Direct Lightning Protection

An audit to recommend lightning protection system for structures calls for understanding the characteristics and effects of lighting strike and methods to be adopted to safe guard the assets and live stocks.
We advocate Lightning Protection design based on the characters of the lightning, strikes per year and geographic location of the structure.
We follow the methodology and procedure to audit the site/structure as per BS 6651:1999.

Structures with inherent explosive risks, e.g. explosives factories, stores and dumps and fuel tanks, usually need the highest possible class of lightning protection system.
The only question remaining is whether protection is necessary or not for all the other structures. In many cases, the need for protection may be self-evident, such as:

1. Large numbers of people congregate
2. Essential public services are concerned
3. Area is one in which lightning is prevalent
4. There are very tall or isolated structures
5. There are structures of historic or cultural importance
6. There are structures with explosive or flammable contents

An assessment based on the exposure risk of the structure being struck are:

1. The use to which the structure is put
2. The nature of its construction
3. The value of its contents or consequential effects
4. The location of the structure
5. The height of the structure (in the case of composite structures, the overall height)

The zone of protection is the volume within which a lightning conductor gives protection against a direct lightning strike by directing the strike to it. The size and shape of the zone varies according to the height of the building or vertical conductor.

Generally, for structures not exceeding 20 m in height:
For a vertical conductor rising from ground level, the zone is defined as a cone with its apex at the tip of the conductor and its base on the ground.
For a horizontal conductor, the zone is defined as the volume generated by a cone with its apex on the horizontal conductor moving from end-to-end.

The structures exceeding 20 m in height:
The above zones are not necessarily applicable and it is recommended that additional lightning protection conductors be provided to protect against strikes on the side of the building.

Characteristics of lightning

Current in a lightning stroke:
The important part of a lightning flash with regard to the resulting damage is the return stroke. The current in this return stroke ranges from about 2 000 A to about 200 000 A and its distribution of values is of the form which occurs frequently in nature, the so-called log normal distribution, as follows:

1 % of strokes exceed 200 000 A;
10 % of strokes exceed 80 000 A;
50 % of strokes exceed 28 000 A;
90 % of strokes exceed 8 000 A;
99 % of strokes exceed 3 000 A;
As the current is discharged through the resistance of the earth electrode of the lightning protection system, it produces a resistive voltage drop which may momentarily raise the potential of the protection system to a high value relative to true earth.

Side-flashing
The point of strike on the protection system may be raised to a high potential with respect to adjacent metal. There is therefore a risk of flashover from the protection system to any other metal on or in the structure.

The thermal effect of a lightning discharge is confined to the temperature rise of the conductor through which the current passes. Although the current is high, its duration is short and the thermal effect on the protection system is usually negligible.
Where a high current is discharged along parallel conductors in close proximity or along a single conductor with sharp bends, considerable mechanical forces are produced.
Consultation should take place between the designer of the lightning protection system and the interested parties such as Architect, Builder, Fire and Safety Officers, before and during all stages of design.

In the case of structures having no suitable metallic members, it is important to consider the positioning of the entire component parts of the lightning protection system so that they perform their function without detracting from the appearance of the structure.

Modern buildings use metal extensively in their structure and there is considerable benefit in utilizing such metal parts to maximize the number of parallel conducting paths; often the lightning protection is improved, worthwhile cost savings may result and the aesthetic appearance of the structure preserved.

However, it should be borne in mind that a lightning strike to such a metal part, especially if it is beneath the surface, may damage the covering and cause masonry to fall. This risk can be reduced, but not eliminated, by use of a surface-mounted lightning protection system.

The metal parts which should be incorporated into lightning protection systems are: steel frames, concrete reinforcing bars, metal in or on a roof, window cleaning rails and handrails. Some metal within a structure may be used as a component of the lightning protection system; for example, sheet piling, being in contact with the general mass of earth, may be used as an earth electrode and is unlikely to be improved upon by the addition of rods or tapes. The whole structure should be provided with a fully interconnected lightning protection system, i.e. no part of the structure should be protected in isolation.