Separation distance calculator

When lightning current runs down a conductor, the voltage along it rises with the length the current has travelled since the last bonding point. A metal pipe, a cable tray or a handrail nearby sits at a different potential, and if the gap between them is too short, the air or masonry breaks down and a spark jumps across. Clause 6.3 of IEC 62305-3:2024 sizes the gap that prevents it.

k_i, the class of LPS. Table 11 sets k_i by class. The stricter the class, the steeper the current it is designed for, so class I has the largest k_i and needs the widest gap for the same layout.

k_m, the material in the gap. Table 12 gives air the reference value and concrete, brick and wood a lower one, which makes s larger: a solid wall breaks down more easily than the same thickness of air. Table 12 NOTE 1 adds a value for an insulating stand-off of FRP, PE or PVC, only from a set length upwards, and sends every other material to its manufacturer. Where several materials sit in series, NOTE 2 takes the lowest k_m.

k_c, how the current divides. With several down conductors, each carries only a share of the current. Table 13 gives an approximate share by the number of down conductors, and a single down conductor is allowed only for an isolated LPS. For Type A earthing the note under Table 13 sets k_c to 1 when neighbouring electrodes differ in resistance by more than a factor of 2, so the calculator asks for those resistances. A value from a detailed calculation may replace the table under clause 6.3.1 NOTE 2, with its source recorded.

l, the length. Measure it along the lightning conductors, air termination first, then down conductor, starting where you are checking and stopping at whichever comes first: a bonding point or the earthing. The longer the path, the larger s, which is why the gap needs to grow towards the top of a building. Clause 6.3.2 NOTE 1 lets the structure's height stand in for l on a building whose smaller plan dimension is no more than three times its height.

This is the simplified approach of clause 6.3.2, one k_c for the whole length. The general approach of clause 6.3.1, equation (5), splits the path into segments with a k_c each, from the ring conductor and meshed air termination methods of Annex B, so it follows the real share of current in each part of the path. An electrically insulated LPS is judged on its manufacturer's equivalent separation distance instead. High installations need the altitude correction of clause 6.3.2, and a loop in a down conductor follows clause 5.3.4. The full assessment handles all of these.