Lightning Protection System
A well designed lightning protection system will carry a lightning charge through lightning rods and cables on your home down to the ground and safely dissipate it. An effective system should even extend to nearby trees, outbuildings, and other structures that might attract a lightning strike. Another concern, however, is the possibility of damage caused by a power surge through your utility lines. A power surge from a lightning strike miles away can still damage your electronic equipment and telephone system.
To protect against a power surge, it is necessary to stop the surge from entering the house wiring at the main panel. This can he accomplished by installing a whole-house surge arrestor at the main panel and using individual surge arrestors, or suppressors, at points of use that protect each device or appliance at its outlet. To work properly, a surge protection system must also be well grounded because excess current is diverted back through your home's grounding system and into the earth. A good lightning and surge suppression system will offer little comfort, though, if you're hit by a major power outage lasting for days. For this, it is wise to have an optional standby generator as a backup source of emergency power.
A lightning strike occurs after a buildup of negative charges of electrical energy in a cloud and positive charges of electrical energy in the earth. As the dry air between the cloud and the ground becomes moist, negative charges move downward to meet positive charges moving upward, creating a lightning bolt. Lightning descends to earth in 150-foot steps. When a negatively charged strike is within 150 feet of a lightning rods, the positive charges in the earth surge upward through the lightning protection system to meet and neutralize the strike. An effective lightning protection system creates a cone of protection around a house. The positive charges flow safely from the ground through the cable to the lightning rod, then jump to the negatively charged lightning strike from the rod, not the surface of the house. Lightning rods are usually from 10 to 12 inches long, and contrary to myth, don't attract lightning to your home because they're not much higher than the roofline.
A lightning protection system provides a clear path for lightning to travel directly to the ground without causing injury or destruction to life or property. It consists of three major components: (1) lightning rods, or air terminals; (2) grounding rods, or grounding terminals; and (3) copper or aluminum low-resistance conductor cable to connect the terminals. Copper and aluminum components are used not only because they are excellent conductors of electricity but also because they are highly resistant to corrosion. Copper is preferred because it conducts electricity better than aluminum and less is needed to carry the same amount of current. However, aluminum is necessary on an aluminum or steel roof because copper coming into contact with aluminum or steel can cause corrosion. Nevertheless, even where aluminum is used, the grounding system must be copper. Aluminum cannot be used underground and must be spliced into the grounding wire at least 18 inches above the ground [NEC Section 250.64(A)].