Lately, I’ve received a number of inquiries from contractors concerning the installation of lightning protection systems. Those electrical contractors’ main concerns are how to begin to design and install such a protective system and who or what entity requires or regulates its installation. The information in this article is not meant to cover in detail all the installation requirements of the National Fire Protection Association’s 780, but it can serve as a starting point to begin designing a lightning protection system.
NFPA 70 and NFPA 780
How does NFPA 70, the National Electrical Code (NEC), relate to NFPA 780, the Standard for the Installation of Lightning Protection Systems? Let me say that the NEC does not require a lightning protection system to be installed. However, if lightning protection is installed, 250.60 and the fine print notes (FPNs), in addition to 250.106 and associated FPNs, refer users to NFPA 780. As far as when lightning protection systems are required, it has been my experience that state, county, or city government and/or facility owners are the authorities that usually insist on installing a lightning protection system.
A lightning protection system consists of five main components, and they are as follows: air strike terminations on the roof, roof conductors and down conductors, connecting means of conductors to devices and electrodes, earth-grounding electrodes, and surge protection systems, where needed. When installed correctly, these components will provide a safe continuous path to ground for dissipating lightning current.
Starting at the roof and based on the type of roof, the number of strike--termination devices can be determined. In Figure 4.1.2 of NFPA 780, the roof types are described and the minimum number of devices recommended. The following styles are some roofs and roof-based supplementary items to consider: flat or gently sloping roofs; dormers; domed roofs; and roofs with ridges, wells, chimneys or vents. Most standards highly recommend that these striking devices be sized and positioned in accordance with the design concept and theory of a 150-foot-radius that is known in the industry as a rolling ball (sphere).
Connecting conductors to electrodes
In many installations, properly sized lightning system conductors are used to connect strike-termination devices on the roof to the earth-grounding electrodes. However, it is not unusual to connect the strike terminals to the structural steel columns or to concrete-reinforcing rebars. These metal items are then used instead of conductors to provide the low-impedance path for dissipating the lightning current to the earth electrodes. Designers and installers must remember that strike-termination devices can be constructed using air terminals, metal masts and permanent metal parts of structures and overhead ground wires as well as any combination of such.
Routing down conductors
To ensure a lightning strike current is safely carried to the earth-grounding electrodes, conductors must be run without shape bends and installed in a horizontal or sloping-downward direction. They must be sized at least as large as required by Tables 18.104.22.168(A) and (B) of NFPA 780 based on the minimum class of material used for construction purposes. The NEC and NFPA 780 require the appropriate bonding of the down conductors and structural steel and other metal bodies to prevent side flashes.
Conductor and electrode connection
The down conductors must be attached permanently to the earth-grounding electrodes by bolding, brazing, welding, high-compression connectors listed for the purpose, and clamps that are suitable for direct burial. If these connections are not performed efficiently, the integrity of the lightning protection system is jeopardized. When selecting connectors and fittings, it is important that they be compatible for terminating the conductors to the devices and electrodes and for meeting the condition in which they are used.
Down conductors must be connected to earth-grounding electrodes that are dedicated to the lightning protection system. In other words, the grounding-electrode system for the service equipment for the facility, as well as that used for grounding telecommunications and cable television systems, must not be used in lieu of lightning protection electrodes.
Electrodes that are suitable for lightning protection systems are ground rods, concrete-encased electrodes, ground-ring electrodes, radial electrodes or plate electrodes. Installing multiple rods or driving longer rods deeper into the soil of the earth can decrease resistance; long grounding conductors buried in the earth can also be used. Note that other methods—such as salt, chemical and other commercial type techniques—are available and can be used to obtain a lower earth (soil) resistance.
Neither NFPA 780 nor 70 implies that lightning strikes can be prevented, but injury to personnel and damage to facilities and installation materials can certainly be reduced with a properly designed and installed lightning protection system.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.