One of them is an electrical surge that is greater than the equipment is capable of sustaining. A lightning strike is capable of this.

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There are many reasons why a lightning arrestor fails. Poor grounding, insulation contamination, poor seal allowing moisture internally, a direct strike, poor manufacturing quality, etc., etc.

You need to provide the usual information such as location, atmospheric conditions, age of equipment, frequency of failure, confined to one manufacturer or batch, what the manufacturer says, etc. More info from you, better answers from us.

I thank you very much for your prompt reply for my quey.rThe lightning which is failed is located at the incoming side of my factory 11KV DP structure before the AB switch in 'Y' phase.There was thunder or rain in the location.My factory is located in Pennya Industrial Area 3rd phase in Bangalore.Ofcourse there is construction activity opposite to this DP structure which produces lot of cement dust.This particular lightening arrester has failed after three months of installation.Before that this particular 'Y' phase lightening arrester have not failed for atleast 1 to 11/2 years.But the nearby construction activity is going on for last 4 to 5 months.Whether contamination dust from the nearby construction on the disk's surface could be one of the reason.Please clarfy-regards-S.Srinivasan

Properly specified lightning arrestors should not fail so frequently. You did not say how the arrestor failed, internally or externally. More than likely the dust build-up combined with the light precipitation caused the arc-over. You might try those suitable for high contamination areas, the housing has larger creep paths created through a combination of wider skirts and additional ridges under them. There are also coatings that can be applied but that is not a job for inexperienced personnel.