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Getting It Right

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NGK-Locke, Inc.


Station Post Insulators: Why Just Being Solid-core Is Not Enough : by Jerry Stewart

We all know the virtues of solid-core post insulators. They cannot be punctured under any circumstance. There is no cavity and therefore there is no reason to worry if the seal will be compromised, which can lead to RIV, ring flashover or puncture. Also, the length of the porcelain is not limited as it is in cavity-core posts. But merely filling the cavity with more porcelain is not getting it right.

It can be reasonably argued that designing for a single mode of failure is desirable, because this will result in a tighter distribution of ultimate mechanical strength values. This then allows for better prediction of the lowest value in the population and, therefore, more confidence in the margin of safety that you employ.

Having a tight distribution of ultimate strength values also aids the quality assurance in the factory. When conducting conformance tests, problems or trends may more quickly be identified if a larger standard deviation is observed.

Porcelain insulators have three major components that see stress under mechanical loading: the porcelain, the cement and the hardware. For post insulators, the hardware is usually the strongest component and seldom fails under extreme mechanical loading. This is generally because the wall thickness of the cap is determined more by the limitation of the casting process than by stress analysis. This leaves the porcelain and cement.

Porcelain is the most prevalent material in the load path. It is used primarily for its dielectric properties. Porcelain is exceedingly strong in compression and, therefore, will fail at or near the point of highest tensile stress. Being a brittle material, porcelain strength is dependent upon the size and location of the largest defect or stress riser.

The cement fails in shear and, therefore, it is best to minimize the thickness of the cement layer. If the cement is allowed to fail before the porcelain, the result will be that the porcelain will then be subjected to point loading and will fail below its intrinsic strength. Increasing the shear area can minimize the shear stress in the cement. This is accomplished by optimizing the ratio of depth of engagement of the porcelain in the hardware to the diameter of the root core of the porcelain. The optimum depth to diameter ratio is directly proportional to the bending strength of the porcelain.

By using a high strength porcelain body, the designer can optimize the depth to diameter ratio while minimizing the size of the hardware. Since the height of the station post insulator is fixed, minimizing the size of the hardware provides for the maximum dry arcing distance without compromising on the mechanical strength or allowing for multiple modes of mechanical failure.

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