The protective angle method is one of the three routes for lightning protection system design defined by IEC 62305, the international standard for lightning protection design. After the first step of determining the class of lightning protection decided by the Risk Assessment, the lightning protection system design can be done using the Rolling Sphere, Protection Angle method or the Mesh method. The system design will provide information such as location of the lightning arresters, down conductors, earth electrodes, other equipment and the complete Bill of Material.
Protective Angle Method
The protective angle method is best used on simple structures. Additionally, the Protective Angle Method is only valid up to heights equal to the radius of the rolling sphere as defined by the class of LPS defined for the structure. For structures with a protruding metallic structure, the Protective Angle Method is generally used as a supplement to the Mesh Method.
The protective angle method is commonly referred to as a mathematical simplification of the rolling sphere method. The protective angle is determined in such a way that when the slope intersects the rolling sphere, the additional area under the protective angle (marked green in the figure below) and the area that is now not under the protective angle (marked red in the figure below) are equal.
The protective angle method is a 3D concept as shown in the figure below. The protected area is the cone that is swept by the line that emerges from the tip of the air termination rod and ends at the surface of the structure. The protected area will be cone-shaped in the case of a simple air termination rod but it can also be a tent-shaped protected area in case of a catenary wire as shown in the figure below
The protective angle (a) depends on the Class
of LPS and the height of the air rod. The angle is determined from Table 2 from
IEC 62305-3. For simplicity’s sake, the table below shows the protective angle
for air termination rods that are up to 2m in length.
The table below is a more comprehensive guide for different heights of the air termination rod and the corresponding protective angles (a).