NFPA 780 Simplified Risk Calculator By: Dr. C.T. Mata c.mata@sls-us.com Jan 22, 2016
CONTENTS Contents 1 Introduction 3 2 Using the Simplified Lightning Risk Calculator 3 3 Equivalent Collection Area 6 4 Lightning Ground Flash Density N g 7 5 Annual Threat of Occurrence N d 9 6 Tolerable Lightning Frequency N c 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 A e 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 A e, 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@slsus.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
2 USING THE SIMPLIFIED LIGHTNING RISK CALCULATOR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Figure 1: Graphical user interphase of the simplified lightning risk calculator. Page 4 of 15
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 A e and displays the result in this field (see Section 4). 6. If the ground flash density (in flashes/km 2 /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 C 1 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 C 2. 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 C 2. 11. This drop down menu is used to determine the value of the Structure Contents Coefficient C 3 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 C 4 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 C 5 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 A e 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 (ft 2 or m 2 ). 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 N G 4 Lightning Ground Flash Density N g The lightning ground flash density N g, in flashes per km 2 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
4 LIGHTNING GROUND FLASH DENSITY NG Page 8 of 15 Figure 2: Ground flash density Ng from 2005 to 2012 in flashes per km2 per year.
6 TOLERABLE LIGHTNING FREQUENCY N C 5 Annual Threat of Occurrence N d The annual threat of occurrence N d or lightning strike frequency to the structure per year is defined in Section L.3 of NFPA 780 [2014] and given by: N d = N g A e C 1 10 6 (1) where N g is obtained as explained in Section 4, A e is calculated as explained in Section 3, and C 1 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, C 1 = 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, C 1 = 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, C 1 = 1, and if the structure being evaluated is an isolated structure located on a hilltop, C 1 = 2. 6 Tolerable Lightning Frequency N c The tolerable lightning frequency N c 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: N c = 1.5 10 3 C 2 C 3 C 4 C 5 (2) Page 9 of 15
7 RISK CALCULATION where the structural coefficients C 2, C 3, C 4, and C 5, 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 N c is compared with the annual threat occurrence N d. The result of this comparison is used to decide if a lightning protection system is needed. The following procedure is used: If N d N c, a lightning protection system can be optional, If N d > N c, 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. 10.0 32.0 20.0 Figure 3: Rectangular building with flat roof. Units are in meters. Page 11 of 15
Page 12 of 15 (a) (c) Figure 4: Calculating the effective collection area of the building shown in Figure 3. (a) start by defining the vertices with the building dimensions. (b) the polygon drawing process ends when the user clicks on the first vertex that was entered. (c) when the user clicks on the first vertex that was entered, the tool asks for the height of the polygon that was just entered. (d) the tool then shows the area (in black) defined by a line drawn from the edge of the roof of the structure and extending three times the structure height. The effective collection area is calculated and shown in the web application. (b) (d) A EQUIVALENT COLLECTION AREA CALCULATOR
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 32.0 10.0 20.0 Figure 5: Rectangular building with pitched roof. Units are in meters. Page 13 of 15
Page 14 of 15 (a) (c) Figure 6: Calculating the effective collection area of the building shown in Figure 5. Building on the model shown in Figure 4, we add the model of the roof ridge as follows: (a) start by defining the vertices of the roof ridge, (b) the polygon drawing process ends when the user clicks on the first vertex that was entered, (c) when the user clicks on the 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. (b) (d) A EQUIVALENT COLLECTION AREA CALCULATOR
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