Separation distance calculation under IEC 62305-3

IEC 62305-3:2024 gives a simplified approach and a general one. Both multiply the same class coefficient by the length of conductor the current has travelled, and both divide by the same material coefficient. They differ in how they treat the current sharing.

• Simplified approach, clause 6.3.2, equation (6): s = (k_i / k_m) × k_c × l. One current-sharing coefficient k_c applies to the whole length l.
• General approach, clause 6.3.1, equation (5): s = (k_i / k_m) × (k_c1 × l₁ + k_c2 × l₂ + ... + k_cn × l_n). The path is split into segments, and each segment carries its own share of the current.

The simplified approach is the one most designs use, because Table 13 gives k_c from the number of down conductors alone. The general approach earns its extra work on tall or meshed systems, where the current spreads out as it travels down and a single k_c would overstate the distance needed lower on the structure.

The building in this example is invented to show the arithmetic. The coefficients are the IEC 62305-3:2024 values in the table above.

• The risk assessment calls for a Class II lightning protection system, so k_i = 0.06 (Table 11).
• The system is attached to the building and has four down conductors, so the simplified approach takes k_c = 0.44 for three or more (Table 13). The earthing is Type B, a ring electrode, so the note on Type A electrodes does not apply.
• An unbonded metal air handler stands on the roof. The gap between it and the nearest roof conductor is open air, so k_m = 1 (Table 12).
• The conductor path from the point nearest the air handler, along the roof conductor and down to the earth ring, is 10 m long, so l = 10 m.

Then s = (0.06 / 1) × 0.44 × 10 = 0.264 m. The air handler must sit more than about 26 cm from the roof conductor. If it sits closer, it is bonded to the system instead.

Now suppose the same check runs through a brick parapet rather than open air. Table 12 gives k_m = 0.5, and the distance doubles: s = (0.06 / 0.5) × 0.44 × 10 = 0.528 m. Solid material holds off less voltage per metre than air, so it needs a wider gap. With only two down conductors, k_c would rise to 0.66 and the open-air distance would grow to 0.396 m, which shows why adding down conductors is a real design lever. To run the same check on your own figures, use the free separation distance calculator.

A few rules change how the inputs are read. Each one has caught designs out.

• Structure height as the length. Clause 6.3.2 NOTE 1 lets the height of the structure stand in for l, but only where the smaller of its length and width is no more than three times its height.
• Altitude. Clause 6.3.2 asks for an altitude correction when the system is installed at higher elevations, by reference to IEC 60071-2. A correction for elevation only ever lengthens the distance.
• A single down conductor. Table 13 allows k_c = 1 for one down conductor only on an isolated system, one held clear of the structure.
• A declared k_c. Clause 6.3.1 NOTE 2 accepts k_c from a more detailed calculation. It is a share of the lightning current, so it sits above 0 and no higher than 1, and the calculation behind it should be on file.

Annex B of IEC 62305-3:2024 refines k_c from the layout. For a wire air termination between two masts, Figure B.1 gives k_c = (h + c) / (2h + c), with h the mast height and c the distance between the masts. For a system with four or more down conductors, Figure B.2 gives k_c = 1/(2n) + 0.1 + 0.2 × the cube root of (c/h), where n is the number of down conductors, c the distance to the next one and h the spacing between ring conductors, or the height where there are no rings. It holds for c and h between 3 m and 20 m, and the standard describes it as an approximation for roughly cubic structures. Figures B.3 and B.4 give per-segment values for ring conductors at several levels and for meshed air terminations, which is where the general approach of equation (5) comes in.

In Lumex^™ the class of LPS comes out of the IEC 62305-2:2024 risk assessment, and the separation distance is worked from the layout you record: the down conductors, the earthing arrangement, the materials in each gap and the conductor lengths. Each check names the table or figure behind every coefficient and the equation it used.

A check the inputs cannot support is refused with a message saying what is missing, rather than computed from a guess. A separation distance that looks satisfied because a value was assumed is the very hazard clause 6.3 exists to prevent. For the rest of the internal LPS, read about lightning protection zones and bonding and SPD types, or see the full list of terms in the glossary.