ERICO Practical Guide to Electrical Grounding - Manual (Page 35)

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Calculation of the Protective Coverage
offered by an air terminal
Collection Volume Design Method
A more efficient air terminal demands a new design
philosophy and discipline. ERICO has developed an
alternative design method matched to the performance of
the System 3000
™ lightning protection system. This method
is based on the work of Dr. A. J. Eriksson, the noted
lightning researcher. A detailed description can be found in
the Australian Lightning Protection Standards
NZS/AS1768-1991, section A8.
The Collection Volume method provides an empirical and
quantitative method based on design parameters such as, the
structure height, field intensification of structural projections,
leader charge, site height and relative propagation velocities
of the intercepting leaders. The model can be developed for
three dimension structures and offers a more rigorous
approach to lightning protection design.
Table 1 (Table A1 NZS/AS1768-1991)
Distribution of the Main Characteristics of the
Lightning Flash to Ground
Table 1 (taken from NZS/AS1768-1991) illustrates the
statistical distribution of lightning parameters. Item 3 in the
table can be used in determining the statistical levels of
protection. Using the equation below, protection levels
directly relating to peak current discharge, I, and the
corresponding leader charge, Q, are derived:
I = 10.6 Q 0.7
where I is measured in kA and Q in coulombs. From Table
2 a discharge having a peak current of 5kA would
correspond to a leader charge of approximately 0.5
coulombs. Further calculation and extrapolation from Table
1 are shown in Table 2.
Table 2 - Statistical probability of a down-leader
exceeding the peak current indicated
Figure 3 shows a downward leader approaching an isolated
ground point. A striking distance hemisphere is set up about
this point. The radius is dependent on the charge on the
leader head and corresponds to the distance where the
electric field strength will exceed critical value. That is, the
field strength becomes adequate to launch an intercepting
upward leader.
Fig. 2-3 Spherical Surface with
Striking Distance radius about point A
The striking distance hemisphere reveals that lightning
leaders with weak electric charge approach much closer to
the ground point before achieving the critical conditions for
initiation of the upward leader. The higher the magnitude of
charge, the greater the distance between leader and ground
point when critical conditions are achieved. For design
purposes a hemisphere radius can be selected which relates
to a desired level of protection. The Collection Volume
method takes into account the relative velocities of the
upward and downward leaders. Not all leaders that enter a
striking distance hemisphere will proceed to interception.
Leaders entering the outer periphery of the hemispheres are
likely to continue their downward movement and to
intercept a different upward leader (issuing from an
Chapter 2: Building Lightning Protection
Leader Peak Percent Protection
Charge Current (I) Exceeding Level
(Q) Value
0.5C 6.5kA 98% High
0.9C 10kA 93% Medium
1.5C 16kA 85% Standard
Item Lightning Percentage of events having Unit
Characteristic value of characteristic
99 90 75 50 25 10 1
1 Number of 1 1 2 3 5 7 12
component
strokes
2 Time Interval 10 25 35 55 90 150 400 ms
between
strokes
3 First stroke 5 12 20 30 50 80 130 k A
current I
max
4 Subsequent 3 6 10 15 20 30 40 k A
stroke peak
current I
max
5 First stroke 6 10 15 25 30 40 70 GA /s
between
strokes (dI/dt)
max
6 Subsequent 6 15 25 45 80 100 200 GA /s
stroke (dI/dt) max
7 Total charge 1 3 6 15 40 70 200 C
delivered
8 Continuing 6 10 20 30 40 70 100 C
current charge
9 Continuing 30 50 80 100 150 200 400 A
current I
max
10 Overall duration 50 100 250 400 600 900 1500 ms
of flash
11 Action integral 10 2 3x10 2 10 3 5x10 3 3x10 4 10 5 5x10 5 A 2 .s
Spherical
Surface
Ground
Lightning
Leader
B
C
Striking
Distance
Grounding Book 4/14/99 10/5/99 6:01 PM Page 27 (Black plate)