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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ANSI/UL2737 (Withdrawn)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MIL-L-24410 (Withdrawn)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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385 Figure 1. Current distribution when a crane contacts a transmission line. Consequently, flashover produces a ground fault if the crane is grounded through an outrigger. If the outrigger is not used, the tier will flashover and cause a ground fault. The line to ground voltage will drive a current through the crane as shown in Figure 1. The current will be distributed between the ground and the worker. As an example, Table 1 shows the line current when a crane touches a five mile long line. The line impedance is 0.05+0.45j ohm/mile. 1 The calculation indicates that the current through the worker is between 10-1000A. Lethal current is around 100 mA. This simple analysis indicates that grounding of the crane will not protect the worker. The ground fault protection of the distribution lines is adjusted around 300-400 A. Higher voltage lines (66 kV and 120 kV) are equipped with more sensitive ground fault protection. Figure 2. Clean fog test of insulator link. Fast operation of ground fault protection is beneficial, but would not eliminate the danger of electrocution. The accident analysis revealed that the arc may burn the line, which de-energizes the crane within seconds. The arcing frightens the crane operator, who will move the crane away from the line within few seconds. Insulator Link From the presented discussion, clearly the rigger can be protected by an insulator link inserted in the crane cable just above the hook. The insulator can replace the headache ball. Two insulator links are available commercially. Figure 2 shows one of the links installed in Arizona State University's fog chamber. The links are rated up to 50 kV voltage and 3-60 ton mechanical load. One of the links uses screw-like metal blocks embedded in dielectric material. The dielectric is compressed when loaded. The other unit is built with two metal spokes, which are held together by resin impregnated fiberglass. Both links are covered by a protective dielectric coating. The important difference between the link and a composite insulator used by utilities is the lack of skirts. Measurements of the leakage and strike distance ratios for a link was between 1-1.55. For a utility insulator this ratio is 2-3. The shorter leakage distance of the links results in low flashover voltage in wet and polluted conditions. The data in the Table 1 shows that at all Voltage levels, the line protection would switch Off the line within a few cycles if the ground is wet or moist. In conditions, the distribution lines (13.4 kV or 24 kV) ground fault protection would not operate. However, the more sensitive protection of the higher voltage lines (66 kV-120 kV) would operate. Tests perform& by different laboratories [g, 10, 113 proved the poor pollution performance theory. AS an example, Ontario H~&~ 11 11 tested different insulator links by the IEC cl= fog method [12]. Test results showed that the heavily contaminated esDD 0.3 mg/cm2) insulator links were flashed over at less than 10 kV voltage. The clean or even the wet insulator links withstood test voltage. Leakage current during the wet tests was
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