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Property protection and preparation

Lightning is a natural phenomenon that happens all across the United States. Severe storms occur more frequently in some parts of the country than others, but every business needs to take precautions. Direct lightning strikes can result in a building fire, but lightning doesn’t need to hit a building to cause damage. Tall structures, structural steel buildings, reinforced concrete structures and metal buildings are especially vulnerable, as are flammable liquid and gas tanks.

The fact that lightning strikes cannot be accurately predicted makes absolute safety from their effects almost impossible. However, hazard levels can be determined and adequate protection taken to lessen the potential for loss. A correct assessment of the risk and resulting installation of lightning protection equipment by a qualified contractor are essential aspects of controlling loss of life and property damage.

The following information is intended to inform readers of the risks associated with lightning and to familiarize them with the principles of lightning protection systems for protection of buildings and equipment inside the buildings.

Social and economic costs of lightning

Lightning can be one of the most dangerous and frequently encountered weather hazards. Deaths caused by lightning are second only to those weather-related deaths resulting from floods and flash floods. Many lightning victims are individuals engaged in recreation or work. Although most survive, survivors generally suffer long-term effects, including memory problems, numbness, attention deficits, sleep disorders, confusion and general loss of strength. Many also are left with a storm phobia.

Annual property losses caused by lightning regularly total in the hundreds of millions of dollars. Communication equipment and computer systems/networks are getting more sophisticated and businesses rely on them quite heavily. The loss of a computer system and communication system can result in large business income losses in addition to the physical damage to the equipment and structures. With the rapid growth of devices controlled by microprocessors across our society, this will tend to accelerate costs and potential losses if we fail to adequately assess lightning hazards and adopt standard practices of protection.

Facility protection and preparation

Basics of lightning protection systems

The fundamental principle in the protection of life and property against lightning is to provide a means by which the lightning’s discharge can enter the earth without causing damage. When lightning follows a path with a relatively high electrical resistance, damage is caused by the heat and mechanical forces that are generated during the passage of the discharge.

An effective lightning protection system must: (1) intercept the lightning discharge before it can strike the object being protected and (2) discharge the lightning current harmlessly to earth.

In order to accomplish this, a low resistance path consisting of an adequately sized metal conductor that is continuous from the air terminal to the ground terminal must be installed. Because metals are good electrical conductors, they are virtually unaffected by either the heat or the mechanical forces as long as they are of sufficient size to carry the current.

Building construction types also will influence the degree of lightning protection that is required. For example, a non-combustible building constructed of structural steel framing with a sufficient cross-sectional area that is electrically continuous may be protected by installing air terminals on the roof, connecting the terminals to the metal framing and grounding the framing at the base.

Reinforced concrete structures, where the reinforcing rods are electrically bonded and grounded together, can be similarly protected. Metal clad on wood frame buildings need to be equipped with complete lightning protection systems due to the likelihood that lightning will puncture the thin sheet metal and ignite the wood framing. This protection can only be waived when the building is constructed of 3/16-inch minimum sheet metal that has been made electrically continuous by bonding.

A complete lightning protection system consists of air terminals on the roof or other elevated structures, ground terminals, the conductor system that connects the air terminals to the ground terminals, a bonding system, and lightning surge arrestors and voltage surge suppressors. When these devices are properly located and installed, although protection cannot be guaranteed, the probability of lightning damage can be reduced.

Air terminals

The component of a lightning protection system designed to intercept the lightning flash is called the “air terminal.” The structure should be examined and the installation of air terminals should be planned for all areas that are likely to be struck by direct lightning flashes. Air terminals should be placed on the tallest items and should be placed around the perimeter of flat roofs at intervals not exceeding 20 feet. The air terminals should be placed high enough above the structure to eliminate the danger of fire caused by the electrical arc. The zone of protection provided by the air terminal is generally assumed to be a function of the shielding angle, sometimes expressed as the ratio of the horizontal distance protected to the height of the mast. Depending on the code that is used, the shielding angle typically ranges between 30 to 45 degrees.

Down conductors

The vertical conductors attaching the air terminals to the ground terminals are referred to as “down conductors.” These conductors should be coursed around the roof edges and properly connected to the air terminals and the grounding rod. Down conductors should be installed to offer the least resistance to the passage of the current. The most direct path is best and there should be no sharp bends or narrow loops to cause blocking (choke) of the electrical current. Conductors should be securely fastened to the structure at intervals not exceeding 3 feet. At least two down conductors should be provided on all buildings and the average distance between the down conductors should not exceed 100 feet.

Bonding system

A major concern in the protection of a building is the ability to maintain an equal electrical potential between the conductors of the lightning protection system and any other grounded metal bodies or wires located within the building. When a building has metal objects of considerable size located within a few feet of a lightning conductor, there will be a tendency for sparks or side flashes to jump between the metal objects or wiring and the conductor. To reduce the possibility of sparking or side flashing, it is necessary to interconnect the grounded metal bodies and wiring to the lightning protection system. This electrical connection will significantly reduce the potential differences that can be created by lightning currents.

Ground terminals

Ground connections (ground rods, ground plates or ground conductors) provide electrical contact with the earth and are essential to the effectiveness of a lightning protection system. These must be properly designed to provide ample contact with the earth. The resistance and surge impedance of the ground terminal must be low enough to prevent side flashes between the down conductor and other grounded objects.

