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Maxwell Lightning Protection Co.

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Note: An updated NFPA 780, LPI 175 and UL 96A Risk Assessment is forthcoming.

Lightning loss risk assessment involves the evaluation of various criteria to determine the risk of loss due to lightning. This guide is designed to assist in that determination. As a guide, it is not possible to cover each special design element that may render a structure more or less susceptible to lightning damage. In these special cases it is recommended the user seek professional advice. Personal and economic factors often are very important and must be considered in addition to the assessment obtained by use of this guide.

To determine the need for protection against lightning for a given building or structure, the following variables are to be considered:

Step 1 - Lightning Flash Density
Step 2 - Structures Collective Area
Step 3 - Relative Structure Location
Step 4 - Type of Construction
Step 5 - Structure Contents
Step 6 - Occupancy
Step 7 - Lightning Consequence
Step 8 - Submit Your Answers


Please select the option that best applies your situation in the following questions. After you have answered all the questions, use the "Calculate My Risk" button at the bottom of the page. Your risk assessment will be immediately returned.

» step 1 - lightning flash density


Few areas in the United States are free from thunderstorms and their attendant hazards, but some areas have more storms than others. The map shows the average lightning flash density in flashes/km2/year - for the United States. Please select the area where the structure is located.

10-year Flash Density Map - U.S. 1989-1999 Average U.S.
USA Flash Density Map
select one:   0.1


» step 2 - structures collective area


Select the type of structure that is relative to your circumstances and enter required values.

Rectangular structure
Enter values in (ft)
L =      W =       H =
Structure where a prominent part encompasses all portions of the lower part 
      of the structure
Enter value in (ft)
H =
Structure where a prominent part encompasses all portions of the structure
For complex structures user enter equivalent collective area in units of square (ft)
Area =


» step 3 - relative structure location


Select relative structure location based on appropriate drawing above. (select one)

Structure located within a space containing structures or trees of the same height or taller within
        a distance of 3H
Structure surrounded by smaller structures within a distance of 3H
Isolated structure, no other structures located within a distance of 3H
Isolated structure on a hilltop


» step 4 - type of construction


Framework(select one)

Roof (select one)


» step 5 - structure contents


(select one)

Low value and nonflammable
Standard value and nonflammable
High value, moderate flammability
Exceptional value, flammable, computer or electronics
Exceptional value, irreplaceable cultural items


» step 6 - occupancy


(select one)

Normally occupied
Difficult to evacuate or risk of panic


» step 7 - lightning consequence


(select one)

Continuity of facility services not required, no environmental impact
Continuity of facility services required, no environmental impact
Consequences to the environment


» step 8 - submit your answers


Please verify that all questions have been answered and then click the following button to submit your answers.


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Before hiring an lightning protection company, verify that the installer is UL Listed. Call the Follow-Up Services department at UL's office:

333 Pfingsten Road
Northbrook, IL 60062
(847) 272-8800