Building Construction, Renovation, Maintenance & Advice

Lightning Rods and Conductors

Thunder, Lightning and Lightning Rods and Conductors

Lightning conductors or lightning rods are used to protect buildings from lightning strikes. Here we look at what lightning is, the different types of lightning, thunder, fear of lightning and how we protect buildings by installing lightning rods or conductors with a very good connection to earth.


See the full Fixed Abode article "The Grilled Fulminologist" here


Thunder and Lightning

People who study lightning are known as fulminologists and probably the most well known fulminologist was Benjamin Franklin who famously went flying a kite in a thunderstorm.

It isn’t easy studying lightening. You can never predict when or where a lightning strike will occur and even if you could you wouldn’t want to get too close now would you?

What is Lightning?

What we do know is that lightning is caused by electrical discharge. A difference of electrical energy builds up between clouds and the ground (or between clouds and clouds) until the potential difference is large enough for an electrical discharge to take place in the form of a bolt of lightning. Usually the ground is positively charged while the clouds are negative. The lightning takes the path of least resistance usually between tall objects (such as trees and tall buildings) that are electrically connected to the ground but closest to the base of the clouds. A current of anything between 30,000 to 120,000 volts flows from the ground to the clouds. This is negative lightning but in some cases lightning can be positive flowing from the clouds to the ground. In positive lightning the current can be far greater and may be as much as 300,000 volts.

Most lightning strikes are in fact made up of 3 or 4 (or maybe more) ‘strokes’ each about 40 to 50 milliseconds apart.

Types of Lightning

There are different types of lightning.

  • Cloud to cloud lightning is the most common type and is an electrical discharge which may be within one cloud or between clouds.
  • The second most common type of lightning and the best known is Cloud to Ground lightning (or Ground to Cloud lightning depending on which way the electricity flows) which is an electrical discharge between a cumulonimbus cloud and the ground. This is the type that poses most threat to people and property because, surprise surprise, it strikes the ground or anything connected to the ground.
  • Forked lightning is cloud to ground lightning that branches into several paths.
  • Sheet lightning is a name for cloud to cloud lightning where the discharge itself cannot be seen but the cloud appears to light up as a sheet of light.

There are other types of lightning such as ball lightning, bead lightning and ribbon lightning but we won’t bother with those today.

Astraphobia - Fear of lightning

Astraphobia is the ‘irrational fear of lightning’ (I’m not so sure it can be considered irrational to be a tad disconcerted at the thought of 300,000 prime herberts up your chuff but that is the definition).

Believe it or not astraphobia is even more prevalent than Politicophobia (the fear of politics) although the symptoms are similar and include sweating, trembling, crying, panic attacks, feelings of dread, and rapid heartbeat. The victim might hide underneath a bed, under the covers, in a closet, in the cellar or any other place where they feel safe. Efforts are usually made to smother the sound of the thunder (or the politician); the person may cover their ears or curtain the windows. A sign that someone has astraphobia is a heightened interest in weather forecasts.

Between 15 and 30 percent of dogs frequently exhibit severe anxiety during thunderstorms.

Thunder

When you get lightning you will also get thunder.

Some people think that thunder is the sound made by lightning. They say that the air in and around a bolt of lightning gets really, really hot (about 20,000 degrees C) and expands rapidly causing a shock wave which travels at the speed of sound, a sonic boom no less, which, if you are close by, will sound like a sharp crack while further away will sound more like a rumble. Light (299,792,000 metres per second) travels a lot faster than sound (343 metres per second) so there will be a time delay between seeing the lightning and hearing the thunder that follows depending on how far away the lightning is. A distance of one kilometre away will give a delay of about 3 seconds but if you are very close, the sound is almost instantaneous and you may even smell ozone. Any closer and you won’t hear or see much at all, ever again.

Early work on lightning

Tall church spires have historically been fairly obvious prime targets for lightning strikes.

In 1745 Peter Ahlwardts wrote a paper “Reasonable and Theological Considerations about Thunder and Lightning" which advised individuals seeking cover from lightning to go anywhere except in or around a church.

In 1749, Benjamin Franklin invented the lightning conductor.

The purpose of a lightning conductor (or lightning rod) is quite simple, to provide a safe pathway for lightning to discharge to earth that avoids damage being caused to structures or people.

Design of a lightning conductor

A lightning conductor has three important elements:

  1. A spike on the top which assists in generating an electric field. The tip of the spike should be rounded but not too blunt to be most effective.
  2. A copper cable to connect the spike to earth by the most direct route possible. A good solid cable is needed usually about a centimetre in diameter.
  3. A very good earth connection into the ground. A good earth is a centimetre diameter copper rod 4 metres long pushed into the ground. The earth connection needs to be readily accessible so that the effectiveness can be checked from time to time. The Earth should be well installed and tested to have a very good electrical contact with earth. Earth connections are checked by measuring the electrical resistance between the copper rod and the surrounding ground.

Placement of a lightning conductor

Lightning conductors should be placed close to or on the structure they are to protect and the top of the lightning conductor must be situated higher than the top of the structure. If it is mounted on the structure the cable to earth should be as straight as possible and placed on the outside surface rather than within the structure.

In some cases in the past the framework of steel reinforcing bar has been used to carry the electric current to ground. This is not a good idea. The concrete and steel in reinforced concrete combine to form a synergistic relationship (the sum is greater than the combined strength of the parts) which is dependent on the concrete’s ability to encase the steel. The huge currents involved in lightning create enormous heat which will make the steel expand and is bound to damage the integrity of the concrete’s encasement of the steel thereby weakening the structure.

Do not connect other things to lightning conductors

It is also important that the lightning conductor should not be connected to other items such as electrical circuits, television aerials, computer equipment or little boys who have a compulsion to take things apart or poke their fingers where they don’t belong.

A friend called me a while ago. He climbed onto his roof and was shocked with what he found. He rang his internet service provider:
“Excuse me but do you realise your internet dish has been bolted to my lightning conductor.”
“Oh yes sir we often do that.”
“Do you think that is a good idea?”
“No one’s complained so far.”

We simply cannot comprehend how much power is released when lightning strikes. Make no mistake lightning is very powerful. Once released it will take any easy path it can to get to earth and will frizzle, fry, toast, bake, roast, grill, poach or barbeque anything in its path.

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Copyright © Phil Wilson April 2012
This article or any part of it cannot be copied or reproduced without permission from the copyright owner.

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