7.2.2 Calculation of the separation distance using nodal analysis

Kirchhoff's first law defines that at any node the sum of the currents flowing into that node is equal to the sum of currents flowing out of that node (nodal rule) (Figure 7.2.2.1).
This rule can also be used for buildings with an external lightning protection system (LPS). If you have a simple building with one air-termination rod (Figure 7.2.2.2), the lightning current divides at the base when it strikes. This lightning current distribution depends on the number of down conductors, also referred to as current paths. Figure 7.2.2.2 shows a node with four outgoing conductors (current paths).
In case of a meshed external lightning protection system with down conductors, the lightning current is distributed in each junction and at the connection point of the air-termination system. This requires a clamp connection according to IEC 62305-3. The closer the mesh and thus the higher the number of nodes, the better the lightning current distribution. The same applies to the remainder of the conductor length (Figure 7.2.2.3). Nodal analysis is used to calculate the exact lightning current distribution and the resulting separation distances. This method is used for network analysis in electrical engineering and is a theoretical transmission line method. If nodal analysis is used for a building with external lightning protection system, each line (current path) is shown in the form of a resistor. Thus, the variety of meshes and down conductors in a lightning protection system forms the basis for nodal analysis. The lines of a lightning protection system, for example of a mesh, are typically divided into many individual line sections by means of nodes (junctions). Each line section represents an electric resistance R (Figure 7.2.2.4).