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How Does a Lightning Protection System Work?

There is an old axiom, which states that ‘lightning never strikes the same place twice’.

Where this expression comes from is hard to determine, although it is quite safe to state that it is wrong.

Why is it wrong?

Ask the Blackpool Tower.

Home of the iconic Tower Ballroom this 19th century structure has been hit more than once. The most significant strike happened as recently as July 2016.

So if such iconic buildings can be struck, repeatedly as it turns out why don’t we hear more about it in the news?

The simple response is that they have well designed and maintained lightning protection systems.

In very simple terms a lightning conductor (or rod) is often a copper based alloy mounted at the highest point of a structure and then connected to a grounding point via heavy-duty conductor cables. While this passive approach to lightning protection provides an effective means of preventing excess damage to a structure, the risk of damage to electrical and electronic devices can remain a threat.

Although buildings with a steel frame do seem to fare better during a lightning strike, the risk of fire still remains. This is because loft spaces and wall insulation can begin to burn and it is not until there is a sufficient build up of heat that a fire alarm will sound. The main reason that steel framed buildings fare slightly better that those without is because the steels act as conductors to the ground.

Having a steel frame will not guarantee safety though, as pointed out above the risk of fire remains. To this end, it is always going to be good practice to install some form of lightning protection system:

Passive systems are relatively inexpensive to install and with regular inspect/maintenance can easily last the life of the building they are installed on.

A financial benefit of a passive system is there is little to wrong, the lightning conductor itself is a sturdy unit as you can imagine. The grounding points are equally robust so it is the conductor cables that require the closest inspection. The conductor cables need to be of low impedance to work efficiently and reduce the risk of damage to electronic devices.

Some structures are fitted with an external wire mesh, or Faraday cage which, can further reduce the risk of harm to not only the structure of the building but also the contents and occupants. The Faraday cage works by distributing the electrical charge around the exterior of the cage, so the massive charge from the lightning leaves the interior of the ‘cage’ unharmed.

The principal of the Faraday cage was adapted by Dr Austin Richards to create a Faraday suit. From this point he used a Tesla coil to create a show called Dr Megavolt.

Early Streamer Emission (ESE) Lightning Protection Systems

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.