Test results on the performance under heavy contamination conditions Salt fog aging cycle test
(accelerated aging test to evaluate long-term performance under contamination conditions)
Certain kind of insulator suffers from severe surface erosion resulting in failure.
No erosion on porcelain insulators
( white part: salt deposit )
Sever erosion around cap pin,
and failure of glass insulators
Failure rate of the insulators on DC line is much higher than on AC line. The following causes are considered by investigation in the world.
Ion migration: Ions such as Na+ in the insulating body move to negative side under DC stress.
The accumulation of ions at one side causes deterioration in electrical and mechanical performance.
The current passing through the insulating body increases by heating of insulator due to repeated unit flashover, direct sunshine or the like.
Increase current causes heating again and this process is repeated resulting in electrical puncture.
EC 61325(test on DC insulators) specifies "Ion migration test" and "Thermal runaway test" as the special test for DC insulators. NGK high quality DC fog type insulator satisfies the requirement.
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