2011 EMC Directory & Design Guide - Interference Technology

surge & transients
A Risk A s s e s s m e n t f o r L i g h t ning Prote c t i o n S y s t e m
Figure 2. U.S. lightning flash density map. (Map provided by Vaisala-GAI. Lightning data provided
by the U.S. National Lightning Detection Network.)
Surrounding Environment
Structure surrounded by
similar sized structures
Structure surrounded by
smaller sized structures
Coefficient
0.25
0.5
Isolated structure – level ground 1.0
Isolated structure – hilltop 3.0
Eqn. 2
Where L is the length of the structure,
W is the width of the structure,
and H is the height of the structure.
The surrounding environment of
the facility has an integral affect on if
and how lightning is going to strike a
structure. Isolated structures located
on a hilltop or mountain top are more
vulnerable to lightning strikes than a
structure located amongst similar sized
structures. Determining the surrounding
environment coefficient is done by
choosing the appropriate values from
Table 1.
The final parameter needed to calculate
the environmental factors associated
with the facility is the lightning flash density. The
lightning flash density is the amount of lightning flashes
that occur per year per kilometer. This value can be obtained
through a variety of sources. However, it is important to
understand that averages can change over time. Therefore,
one should obtain not only the average of an extended period,
e.g. ten years, but also maximum and minimal values
over a short period of time, e.g. three months. A lightning
flash density map is shown in Figure 2.
The tolerable risk of the facility (Nc) is determined by
equation EQ3 and is dependent on the type of structure
(C2), the contents within the structure (C3), the structure
occupancy (C4), and the consequence of the loss of operations
of the structure (C5).
Table 1. Coefficients of surrounding environment (C1).
Eqn. 3
system requires the engineer to compare environmental
factors (Nd) to the tolerable risk factors (Nc). Comparison
is conducted by a ratio between the environmental factors
and the tolerable risk. If the calculated ratio is 1.0 or greater,
then a lightning protection system, which includes SPDs,
is required. If the calculated ratio is less than 1.0, then a
lightning protection system is not required.
The environmental factors are calculated using the equation
of Eqn. 1.
Eqn. 1
The environmental factors consist of the collective area
of the facility (Ae), its surrounding environment (C1) and
the lightning flash density (Ng) of the area. There are different
equations to determine the collective area of the facility
based on the type of structure: standard rectangular structure,
rectangular structure with prominent riser, rectangular
structure with small riser. The collective area for a standard
rectangular structure is calculated using equation Eqn. 2.
The type of structure is either metal with a non-metallic
roof or metal with a metallic roof. Structures with other
construction are not considered in this risk assessment. The
coefficients for the type of structure are shown in Table 2.
The content of the structure is the second parameter
to be determined. The structure contents range from low
value, nonflammable contents to those of exceptional value,
irreplaceable cultural items. The coefficients associated with
each parameter are denoted in Table 3.
The occupancy of the structure is the third parameter
that is determined. The definition of structure occupancies
are: unoccupied; normally occupied; or difficult to evacuate.
The coefficients associated with each parameter are
denoted in Table 4.
The consequence of an interruption of service as a result
of lightning is the fourth parameter to be determined. The
definitions are: continuity of service is not required, no
environmental impact; the continuity of service is required,
no environmental impact; or the there are consequences
to the environment. The coefficients associated with each
102 interference technology emc Directory & design guide 2011