for INMR WEEKLY TECHNICAL REVIEW
An example of failure of a porcelain-housed cutout due to moisture ingress followed by repeated freeze-thaw cycles
What does the photo show?
This is an example of failure of a porcelain-housed cutout due to moisture ingress followed by repeated freeze-thaw cycles.
Where was the photo taken?
This cutout was removed from the overhead network at a power utility located in western Canada.
Why is it interesting?
An investigation conducted into this type of problem revealed that of circa 32,700 cutouts installed on the distribution network at this power utility, there were 132 similar cases since 2004, representing a cumulative failure rate of 0.4%. Correlation of this rate against date of manufacture of the failed cutouts showed that the relative age of the affected units did not significantly impact incidence of failure. Units that had been in service only a few years were failing at about the same rate as much older cutouts. The conclusion was that failures were as much related to poor design and quality control problems during manufacture as to ageing under local service conditions. For example, if the metallic fittings cemented into the cutout housing come into direct contact with the porcelain through improper manufacturing, an inherent weakness is created.
When the cutout is exposed to high temperature, the metal expands and can crack the porcelain. A second and more common mode of failure involves the paint or epoxy sealant used to prevent moisture from entering the unit. If this sealant deteriorates, there can be moisture ingress. Subsequent freeze-thaw cycles will then lead to cracks and eventual failure. A further cause of failures is damage occurring due to improper handling or installation, causing chips or creating stresses in the porcelain body.
Porcelain cutouts in the Okanagan have experienced cracking due to cycles of moisture penetration combined with cold temperatures.
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