Polymer line post insulator Load Definitions

Polymer line post insulator Load Definitions includes Cantilever Breaking Load (CBL), Specified Cantilever Load (SCL), Elastic Limit or Damage Limit, and Maximum Design Cantilever Load (MDCL).

Polymer line post insulator

Polymer line post insulator Load Definitions:
Cantilever Breaking Load (CBL) is defined as the maximum load that is reached during a cantilever breaking test. Specified Cantilever Load (SCL) is defined as the cantilever load rating that is assigned by the manufacturer. The CBL should be equal to or larger than the SCL. ORIENT includes a specific safety margin on the average CBL when defining the SCL rating. That safety margin ensures that the insulators satisfy the SCL rating with more than a 90% probability. Both CBL and SCL are terms that define the mechanical failure point of polymer insulators.

The Damage Limit or Elastic Limit for polymer line post is a concept similar to the yield point of metal materials. A specific bending stress range exists for which the slope of the Deflection - Time curve begins to rapidly increase. This is illustrated in the related curves shown at the bottom of this page.
The first chart shows different deflection levels of a polymer line post over time, while under several different bending stress levels. The second plot is essentially a plot of the slopes of the curves on the first chart. This plot clearly shows a transition point where the slope changes dramatically. This transition point corresponds to the Elastic or Damage Limit. Once this Limit is exceeded, plastic (permanent) deformation (damage) is considered to have occurred. The Limit is in the range of 60% ~ 80% of CBL and depends on the end fitting material and design, the core material, and the particular combination of the two. The MDCL is the maximum working load and must not be exceeded in service. The sealing failure of a polymer line post insulator and the Elastic Limit are important considerations for the establishment of the MDCL.

The Relationship of the Previous Definitions:
ORIENT suggests setting the MDCL as 50% of the SCL for our polymer line post applications which also agrees with the 2007 version of the U.S. National Electric Safety Code. That MDCL level has significant margin from the Elastic Limit. Therefore, the MDCL level is considered as safe from the Permanent Damage Zone. Also, the MDCL level has margin from the Sealing Failure Zone. In order to maintain these safety margins, the MDCL must not be exceeded in service.
Even if a load applied to a polymer line post application exceeds the MDCL, mechanical failure does not occur until the SCL is reached. However, the insulator may be damaged. Therefore, careful inspection and possible replacement is required in the event that a load greater than the MDCL may have been applied to the insulator.

Mechanical Rating of polymer line post is generally defined by the MDCL. A long insulator has a lower MDCL than a short one, due to the increased moment arm for the same core size. Therefore, the mechanical strength of a polymer line post takes various different ratings according to the insulator length, even though it has the same core diameter. The MDCL vs. line post length chart shows how the MDCL changes with length.

Deflection occurs to a polymer post insulator when exposed to a cantilever load. This deflection is proportional to the insulator length cubed. In some cases, the selection of polymer line post should be based not only on the MDCL, but also on the deflection while loaded.

Combined Loading of vertical (conductor, accessories, ice), longitudinal (unbalanced), and transverse (wind) loads should also be considered for polymer line post applications. The combined loading should not induce a stress level in the core that is greater than the equivalent stress level of the MDCL. Combined loading charts for polymer line posts are available from the factory.


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