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Lightning Protection System

by Ron Kurtus (revised 14 April 2011)

Lightning is a huge static electric spark created during a thunderstorm. Lightning strikes can be damaging to buildings and equipment, as well as dangerous to people.

Buildings often use a lightning protection system consisting of a lightning rod (also called a lightning conductor) and metal cables to divert and conduct the electrical charges safely into the ground. Another form of lightning protection system creates a short circuit to prevent damage to equipment. The electrically conducting metal skin of commercial aircraft is isolated from the interior of to protect passengers and equipment.

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Protecting buildings

When lightning jumps toward the earth from a cloud during a thunderstorm, it will seek out the highest structure that has collected electrical charges opposite of the static electric charges in the cloud. A high tree in a field or a tall building can be likely lightning targets.

Lightning rod invented

In 1752, Benjamin Franklin invented the lightning rod, which was meant to extend above a building and attract the lightning bolt to the rod, where it could be diverted harmlessly to the ground.

Lighting diversion system

A lightning rod (also called lightning conductor) is just part of a lighting diversion system meant to protect building from damage due to a lightning strike. Included in the system is the metal cable or rod that extends down into the ground and electrodes in the earth to safely dissipate the energy away. Often buildings prone to being struck by lightning will have a network of lightning rods and conductors.

Lightning rods on the roof of a barn

Lightning rods on the roof of a barn

The idea is that the electricity will take the path of least resistance and thus bypass the structure of the building as it travels to the ground. But also, the configuration of tip of the lightning rod is such that it is a good receptor for lightning.

Round tip better

When Franklin invented the lightning rod, he felt that a sharp tip would be best for attracting electrical charges and thus lightning. It wasn't until 2000 that scientists at the Langmuir Laboratory for Atmospheric Research in New Mexico proved that a rod with a rounded or spherical end works better.

Protecting sensitive equipment

When lightning strikes an electrical power line or communication line external to a building or when it strikes a building and jumps to one of those lines, it can create a surge of current that will burn out or damage electronic instrument and computers.

Lightning arrester is at input to building

A lightning arrester is a device that is connected between each electrical conductor in a power and a communications system and the Earth or ground. They create a means to short circuit the surge in power, limiting the rise in voltage. Usually, lightning arresters are placed where the power and communication lines enter the building.

Surge protector is near equipment

Surge protectors are often used between the power and communications outlets and the computer or other electronic device. They add an extra security. In most residential buildings, there is not lightning arrester, so surge protectors are important.

Protecting an airplane in a storm

Although most commercial airliners try to avoid flying through a thunderstorm they occasionally have no choice. In such a situation, it would seem that the airplane could be in danger of being struck by lightning and damaged. In fact, individual airliners are struck by lightning an average of once a year.

The way commercial aircraft are protected from lightning damage is by keeping the lighting current in the outer skin of the airplane. This system works so effectively that the last crash due to lightning was over 40 years ago.

Keeping lightning current on the outside

Since most commercial aircraft skins are primarily made of aluminum, a very good conductor of electricity, most of the lightning current remains on the exterior skin of the aircraft. The aircraft are designed to make sure that there are no gaps in this conductive path.

Since the aircraft is speeding through the clouds, the excess charges will then be dissipated into a region of cloud that has an opposite charge.

Protecting electronics

However, when the lightning is traveling along the exterior of the aircraft, transient effects, similar to electrostatic induction, can create power surges in the aircraft wiring that could damage electronics and aircraft computers. By shielding wiring and equipment and adding proper grounding and surge suppression devices. The problems caused by these lightning effects can be averted.

(See Electrostatic Induction for more information.)

Preventing fuel explosions

Likewise, the fuel system must be protected from even tiny sparks caused the lightning currents in the aircraft exterior that could result in an explosion. The skin around the tanks, fasteners and fuel filler caps must be able to prevent sparks near the fuel.

Misconception on static wicks

A common misconception is that the static wicks on the edges of the wings. Since they look like little lightning rods, many people think they are used to attract lightning to them, as a way to protect the aircraft.

Static wicks on Boeing 737 wings

Static wicks on Boeing 737 wings

From Boeing 737 Technical Site

These static wicks actually encourage the static buildup on the airframe, during normal flying conditions, to bleed off so that the charges will not accumulate and cause radio interference.

Protecting sensitive equipment

Although the electrical charges are connected through the metal skin of the airplane, there still is the possibility that some current can leak into the electrical system and cause damage to sensitive electrical and computer equipment. To protect against that, the circuits employ surge protectors to either block or short circuit excess current.

Summary

Lightning strikes can be damaging to buildings and equipment, as well as dangerous to people. Buildings often use a lightning protection system consisting of a lightning rod and metal cables to divert and conduct the electrical charges safely into the ground. Lightning arresters and surge protectors prevent damage to equipment. The electrically conducting metal skin of commercial aircraft is isolated from the interior of to protect passengers and equipment.


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Resources and references

Ron Kurtus' Credentials

Websites

Fundamentals of Lightning Protection - National Lightning Safety Institute

Lightning Protection for Buildings - eHow.com

Invention of the Lightning Rod - The Great Idea Finder

Lightning Protection System - Wikipedia

Lightning Rod - Wikipedia

Protecting Airplane in a Storm -Physlink.com

Lightning Testing and Protection

Static Electricity Resources

Books

Top-rated books on Lightning Protection


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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