How likely is it that lightning will strike a building?

This cannot be precisely determined, but some rough predictions can be made. The physics involved are too complex for this FAQ, but the following examples should give a few ideas. The variables involved are many: the local terrain, the shape and composition of the structure, nearby structures or trees, and, of course, the overall local frequency of thunderstorms. Frequency of thunderstorms varies with the weather from year to year, but long-term averages can be used. Statistics are kept on the number of days that locations can expect to hear thunder. This is called the isokeraunic level. In addition to the isokeraunic level, the density of flashes to ground in a typical storm also can be calculated per square kilometer according to the local latitude. The other main factors are the height and area of the structure. As an example, calculations based on these factors show that an 80-foot tall building in Newport, Rhode Island, USA can expect to be struck by lightning approximately every 30 years, but a similar building in central Florida might expect a strike every six years. Put that Florida building in the shadow of a 1,000-foot skyscraper and the chance of it being struck is very low, if not zero.

It is tempting to conclude that for many parts of the world, where lightning levels are low, the threat is negligible. But it is very important to remember that lightning need not strike a building directly to cause serious damage to any sensitive electronic equipment that it might contain. It is common for lightning strikes several kilometers away to cause damaging electrical surges. And even in Newport, Rhode Island, where there are only about 20 thunderstorms a year, each square kilometer is struck an average of twice a year. This is enough to be of concern to facilities where damage can result in expensive downtime, the loss of important data or the potential to lose control over hazardous processes - not to mention the actual damage to costly electronic hardware.

How does lightning affect a building?

Direct and indirect effects are the two broad categories. Direct effects include the physical damage produced by lightning. Ignition of fires is a clear example caused by contact of the 20,000°C lightning channel. Other direct effects include shattering of wood, windows, masonry and other poorly conductive materials. Direct effects also include the burnout of electrical power and distribution equipment caused when lightning injects high currents and voltages into a power distribution line. A common example of this is the explosion of power distribution transformers.

Lightning also causes a variety of indirect effects. These result from earth-voltage rises caused when the flash dumps thousands of amperes into the earth and from the electromagnetic fields generated by the lightning stroke currents. These induce voltage and current surges in electric power and signal circuits, which may, in turn, burn out electrical equipment connected by these circuits. Solid-state electronics are especially vulnerable to these surges unless properly protected. Of particular concern are facilities with several buildings or installations that are interconnected with above or below ground cables. These cables can experience significant induced transients. Power, telephone, data and even underground plumbing have the potential to transfer damaging lightning surges into a building. Even some fiber-optic cables can be susceptible to damage from lightning. Although the fiber-optic signal lines themselves are nonconductive, the cables are often constructed with a conductive metal sheath for strength purposes. Fiber-optic cable sheathes can attract lightning and its blast pressures may crush optical fibers.

How are buildings protected from direct effects?

Since the time of Ben Franklin, the lightning rod, or air terminal, has been the front line of defense against lightning. Its basic concept is to provide a preferential terminal for lightning that would have otherwise hit a vulnerable part of the structure. An air terminal only will protect a portion of a building, so most structures will have several lightning terminals. The spacing and position of air terminals has been well understood for many years and the proper configuration and installation of air terminals is detailed in well-known standards, such as NFPA 780 (National Fire Protection Association). Basic direct-effects protection also includes a system of down conductors connecting the air terminals to the grounding system.

The configuration of the grounding system is very important and depends upon soil conditions, building construction and the presence of other underground conductors. Grounding systems can be created with driven ground rods, plates and possibly a counterpoise, which is a buried cable encircling the site. A counterpoise adds greatly to the protection from earth voltage rises that may injure people standing on the ground.

The interconnection (bonding) of other metallic items in the building is important to prevent sparkover from a lightning conductor to other conductive items, such as water pipes, roof edgings, vent stacks or HVAC equipment, depending on their locations.

How are buildings protected from indirect effects?

Lightning can cause damaging transient voltages and current surges in equipment through a direct strike to a wire, through earth voltage rise and through magnetic field and capacitive coupling.

