Basis of calculation: IEC 62305-3:2024 (Ed. 3.0)
| LPS class | II |
| Method | Mesh |
| Maximum mesh size | 10 m × 10 m |
Mesh conductors on the roof perimeter and ridges; no metal installation outside the protected volume.
One dimension of a mesh cell may be increased by up to +25 % provided the cell perimeter is not increased (Table 2 provision).
| Perimeter | 128 m |
| Typical spacing | 10 m |
| Required down conductors | 13 |
Fit a test joint on each of the 13 down conductors at its connection to the earth termination, openable for measurement (§5.3.6).
| k_i (class) | 0.06 |
| k_c (current division) | 0.44 — 13 down conductor(s) |
| k_m (material) | 1 — Air |
| Length l | 20 m |
| Point considered | 20 m above ground |
| Per-level k_c | level 1: 0.337, level 2: 0.177 |
| Required separation s (general) | 0.309 m |
| Simplified comparison | 0.528 m (general saves 42 %) |
General approach assumes all down conductors interconnected by ring conductors at the stated spacing (§5.3.4, Figure B.3).
The segment sum runs to the earth termination (bonding at ground level only — conservative). Where the internal installation is bonded at a ring level, only the segments above that level apply (Figure B.3, cases a–e).
Every internal metal installation closer than s to a down conductor must be bonded or the distance increased.
For an LPS installed at high elevations an altitude correction applies (§6.3.2, per IEC 60071-2).
| Arrangement | Type B (ring electrode) |
| Soil resistivity ρ | 1500 Ω·m |
| Minimum length l_1 | 19.0 m |
| Ring requirement | mean radius of enclosed area r_e ≥ 19.0 m |
| Effective ring radius r_e | 18.6 m (from footprint, ring ≈ 1 m out) |
| Ring criterion | not met — additional electrodes required |
| Additional electrodes | 13 × horizontal ≥ 0.4 m, or vertical ≥ 0.2 m |
The minimum lengths may be disregarded where the earth-termination resistance achieved is below 10 Ω (measured at low frequency).
Ring in contact with the soil for at least 80 % of its length, buried ≥ 0,5 m deep, about 1 m from the external walls (§5.4.3).
Connect the additional electrodes to the ring where the down conductors connect, as equidistantly as possible.
| LPS conductor material | Copper / tin-plated copper |
| Solid tape, minimum | 50 mm² |
| Solid round, minimum | 50 mm² |
| Stranded, minimum | 50 mm² |
| Air-termination rods | 176 mm² solid round |
| Earth-electrode material | Copper / tin-plated copper |
| Earth rod (solid round) | Ø 15 mm |
| Earth rod (tubular) | Ø 20 mm |
| Buried earth conductor | 50 mm² |
| Earth plate | 500 × 500 mm |
All LPS components (conductors, rods, electrodes, clamps) shall meet IEC 62561-2 mechanical/electrical/corrosion requirements (Table 7/8 note a).
Air-termination rods where mechanical stress such as wind loading is not critical may be reduced to 9,5 mm diameter, 1 m long (Table 7 note c).
Copper solid round 50 mm² (8 mm Ø) may be reduced to 28 mm² (6 mm Ø) where mechanical strength is not essential; reduce fastener spacing accordingly (Table 7 note b).
A buried copper earth electrode shall not be used in contact with the structure's steel members, pipes or piles — corrosion (Table 8 note g).
| Bonding conductor material | Copper |
| Bars ↔ bars / bar ↔ earth termination | 16 mm² (Table 9) |
| Internal installations ↔ bonding bar | 6 mm² (Table 10) |
Install the main bonding bar in the basement or at ground level, accessible for inspection, and connect it to the earth-termination system (§6.2.2 a).
Structure length 40 m exceeds 20 m — use a ring bonding bar or several interconnected bonding bars (§6.2.2 a).
Bond additionally at every point where the separation distance s of the section above cannot be kept (§6.2.2 b). Keep bonding connections as direct and straight as possible.
