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An understanding of the NESC insulator strength requirements and their application in PLS-CADD, PLS-POLE, and TOWER is critical in order to insure that correct results are obtained. This TechNote will briefly discuss one possible interpretation of the NESC and how to translate this into settings for criteria and loads within our software.
First, let's cover the NESC requirements for insulator strengths. Rule 277 of the NESC governs the mechanical strength of insulators. It can be found on page 188 of the 2002 NESC (page 223 of the PDF version) and is as follows:
Careful reading of this Rule indicates that all applicable loads from Rules 250, 251, and 252 should be used, except those of Rule 250C. Rule 250 is General Loadings and Maps, Rule 251 is Conductor Loading, Rule 252 is Loads on Line Supports, and Rule 250C is the Extreme Wind Loading condition. It should be noted that Rule 253 is NOT required to be followed. Rule 253 is Overload Factors for Structures, Crossarms, Support Hardware, Guys, Foundations, and Anchors.
Literal interpretation of these requirements is that the NESC Loading District loadings as specified in Rule 250B, with conductor tensions adjusted to account for the constant specified in Rule 251, and applied to the structure as specified in Rule 252 should be used. Note that literal interpretation of these requirements does NOT specify that the overload factors specified in Rule 253 should be used. This is further reinforced by the specific mentioning of insulators as a component to be considered in Rule 252, while support hardware is listed in Rule 252, insulators are ominously absent from this listing. To further complicate the issue, the phrase "Proper allowance should be made for the loads in Rule 250C." leaves much of the interpretation up to each line designer.
With that background let's cover the application of these Rules in PLS-CADD, PLS-POLE and TOWER. In PLS-CADD (both the full version and in Lite), under Criteria / Loads, District Loading conditions should be specified for the insulators. The vertical, wind, tension, and structure weight load factors should be set to 1.0 for the insulator loading conditions for literal interpretation of the NESC. The Strength Factor for Insulators should correspond to the value used in the PLS-POLE / TOWER Insulator Component library. In other words, if you specify the full rated ultimate strength of the insulators in the Component library, you need to take the strength reduction factor of Rule 277 in the Strength Factor for Insulators in PLS-CADD. The other option, which we recommend, is to use the reduced published rated ultimate strength of the insulators in the Component library, and use a value of 1.0 for the Strength Factor for Insulators in PLS-CADD. Whichever method you chose, we recommend that, for any loading condition used to check insulators (with all load factors equal to 1.0) and concurrently with the strength factor which you chose for insulators, you also use 1.0 for all the other strength factors (including Steel, Wood, and Concrete Poles). This will not affect the checking of the structures or non insulator components (since these are normally checked with other loading conditions that include load factors larger or equal to 1.0), but it will ensure proper calculation by TOWER or PLS-POLE of the allowable wind and weight spans for the structure.
This should be the end of this TechNote. However, there is an important issue currently being debated in the industry that should be noted. You may have noticed the frequent use of the phrase "literal interpretation" above. This is because there are some in the industry who have said that the intended application of the NESC is not to use unfactored loads. This issue was further complicated by a recent presentation at an NESC meeting of a series of polymer insulators of various ages that were pulled out of service and tested to destruction (January 2003 NESC meeting minutes). Approximately 90% of them failed below the rated ultimate strength. During the ensuing discussion of this presention, some stated that the NESC insulator strength reduction factors of Rule 277 are the actual capacities of the insulators and thus should be used for all design loading conditions, not just the NESC District Loadings. The discussion was based around the premise that even if the insulators don't experience catastrophic failure, they may develop stress cracks and other problems at loads below the specified rated ultimate strengths and catastrophic failure could then occur at a later time under another less stressful load. It was further clarified that that this is true under any design load conditions and not just the NESC Loading District and that the NESC Overload Factors of Rule 253 should be used in addition to the strength reduction factors of Rule 277. There was discussion of whether or not this should be changed in the 2007 NESC and even if an emergency TIA should be issued to modify the existing 2002 NESC to reflect these problems. The exact status of the TIA is not known to Power Line Systems at the present time.
As a software vendor, Power Line Systems cannot take responsibility for deciphering codes, their intentions, or recommending practices with respect to them. This is the responsibility of the Engineer of Record of the project. We are providing this TechNote as an example of how one can set up PLS-CADD criteria to match one's interpretation of the code. Since parts of the code are vague and subject to interpretation we strongly recommend that you contact your NESC representatives both to discuss the latest direction of the committee and to request clarification as to how the insulator provisions of the code should be interpreted. Further, we recommend that you become active not only with the NESC, but also other IEEE and ASCE codes and standards to ensure that the voice of the industry is heard and a broad industry consensus based on engineering facts is implemented.