There are four engineering concepts that, when properly applied, can comprise a total lightning protection plan, whether for a single piece of equipment or a complex of buildings. These concepts are grounding, bonding, shielding and surge suppression.

There is no magic bullet for lightning protection. Protection is achieved only through a careful investigation to identify all sensitive components and all possible paths for lightning currents and voltages, followed by the design, specification, installation and maintenance of a protection system.

Grounding and bonding improvements are made to provide additional paths for lightning currents to flow to earth, thereby minimizing surges. These improvements usually involve interconecting adjacent conductors, such as structural steel, conduits and ground conductors. Commonly overlooked grounding problems include conduits, metal equipment cabinets and individual components within computer rooms.

Shielding of cables helps to reduce surges by providing a preferential path for lightning currents rather than the actual circuits. To be most effective, shielding must be completely continuous, and grounded or terminated to equipment housings at both ends.

In some cases, grounding, bonding and shielding provide a sufficient reduction in indirect effects. However, critical sensitive equipment, and any equipment interconnected by cables over long distances, requires the installation of surge suppressers. There are many types and many manufacturers of surge-suppression equipment. Care must be taken to select the most cost-effective device that will handle the currents and voltages expected from a severe strike. Surge suppressors should be installed where they readily can be inspected and replaced when damaged by a severe strike. In many cases, the most expensive surge suppressors turn out to be the least effective or appropriate for a particular application.

How should I protect my home and small business computers from lightning?

By far the best way to protect your computer from lightning is to disconnect it from both the power line and telephone line at the approach of a thunderstorm. Commonly available surge strips do provide protection against some of the surges caused by lightning, specifically voltages induced between the positive and negative lines of the power supply. Surge strips do not always protect against voltages that arise between the power line and house, or system ground or the telephone lines. The larger concern, at least for any one capable of reading this FAQ, is the modem. Simply stated, your computer effectively forms a circuit between your local power company and your local telephone company. The interface between these two widely distributed networks is your computer. Lightning commonly causes serious voltages to arise between circuits at different distances from the strike and/or referenced to earth ground at different locations. These voltages appear at the interfaces between these systems - your computer. It is a misconception to think that the lightning voltages are carried down the telephone lines to your computer. Rather, the voltages appear between the components of your computer. That is why a surge suppressor designed for a modem would be ineffective in most cases. This effect can be mitigated by providing a common ground reference for every device connected to your computer. In such a system, everything that is connected to the computer that has its own power or telephone connection is first connected to a device that provides a single ground path. This device is based on the concept of an equipotential plane, commonly used for lightning protection of aircraft and other advanced systems.

What about golf courses?

Because public safety is the highest concern in any lightning protection program, the most important installation for a golf course is an early warning system. Early warning systems can include dedicated systems installed at the golf course or connections to realtime lightning data from a commercial network. In addition, strategically placed shelters should be designed and constructed to withstand all lightning effects. Please see our article on golf shelters available on this web site.

Golf courses often have sophisticated irrigation systems with electronic controls. Because these systems have power and control cables distributed over many acres, they are highly susceptible to lightning-induced surges. Careful grounding and shielding and the installation of surge suppression devices can protect these systems.

There are devices being marketed with claims that they actually reduce the likelihood of a lightning strike and others with claims to provide increased effectiveness as air terminals. What are the merits of these devices?

Lightning Technologies, Inc. has no firsthand experience with these devices. Most such items are not available for purchase or installation other than directly through manufacturers and their licensees, often making it difficult for independent evaluation. Moreover, LTI has had very good success with commonly available conventional parts and materials, and so has not found it necessary or useful to use expensive, unproven and sometimes controversial proprietary protection devices. To our knowledge, no independent studies have yielded any conclusive evidence of the effectiveness of many of these devices. Total lightning protection can be achieved with the proper planning and installation of conventional lightning-protection hardware and improvements to grounding, bonding, shielding, circuit design and surge suppression. Conventional lightning-protection hardware is relatively inexpensive.

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