How far from a lightning protection installation are we still safe?

It is said that lightning protection installed on e. g. church tower enables protection for at least half of the village. It is also believed that the conductor on our neighbour's house protects us from a lightning strike.

Lightning protection installations have been known from 1752 on a building when Benjamin Franklin proved lightning to be an electrical discharge, and that a metal conductor or a collector – Franklin's rod – could protect the building from a lightning strike. The question that arises here is how wide the protected area offered by such a rod is. The information is crucial for everyone dealing with lightning installation design.

According to 18th century knowledge the protected area of such a rod extends round the rod in the shape of a cone. The diameter of the cone plane has been changing over the years. In 1892 Sir Oliver Lodge published a review of different protected area concepts. The concepts differed a lot; protected angles varied from 90°to 30°. The angles are still in use.

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Picture 1: lightning rod protection area

In 1976 an electrogeometrical model of a lightning stroke was made, which enables to define the protected area with more accuracy.

The concept of a jump distance is essential for the electrogeometrical model or the conception of the protected area. A storm cloud causes an electric field to be formed. It can reach more than 100 kV/m. Raindrops with better conductivity can cause a leading spark, which then travels to the earth coincidentally and uncontrolled. The speed of the forming of such a spark is about one thousandth of speed of light (300 km/s). About 10 − 100 m above the earth the electric field on trees' branches or spires is enlarged. When the value of the electric field overcomes the breakthrough hardness of air, a counter-spark from the point of a lightning strike unites with the leading spark from the cloud thus forming a conducting canal. The distance in which the electric field starts rising depends on the cloud charge or the lightning current flowing from the cloud to the ground. It is called a final jump distance.

For the calculation of final jump distance, where the leading spark and the counter-spark are united, Lowe's equation may be used:

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In accordance with the standard for building protection against lightning strikes IEC 61024-1 buildings are classified into four protective levels with regard to their type, use, size, location and other characteristics.

In compliance with the protective levels minimum values of lightning current amplitude are determined which enables to calculate the final jump distance for the protective levels.

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Table 1: Values of lightning current and breakthrough distance for different protective levels

In IEC 61024-1 standard there is a rollingsphere method used for the lightning protection installation design. The sphere with a radius equal to the final jump distance D is rolled all over the building that is to be protected with a lightning protection installation. Lightning strikes are possible to occur at the points where the sphere touches the building – the points are usually roof ridges and edges.

What is the protection offered by a lightning conductor, installed on the building?

Several cases of a protected area are described below.

Protected area on a church whose dimensions are:

aisle size 15 m x 30 m, ridge height up to 25 m,

church tower size 6 m x 6 m, height 40 m

In many cases the only collector on a church happens to be the cross, the aisle itself not being protected.

Protective angle 45°

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Picture 2

Protected area in accordance with IEC 61024-1, standard III protected level (sphere radius R=45 m)

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Picture 3

Picture 2 shows that in the case of a protected angle of 45° a part of the church aisle remains unprotected (outside the hatched area). Picture 3 shows a protected area (the area within the green hatched area) according to the rollingsphere method in accordance with IEC 61024-1 standard for III protective level. Also this picture suggests that the lightning conductor on the church tower does not protect the aisle. The simulation was carried out by means of the ŠČIT programme.

Protected area on a house of 10 m x 13 m, roof ridge height 9 m, roof inclination 35°

Protective angle 45°

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Picture 4

Protected area in accordance with IEC 61024-1, standard III protected level (sphere radius R=45 m)

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Picture 5

Pictures 4 and 5 show that the whole building is within the protected area offered by the lightning protection installation.

Protected area on two houses of the same size (10 m x 13 m): the house on the left has a lightning conductor installed while the one on the right does not. The houses are situated 15 m from each other.

Protective angle 45°

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Picture 6

Protected area in accordance with IEC 61024-1, standard III protected level (sphere radius R=45 m)

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Picture 7

Pictures 6 and 7 suggest that only the house with a lightning conductor installed is actually protected while the other one remains unprotected.

From the examples given above it may be seen that a lightning protection installation on the church tower does not always protect the aisle, let alone the whole village. The same can be concluded from the case of two neighbouring houses mentioned above.

Janez Podlipnik, BSc