Cantilever Loading of Post Insulators

Whereas tensile loading is straightforward and simple, cantilever loading is not, with many layers of complexity. Again, there are ultimate and working ratings expressed respectively as SCL (Specified Cantilever Load) and MDCL (Maximum Design Cantilever Load). But this is where all similarity to suspension mechanical loading ends and cantilever loading becomes complicated.

Line post and braced post insulators are not selected by SCL rating or working load. Rather, the mechanical rating of a post is limited by section length such that, as a post increases in length (voltage), its ultimate cantilever rating is reduced. Conversely, as a post is shortened it gains mechanical strength but also loses electrical capability. Therefore, when selecting a post-insulator, there needs to be the right balance between mechanical and electrical performance.

The mechanical rating of a post is impacted by the type of end fittings and connection base. Both are part of standards but not so well defined as the Go/No Go criteria used for tension insulators. These variables aside, the ‘Load Curve’ or interaction diagram is by far the most complex part of a post-application. Unlike a tension insulator that withstands a single tensile load no matter the application, line posts, and braced posts are subject to a variety of combined loads, comprised of vertical, transverse (compression and tension), and longitudinal loads. The purpose of the load curve is then to consider these various load conditions and provide some Go/No Go criteria for that post-insulator. All required loading conditions must fall on or below the MDCL (working) curve. Any load above the MDCL has the potential to cause issues and should be reviewed closely before proceeding.

No single cantilever test validates the ultimate and working ratings of a post. For example, a combined vertical and compression load will have a lower buckling limit than a combined vertical and tension load. Each of these load cases will therefore have different results with regard to SCL and MDCL. Testing can be completed at the factory for most line post applications along with some braced post applications. However, cantilever testing cannot be completed as part of the manufacturing process. Fortunately, various 3rd party test laboratories are able to perform cantilever testing. Some may be limited to single vector load testing (i.e. the traditional test method) while others have the capability to test multi-axis loading, which provides a better representation of actual load conditions an insulator can be exposed to in service.

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