ERICO Practical Guide to Electrical Grounding - Manual (Page 36)

alternative structure or feature on the ground). This leads to
the development of a limiting parabola. The enclosed
volume is known as the Collection Volume. A downward
progressing leader entering this volume is assured of
interception. Figure 4 shows how the velocity parabola
limits the size of the Collection Volume.
Fig. 2-4 Collection Volume formed by equal-
probability locus and spherical surface
Designing with Collection Volumes using statistically
derived lightning parameters as in Table 2 will provide
designers with better risk analysis. Magnitudes of
Collection Volumes are determined according to peak
current. That is, if the designer desires a high level of
protection (peak current 6.5kA), 98% of all lightning
exceeds this value. Discharges of greater magnitude will
have larger Collection Volumes that create greater overlap
in the capture area of air terminals. A design based on
lightning with small peak current can be considered conser-
vative. The design performed to 98% High level does not
mean that all lightning less than that level will miss an air
termination. There is simply a statistical chance some
lightning may not intercept with an upward leader
emanating from within the Collection Volume.
The Collection Volume model assumes all points on the
structure are potential strike points, and as such exhibit
natural Collection Volumes.
ERICO, Inc. has developed a computer program that
evaluates the corresponding electric field intensity at each
stage and compares the electric field intensification of
competing points (building corners and edges, antennae,
equipment, masts etc). The program then evaluates which
point will first generate the upward moving leader which
meets the downward leader. The main discharge return
stroke follows the upward/downward leader path. An
attractive radius for each relevant point can then
be calculated.
The larger collection volumes of enhanced air terminals
means that fewer such terminals are required on a structure.
They should be positioned such that their collection
volumes overlap the natural small Collection Volumes of
the structure projections.
This method is visually more attractive and convenient to
apply by consultants in lightning protection design. Figure
5 shows the Collection Volume Concept when applied to a
structure.
Fig. 2-5 The Collection Volume Design Concept
The design discipline employed in lightning protection
design is critical to reliable systems. Erico’s system has
been tested and has been used in the accomplishment of
over 7000 successful installations around the globe over the
past 15 years. Many of these installations are on high risk
structures in some of the most active lightning
environments on the planet.
LIGHTNING PROTECTION
COMPONENTS
A lightning protection system is comprised of a chain of
components properly specified and properly installed to
provide a safe path to ground for the lightning current. The
lightning protection system provides an uninterrupted
conductive (low impedance) path to earth. Lightning does
not always strike the highest point. The rolling ball theory
of determining what is protected from lightning strikes,
described below, is widely accepted as a sound approach to
sizing and positioning air terminals on the top of structures,
and for tall structures, on the sides of the structure.
Properly designed lightning protection systems based on
existing standards ensure adequate conducting and surge
diverting paths which have been proven safe for people,
structures and equipment in the great majority of cases.
Other systems exist which are not covered in standards.
These systems, which claim to prevent lightning strikes,
must be considered carefully before installation.
28 Practical Guide to Electrical Grounding
Spherical
Surface
Ground
Lightning
Leader
B
C
Equal
Probability
Locus
Collection
Volume
Striking
Distance
A
A
B
Strike 2
Strike 1
Grounding Book 4/14/99 10/5/99 6:01 PM Page 28 (Black plate)