Live conductors of internal systems are bonded via SPDs — coordinate per IEC 62305-4.
| People within 3 m of down conductors | yes (assumed) |
| Assessment | Protection measures required |
| Touch-voltage measures | Insulate exposed down conductors to 100 kV, 1,2/50 µs impulse withstand (e.g. ≥ 3 mm cross-linked polyethylene), or apply physical restrictions / warning notices |
| Step-voltage measures | Equipotentialize with a meshed earth-termination system, or apply physical restrictions / warning notices within 3 m of the down conductors |
The insulation withstand voltage should be considered in wet conditions (§8.1). Signage shall conform to ISO 3864-1.
Alternative exemption (§8.1 c / §8.2 c): a measured contact resistance of the soil surface layer within 3 m of the down conductors of at least 100 kΩ (over a 0,02 m² footprint) also brings the hazard to a tolerable level — a 5 cm layer of insulating material, e.g. asphalt, generally achieves this.
| Site environment | Sensitive internal systems / high occupancy |
| Visual inspection | every year |
| Complete inspection (incl. electrical testing) | every year |
Also inspect: during construction (components that become concealed), after LPS installation, after alterations or repairs, and after a known or suspected lightning strike to the structure (§7.4).
Periodic checks: deterioration and corrosion of air-termination elements, conductors and connections; earthing resistance of the earth-termination system; condition of connections, equipotential bonding and fixings (§7.4).
Testing on a 14–15-month cycle is acceptable where earth-resistance readings across different seasons are wanted (D.7.2.2). Consider improving the earthing system when measured resistance rises over successive inspections.
Record all inspection and test results and keep them with the LPS design documentation for the property manager (§7.4).
| Step | Formula | Values | Result | Reference |
|---|---|---|---|---|
| Mesh size | w_m = w_m(class) | class II | 10 m × 10 m | IEC 62305-3:2024 §5.2.2 (values: Ed. 2 Table 2) |
| Down-conductor count | n = max(2, ⌈perimeter / spacing⌉) | perimeter = 128 m, spacing = 10 m (class II) | n = 13 | IEC 62305-3:2024 §5.3.3 (values: Ed. 2 Table 4) |
| Separation distance (simplified) | s = k_i · k_c · l / k_m | k_i = 0.06, k_c = 0.44, l = 20 m, k_m = 1 | s = 0.528 m | IEC 62305-3:2024 §6.3.2, eq. (6), Table 13 |
| k_c per level (ring-interconnected down conductors) | k_c1 = 1/(2n) + 0.1 + 0.2·∛(c/h₁); k_c2 = 1/n + 0.1; k_c3 = 1/n + 0.01; k_c4… = 1/n | n = 13, c = 9.8 m | k_c1 = 0.337, k_c2 = 0.177 | IEC 62305-3:2024 Annex B, Figure B.3 |
| Separation distance (general approach) | s = k_i/k_m · Σ (k_cn · l_n) | k_i = 0.06, k_m = 1; Σ = 0.337·10 + 0.177·10 | s = 0.309 m | IEC 62305-3:2024 §6.3.1, eq. (5); Annex B |
| Minimum electrode length | l_1 = l_1(class, ρ) | class II, ρ = 1500 Ω·m | l_1 = 19.0 m | IEC 62305-3:2024 §5.4.2, Figure 5 |
| Type B ring criterion | r_e ≥ l_1 | l_1 = 19.0 m | r_e ≥ 19.0 m | IEC 62305-3:2024 §5.4.2.3, eq. (1) |
| Effective ring radius | r_e = √(A/π), A = (L+2)·(W+2) | L = 40 m, W = 24 m | r_e = 18.6 m | IEC 62305-3:2024 §5.4.2.3 (r_e of the enclosed area) |
| Type B additional electrodes | l_r = l_1 − r_e ; l_v = (l_1 − r_e) / 2 | l_1 = 19.0 m, r_e = 18.6 m | l_r ≥ 0.4 m or l_v ≥ 0.2 m, × 13 | IEC 62305-3:2024 §5.4.2.3, eq. (2)/(3) |