Lightning hit prompts words from the wise

By John Satterley

While the two elderly occupants (unhurt) deal with a $100,000 damage bill, this writer asked an expert to explain the whys and wherefores of such a random, dangerous event.
Dr Franco D’Alessandro is MD and principal consultant of PhysElec Solutions p/l, a company that provides consulting services in lightning protection. He has 13 years of experience in lightning protection and gave a paper on the topic at the recent AusRail Plus 2009 conference in Adelaide.
D’Alessandro says the probability of a direct lightning strike depends on factors such as the height of structures, the site footprint and, importantly, the “ground flash density” of the region. He defines the latter as the number of flashes/km2/year: the higher the figure the bigger the likelihood of a lightning strike and hence damage.
For the Adelaide region, the ground flash density is relatively low, around 0.5 flashes/km2/year. This compares with 2-3 flashes/km2/year around Sydney, 3-4 flashes/km2/year around Brisbane and 6-8 flashes/km2/year around Darwin.
Even though the lightning activity around Adelaide is lower than other areas of Australia, the lightning damage seen on December 16 is a reminder that lightning protection should not be an afterthought.
D’Alessandro says each year lightning and overvoltage transients cause millions of dollars of damage in different industries. Although 100% protection is not practicable a risk management approach can provide comprehensive protection to a predetermined level.
He recommends a checklist of six steps that start with an effective means to capture, conduct to ground and safely dissipate the energy in a direct strike.
Another important one is clamping and diverting transients that are induced in the electrical systems or which arrive at the site from external sources.
In general, the tallest and most exposed points on a site are the most vulnerable. And key to direct-strike protection is the lightning rod or air terminal that is positioned to capture the lightning flash.
The correct installation of air terminals near these vulnerable points cannot be over-emphasised, he adds.
Enquiries: info@physelec.com