Introduction:
ESE Air Terminal Rod is an active type air terminal which stimulates continuous upward leader before any other object does within its radius of protection. Due to its earlier triggering of upward leader, the area of protection is much larger when compared to a simple conventional air terminal rod. The area protected by an ESE terminal depends upon the time difference between the streamer raised from an ESE terminal and the streamer raised from other passive components located at same height. If this time difference is higher, the area protected by the air terminal will also be higher. That time difference is generally termed as triggering advance time and it is always expressed in micro seconds (10-6s or us).
Triggering Advance Time (t):
The triggering advance time is defined as the difference in triggering time of an early streamer lightning rod and a simple conventional air terminal rod obtained when both rods are exposed to the same atmospheric & electrical conditions.
Δt= TSRAT - TESEAT
Where,
TSRAT = The mean triggering time of the upward leader of a simple conventional air terminal rod.
TESEAT = The mean triggering time of the upward leader of an ESE air terminal rod.
As per NFC 17-102/2011, the value of 'Δt' should be between 10us and 60us. An air terminal is considered as ESEAT only if the triggering advance time is greater than 1Ous also the value 'Δt' being greater than 60us, it is to be still considered as maximum 60us for all design calculations.
Radius of Protection - NFC 17-102/2011:
The radius of protection is the distance between the point where you want to place the (ESE) air terminal rod and the farthest point from the structure or building to be protected. For calculating the radius of protection, it is very important to get the triggering advance time of the device and the height of the mast on which the ESE air terminal rod is mounted upon. The ESE Air terminal should be installed at least 2 meters over the surface of the structure to be protected.
Calculation for Radius of Protection:
For Early Streamer Emission air terminal, the radius of protection for different levels can be calculated using the following equation:
For h ≥ 5m
Rp(h) =√((2rh – h²) + Δ(2r + Δ)
For 2m ≤ h ≤ 5m
Rp(h) = h * Rp(5)/5
h - Height (m) of the mast above the surface to be protected
Δ - Δ= Δt x 106s
r - Depends on the selected level of protection (in meter)
r = 20 for Level 1 protection ; r = 30 for Level 2 protection
r = 45 for Level 3 protection; r = 60 for Level 4 protection
The radius of protection provided by an ESE air terminal having triggering advance time of 25us, 40us and 6Ous at different heights calculated based on the above formula are tabulated below.
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