 79 | 79 | 80 | 80 |
| 60 µs | Level II | 35 | 52 | 69 | 86 | 87 | 87 | 88 | 89 | 89 |
| 60 µs | Level III | 39 | 58 | 78 | 97 | 97 | 98 | 99 | 101 | 102 |
| 60 µs | Level IV | 43 | 64 | 85 | 107 | 107 | 108 | 109 | 111 | 113 |
The protection level comes from a lightning risk assessment, to IEC 62305-2 or the method in NF C 17-102 Annex A. It depends on the lightning density at the site, the building size and use, its contents and the consequences of damage. Level I is the most stringent (hospitals, fuel and explosive stores, critical facilities); Level IV the least.
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Use the NF C 17-102 formula Rp = √(2rh − h² + ΔL(2r + ΔL)) for heights of 5 m or more, where r is 20, 30, 45 or 60 m for protection levels I to IV, h is the tip height above the protected surface and ΔL in metres equals the ESE efficiency ΔT in microseconds. Below 5 m, Rp = h × Rp(5) ÷ 5.
ΔT is how much earlier, in microseconds, the ESE terminal launches its upward leader compared with a simple rod under the same conditions. It is measured in a high-voltage laboratory and stated on the test certificate. NF C 17-102 caps the value used for design at 60 µs.
About 79 m for Level I, 87 m for Level II, 97 m for Level III and 107 m for Level IV.
Up to 5 m it does strongly, because below 5 m the radius grows in proportion to height. Above 5 m the gain is small: for Level I and 60 µs the radius only grows from 79 m at 5 m to 80 m at 20 m.
No. ESE terminals are designed to NF C 17-102, UNE 21186 and similar national standards, and are widely used in Europe, the Middle East and Asia, but IEC 62305 and NFPA 780 do not recognise an enlarged protection radius. Always confirm that the local authority accepts ESE.
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