![]() ![]() What percentage above the SML should the lower control limit load be?.This is where there are critical questions: Once average failure load is defined, SPC can be used to determine the lower control limit for that insulator and manufacturing process. The goal for any manufacturer would therefore be to have a tighter range of failures, which is evidence of a process that is in control. The wider this range, the less in control is the manufacturing process. When validating SML for a suspension application, data from tensile tests should provide a range of failure loads. This is typically supported and defined by some form of Statistical Process Control (SPC). Lower Control Limit = A design limitation imposed by the manufacturer as part the process of validating the insulator’s tensile rating. This includes stretching or breaking of individual fiberglass strands within the core rod as well as elongation and deformation of end fittings. Damage Limit = Load at which the insulator begins to stretch, elongate or deform. The limit of the rod is a function of its diameterįig. This is the least common failure mode, i.e. The end fitting limit is directly tied to what specific end fitting type is being used and also by the standards that govern that design (e.g. This is the next most common failure mode, with ball type end fittings potentially having the lowest breaking loads. This is the most common failure mode and the one that can be controlled and predicted as a function of the crimping process during production. Tensile Failure Load = TFL = Load at which the insulator fails during tensile loading by one of 3 possible failure modes: (Note: Service conditions should include safety factors for ice and wind loading.) RTL = Rated Tensile Load = Working tensile load for an insulator in service. SML = Specified Mechanical Load = Ultimate rated tensile load for an insulator. While 30 klb and 50 klb SMLs have been among the most used ultimate strengths for polymer insulators, suspension applications can actually go up to 120 klb SML and beyond, if required. Indeed, one of the key elements during crimping is achieving the highest possible tensile output yet without cracking or otherwise overstressing the rod. Maximizing tensile performance of a suspension insulator is a function of increasing rod diameter, upgrading end fittings and optimizing compression of end fittings onto the core rod during crimping. Testing can then be completed as part of the manufacturing process in the factory or at a 3rd party laboratory where test set-up is not difficult and the testing process is standard. Moreover, for whatever the specified SML rating, an insulator is not limited by section length. Regardless of application, the primary load applied to an insulator is tensile. Mechanical loading of suspension insulators is straightforward and definitions of Specified Mechanical Load (SML) and Rated Tensile Load (RTL) are well understood and also easily quantifiable. This past edited contribution to INMR by Edward Niedospial, Technical Director Transmission at MacLean Power Systems in the United States discussed how insulator applications can be improved to achieve higher capabilities and how these can be tested and validated. The next challenge is to verify that these will indeed work and that the new approach is worth the effort. But achieving this efficiency requires going beyond traditional thinking in line design and studying new solutions. Power system projects with ‘High Strength’ within their scope typically receive special attention since high strength insulators have the potential to improve project efficiency through shorter duration and lower total cost. ![]()
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