SS 555-3 gives three methods for positioning air terminals: the rolling sphere, the protective angle, and the mesh method. Each models the same physical reality — the volume of space a strike is statistically unlikely to bypass — but they suit different building shapes.
Rolling sphere
The rolling sphere method models a sphere of a radius set by the lightning protection level rolling over the building. Anywhere the sphere can touch without being intercepted by an air terminal is unprotected. It is the most universally applicable method and the one we default to on buildings with complex rooflines, chimneys, parapets and rooftop plant.
Protective angle
The protective angle method is simpler and faster to apply to straightforward structures — a cone of protection projected from each air terminal at an angle set by its height and the protection level. It works well on simple, low-rise roofs but loses accuracy on tall or irregular buildings, where the angle assumption breaks down.
Mesh
The mesh method covers flat or near-flat roofs with a conductor grid at spacing set by the protection level — the standard choice for large flat industrial and commercial roofs, where terminals alone would leave gaps.
Most real buildings need a combination of all three, applied section by section. Choosing correctly is what separates a system that is compliant on paper from one that actually protects the building.