Lightning Protection System Design - Mesh Method (IEC62305)

Introduction:

The main and most effective measure for protection of structures against physical damage is considered to be the lightning protection system (LPS). IEC 62305 explains about the Internal and External lightning protection system. An External LPS provides protection against physical damage and life hazards whereas internal LPS provides protection for electrical and electronic systems.

The air termination system of an external LPS can be composed of rods, catenary wires or meshed conductors.

The following methods are specified by IEC 62305-3 to determine the position of air termination system.

  1. Protection Angle Method
  2. Rolling Sphere Method and
  3. Mesh Method

Mesh Method:

Mesh method of lightning protection is suitable for structures with flat surfaces and sheds having slope less than  1/10. Mesh method should not be used for curved surfaces. The size of mesh for different classes of LPS are as  follows.

As per IS/IEC 62305, for the purposes of protecting flat surfaces using mesh method, all of the following conditions should be fulfilled:

  1. Air termination conductors are positioned
    - On roof edge lines,
    - On roof overhangs,
    - On roof ridge lines (slope < 1/10)
  2. The mesh dimensions are not greater than the values specified by standard.
  3. The network is constructed in such a way that the lightning current will always encounter at least two distinct metal routes to earth termination.
  4. No metal installation protrudes outside the volume protected by this method.
  5. The air termination conductors follow, as far as possible, the shortest and most direct route.

For buildings with some items protruding from the flat surface, PAM is commonly used to supplement the mesh method. The protection provided by the air termination rod can be calculated by using their physical height above the structure to be protected and the angle for that specific height as specified IEC 62305-1.

If the structure to be protected has different roof levels, we have to install mesh of suitable size on all the roof levels and the mesh of different roofs should be interconnected.
Let us consider a residential building, which utilises the natural components (reinforcement bars) of the building as down conductor with external ring earthing being installed below the ground level. For this condition, the down conductors should be connected to reinforcement bars near the roof of the building and the connections to external ring earth electrodes are taken from the reinforcement bars at a height of 1.5m above the ground level.

Conclusion:

From the above analysis, we can conclude that the mesh method of protection cannot be used for complex buildings. The mesh size purely depends on the level of protection considered.

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