Two major standards bodies, the International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI), define the test methods to be used in demonstrating the adequacy of tension ratings. The ratings themselves are assigned by each manufacturer.
The IEC and ANSI test methods are essentially the same. The procedure requires that the sample insulators be subjected to a tensile load that is increased rapidly but smoothly from zero to 75% of the Specified Mechanical Load (SML) and then gradually up to 100% of the SML in a time between 30 and 90 seconds.
If 100% of the SML is reached in less than 90 seconds, the load is maintained at SML for the remainder of the 90 seconds. The insulator is considered to have passed if no failure occurs.
Following this 90-second period, ANSI mandates (while IEC leaves as optional) that the load be increased until failure occurs. The load at which failure occurs is recorded and used to justify the manufacturer’s choice of SML.
The IEC and ANSI definitions are identical. The Routine Test Load (RTL) is a rating equal to 50% of the SML. The actual routine load applied to each insulator shall be at least the RTL and shall be held for at least 10 seconds.
If the actual routine load applied to each insulator exceeds the RTL, it should be established that the routine tests are not causing damage to the insulators.
NOTE— Ratings for composite insulators closely parallel those for ceramic insulators. Compliance with the Mechanical and Electrical (M&E) rating for ceramic insulators is demonstrated by failing insulators and statistically demonstrating the required margin between the rating and the ultimate strength.
Compliance with the withstand rating for composite insulators is demonstrated by a withstand test for 90 seconds at the rating. Both ceramic and composite insulators must withstand a routine test of at least 50% of the tension rating; ceramic insulators for 3 seconds, composite insulators for 10 seconds. Both ceramic and composite insulators may be used continuously at loads up to the RTL.
Suspension insulators do not ordinarily have compression ratings. Not all designs have the same compression capability. On some types of suspension insulators, the grip of the metal end fitting to the core has a very low compression load-carrying ability.
Generally, for very long insulators, the compression ratings are limited by buckling. A large variation in compressive strength can be found depending upon how the ends are restrained.
Suspension insulators normally do not have bending ratings. In some applications, for example, when used as phase-to-phase spacers, suspension insulators will experience bending loads. In this application, it will be necessary to obtain the bending capabilities of the specific insulator from the manufacturer because not all types of tension insulators have the same bending capabilities.
The torsion strength of a composite suspension insulator is determined in a manner similar to that used to determine the torsion strength of a porcelain cap and pin or post type apparatus insulator. The only difference between the two is that composite insulators are examined following the test to determine if any damage has occurred, while porcelain units are judged on a break or no-break basis.
Composite suspension insulators have been damaged in dead-end applications by excessive twisting. The torsion capability of suspension insulators depends on the combined torsional strength of the rod and on the method of attaching the end fittings to the core.
There is a wide variation in the torsional load capability of various insulators. Caution is advised in using single insulators to dead-end long conductor spans as the torsional limits of composite insulators can definitely be exceeded. Such situations can be remedied through the use of parallel units or stress relieving hardware.
Combined loadsSuspension insulators do not carry combined ratings. For applications such as phase-to-phase or V string insulators where combined loads may occur it is advisable to consult with the manufacturer.