As per legislation on use Electricity, it is mandatory to provide Lightening arrestor by earthing the oil tanks in on shore oil fields.
In a few cases, it has caused fire because of Lightening strike. Is this due to faulty design of Lightening arrestor considering the zone of protection or the provision of lightening arrestor has caused the least path for the current to flow and create spark due to lightening?

it has caused fire because of lightning strike.
How is this known?
Lightning often causes fires where there is no protection at all.