Lightning Protection Systems: US NFPA 780 vs BS EN 62305:2006
Lightning Protection Systems are not only one of the most expensive infrastructure components of a building, but is also one of the least understood. In the United States, most industry and the government facilities are protected by NFPA 780 Standard for the Installation of Lightning Protection Systems. This tried and true standard issued by the same group that writes the National Electric Code (The NFPA), provides an excellent guideline for installing a straightforward one-size-fits-all lightning protection system.
US NFPA 780
NFPA 780 provides guidelines for how often to place air terminals, spacings for cross and down conductors, ground rod and loop requirements, surge-protection requirements, and how to install protection for trees, towers, etc. The standard however has two primary short falls in that it does not analyze the installed systems ability to handle an actual lightning strike, nor does it take into consideration what the system is protecting. In other words, NFPA 780 has the same requirements for a work shed as it does for a billion-dollar computer farm. These short falls along with virtually no legal and/or insurance industry requirements for lightning protection, has led many facilities managers to simply take their chances and forgo protecting their buildings.
BS EN 62305:2006
In 1752 Benjamin Franklin installed the world’s first lightning protection system on the Academy of Philadelphia (later the University of Pennsylvania) and the Pennsylvania State House (later Independence Hall). Within a few years he had installed lightning protection systems on several major public buildings in London and was one of the few 18th century Americans to receive the Royal Societies, Copley Medal for his work. Ever since, England has been a world-leader in the development of effective lightning protection systems. Today, there is no better standard than the BS EN 62305:2006 for lightning protection.
We can learn a lot about lightning protection by looking at the requirements of BS EN 62305:2006 which is significantly different and more demanding than the US NFPA 780 standard.
The Basic Requirements of BS EN 62305:2006:
1. It requires a highly complex risk factor assessment which determines the level of required lightning protection. This risk assessment is quite complex and software is almost always used to perform the calculation. The calc includes, Human Life, Public Services, Cultural Heritage, Economic risk, and occupancy issues. This risk assessment is both good and bad in that work-sheds will have fewer requirements than found in NFPA 780, but billion-dollar computer farms will have greater requirements.
2. The BS EN 62305:2006 standard requires an actual assessment of the lightning protection system to insure that it is capable of handling a lightning strike. The lightning strike calculations are far more significant for both the time domain parameter and the actual strike amperages (100kA to 200kA) than the US industry standard (often only 15kA).
The Basic Requirements of BS EN 62305:2006:
1. Expected amount of current to be carried on individual conductors in DC amps to ensure that current carrying capacity is not exceeded.
2. Rolling-ball theory of lightning protection tested against 3D computer models of the structure and surrounding area.
3. Spark gap and arc-flash calculations from the lightning protection system to adjacent conductive utilities.
4. Time-domain of the lightning strike on the specific structure. This is critical to understanding the amperage carrying capacity of the conductors. Without an actual calculation the BS EN 62305:2006 default time-domain could force the unnecessary installation of additional conductors.
5.Frequency Spectrum of the lightning strike on the specific structure. This data is needed for both surge-protection and for timing of Circuit Breakers to prevent power outages.
BS EN 62305:2006 has requirements that are far greater than NFPA 780:
In general, BS EN 62305:2006 has physical construction and installation requirements that are far greater than NFPA 780.
1. And down conductors vary from 10-meter to 20-meter spacing’s, in the US we use a one-size-fits-all 30-meter spacing.
2. Also the Zone of Protection or rolling ball theory in the EN standard varies the angle required based on the risk assessment, which can impact placement of certain types of aerials from 20-meter to 60-meter heights.
3. Concrete columns that are used for down conductors must be tested a 0.2 ohms or less continuity, and rebar must be welded with 20x diameter overlaps. These must be bonded to the floor slab.
4. Ground rings are required for all non-conductive buildings, buildings housing electronic systems, and certain risk factors. Individual rod installations (no ground ring) must be tested so that each electrode is at the same resistance to ground.
5. Spark gaps between lightning conductors and other metallic objects must be considered.
6. Incoming utility services (such as overhead power lines) and adjoining public spaces may also be required to have protection systems installed, based on the risk assessment.
7. Both internal and external lightning surge protection systems are mandatory.
The BS EN 62305:2006 has stringent requirements for annual testing and inspection of the lightning protection systems. This of course, goes along with mandatory maintenance requirements.
BS EN 62305:2006 has requirements that are far greater than NFPA 780
This is of course only a small list of differences. Bottom line, BS EN 62305:2006 is the grown-up version of the lightning protection compared to the watered down child’s play version the US uses. Also of note, the EN/IEC and BS versions of the lightning standards differ in the risk assessment calculation.
Rolling Sphere Studies, Time Domain Studies and Frequency Spectrum Studies
E&S Grounding Solutions can perform Rolling Sphere studies in compliance with BS EN 62305:2006 standards, along with the Time Domain and Frequency Spectrum studies. The BS standards have many exceptions for reducing the physical installation requirements. The Rolling Ball and Lightning Transient studies will be critical in the Risk Assessment process for reducing over-engineered requirements found in the BS EN 62305:2006 standard.