I was writing a series about lightning, thunderstorms early warning and tracking systems and lightning protection when this came to my mind, my current occupation has implied one of the most drastic changes of subject in my career. Most of my previous occupations involved working with nearly harmless and relatively predictable low level voltages and currents. The exception to that was the while I spent designing and programming smartcard manufacture automation machines. I continue designing and programming embedded systems and sensors, but they are no longer destined to intrinsically or partially safe environments. Perhaps it could be enough saying that many of our current designs must not only survive but continue operating after being impacted by lightning.
The figure below shows one simplified lightning protection system (LPS) installation. It depicts one air terminal (lightning rod), the down-conductor and the ground installation. All other protection elements have been intentionally omitted. There are several other LPS installation schemes but most of them share these three basic elements. Needless to say that designing devices for being installed at any of these three locations represented a real challenge for me at the beginning. After digging a lot into many manuals and application notes from several surge and ESD protection device manufactures I found myself in front of one tangible fact: there are no magic rules, neither a one-fits-all solution. I also observed how several wireless communication SoC manufacturers made a step back when the antenna circuit protection subject came up.
Simplified lightning protection system showing the air terminal (1), the down-conductor (2) and the ground installation (3).
Lightning protection for exposed devices, from the electronic devices designer’s perspective, must be considered more like a strategy than a simple method. It somehow resembles a chess play. You need to find out all (or most) possible lightning surge current paths into your system and create the appropriated protection barriers. You need to design defensively and consider that under a lightning strike condition your system’s earth reference is no longer the beloved and lifesaving reference potential. Any power, data or analog input line is a potential surge path into the system. We must take into account that ESD protection devices, gas discharge tubes (GDT), transient voltage suppressors (TVS), metal oxide visitors and surge arrestors provide a means for dissipating the lightning surge current entering into your system, but none is perfect. Almost all of them allow for a residual impulse to continue traveling, yet carrying much less energy.
Lightning current impulses can have current peaks ranging from several tens of amps to hundreds of kilo amps in either polarity. And as it is supposed to happen with any time-varying current, the electromagnetic impulse generated by the lightning current impulse has also a far from negligible magnitude. The same happens with the electric field, especially for devices installed directly in the down-conductor or at the air terminal. These two pictures below correspond to one cloud-to-ground flash. The one at the left shows the ramifications of the stepped leaders just before the attachment occurs. The ionized channel is still visible in the picture on the right, taken after the occurrence of the first return stroke.
Instants before (left) and after (right) the attachment occurs. (Images source: Ingesco)
In the next part of this series I will cover how we can take advantage of several aspects of the lightning physics for improving safety, for generating early warning and alert signals and for monitoring the evolution of active thunderstorms.
It will be interesting hearing about your experiences in circuits protection against lightning and changes in occupation that implied a deep dive into new areas of knowledge and research.