Lightning is a natural hazard, being the discharge of static electricity. Some of them cause damage to buildings or equipment and a few even kill or injure people and animals directly or indirectly by causing fire and explosions. The important part of a lightning flash, from the resulting damage point of view is the "Return Stroke". This is the part in which the a charged cell in a cloud is discharged to earth. 

Effects of Lightning Stroke:

As the current is discharged through the resistance of the earth electrode of the lightning protective system, it produces a resistive voltage drop which raise the potential of the protective earth system, to a value higher than the true earth potential. It may also produce around the earth electrodes, a high potential gradient which causes damage to persons and animals.

The effects of a lightning discharge is confined to temperature rise of the conductor through which current passes. Although the current is high, the duration is short and hence the thermal effect on the protective system is usually negligible.

Zone of Protection:

The term Zone of protection can be explained simply as the area within which the lightning conductor gives protection against direct lightning strokes by directing the stroke to itself. 

A Substation has to be shielded from direct lightning strokes either by using Shield wires / Earth Screen wires or by using Spikes (on masts). The procedure followed is by suitably placing the shield wires by forming an 'air termination network'. An acceptable degree of protection is defined as 45°, considering the zone protected by only one mast. For zones that are protected by more than one masts, the angle of protection between the two masts (the distance between the two masts is twice that of the height of one mast) is 60°. ( Figure - 2)

Components of the Protective System: 

The components of the the lightning protective system includes the following;
1. Air terminations
2. Down Conductors
3. Joints and Bonds
4. Testing Joints
5. Earth Electrodes

The 'air terminations' include both horizontal and vertical conductors. The earth screen wires or the shield wires that are strung across the masts forming a protective zone, are the air termination network.

The purpose of a down conductor is to provide a low impedance path from the air termination to the earth electrode so that the lightning current can be safely conducted to earth. A down conductor should follow the most direct path between the air termination network and the earth termination network.

The bonds and joints in a lightning protective system has to be electrically and mechanically effective and has to be reduced to the maximum extent. Any other joints other than being welded represents a discontinuity in the current carrying system. 

Each down conductor should be connected to the earth termination network through test joints. Each of the earths should have a resistance not exceeding the product obtained by  multiplying 10 ohms by the total number of earth electrodes. 

Procedure for Lightning Protection:

Consider a simple area that is to be protected by two masts (Figure - 1). As stated above, the acceptable degree of protection for area outside the structure is 45° and for the area between the two masts is 60°. The height of the protective zone is to taken. In general, the height of the Circuit breaker is taken as the height of the equipment bus since the height of the circuit breaker is more than other equipment. A margin of 0.2 meters is added to the equipment bus height and is taken as the height of the zone to be protected. 

The total height (h1)of the supporting tower for protection is considered including the height of the plinth. The height of the object to protected(h2) is the sum of equipment bus height and a marginal height of 0.2m. From this, the height above the equipment (h3) can be found out as (h1-h2). 

The zone of protection (in distance) outside the mast can be found using the formula : [ h3 x tan(45) ]

Similarly, the zone of protection between two or more masts can be found using the formula : [ h3 x tan(60) ]

For instance;
h1 = 16.3 m
h2 = 5.70 m (5.50 + 0.2)
h3 = (h1-h2) = (16.3 - 5.70) = 10.60

As per above stated formula, the protection zone outside the mast is : 10.60 x tan(45) = 10.60
and, the protection zone between masts is : 10.60 x tan(60) = 18.36

Layout Preparation:   (Figure-3 to Figure-6)

The Lightning Mats (LM) has to be placed in suitable position maintaining the clearance required and as per site conditions. The protection zone is to be calculated as per the aforementioned procedure. From the above example, we get the protection zone outside the mast as 10.60m of radius. Considering this, a circle of 10.60m radius is to be drawn from center of the mast. (Figure - 3) 

Similarly, the protection between the masts are 18.36m of radius (Figure - 4). So, circles of 18.6m radius is drawn from the center of the mast. The interference portion of the circles drawn are joined by tangent lines (Figure - 5). This forms the 'protective zone'. (Figure - 6)

If the equipment to be protected falls well within this zone, the position of the LMs are correct and if not the position and/or the number LMs needs to adjusted and checked again. 


   Figure - 1 [ A simple Protective Zone of two masts ]


   Figure - 2 [ Angle of protection for two or more masts ]


 
 Figure - 3 [ Two LM with circle of 10.6m (45deg protection)

 
  Figure - 4 [ Intersection of 60deg protection circle ] 



   Figure - 5 [ Tangent lines for portion between two LM ]



  Figure - 6 [ Zone of Protection between two LMs ]