When structures are containing protruding or flush-mounted structures, the Lightning protection is designed in such a way that those parts lie within the protection given by air terminal rods mounted over the structure.
The above picture depicts that the protruding structure lies within the region of protection given by the lightning rods. However, additional protection is required if
- The total area of the protruding structure is more than 1 m²
- The length of the protruding structure is more than 2 m.
- The height of the protrusion is above 0.3 m from the roof level.
If those protruding structures are non-conductive, then additional protection from the lightning rods is not required if they are within 0.5 m height.
When a Chimney is made of non-conductive material, the ash or soot deposits at the inner wall can cause conductivity and can even result in forming upward leaders of greater length resulting in a lightning strike on the Chimney. This can happen even in the absence of rain, and so the lightning rod over the Chimney should be at such a height that it lies under the area of protection given by the lightning rod.