NFPA 780 Simplified Risk Calculator

Version 2.0

NFPA 780 Simplified Risk Calculator

By: Dr. C.T. Mata

[email protected]

May, 2016

CONTENTS

Contents

1 Introduction 3

2 Using the Simplified Lightning Risk Calculator 3

3 Equivalent Collection Area 6

4 Lightning Ground Flash Density Ng 11

5 Annual Threat of Occurrence Nd 13

6 Tolerable Lightning Frequency Nc 13

7 Risk Calculation 14

8 Report Calculation 14

A Equivalent Collection Area Calculator 15

A.1 A flat roof building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

A.2 A pitched roof building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

B Sample Report 19

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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, Inc. (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 an

easy way to determine 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 explicitly, which is a difficult parameter

to obtain in complex structures and required for the more comprehensive lightning risk

assessment presented in either NFPA 780 [2014] or IEC 62305-2 [2010].

This web application has been developed to be compatible with all OS and web browsers.

If you have any questions or suggestions, please send them to Dr. Carlos T. Mata, SLS at

the following email address: [email protected]

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.

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mailto:[email protected]

2USIN

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IED

LIG

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1 2 3

4

5

910

1314151617

678

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Figure 1: Graphical user interphase of the simplified lightning risk calculator.

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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. Enter here the Project Name, which will be shown on the report.

6. Enter here the name of the person performing the risk assessment, which will be

shown on the report as “Prepared by.”

7. Enter here the Project Description, which will be shown on the report.

8. After the structure has been drawn, the tool calculates the equivalent collection area

Ae and displays the result in this field (see Section 4).

9. 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 10.

10. 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

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3 EQUIVALENT COLLECTION AREA

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).

11. This drop down menu is used to determine the value of the Location Factor C1 in

Table L.4.2 of NFPA 780 [2014].

12. 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.

13. 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.

14. 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].

15. 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].

16. 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].

17. 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

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3 EQUIVALENT COLLECTION AREA

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 structure,

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.

The drawing canvas creates a legend on the left hand side, in which the different heights

are represented by different shades of orange (see Figure 2). As the user hovers the mouse

over the legend, the height of structure is shown and the polygon representing that par-

ticular structure is highlighted in the canvas (see Figure 3 in which the polygon of height

10 ft is being highlighted). Note that the polygon is highlighted and a red “x” next to the

height shows up. Pressing the “x” will delete the polygon from the canvas. The user has

the option to also click on the number and modify the height of the polygon (see Figure 4).

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Figure 2: Graphical user interphase of the simplified lightning risk calculator showing the legend on the left handside of the drawing canvas in shades of orange. The darker orange represents the shortest structure while the lightestorange color represents the tallest structure.

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3EQUIV

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Figure 3: Hovering the mouse over the legend on the left hand side of the canvas will show the height of the structure,highlight the polygon that represents that structure, and allow the user to delete the polygon or change its height.

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Figure 4: Clicking on the number that represents the height of the structure in the legend will allow the user tochange the height of the structure. The user can either accept the change by clicking on the checkmark or cancel theinput by pressing “cancel.”

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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 5. 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.

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4LIG

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Figure 5: Ground flash density Ng from 2005 to 2012 in flashes per km2 per year.

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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 contents, 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)

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8 REPORT 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.

8 Report Calculation

When the user clicks on the “CALCULATE” button, on the lower left corner of the GUI,

either a new tab is opened and a PDF report showing the inputs of the user is generated

or the web browser asks the user where to save the PDF report. The behavior is different

depending on the web browser being used. See Appendix B for an example of the report.

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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 6.

20.0

10.0

32.0

Figure 6: Rectangular building with flat roof. Units are in meters.

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(a) (b)

(c) (d)

Figure 7: Calculating the effective collection area of the building shown in Figure 6. (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.

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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 8. In this case, the only thing that has to be done is to model the ridge

of the roof as shown in Figure 9. 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 8: Rectangular building with pitched roof. Units are in meters.

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(a) (b)

(c) (d)

Figure 9: Calculating the effective collection area of the building shown in Figure 8. Building on the model shownin Figure 7, we add the model of the roof ridge as follows: (a) click to create the first and second vertex of the roofridge, (b) click again on the first vertex of the roof ridge, (c) enter the height of the roof ridge, and (d) the tool thenshows the area (in black) defined by a line drawn from the edge of each of the structures and extending three timestheir respective heights. The effective collection area is calculated and shown in the web application.

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B SAMPLE REPORT

B Sample Report

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ExampleProject Engineer: C. Mata

Description:Example use to show the legend and the multi polygon drawing capability.

Project Layout

Risk Determination

It is recommended that a lightning protection system be installed.Calculations on next page

NFPA 780 [2014] Simplified Lightning Risk Report - 05/18/2016

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Calculation Metrics

Total Collection Area (Ae)

29355 sq ft

Lightning Ground Flash Density (Ng)

18 Flashes / sq km / yr

Surroundings (C1)

Structures/trees of greater height within collection area (0.25)

Structure Material Type (C2a)

Metal (0)

Roof Material Type (C2b)

Metal (0)

Structure Contents (C3)

Low value and noncombustible (0.5)

Structure Occupancy (C4)

Unoccupied (0.5)

Lightning Consequence (C5)

Continuity of facility services not required, no environmental impact (1)

Calculations used

Annual Threat of Occurence (Nd)

Nd = 0.012

Tolerable Lightning Frequency (Nc)

Nc = 0.012

If Nd is less than or equal to Nc: A lightning protection system can be optional

If Nd is greater than Nc: It is recommended that a lightning protection system be installed

All calculations are performed in accordance with NFPA 780 [2014].

NFPA 780 [2014] Simplified Lightning Risk Report - 05/18/2016

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BSAMPLE

REPORT

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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.

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NFPA 780 Simplified Risk Calculator

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