In its 16 years of experience in installing lightning protection systems, Lightning Elimination Systems (LES) has worked with three distinct air terminal technologies; there are also various types of air terminals within each technology. While most customers are correct in understanding that air terminals provide protection from lightning, they do not really understand the physics behind the operation of air terminals, much less the various technologies.
Many controversies exist among scientists and engineers working in the lightning protection field about air terminal technologies, particularly those that are referred to as unconventional.
Air terminals are installed on a roof as network of interconnected rods and grounded to an appropriately designed grounding system. As the atmospheric electrostatic field develops during a thunderstorm, the surface of the earth becomes charged. The air terminals gain the same potential as the surface of the earth, practically elevating the ground plane, and thus become the preferred point for a potential lightning strike. Once struck, the energy from the lightning charge is safely grounded, protecting the facility from damage.
A conventional air terminal relies on technology that has existed since the days of Benjamin Franklin, so these terminals are also called Franklin rods. Rods can have a blunt or sharp tip and are made out of aluminum, copper or stainless steel. They are evenly spaced on the roof of a structure to be protected. United Laboratories (UL) defines the size of the air terminals based on the height of the structure and suggests the optimal spacing. In the international market, the International Electrotechnical Commission (IEC) uses the same Franklin rod technology and has similar recommendations for size and spacing; however, they recommend four levels of protection. Many studies have been conducted to compare the effectiveness of blunt- vs. sharp-point rods.
Unconventional air terminals are simply conventional air terminals modified to change their responsiveness to lightning strikes. One type of air terminal is called an early streamer emitting (ESE) device; another one is called a charge dissipation (CDT) device.
Some ESE devices are constructed with built-in electronic modules and some without electronics. They are designed to be capable of faster reactions than conventional air terminals, and with larger strike distances. A larger strike distance technically means that the collection radius of the terminals is larger; this directly relates to a larger protection area. International standards NFC 17-102 (from France) and UNE 21186 (from Spain) recommend specific testing procedures to qualify, design and install lightning protection systems using these devices.
CDTs are based on the theory of static charge dissipation, where the air terminal is constructed in a way that the terminal does not respond to an approaching strike. These terminals prevent the formation of upward streamers that intercept a downward leader. Manufacturers of these terminals claim to eliminate, mitigate or reduce the probability of lightning strikes on the facility where CDTs are installed. These terminals have struggled to find acceptance among international standards bodies, so manufacturers of these devices recommend their own test and installation procedures, most following the UL recommendations.
UL and the National Fire Protection Association (NFPA) recognize only conventional air terminals. The American Petroleum Institute (API) accepts Franklin rods, ESE devices and CDTs.
Lightning Elimination Systems works with all three technologies. We took it upon ourselves to conduct tests in laboratory conditions to actually compare the performance of air terminals from different manufacturers. Blunt air terminals reacted faster than sharp air terminals and received more strikes. Some ESE devices were no different than conventional air terminals, while others reacted faster than blunt air terminals. Some CDTs were no different than regular air terminals. Others reacted even faster than ESE devices, while still others took no strike when compared with a sharp air terminal.
We are not research scientists, but we saw that the technologies do work in controlled laboratory environments. We have installed all three technologies at various sites and the systems have performed. We test and qualify the air terminals before any project to make sure the product does what its manufacturer claims and then use our best judgment to install the terminals so that they comply and exceed industry recommendations.
We find that the industry lacks a common platform to regulate air terminal manufacturers with performance parameters. Groups oppose the claims of unconventional air terminal manufacturers, but the industry has yet to establish common test procedures to define specifications that could lead to acceptance.
In our experience, CDTs work – but the terminals have to be of the right construction and their effect is localized. CDTs installed on facilities with small footprints do give the desired effect of reducing lightning strikes.
In our experience, ESE devices also work. Again, the terminals have to be of the right construction but the protection works for all facilities, small or large. Adequate shielding from lightning-caused electromagnetic pulses is compromised, however, as an adequate network of conductors does not exist.
For LES, the ideal lightning protection system is a hybrid one where we use:
CDTs on communication antennas, storage tanks and for storage of explosive materials.
Conventional air terminals on instrumentation buildings and structures housing critical electronics.
ESE devices for multilevel buildings and large open areas.
Lightning Elimination Systems