Probabilistic method to assess insulating link performance for protection of crane workers

Accidental contact between the power line and crane causes a ground fault. The line to ground voltage drives a high current through the crane cable and the worker, if he touches the load. This current may cause fatal electrocution. The probability of injuries can be reduced by inserting an insulating link in the crane cable. But the effectiveness of the protection depends upon the insulating link condition. As an example, a wet and heavily polluted link may flashover and fail to protect the worker. The purpose of this paper is: a) analysis of the electrical stress of the link and development of a test procedure simulating actual operating conditions, b) introduction of a new, more effective insulator link and presentation of flashovertests results of links c) estimation of expected link performance using a probabilistic risk assessment technique. Figure 1 shows the current distribution when the crane touches a line. The calculation shows that the crane current is between 100A-10kAand the current through the worker is between 10-1OOOA. This simple analysis indicates that the grounding of the crane will not protect the worker. However, the large current activates the line protection which switches off the line within a few seconds and consequently the link is energized to the line to ground voltage only for short period of time (0.06-2 sec). This may occur only one or two times in its service life. The once in a lifetime short duration exposure to 60 Hz line to ground voltage, produces significantly different stresses on the insulating link than on utility insulators. This also suggests different testing techniques are required for insulating links than utility insulators. A new test procedure is developed. The insulator link is contaminated by the IEC method and tested in a fog chamber. When fully wetted, the test voltage is applied 25 times for 1 second duration with 20-40 second waiting period between each energization. By using repeated application of the voltage, typical flashover probability curves are shown in Figure 2 for different links. The leakage distance of presently available links is short which reduces the pollution performance. To improve the pollution per-formance a new more effective insulator link is developed. This link is rated to 50 kV and 30 ton (300 kN). Two heavy hot dip galvanized forged steel end-fittings are compressed, crimped on a 38 mm (around 1.5 inch) impregnated fiberglass rod. The rod is covered by an injected EPDM housing with seven skirts. The leakage distance to strike distance ratio is 2.22. The EPDM housing is elastic and impact resistant. The link performance can be estimated by using the probability curves. The link should provide 100 percent protection. As an example, Figure 2 indicates that the link 3 provides 100 percent protection if the line to ground voltage is less than 40 kV. In practical terms, it means that a heavily polluted new link protects the operator if the line voltage is less than 69 kV. The major contribution of this paper is: • The development of a modified test method which measures the flashover probability of different insulator links in contaminated conditions. • Introduction of a new better insulating link which is similar to power line insulators. • A new probabilistic risk analysis method which is proposed to assess insulator link efficiency. This method uses pollution, wetting and flashover probability to calculate the risk of link failure. The paper concludes that the risk of failure can not be determined using the results of standard pollution tests.