Separation distance in LPS – Basic assumptions for the calculation according to IEC 62305-3 and possible extensions

Separation distance, or bonding distance as known in some countries, is a key factor of modern lightning protection systems. The separation distance equation was introduced in IEC 61024-1 in 1990. The factors ki (induction coefficient) and kc (configuration coefficient) were established at that time, with the factor km (insulation material coefficient). km is also an important factor where insulating components (IEC 62561-8) are used. Values of ki, kc, and km are tabulated, relative to IEC 62305-3 specific scenarios. For standards such as NFPA 780 in the US, where down conductor spacing can be up to 30 meters and not vary with lightning protection level, there can be some uncertainty in how to use the IEC separation distance equation. This paper will provide a detailed explanation of ki and kc factors based on the work of Zischank to its entry into IEC 61024-1 and later IEC 62305-3. It also addresses the procedure to assess values to be used for 30 meter down conductor spacing and when the down conductor spacing does not change with lightning protection level.