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Standards for Insulating links
ANSI/CPLSO-14
ANSI/UL2737 (Withdrawn)
ASTM F2973
MIL-L-24410 (Withdrawn)
Tests by Independent Organizations
Load Insulator
Miller & Hirtzer
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386 less than the lethal (100 mA) leakage current. These results show that a clean link, either dry or wet, provides sufficient protection for the crane worker. Electrical Stresses Transmission line insulators are continuously subjected to line to ground voltage and effects of the environment. Inland wind driven dust and industrial air pollution contaminate the insulators. Rain cleans the insulator. Contamination causes flashover when the contaminated insulator is wetted by fog or drizzle. Near to the sea, wind driven sea water contaminates the insulators. Most insulators are not cleaned regularly, except in extremely polluted areas, where insulators are washed or coated by silicon grease to avoid contamination flashover. Furthermore, transmission line insulators are subjected to lightning and switching surges. In contrast, the insulator link is energized to the line to ground voltage only for a short period of time (0.06-2 sec) one or two times in its service life. The once in a life time short duration exposure to 60 Hz line to ground voltage, results in significantly different operating conditions for the insulating link than continuously stressed transmission line insulators. Short duration of exposure to transmission line doesn’t generate the dry band formations on insulating link and causes the higher flashover voltage compared to the transmission line insulators. The probability that a lightning or switching surge striking during the short duration energization is negligible. Therefore, the link is not subjected to these types of stresses. The build up of atmospheric contamination on an insulating link is limited by changing work locations. In addition, the manufacturer recommends regular maintenance of the link, including cleaning. The most probable cause of link Contamination is negligent handling, which may include dragging the link in dust or mud. This type of contamination cannot produce a uniform contamination layer which is observed frequently on transmission line insulators. Dragging causes contaminated patches and regions along the link. To prove this assumption, Arizona State University tested the contamination process of an insulating link. The insulator link described in the next paragraph was polluted by thee different ways: dipped in a slurry, rolled into mud and dust. Subsequently, contamination distribution on the link was observed and the equivalent salt deposit density (ESDD) was measured. First, a standard contamination slurry was prepared following the recommendation of IEC Standard 507 for clean fog testing of insulators. The slurry contained 15 &liter salt (NaCl), 40 @iter kaolin and distilled water. The clean insulator link was dipped in this slurry to produce light pollution. After dipping, the links were dried in a vertical position and the ESDD was measured. Uniform contamination resulted on the surface of the links by using this method. Second, a dust mixture consisting of salt (NaCl) and kaolin was placed in a narrow box with a depth of about 2.5 inches. The kaolin and salt ratio was 40115 = 2.66. The clean insulating link was placed in the box and rolled five times. After rolling, the links ESDD was measured. Third, water (1.36 I/kg) was added to the dust mixture to produce mud. This resulted in a moist, but not liquid, mixture. The mud was placed in the same narrow box and the cleaned insulator link was rolled five times in this mud. After rolling, the links ESDD was measured. The last two tests were repeated using natural soil, collected from the garden. This natural soil contend only 36.36 g/kg salt. In order to obtain the same salinity with the kaolin mixture, additional salt was added into the soil mixture to make the results easily comparable. The equivalent salt content was 272.72 gkg. The ESDD was measured using the method described in EC Standard 507. Each test was repeated two or three times. Results of contamination test are summarized in Table 2. Table 2 Results of contamination test The test results showed that dust resulted in less contamination than mud. Natural soil caused slightly lower levels of contamination than the kaolin mixture. In spite of that, the salt content was the same. Dipping causes slightly lower levels of contamination than the mud rolling. Dipping produced uniform and non-sticking contamination. The dust method produced a very thin layer of non-sticking deposits. However, patches of contamination were observed on the surface. This contamination can easily be removed by shaking the insulator. Mud produced thick and non-uniform layers of deposits. These deposits were observed as lump forms. The flashover tests were performed on the insulators which were contaminated uniformly by the standard slurry. This internationally accepted method of pollution produces the lowest flashover voltages [ 131. The actual link contaminated by mud will have slightly higher flashover voltage. According to this test, light contamination is expected on a maintained insulator link. Utility insulators are exposed to more lightning and switching surges and the insulating link is not. The once in a lifetime, short duration exposure to 60 Hz line to ground voltage, produces different operating conditions on the insulating link than on utility insulators. This suggests different testing techniques are required for insulating links than insulators.
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