Service entrance protection/equipotential bonding – Type 1 SPDs – lightning current arresters (mains supply)
Only Type 1 SPDs are selected using the LPL and LPS calculated from BS:EN 62305.
When lightning (200kA) strikes a building with structural lightning protection, it is assumed that 50% of the current (100kA) flows directly to earth through the building’s lightning protection conductors. The rest is assumed to flow through the metallic services. So, if there was only one metallic service supplying the building, 50% of the current (100kA) would be assumed to flow through it.
If that metallic service was a three phase electricity supply, then the 100kA would be equally split between each of the modes (lightning current flows to earth so, in a three phase system, there are four modes (or ways) in which lightning will flow – L1 to E; L2 to E; L3 to E and N to E – known as “common mode”).
If there is more than one metallic service entering a building, the 100kA is split equally between each service. If that second service happens to be a power supply, then it is further split by mode, as illustrated below: