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384 Probabilistic Method to Assess Insulating Link George G. Karady, Fellow Minesh Shah, Arizona State University Tempe, AZ 85287-5706 IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996 Performance for Protection of Crane Workers Abstract: Contact between cranes and transmission lines is the most frequent cause of accidents, which may lead to electrocution of an operator or rigger. This accident can be prevented by inserting an insulating link in the crane’s cable. This paper analyzes currents during accidents and proposes a modified test method for contaminated insulating links. A new and better insulating link is also introduced. Flashover probability of contaminated insulator links is measured. The test results are evaluated with a new probabilistic method which leads to better assessment of link efficiency. The paper concludes that the risk of link failure cannot be determined without the new flashover probability measurement. Kev Words: Composite insulators, crane, pollution flashover, electrical accident, risk analysis. INTRODUCTION Accidental contact between the power line and crane is the most serious of all types of crane accidents [l]. Power line contact caused 44% of construction industry mobile crane fatalities between 1969-1990 [2]. Abramo investigated 379 crane accidents and found that 143 (37.7%) of them were caused by power line contact [3]. These accidents resulted in injury or death of 197 workers. The most frequent victim (33.5%) is the rigger or helper who guides the load. Truck mounted cranes and hydraulic cranes caused 61.5% of the accidents [3]. Paques evaluated 262 accidents in the USA, Canada and France [41. He found that voltage was less than 700 V at 2% of the accidents. Voltage was between 7 kV and 44 kV at 74% of the accidents. Only 18% of the accidents occurred at higher than 44 kV. In the remaining 6%, voltage is unknown. It can be concluded that most accidents occur at 15 kV (7.2 kV ground-to-line) distribution lines. These lines are the most frequently used in the United States. Contact between the crane and the distribution line causes a ground fault. In the worst case scenario, the line to ground voltage drives a high current through the crane cable and the worker, if he touches the load. The probability of injuries can be reduced by an insulating link which is inserted in the crane cable. This insulator reduces the current and protects the worker. The effectiveness of the protection depends on the condition of the insulating link. 95 WM 235-2 PWRD A paper recommended and approved by the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for presentat- ion at the 1995 IEEE/PES Winter Meeting, January 29, to February 2, 1995, Nev York, NY. Manuscript sub- mitted July 15, 1994; made available for printing December 13, 1994. Student Member D. Dumora, Member SEDIVER Saint Yorre, France As an example, a wet and heavily polluted link may flashover and fail to protect the worker. The author of this paper analyzed the operating conditions of an insulating link and demonstrated that both the steady state and transient current must be considered at the evaluation of the link’s protection effectiveness 151. The insulating link was patented more than 35 years ago [61. Today industry offers only two types of insulating links [7, 81. Several tests were performed on these links to determine their effectiveness under polluted conditions [9, 10, 111. The test reports show that flashover voltage of heavily polluted and wet links are less than their rated voltage. On the basis of these findings, the effectiveness of these links is in question. The purpose of this paper is: a) analysis of the electrical stress of the link when a crane contacts a transmission line and development of a test procedure simulating actual operating conditions. introduction of a new, more effective insulator link and presentation of flashover tests results of links using the new method. b) c) estimation of expected link performance using a probabilistic risk assessment technique. ELECTRICAL STRESSES OF AN INSULATING LINK Currents during: the accident The majority of cranes are not grounded directly during operation. However, the outrigger unintentionally grounds the crane in most cases. Outrigger provided grounding resistance is estimated to be about 50-150 ohms in dry conditions and about 10-20 ohms in wet conditions. The crane rigger, who adjusts the load, is grounded through his shoes. Estimated grounding resistance in dry conditions is 500- lo00 ohms and in wet conditions, the resistance may reduced to 50-130 ohms. Figure 1 shows a typical accident scene. The crane is grounded through the outrigger and the rigger directs the load when the cable touches the power line. Analysis of different accidents shows that a flashover occurs between the line and the crane’s body when the crane approaches a transmission line [3]. The neutral point of most transmission lines in the USA is directly grounded. 0885-8977/96/$05.00 0 1995 IEEE
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