How does a lightning protection system actually work?

This article describes how a lightning protection system conducts the electrical charge of a lightning strike to earth to reduce the chances of costly damage to a building or its systems.

This article series describes common lightning protection systems, certification, installation, and lightning protection system inspection.

We provide information about lightning strikes, lightning hazards, related equipment, sources of lightning protection system installers, and lightning strike risk assessment

How do Lightning Protection Systems Work?

This article series , in two parts, describes first how a lightning strike forms and moves between a storm cloud and objects on the earth's surface, and second, how a lightning protection system installed on a church, tree, tower or other tall object reduces the chance of lightning damage by conducing the lightning strike more-safely to earth.

Our photo shows the ornate lightning rod atop the Saxon Tower in Oxford in the U.K.

[Click to enlarge any image]

The actual lightning rod is mounted at the apex of a metal (lead) roof capping part of the Saxon Tower roof.

A thick braided copper cable is bonded to the bottom of the lightning rod.

That cable in turn extends down the tower to earth where it is bonded to a driven electrode that enables the system to conduct a lightning strike to earth while minimizing the risk of lightning damage to the tower itself.

How a Lightning Protection System More-Safely Conducts Lightning Strikes to Earth

Here we use the lightning protection system installed on a church tower as an example to illustrate the simple components of a lightning protection system.

Other articles in this series discuss these components in various installations and in added detail.

Our photo of la Iglesia Valencia in Guanajuato, Mexico, shows a large iron filigree cross atop the church bell tower.

[Click to enlarge any image]

Our photo above shows the lightning protection system cabling running along the corner of the Valencia Church steeple in Guanajuato, Mexico.

This church, la Valencia, is located on a high hilltop in a mountainous area where lightning strikes are common.

Although there is no record of his having traveled to Mexico in the course of his tests involving lightning, the lightning protection system was first invented by Ben Franklin in 1752.

Franklin proposed two purposes of a lightning rod:

1. Lightning dissipation:
   
   the lightning rod may prevent a cloud from releasing a lightning bolt at all by reducing the earth-level static charge being formed by the storm and lightning process described above.
   
   By conducing the developing static charge at the earth's surface - at least in the vicinity of the tall object being protected by the lightning rod - the static charge is reduced and we slow or stop entirely the formation of the rising streamer.
   
   Franklin and other early lightning researchers thought that lightning dissipation was the most-important function of lightning protection systems. Modern physicists disagree, pointing out that the ionized air charge dissipation effect of a lightning protection system only affects the air a few meters around the tip of the lightning rod itself.
   
   This area is trivially small compared with a kilometer-sized and electrically-charged space between the lightning rod and the underside of the storm cloud.
2. Lightning diversion:
   
   the lightning rod combined with cable and ground rod (parts we detail just below) provides a very good conductor (having very low electrical resistance) between the lightning rod atop the protected object (church steeple or something else) and the earth.
   
   Thus if a lightning strike does begin to form between the protected object and the cloud, the lightning protection system conducts that electrical energy safely to earth.

Otherwise that same energy would pass through the protected object itself, almost certainly causing more serious damage or perhaps a fire as well.

As a child living in Dunnsville on the Rappahannock river in Virginia I sat on our screened porch (on the house shown here) on a high riverbank as we watched thunderstorms move up the river from Tappahannock.

More than once we watched as lightning struck and literally exploded trees on the riverbank near our home. Without a lightning protection system on the tree, when it participated in a lightning strike the heat caused by the tremendous electrical energy flowing through the tree vaporized its sap and blew the tree in half or into smithereens.

The same energy that causes air to heat and expand to create thunder vaporizes a tree or blows apart masonry blocks of a church steeple if it participates in a lightning strike unprotected by a lightning protection system.

More than a century ago the Valenciana church in Guanajuato (shown above and earlier on this page) installed a lightning protection system which encompasses these simple parts.

1. An iron cross atop the church steeple
   
   (photo at the start of this section) acts as an air terminal. When weather conditions create a lightning strike that would occur on or close to the church steeple, the air terminal provides an attracting strike point.
   
   Air terminals (rods - the sketch at page top is of a lightning protection system terminal rod tip) are fastened to the building to intercept electric discharges which might otherwise strike a building component itself, such as a chimney or metal roof.
2. Cabling from the iron cross
   
   is routed to ground as we show in a photograph just below. That heavy cabling safely conducts the very high voltage of the lightning strike safely to earth.

Electrical discharges striking the air terminal are directed through heavy duty metal conductors to a grounding system (rods driven into the soil) and thence into the earth.

In our photo below, the yellow arrow points to the lightning protection system cable near the base of the church tower, while the red arrow shows how the tower masons constructed a hollow tubular corner on the tower through which the lightning arrestor cable passes down to reach contact with the earth below the paved area around the tower.

In sum, lightning protection systems control electrical discharges by directing them through a low-resistance path to the ground, avoiding passage through parts of a structure and reducing risk of fire or other damage.

Proper electrical bonding of all of the metal components of the lightning protection system, including lightning rod to cable and cable to ground rod that is driven into the earth, is critical for safe system operation. [Click to enlarge any image]

Watch out: Our OPINION is that if the bonding of a lightning rod system may have been interrupted the presence of the thus incomplete lightning protection system on the building might increase the risk that the building will be damaged by a lightning strike.

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When I took these lightning protection system photos (in 2012) someone had pulled out a loose loop in the lightning protection cable and was using it to secure the end of the bell pull rope.

When I see a loose loop in a lightning protection system conductor and when I can't see the grounded end of the cable I'm worried that the system may have been damaged and may be ineffective. Happily, when we last checked in 2016, the tower remained intact.

Examples of proper and improper lightning protection system installation are given throughout this article series.

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Continue reading at LIGHTNING PROTECTION SYSTEMS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• LIGHTNING BOLT FORMATION & MOVEMENT
• LIGHTNING PROTECTION SYSTEM DAMAGED, UNSAFE

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