LINE POST INSULATORS MECHANICAL RATINGS AND CONSIDERATIONS BASIC TUTORIALS

Mechanical Rating Criterion
Maximum Design Cantilever Load (MDCL) is a mechanical characteristic by which Composite Line Post Insulators (CLPI) are rated. This is the maximum cantilever load that the insulator is designed to withstand continuously throughout its service life.

At this level of load, which generally represents 40% to 50% of failing load (as defined by failure data provided by manufacturers), there will be no reduction in strength due to mechanical damage of the insulator’s components.

At this point in time, universally accepted definitions of cantilever load characteristics, including failure and failing load, have not been agreed upon and are under consideration by ANSI and IEC.

Tension loads
Tensile strength of composite line post insulators is dependent on the mechanical characteristics of the end fittings, and on those of the core-to-end fitting interfaces.

Compression loads
Compression loads on line post insulators do not usually result in damaging stresses until buckling occurs. Generally, elastic buckling of long posts, while not desirable, is not damaging. However, for further information on this, the manufacturer should be consulted.

Torsion loads
Some designs of line post insulators may have low torsional capability. In this event, cracking of the fibreglass rod or permanent rotation of the end fittings can occur by twisting. The manufacturer should be consulted to determine the torsional capability of a given insulator.

Combined loads
Actual line loads are rarely single loads. Usually, they are a combination of loads. When comparing actual mechanical load conditions with composite line post mechanical strength ratings, it must be recognized that vertical transverse and longitudinal loads each contribute to the total bending moment on the insulator, and therefore to total cantilever load.

To aid designers in determining how the combined mechanical loads in an intended application compare with the cantilever strength capability of a particular composite line post insulator, some manufacturers provide application curves and instruction on their proper use. At this point in time, universally accepted definitions of cantilever load characteristics have not been agreed upon and are under consideration by ANSI and IEC.

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