What is the Rolling Sphere Method
A common question on Lightning Protection Design that crops up repeatedly is – “What is the Rolling Sphere Method of protection?”
Suppose you’re looking for a convenient definition in BS EN 62305. I can save you the hassle because there isn’t one! The closest you will get to a conveniently packaged definition is in para E.5.2.2.2.
The positioning of an air-termination system is adequate if no point of the volume to be protected is in contact with a sphere of radius, r, rolling on the ground, around and on top of the structure in all possible directions. Therefore, the sphere should touch only the ground and/or the air-termination system.
Hmmm, not exactly succinct or useful, right?

The Rolling Sphere Method concept
First, let’s take a look at what the Sphere represents. The gym ball, as shown in the image, represents the ‘reach’ of a given lightning stroke. Lightning strokes are calculated to vary depending on the vulnerability or degree of risk considered. So, the sphere radius is at its smallest for a high-risk facility, e.g. 20m or a 40m diameter ball. The smallest ball means the amount of protection installed will be at its highest. Thus lowering the risk profile and increasing the protection afforded.
For a low-risk scenario method, the sphere radius is at its largest distance, 60m (120m diameter ball), which means less hardware to install.
What’s in a radius?
Without going into detail on the lightning attachment and discharge process itself (as there is plenty of material on the subject), let’s call the centre of the ball Lightning’s “point-of-discrimination” – the point at which the last segment of the Lightning stepped leader is close enough to ground-based objects to start hunting around for something to latch onto, i.e. terminate, attach, etc. Call it what you will.
Although some might argue that the Sphere is a crude representation, the concept works in practical terms for us engineers: it allows us to visualise where a strike attachment might reasonably occur and do something about it.

Rolling a ball around an object or group of objects and tracking where the ball stays in contact determines where the lightning attachment may happen. Thus, where your lightning protection system should be deployed is for a non-isolated system. It gets a little more complicated for an isolated LPS (lightning protection system), but that’s another story.
So, having explained a little about the concept of rolling a ball around some objects, how does this methodology compare with the legacy methods, such as the protection angle and the mesh methods of BS 6651?
Which method of calculating the protection is best?
Compared with the Protection Angle Method, the Rolling Sphere Method more elegantly and technically securely accounts for all types and combinations of structures. The Protection Angle Method is only suitable for the most basic structures/geometries, where the object dimensions do not exceed its comparative rolling sphere radius dimension. So, the Rolling Sphere Method covers everything for simplicity, whereas other methods do not. This point is often overlooked, leaving designs vulnerable and non-compliant.
If In Doubt, Use The Rolling Sphere Method
The downside to the rolling sphere method is that it can be quite demanding to get right. And unless you’ve got a bag of balls hanging around, trying to CAD the contact surface areas manually is time-consuming. Thankfully, there are software tools that can do much of the heavy lifting for the Lightning Design Engineer, but these still require a high degree of competency to get the right answers. I realise, as a specialist, I would say that, wouldn’t I, right? But it is true! I’ve seen it time and time again.
Buyer beware!
Unfortunately, most engineering software tools are not built like consumer software. Consumer software has a lot of ‘intelligence’ built into it for a specific task. Unlike engineering software, which assumes the user is holding all the intelligence!