nfpa 780 simpli ed risk calculator - · pdf file2 using the simplified lightning risk...
TRANSCRIPT
CONTENTS
Contents
1 Introduction 3
2 Using the Simplified Lightning Risk Calculator 3
3 Equivalent Collection Area 6
4 Lightning Ground Flash Density Ng 7
5 Annual Threat of Occurrence Nd 9
6 Tolerable Lightning Frequency Nc 9
7 Risk Calculation 10
A Equivalent Collection Area Calculator 11
A.1 A flat roof building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2 A pitched roof building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Page 2 of 15
2 USING THE SIMPLIFIED LIGHTNING RISK CALCULATOR
1 Introduction
A lightning risk calculator using the simplified risk procedure presented in NFPA 780 [2014]
has been developed and deployed on the East Coast Lightning Equipment, LLC (ECLE)
website (www.ecle.biz/riskcalculator). This risk calculator is intended to be used by
engineers, architects, or people in the lightning protection community and provides and
easy and effective way for them to figure out if the lightning protection for the structure
being evaluated could be optional or is recommended.
This application was developed by Scientific Lightning Solutions, LLC (SLS) for ECLE
and allows the users to easily calculate the equivalent collection area Ae of any structure
and to perform the simplified lightning risk assessment as per NFPA 780 [2014]. The steps
on how to use this simplified risk calculation tool are described in the following sections.
Note that this tool can be also used to calculate Ae, which can be used for the more complex
lightning risk assessment presented in either NFPA 780 [2014] or IEC 62305-2 [2010].
The current version of this simplified risk calculator does not support report generation,
but this capability will be added in future versions. If you have any questions or suggestions,
please send them to Dr. Carlos T. Mata, SLS at the following email address: c.mata@sls-
us.com
2 Using the Simplified Lightning Risk Calculator
The graphical user interphase (GUI) of the web application is shown in Figure 1. The
two main areas are: (a) the drawing area or the canvas (right hand side) and (b) the
calculation panel (left hand side). The description of the function of each icon, sliding bar,
and dropdown menu, circled in red and numbered with red numbers follows.
Page 3 of 15
2USIN
GTHE
SIM
PLIF
IED
LIG
HTNIN
GRISK
CALCULATOR
1 2 3
4
5
678910111213
14
Figure 1: Graphical user interphase of the simplified lightning risk calculator.
Page4of15
2 USING THE SIMPLIFIED LIGHTNING RISK CALCULATOR
1. By default the canvas is in the “Draw Polygon” mode, which is selected by clicking on
the icon. Appendix A presents a brief tutorial that shows how to draw the structures
in the canvas.
2. Clicking this icon puts the canvas in “Move Vertex” mode. In this mode the user can
move any of the vertices already drawn.
3. The user can select the units in which the structure is being drawn, Meters or Feet.
Note that the user can switch between units at any time and the structure already
drawn will not be scaled, but instead, the new selected unit will be used to calculate
the equivalent collection area (see also Section 3).
4. This sliding bar is used to zoom in or zoom out. Simply drag the sliding bar to the
right to zoom in or to the left to zoom out.
5. After the structure has been drawn, the tool calculates the equivalent collection area
Ae and displays the result in this field (see Section 4).
6. If the ground flash density (in flashes/km2/year) of the area where the structure is
located is known, enter it here. If not, click on the hyperlink labeled 7.
7. This is a hyperlink to a flash density map by Vaisala. The user can select the average
value of ground flash density of the area where the structure is or will be located.
It’s recommended that the user selects the higher value of the range provided in the
map for a slightly conservative result (see Section 4).
8. This drop down menu is used to determine the value of the Location Factor C1 in
Table L.4.2 of NFPA 780 [2014].
9. This drop down menu is used to determine which row of Table L.5.1.2(a) of NFPA
780 [2014] is used to determine the value of the construction coefficient C2.
Page 5 of 15
3 EQUIVALENT COLLECTION AREA
10. This drop down menu is used to determine the column of Table L.5.1.2(a) of NFPA
780 [2014] is used to determine the value of the construction coefficient C2.
11. This drop down menu is used to determine the value of the Structure Contents
Coefficient C3 in Table L.5.1.2(b) of NFPA 780 [2014].
12. This drop down menu is used to determine the value of the Structure Occupancy
Coefficient C4 in Table L.5.1.2(c) of NFPA 780 [2014].
13. This drop down menu is used to determine the value of the Lightning Consequence
Coefficient C5 in Table L.5.1.2(d) of NFPA 780 [2014].
14. When the user clicks this push button, the GUI shows the result of the simplified
risk assessment and the calculations are performed as outlined in Sections 5, 6, and
7 and shown by a pop-up window.
3 Equivalent Collection Area
The equivalent collection area Ae is calculated within the web application and the steps,
with some examples, are outlined in Appendix A. The user can define the units used to
draw the structure or structures (ft2 or m2). The user can also zoom in or zoom out by
moving the sliding bar on the lower right corner of the web application. There are two icons
that are used to switch the operating mode of the drawing tool: 1) the “Draw Polygon”
mode, and the 2) “Move Vertex” mode. In the “Draw Polygon” mode, the user can draw
the structures by defining the vertices. When the user closes the shape by clicking on the
first vertex, the tool finishes drawing the polygon and asks for the height of the structured,
assuming a flat roof. In the “Move Vertex” mode, the user can click and drag the vertices
that have been previously entered on any of the structures already drawn.
