Since moving south in June, I have paid a lot more attention to thunderstorms. Thunderstorms in the Southeast are more powerful and frequent than I experienced in the Northeast. The most severe thunderstorms generate tornadoes. I always want to be prepared, so I set up a weather radio next to the bed the first night. Weather radio announces every detected thunderstorm. I didn’t sleep well because I was not ready for the number of times during the night that it went into alarm. The weather alert radio was gone from the bedroom the next day.
Thunderstorms are serious business; they are a fire protection problem and a hazard to electrical systems. Lightning is a natural meteorological phenomenon that is not entirely understood. It is difficult to reproduce, which makes testing some theories difficult.
NFPA 780, the standard for the Installation of Lightning Protection Systems, addresses the fire protection hazards and the protection techniques have stood the test of time. Benjamin Franklin’s work is its foundation and has been revised as more is learned about lightning. The hazards of lightning to electrical systems are well-known. The National Electrical Code has had installation requirements for surge protection since its first edition in 1897.
If lightning strikes your building, the building next door, a tree on the property, a utility pole or even the ground, it can cause a lot of damage inside your building. Most damage to electrical and electronic equipment is due to indirect strikes. A strike to a nearby tree is a large collapsing electrical field. That field will induce current in nearby conductive objects that are capable of providing a path to ground. That path doesn’t have to be a good path. High-impedance paths will usually generate damaging heat.
A common question is how much voltage/current is present in a typical lightning bolt. I Googled the question and found a variety of answers. The simple answer is that the voltage is very high because it takes a very high voltage to ionize that much of the atmosphere. Once the path through the air is established, a lot of current will flow.
Protection of structures is the domain of NFPA 780. Electrical-system protection is the domain of the NEC, and NFPA 780 covers traditional lightning protection systems. It does not cover lightning protection system installation requirements for early streamer emission systems or charge dissipation systems. There are standards from the International Electrotechnical Commission that covers some of these other lightning protection systems.
Lightning protection is not an exact science, but, as mentioned in Annex B of NFPA 780, what is known about lightning is based on over 250 years of observations. That lightning doesn’t strike the same place twice is a old myth that has long been debunked. There is no shortage of photos of the Empire State Building, the Eiffel Tower and Chicago’s Willis Tower being struck by lightning.
The highest structures are the most likely to be struck. Vulnerable structures include water tanks, towers, chimneys, antennas, railings and other metal structures. Lightning is a short-duration but high-frequency event. Therefore, it seeks low-impedance paths to ground. High-impedance paths will often result in heat and sometimes mechanical damage. Some concepts are simple, such as the need for bonding of conductive items that could become energized by lightning.
NFPA 780 provides requirements for lightning protection systems that are based on the cone of protection method or protection angle method and the rolling sphere method.
Lightning protection systems have their own terminology, which is defined in Chapter 3. What was once referred to as lightning rods are now called air terminals. Lightning protection systems can also consist of grounded masts or overhead grounded conductors. Systems consisting of overhead grounded conductors are referred to as catenary lightning protection systems. Air terminals, masts and catenary system overhead conductors are referred to as strike termination devices. Strike termination devices are those components of the lightning protection system that are intended to receive a lightning strike and provide a path to ground. Permanent metal structures can also be strike termination devices.
A lightning protection system consists of strike termination devices, grounding electrodes and conductors to connect the strike termination devices to the grounding electrodes. The conductors from the strike termination devices to the electrodes are kept as short and direct as possible to keep the impedance as low as possible. Many designs call for multiple electrodes around the structure.
Strike termination devices provide a zone of protection, which is an area where lightning should not strike because it will strike the termination device first. Most structures are covered by the requirements of Chapter 4 of NFPA 780. The document also has requirements for miscellaneous structures and special occupancies, heavy-duty stacks, structures containing flammable vapors, flammable gases or liquids that can give off flammable vapors, structure housing explosive materials, wind turbines, watercraft, airfield lighting circuits and solar arrays.
The unique shapes of the roofs of many buildings can present challenges for the design of a lightning protection system. NFPA 780 provides requirements for the location of air terminals for a number of different roof designs.
NEC surge protection requirements
The NEC has had requirements for surge protection installation since the 1897 edition. The requirements were in Articles 280 and 285 for many editions. In the 2020 edition, the two articles have been combined into a new Article 242, “Overvoltage Protection.” Lightning isn’t the only cause for overvoltage in electrical systems, which this new title recognizes.
Lightning surges can enter a building through services, feeders, communications systems, CATV systems and broadband communications systems. Section 250.94 requires intersystem bonding of grounding electrode systems. Some communications and CATV installers have installed separate electrodes for their systems in the belief that the electrode for the electrical system is hazardous. If a lightning strike occurs, the voltage between grounding electrodes that are not bonded together can be very high. This potential difference can cause insulation breakdowns within sensitive communication equipment and computers.
A surge-protective device (SPD) is defined in Article 100 as “A protective device for limiting transient voltages by diverting or limiting surge current; it also prevents continued flow of follow current while remaining capable of repeating these functions and is designated as follows:
- Type 1: Permanently connected SPDs intended for installation between the secondary of the service transformer and the line side of the service disconnect overcurrent device.
- Type 2: Permanently connected SPDs intended for installation on the load side of the service disconnect overcurrent device, including SPDs located at the branch panel.
- Type 3: Point-of-utilization SPDs.
- Type 4: Component SPDs, including discrete components, as well as assemblies.”
Surge protection is not required everywhere. Section 242.3 provides a reference for several articles that contain requirements for surge protection. In recent cycles, the number of systems requiring surge protection has increased. The occupancies or systems currently requiring surge protection include hazardous locations, communications systems, CATV systems, critical operations power systems, emergency systems, fire pumps, industrial machinery, information technology, radio and television equipment and wind turbines.
The grounding and bonding requirements of Article 250 are a key ingredient of surge protection because the requirement intends to keep noncurrent-carrying parts of electrical equipment at the same potential with respect to ground. Article 250 also requires grounded electrical systems to be connected to earth in a manner that will limit the voltage from surges. When the ground potential increases, properly grounded equipment will ride the increase in potential up and back down again without damage to equipment.
Lightning surges will take whatever path they can to get back to ground. Often, the strike occurs nowhere near the service, so the current will flow through the electrical system across the building and back to the service.
Some equipment is particularly vulnerable to surges, and sensitive electronic equipment is particularly vulnerable. As we move further toward the internet of things and power over ethernet, requirements in the NEC for surge protection are likely to increase. The NEC is intended to protect people and property from the hazards arising from the use of electricity. Surge damage could make a smart building useless and difficult to diagnose.
Why aren’t lightning protection systems everywhere? Because they aren’t needed everywhere. Annex L of NFPA 780 provides a detailed risk assessment methodology that can be used to assess the need. NFPA 780 is a standard and not a code. It is written in mandatory language, but it does not require that lightning protection systems be installed. Installation is a choice. If a system is installed, it should comply with the requirements of the standard.
Why isn’t surge protection required everywhere? A key change in the 2017 Code added requirements for surge protection for industrial control panels with safety interlocks. The 2020 NEC will require surge protection for all services supplying dwelling units. As noted in the NEC First Draft Report, this change recognizes “the need for surge protection to protect the sensitive electronics and systems found in most modern appliances, safety devices (such as AFCI, GFCI and smoke alarms) and equipment used in dwellings. Additionally, the expanding use of distributed energy resources within electrical systems often results in more opportunity or greater exposure for the introduction of surges into dwelling unit electrical systems.”