In general, the extent of the grounding arrangements will depend on the earth’s resistivity (measured in ohm-meters). Grounding arrangements will range from a simple extension of the conductor into the ground where the soil is deep and of high conductivity, to an elaborate buried network where poor conductivity results from dry or rocky soil conditions.

Vertical rods or pipes driven into the ground are the most common grounding system. Ground rods should be at least a 1/2 inch in diameter and 8-feet long. Rods should be copper-clad steel, solid copper, hot-dipped galvanized steel or stainless steel. Rods should be free of paint or other non-conductive coatings. In addition, to improve personnel safety during a storm, the ground terminals should be buried and arranged to prevent excessive voltage gradients in the soil surrounding the ground terminal.

Likely lightning targets

Properties most likely to be struck by lightning are those that are located on higher ground or that project above surrounding properties such as chimneys, flagpoles, towers, water tanks, steeples, ridges and parapets. On flat-roofed buildings, the edge of the roof is the most likely area to be struck. Protection from lightning strikes should be provided for any buildings or structures that:

  • Have experienced previous lightning damage
  • Are located in areas of known high thunderstorm frequency
  • Are occupied by hazardous storage or operations
  • Present a significant life safety exposure
  • Have the potential to experience a large property or business interruption loss if struck
  • Contain sensitive electronic equipment

Personal protection

  • Watch for developing thunderstorms when outside.
  • When a thunderstorm has been sited or thunder has been heard, move yourself and employees to a safe shelter indoors and away from windows or inside an enclosed vehicle.

Installation codes and standards

All lightning protection systems and equipment should be installed in accordance with the National Fire Protection Association (NFPA) 780 “Standard for Lightning Protection Systems” and NFPA 70 “National Electric Code” and should comply with the requirements listed below.

Buildings and structures protection

A complete lightning protection system should be installed to intercept the lightning discharge before it can strike the object being protected, and arranged to discharge the lightning current harmlessly to earth. The system should be installed and maintained by qualified and/or certified installers. An example of a certified installation is the “UL Master Label” lightning protection program. These systems are installed by Underwriter’s Laboratories (UL) listed installers, constructed with UL-listed components, and maintained by the UL re-examination service.

Electrical service protection

To protect the interior electronic equipment and communication lines from direct discharges and/or induced currents that occur on overhead transmission lines, lightning arrestors (surge arrestors) should be installed on the power lines coming into the building. The surge arrestors should be installed on the line side of (before) the main circuit breaker or fuse. The arrestors may be installed at the yard pole, at the outside electrical service entrance, or before the interior service connection. The surge arrestors are intended to protect wiring systems and equipment by diverting the energy of the lightning strike directly to earth. However, due to the magnitude of the energy generated by a lightning strike, there is no guarantee that any system will provide protection against a direct lightning strike to the electrical system.

Electric motor protection

To properly protect large electric motors or equipment, additional surge arrestors and/or capacitors should be installed on the load side of (after) the main circuit breaker or fuse, immediately upstream of the equipment being protected. Because of the variety and complexity of the protective devices that are available, the installing electrical contractor must work closely with the equipment manufacturers to verify that the appropriate devices are used.

Sensitive electronic equipment protection

UL-listed transient voltage surge suppressors (TVSS) should be installed on all branch circuits that serve computers or other sensitive electronic equipment. The UL-listed TVSSs are marked with a suppressed voltage rating (SVR). The manufacturer of the equipment that is being protected should be contacted to determine the recommended SVR rating. The voltage surge suppressors can be “hard-wired” by a qualified electrician into the building electrical distribution system or portable plug-in units can be purchased and used to connect the electronic equipment to the building electrical outlets.


Because a lightning protection system is expected to remain in working condition for long periods with minimum attention, the mechanical construction should be sturdy, and the materials used should offer resistance to corrosion and mechanical injury.

In addition, because the adequacy of the grounding system can decrease over time, the grounding system must be tested to ensure it is maintained at five ohms or less.

It is recommended that lightning protection systems be visually inspected at least annually. Complete, in-depth inspections of all systems should be completed every three to five years. Critical systems should be inspected every one to three years depending on the occupancy or the environment in which the protected structure is located. In addition, the lightning protection system should be inspected whenever alterations are made or a lightning discharge to the system has occurred. When conducting a visual inspection, the following items should be assessed:

  • The system is in good repair.
  • There are no loose connections that might result in high- resistance joints.
  • No part of the system has been weakened by corrosion or vibration.
  • All down conductors and ground terminals are intact (non-severed).
  • All conductors and system components are securely fastened to their mounting surfaces and are protected against accidental mechanical displacement.
  • There have not been additions or alterations to the protected structure that require additional protection.
  • There is no visual indication of damage to surge suppression (over voltage) devices.
  • The system complies in all respects with the current edition of the NFPA 70 “National Electrical Code” and the NFPA 780 “Standard for Lightning Protection Systems.”


Although no fail-safe lightning protection system presently exists, steps can be taken to help mitigate the property and life exposure to loss from lightning. Any property containing highly susceptible equipment, or located in a lightning prone area, should take steps to alleviate this loss potential.


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