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Electrical protection and safety
training in lightning protection. Tragic and unnecessary losses
have occurred because of this approach. Through this guide, you
may make a more accurate determination regarding the need for
lightning protection. Once the need for lightning protection has
been established, loss of life and property can be avoided by the
installation of an approved lightning protection system.
• To determine the need for protection
against lightning for a given building
structure, the variables to the right are to
be considered.
• The variables to the right are calculated in
the following formula:
R = (A+B+C+D+E) divided by F*
• Compare the final ‘R’ value to the ‘R’
values in the table below to determine the
level of risk and the corresponding need for
protection against lightning.
A - Type of Structure
B - Type of Construction
C - Relative Exposure
D - Topography
E - Occupancy and Contents
F - Lightning Frequency
(NB - * Lightning Frequency is
obtained from the Isokeraunic
Map for the region)
R = (A+B+C+D+E)
F *
‘R’ Value
Risk Level
Protection
0 – 2
Little
May not be needed
2 - 3
Low
May be advisable
3 - 4
Medium
Advisable
4 - 7
High
Should be required
Over 7
Great
Required
Table 1: Risk values, levels - and need for protection.
Single family residence less than 500 sq m/ 5 382 sq ft
1
Single family residence over 500 sq m/ 5 382 sq ft
2
Residential office or factory less than 17 m in height.
Covering Less than 3 000 sq m. of ground area.
Covering over 3 000 sq m of ground area
-
5
3
Residential, office or factory building from 17 - 20 m/ 182 -215 ft high
4
Residential, office or factory building 20 - 50 m/ 215 - 538 ft high.
5
Residential, office or factory building over 50 m/ 538 ft high
8
Public utility building
7
Libraries, museums, historical structures
8
Barns, stables, out-buildings, golf shelters, other recreational shelters
9
Places of public assembly such as schools, churches, theatres, stadiums,
etc
9
Slender structures such as smokestacks, church steeples, control towers,
lighthouses, etc
10
Hospitals, nursing homes, housing for the elderly or handicapped
10
Buildings housing the manufacture, storage or handling of explosives,
explosive vapours in ingredients, flammable gases, etc
10
Table 2: Lightning Risk Assessment Worksheet. Type of structure (A)
Framework
Roof
Index value
Non Metallic
Wood
Composition
Metal - not continuous
Metal - electrically continuous
5
3
3
1
Wood
Wood
Composition
Metal - not continuous
Metal - electrically continuous
5
3
4
2
Reinforced Concrete
Wood
Composition
Metal - not continuous
Metal - electrically continuous
4
3
4
3
Structural Steel
Wood
Composition
Metal - not continuous
Metal - electrically continuous
3
2
3
1
Table 3: Lightning Risk Assessment Worksheet. Type of construction (B).
Buildings in urban areas among higher structures: Small buildings cover-
ing ground areas of less than 1000 sq m/10 763 sq ft
1
Large buildings covering ground area of more than 1 000 sq m /10 763 sq ft 2
Buildings in suburban areas with no high structures: Small buildings
covering ground area of less than 1 000 sq m/10 763 sq ft
4
Large buildings covering ground area of more than 1 000 sq m/10 763 sq ft
5
Buildings extending up to 18 m above adjacent structures.
6
Buildings located in rural areas, any size
7
Buildings located in open – country (no other structures in immediate area) 10
Buildings extending up more than 18 m above adjacent structures
10
Table 4: Lightning Risk Assessment Worksheet. Relative Exposure (C).
Topography (D)
On flat land
1
On hillside
2
On hilltop
4
On mountaintop
5
Occupancy - Contents (E)
Non - flammable materials - seldom occupied
1
Ordinary furnishings or equipment - small occupancy
2
Livestock
3
Small assembly of people - less than 100
4
Combustible materials
5
Large assembly of people - 100 or more
6
High value materials or equipment
7
Essential services, police, fire, etc
8
Immobile or bedfast persons
8
Flammable liquids or gases - gasoline, hydrogen, etc
8
Historic contents, valuable artwork
10
Explosives and explosive ingredients
10
Table 5: Topography (D)/ Occupancy – Contents (E).
Electricity+Control
September ‘12
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