Arresters are surge diverters for lightning and switching surges that can at times be 

overloaded; resulting in their end of useful life.  If the reason for the failure is not obvious, sometimes it is necessary to do an investigation to determine the root cause of the failure; this process is known as arrester forensics.   Often times the underlying reason for an analysis is that several arresters of similar style have experienced a terminal overload and the organization requesting the analysis needs to know if the problem is isolated or system wide.  If it is a system wide issue and is indeed a failure issue, then perhaps a different mitigation strategy would be used than if it was an isolated event.  Sometimes the purpose of an analysis is to determine if there is a power system issue that is not related to the arresters, but since the arresters are overvoltage protectors, they result in a terminal overload.  The cost of a misdiagnosis can be very expensive.  If the cause of the overload is incorrectly identified as a system issue, literally millions of dollars can be spent trying to correct the wrong thing.     more ...