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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ANSI/UL2737 (Withdrawn)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MIL-L-24410 (Withdrawn)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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317 calculated from the system short circuit duty. Typical short circuit Table 2 Resistance of clean and polluted crane insulators Voltage kV duties and the corresponding source inductances are shown in Table 1. inductance MVA mH The metal crane structure hasvery low resistanceand inductance compared with the source, therefore it is neglected in this study. 12.4 750 ' Sample I Clean and Dry I Polluted and Wet I I250 SZ cm I 150SZcm .54 50 kV, 2-3 ton. Test 1 244,000 1400 I 288 Test 2 278,000 No reading ~ 24.9 69 120 Figure 2. Equivalent circuit Table 1. Typical Short Circuit Duties and Source Inductances I Rated I Short Circuit Dub I Source I .~ 1500 1.10 2500 5.05 4000 9.54 The values in Table 2 show a significant spread. The surface resistance vanes between 250 kohm and 1.4 Mohm in polluted and wet conditions and between 216 and 270 Mohm in clean conditions. For the calculation of insulator capacitance, the electrical field has been plotted by the 'ELECTRO" Electric Field Solver program and was integrated around the high-voltage electrode. A typical field plot is shown in Figure 3. The calculated capacitance values are listed in Table 3. The measured capacitance of an installed 50 kV, 10 tonunitwas4.5pF, whichindicatesareasonableagreement with the calculated value. /I Figure 3 Electrical field of an insulating link Table 3. Capacitance of insulated links The manufacturer requires that the insulator inserted in the crane link is clean and dry. The resistance of a clean and dry insulator is very high. However, if the insulator is polluted and wet, its resistance is reduced significantly. The resistance of a polluted and wet insulator depends on the pollution severity. The industry uses three insulator pollution levels: light (480 Q.cm), medium (250 Rem) and heavy (150 SZ.cm). The calculation of insulator resistance is difficult because the contamination layer thickness is not available. The equivalent insulator resistance can be estimated from the measurements presented by Hamilton and Morgan [l]. The results of these measurementsare summarized in Table 2. The insulatorcan be represented byaresistanceandacapacitance connected in parallel. The capacitance value was varied between
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