Striking Distance

What is Striking Distance?

Striking Distance is termed as the shortest distance between a downward leader and a particular structure or object. It  depends on the amplitude of the lightning current.

Where ‘I’ is the minimum peak current.

‘r’ is the striking distance i.e. radius of Rolling Sphere

The minimum values of peak current is given in IS/IEC 62305-1, which is 3kA, 5 kA, 10kA & 16kA for Level I, II, III, IV respectively.

The Striking Distance Approach:

• At a point the di-electric strength of the air breaks and ionization happens, due to which it starts conducting.
• For lightning flashes to earth, a downward leader grows step-by-step in a series of steps from the cloud towards the earth.
• A further “leader” discharge similar to the downward leader begins to grow towards the head of the downward leader.
• Upward leader will be launched at points of greatest electric field intensity and can move in any direction towards the approaching downward leader.

Relation between LPL and Striking Distance: According to the IS/IEC 62305-1,  it is said that Level I, II, III & IV offers 99%, 97%, 91% & 84% probability that the Lightning Current may be greater than these levels.

Origin  of Rolling Sphere Radius: So, lets take a look how the Rolling Sphere Radius is coming from?

So if we take a quick glance at the last page and look into the formula and put the values of Ip, lets see what we get.

Now, Lets put Ip as 3 kA as per LPL I.

Case Study:

• If we install a Lightning Protection System in accordance to Level I then 99% of all lightning strikes will be intercepted. There is only 1% probability that lightning strike of amplitude less than 3kA may not be intercepted.

Again it is to be noted that lightning strikes lesser than 3kA are very rare and they don’t potentially cause any damage to the structure.

• If we are designing a Lightning Protection System in accordance to Level IV, then the Air Terminal placement will be done using the Rolling Sphere Radius of 60 m. In this case the air terminal will capture the lightning strikes of 16 kA or more than that. Thus it is giving protection percentage to 84% at least.

Conclusion:

To offer a greater lightning protection level a smaller striking distance (Radius of Rolling Sphere) is needed. The lesser the striking distance, the distance between the air terminal decreases thus increases the capability to intercept lightning strikes of lesser amplitude. In this way we can increase the total percentage of lightning strikes captured.