Why Rolling Sphere Method is best

A common question on Lightning Protection Design that crops up time and again, is - "What is the rolling sphere method of protection?"

If 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. ..

... 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?

Rolling Sphere Method

source: WBG Wiesinger

The 'Rolling Sphere Method' concept

First, let's take a look at what the 'Sphere' represents.  The gym ball as shown in the image is meant to represent the 'reach' of a given lightning stroke.  The lightning stroke is calculated to vary depending on the vulnerability or degree of risk being considered.  So, for a high-risk facility, the sphere radius is at its smallest, e.g. 20m or a 40m diameter ball. The smallest size ball means the amount of protection to be installed will be at its highest. Thus, lowering the risk profile and increasing the protection afforded.

For a low-risk scenario, the sphere radius is at its largest distance, 60m (120m diameter ball), which means a lot 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 the sphere is a crude representation. For us engineers, the concept works in practical terms to visualise where a strike-attachment might reasonably occur - and do something about it.

Basically, by 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 - for a non-isolated system, that is.  It gets a little more complicated for an isolated LPS (lightning protection system) but that's another story.

Rolling Sphere Method

Courtesy of Dehn

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 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 accounts for all types and combinations of structures in a more elegant and technically secure way.  Whereas, the Protection Angle Method is only suitable for the most basic of structures/geometries, and where the object dimensions do not exceed its comparative rolling sphere radius dimension. So, for simplicity, the Rolling Sphere Method covers everything, whereas, other methods do not.  This point is often overlooked which in turn leaves designs vulnerable and noncompliant.

If in doubt, use the rolling sphere method.

The downside to using the Rolling Sphere Method is that the method can be quite onerous 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-in to it for a specific task.  Unlike engineering software, which assumes the intelligence is held within the user!

Ian Griffiths SelfieIan Griffiths

This post is written by Ian Griffiths, Principal Engineer at GreyMatters, an Earthing & Lightning Consultant of 27 years, one of the top 1% accredited CDEGS consultants and professional advisor to international utility companies, data centre and infrastructure developers.

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