Lightning Protection Design using SafeGrid Software

We explain lightning protection design principles and perform an example calculation based on the rolling sphere method for a 220 kV switchyard.
The Lightning Protection Module in SafeGrid Earthing Software allows users to overlay lightning protection systems (LPS) such as lightning masts and earth wires, over a PDF schematic. Using the Rolling Sphere Method (RSM), the Lightning Protection Module calculates the area protected by the masts and wires based on factors such as mast height, placement, and the protection level, which determines the sphere radius. To access this module, open the SafeGrid Earthing Software and select the Add-ons tab. Then, select the Lightning Protection Module.
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▬ Contents of this video ▬▬▬▬▬▬▬▬▬▬
0:00 - Introduction
0:14 - Designing Lightning Protection Systems
0:37 - Air Terminals
1:13 - Rolling Sphere Method
1:38 - Lightning Protection Levels from IEC 62305-1
2:17 - Protection with Lightning Masts - Single and Two Masts
2:57 - Protection with Lightning Masts - Multiple Masts and Shield Wires
3:42 - How the Lightning Protection Module Works
4:38 - Import of the Background Drawing
4:52 - Specifying the Protection Level
5:35 - Set the Scale of the Background
6:03 - Specifying the Equipment Height
6:27 - Elevation Drawing Showing the Busbar Height
7:49 - Adding, Specifying and Positioning the Lightning Masts
8:25 - Adding the Overhead Earth Wires
9:04 - Calculating the Protected Area
9:23 - Saving the Results and Printing the Report
Lightning Protection Systems (LPS for short) design for structures involves placement of air terminals and down conductors, specifying earthing systems, conductor sizing, and materials selection. An LPS is installed to intercept lightning-stroke currents based on the probability of their magnitude.
Air terminals are used to intercept lightning flashes, and their effectiveness relies on where air terminals are positioned on structures to achieve the desired protection level. An air terminal may consist of a vertical rod, raised horizontal conductor, or naturally conductive parts such as metal rooves. The rolling sphere method (RSM for short) is used to position the dedicated air terminals and check the adequacy of any natural components used as air terminals.
The principle of the rolling sphere method is that a sphere of radius ‘r’ is rolled up and over the total structure, and any surface in contact with the sphere is prone to lightning strikes. Therefore, for an installation containing vulnerable equipment, the only points which should remain in contact with the rolling sphere are an air terminal or the ground itself.
The standard IEC 62305 defines four lightning protection levels (LPL I, II, III, and IV) with different levels of interception efficiency and provides the sphere radius for use with RSM.
Note the following limitations of the rolling sphere method:
1. Since RSM makes no allowance for electric field intensification at the edges and corners of structures, air terminals should be placed at these vulnerable locations anyway.
2. The RSM is conservative for protecting large flat rooves, so double the sphere radius can be used to place air terminals.
Typically, air terminals are mounted on masts or on structures to protect underlying equipment from direct lightning strikes. As can be seen, for a single mast, the protected zone extends away from the mast and provides protection up to a certain height at a certain distance. The protected zones of two or more masts will interact if the masts are placed close enough to one another. Note that the protective zone for the left and right masts shown here is identical to that of a single mast and that the protective zone is only altered between the masts.
Where multiple masts are in proximity, the protective zone in 3 dimensions can be difficult to visualise but also to calculate. There are simple equations for determining the protective zone provided by up to 4 masts, but only when evenly spaced and at the same heights.
Specialised software tools such as in SafeGrid are required for analysing the protected zones where interactions with multiple air terminals or masts are involved. Shield wires are also commonly used to protect structures from direct lightning strikes, providing a protected zone between the support points.