In this previous article we talked about passive lightning protection systems. In the following article we will be paying more attention to active or early streamer emission (ESE) protection systems.
In very simple terms, the main difference between passive and active systems is that a passive system is a conductor that sits at the highest point of a structure and is connected to the ground terminal with a heavy-duty cable. This type of device is known by a few names such as a lightning conductor or Franklin rod.
These devices are relatively straightforward, in the crudest sense it could be described as a spike pointing skywards. The main difference with an active device is that it uses an ionization system that is activated by electromagnetic field, which is produced by the advancing storm.
An additional bonus of ESE systems is that they provide a greater protection radius than that afforded by a passive lightning conductor.
When the atmospheric conditions are ‘normal’ the device does not appear to work. It simply sits there like any other passive design. However, as a storm approaches the difference in potential between the groundside and the atmospheric side grows.
It is this difference of potential that becomes the power source for the active system.
As this build up peaks the electric field value, which is able to ionise the air around the tip and this happens at a higher speed than with a simple rod, this in turn allows an increase in voltage within the device that is higher than at ground level.
What happens next is the formation of an upward leader from corona discharges (streamers) that propagate towards the downward leader. One of these streamers will then become the upward leader and this will continuously propagate towards the downward leading, thus creating the lightning discharge path.
A distinct advantage that the active systems have over their passive brethren is that they can protect a wider area from lightning strikes. As a consequence of providing a greater area of protection, it becomes possible to use fewer active rods in order to provide the same level of defence as a passive system. An added bonus is that an active system protects not only the structure that it is mounted to, but also surrounding and open areas.
Please Note: At the time of writing, this type of lightning protection system is not part of the current British Standards BS EN 62305 Protection Against Lightning. Whilst they are used effectively in other countries, we always advise you follow best practices and adhere to the standards.