Diagnosis: Severe local overheating and sparking not involving
cellulose.
Nomograph: Heating
This unit had no history of any problem and was scheduled to
handle an overload while a transmission line was reconductored.
However, prior to the overloading, an oil sample was subjected to
DGA and gases were found that indicated severe heating with no
involvement of cellulose. This was confirmed as a partly destroyed
no-load tap changer contact that would have failed in service
shortly, even without an overload. Further investigation of the other
eleven units at this site showed that all had undersized contacts,
which were subsequently replaced.