Tension ratings
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.
Sample tests
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.
Routine tests
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.
Compression loads
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.
Bending
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.
Torsion
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 loads
Suspension 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.
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