Lightning & Surge Technologies Home

Risk Assessment - Direct Strike Protection
 
Home | Technical | Lightning Protection

Those structures deemed to be at a greater risk from direct strike are usually the largest and tallest structures on a site.

The Direct Strike Risk Assessment is based on Points 1, 2 and 3 of the 6 Point Plan, and arrived at by considering the sites specific structural variables as outlined in the Risk Assessment section of AS1768-1991.

Note that AS1768-1991 has since been revised twice, and has been replaced by AS1768-2007 released Jan 2007, however for this preliminary Risk Assessment guideline, the older AS1768-1991 Risk Assessment is still a useful guide in which to identify those key risk factors that make up a brief and preliminary assessment.

We recommend that where this preliminary assessment indicates a risk assessment greater than “Fair”, a more detailed Risk Assessment (using the Microsoft® Excel based Risk Assessment calculator that comes bundled with the current AS1768-2007 Lightning Protection standard) be conducted.

The 5 tables which make up the key risk factors and their associated indices are as follows:

Table A - Type of structure or use
Usage and Contents
Value of index
Protection not justified having regard to occupancy and mission critical contents
-10
Structure and inert contents, infrequent occupation. i.e. domestic outbuilding, shed, metal chimney or mast arrangement
0
Structure containing non mission critical equipment, or small number of people, e.g. Domestic dwelling, shop, small factory
1
Structure with contents of fair importance, e.g. Water tower, shop with valuable contents, office, factory or residential building
2
Cinema, Church, Education campus, marine vessel , monuments
3
Telecommunications, Essential Services, Broadcasting, Utilities, Museum and galleries, sporting and entertainment arenas, telephone exchange, computer data centre, airports and control towers, lighthouse, mineral processing and industrial plant, power stations, or other sites of major importance
4
Petrochemical installations, hospitals
5
Explosives magazines, munitions manufacture and fireworks
15



Table B - Type of construction
Type of Construction
Value of index
Fully metallic structure that is electrically continuous
0
Reinforced concrete or steel frame structure with a ferrous metallic roof
1
Reinforced concrete or steel frame structure with concrete or other non-metallic roof.Small building of timber or masonry with ferrous metallic roof
2
Large area building of timber or masonry with ferrous metallic roof
3
Large area building of timber or masonry with non-metallic roof.
Membrane structures with metallic frames
4



Table C - Height of Structure
Height of Structure (mtrs)
Value of index
Exceeding
Not Exceeding
 
0
5
0
6
11
2
12
16
3
17
24
4
25
34
5
35
49
6
50
69
7
70
100
8



Table D - Situation
Situation
Value of index
On the flat, at any elevation
0
Hillside, or mountainous country up to1000m
1
Mountain top above 1000 m
2



Table E - Prevalence/Thunder Days
Average Thunder Days Annually
Value of index
Exceeding
Not Exceeding
 
0
1
0
2
3
1
4
7
2
8
15
3
16
31
4
32
63
5
64
+
6

Table E reflects average Thunderday data for a particular geographic area, where the Australian Annual Thunderday map produced by the Bureau of Meteorology Thunderday can be used to reference an accurate risk indice value for inclusion in the assessment.

Thunder Day Map
Australian Thunder Day Map

Once each of the identified at risk areas are assessed as per the framework provided by the Risk Assessment Tables, the resultant indices are then summed and compared to the assessment results contained in Table F.
Table F provides an assessment of the risk, and states the need for protection.

Table F - Assessment of Risk and Need for Protection
Risk Index
(= A+B+C+D+E)
Assessment of Risk

Need for Protection

11
Negligible
Not needed
11
Small
Not needed
12
Fair
Might be advisable
13
Medium
Advisable
14
Great
Strongly advisable
>14
Very Great
Essential

Should the preliminary risk assessment indicate there to be “a need for protection”, it is then recommended that a copy of the AS/NZS AS1768-2007 Lightning Protection standard be purchased, and a detailed assessment be conducted using the updated and automated spreadsheet based “Risk Assessment Calculator “.

Where protection is stated as “Not needed”, this does not mean that lightning protection should not be installed. What the Assessment table is suggesting is that based on the given parameters entered into the risk assessment process , statistically the risk is quite low.

It must be remembered that Lightning should always be understood to be a random and unpredictable event, and can still affect structures which are stated as Negligible or Small Risk.

Often the decision to Lightning Protect, or not to Lightning Protect may be as simple as considering “what are the costs and consequences of not lightning protecting.

Disclaimer:
The information contained in this preliminary risk assessment is provided for information purposes only, and no responsibility is taken for any losses or damage which might arise through any use of this information.

Email us for more information

Back To Top