Page 6 of 15
4 LIGHTNING GROUND FLASH DENSITY NG
4 Lightning Ground Flash Density Ng
The lightning ground flash density Ng, in flashes per km2 per year, can be obtained from
one of the ground flash density maps published by VAISALA, such as the one shown in
Figure 2. The simplified lightning risk calculator contains a link, titled “Flash Density
map,” to a ground flash density map that the user can access on the left hand side of the
webpage. The user can select the average ground flash density for the region where the
building is or will be located or select the maximum value of the ground flash density if a
more conservative result is desired.
Page 7 of 15
4LIG
HTNIN
GGROUND
FLASH
DENSIT
YN
G
Figure 2: Ground flash density Ng from 2005 to 2012 in flashes per km2 per year.
Page8of15
6 TOLERABLE LIGHTNING FREQUENCY NC
5 Annual Threat of Occurrence Nd
The annual threat of occurrence Nd or lightning strike frequency to the structure per year
is defined in Section L.3 of NFPA 780 [2014] and given by:
Nd = Ng ×Ae × C1 × 10−6 (1)
where Ng is obtained as explained in Section 4, Ae is calculated as explained in Section 3,
and C1 is the location factor, which is obtained as follows:
• if the structure being evaluated is surrounded by taller structures or trees within a
distance of 3 times the height of the structure, C1 = 0.25,
• if the structure being evaluated is surrounded by structures of equal or lesser height
within a distance of 3 times the height of the structure, C1 = 0.5,
• if the structure being evaluated is an isolated structure, with no other structures
located within a distance of 3 times the height of the structure, C1 = 1, and
• if the structure being evaluated is an isolated structure located on a hilltop, C1 = 2.
6 Tolerable Lightning Frequency Nc
The tolerable lightning frequency Nc is a measure of the risk of damage to the structure,
including factors affecting risks to the structure, to the contents, and of environmental
loss. It is calculated by dividing the acceptable frequency of property losses by various
coefficients relating to the structure, the con- tents, and the consequence of damage.
The tolerable lightning frequency is expressed by the following formula:
Nc =1.5 × 10−3
C2 × C3 × C4 × C5(2)
Page 9 of 15
7 RISK CALCULATION
where the structural coefficients C2, C3, C4, and C5, can be obtained from Tables L.5.1.2(a),
L.5.1.2(b), L.5.1.2(c), and L.5.1.2(d) of NFPA 780 [2014], respectively.
7 Risk Calculation
The tolerable lightning frequency Nc is compared with the annual threat occurrence Nd.
The result of this comparison is used to decide if a lightning protection system is needed.
The following procedure is used:
• If Nd ≤ Nc , a lightning protection system can be optional,
• If Nd > Nc , it is recommended that a lightning protection system be installed.
Page 10 of 15
A EQUIVALENT COLLECTION AREA CALCULATOR
A Equivalent Collection Area Calculator
The equivalent collection area is calculated by a drawing tool deployed within the simplified
risk calculator web application. The user can draw polygons and specify their height. Any
number of polygons can be drawn and superimposed, each with its own height. The first
vertex of the polygon is created the first time the user clicks on the canvas. Subsequent
vertices are created by subsequent mouse clicks on the canvas.
A.1 A flat roof building
In this case we want to calculate the effective collection area of the building shown in
Figure 3.
20.0
10.0
32.0
Figure 3: Rectangular building with flat roof. Units are in meters.
Page 11 of 15
AEQUIV
ALENT
COLLECTIO
NAREA
CALCULATOR
(a) (b)
(c) (d)
Figure 4: Calculating the effective collection area of the building shown in Figure 3. (a) start by defining the verticeswith the building dimensions. (b) the polygon drawing process ends when the user clicks on the first vertex that wasentered. (c) when the user clicks on the first vertex that was entered, the tool asks for the height of the polygon thatwas just entered. (d) the tool then shows the area (in black) defined by a line drawn from the edge of the roof of thestructure and extending three times the structure height. The effective collection area is calculated and shown in theweb application.
Page12of15
A EQUIVALENT COLLECTION AREA CALCULATOR
A.2 A pitched roof building
Building on the example of Appendix A.1, imagine that the roof is not flat, but it is pitched
as shown in Figure 5. In this case, the only thing that has to be done is to model the ridge
of the roof as shown in Figure 6. The user can draw multiple structures and they can
overlap each other, so that complex building and roofs can be modeled. The user can also
draw a line to model the roof ridge by clicking on the canvas three times while in “Polygon
Draw” mode: 1) first click is the first point of the line segment, 2) second click is the end
point, 3) third click should be on the first vertex so that the application knows that the
data entry is complete (the application will ask for the height at this point).
20.0
10.0
32.020.0
Figure 5: Rectangular building with pitched roof. Units are in meters.
Page 13 of 15
AEQUIV
ALENT
COLLECTIO
NAREA
CALCULATOR
(a) (b)
(c) (d)
Figure 6: Calculating the effective collection area of the building shown in Figure 5. Building on the model shownin Figure 4, we add the model of the roof ridge as follows: (a) start by defining the vertices of the roof ridge, (b) thepolygon drawing process ends when the user clicks on the first vertex that was entered, (c) when the user clicks onthe first vertex that was entered, enter the height of the roof ridge, and (d) the tool then shows the area (in black)defined by a line drawn from the edge of each of the structures and extending three times their respective heights.The effective collection area is calculated and shown in the web application.
Page14of15
REFERENCES
References
IEC 62305-2. Protection against lightning – Part 2: Risk management. 2.0 2010-12 edition,
2010.
NFPA 780. Standard for the Installation of Lightning Protection Systems. 2014 edition,
2014.
Page 15 of 15