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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to safety systems and methods in general and, more particularly, to a safety system and method for use with conductive members when used in the presence of power lines. 
     SUMMARY OF THE INVENTION 
     A safety system and method for use with apparatus having a conductive member, where said apparatus changes the attitude and/or altitude of the conductive member during the operation of the apparatus, includes at least one sensor mounted on the conductive member. The sensor will sense an electric field and provide a signal representative of the strength of the sensed electric field. Further changing of the conductive member&#39;s attitude and/or altitude is prevented in response to the signal from the sensor when the electric field strength is greater than a predetermined safe value. 
    
    
     The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial representation of a truck mounted derrick having a safety system constructed in accordance with the present invention. 
     FIG. 2 is a simplified block diagram of the safety system shown in FIG. 1. 
     FIGS. 3A through 3D illustrate schematically different types of sensors which may be used with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     There have been fatal accidents in the oil field with drilling and with well service groups due to raising derricks and masts of drilling and workover rigs into electrical power lines. This safety problem is not unique to the oil industry but is also applicable to other industries where devices such as cherry pickers used by power companies, tree surgeons, plus construction cranes and other conductive masts or devices, accidentally come in contact with power lines. Nor is the present invention restricted to devices located on trucks but is also applicable to a situation where a conductive member may be raised in the presence of a power line. A conductive member for purposes of the present invention is any member whose structure is conductive or whose structure is non-conductive but has other means of conduction such as wires and cables. The present invention provides a method of preventing this type of accident and at the same time warning the operator that the rig is in a hazardous situation. 
     Referring now to FIG. 1, there is shown a typical service conductive member such as a derrick 3 mounted on a truck 1 which is raised to an operating position by conventional means. Located at strategic points on derrick 3 are any number of sensors represented by sensors 7, 7A, and 7n, which are electrically connected to an electronic system 10. As derrick 3 is raised during normal operation and it approaches power lines, sensors 7 through 7n will have voltages induced by the electric field created by the power lines and provides signals to electronic systems 10. When the electric field is strong enough electronic systems 10 will stop the raising of derrick 3 and sound an alarm audibly and visibly as hereinafter explained. 
     With reference to FIG. 2, sensors 7 through 7n in the presence of an electric field provides signals to a multiplexing unit 14 of electronics system 10 which multiplexes the signals to approximately 1 second samples. The multiplexed signal is then provided to a narrow band filter and amplifier 17 which filters the multiplexed signal so that only a signal associated with the power lines is provided to a maximum signal level circuit 23. Circuit 23 is of a conventional type and its detail is not necessary to an understanding of the present invention. Maximum signal level circuit 23 may be overriden by an override signal from an override signal means 25 which may be a simple on/off switch receiving a direct current voltage for application to the maximum signal sensing circuit 23. A sensitivity control signal means provides a signal to maximum signal level circuit 23 for adjusting the sensitivity of maximum signal level circuit 23. Maximum signal level sensing circuit 23 provides a brake signal to actuator means 32, which is part of the conventional raising means, which also receives a hoist enable signal from hoist enable signal means 37 when derrick 3 is being raised. The brake signal from actuator means 32 is provided to brake actuator means 44 and to alarm means 48. Alarm means 48 may provide either an audio alarm, a visual alarm or both. 
     In operation, an operator wishing to raise derrick 3 causes hoist enable signal means 37 to provide a hoist enable signal to actuator means 32 which raises derrick 3. When derrick 3 approaches a power line, the electric field around that power line causes sensors 7 through 7n to provide signals to multiplexing unit 14 having amplitudes corresponding to the intensity of the electric field. Multiplexing unit 14 provides a multiplexed signal to narrow band filter and amplifier 17 which in turn provides the filtered signal to maximum signal level circuit 23. Any one of the sensors 7 through 7n providing a signal that is greater than a predetermined safety level causes maximum signal level circuit 23 to provide a signal to actuator means 32 disabling actuator means 32 and causing actuator means 32 to provide the brake signal to brake actuator means 44 and alarm means 48. Brake actuator means 44 stops the movement of derrick 3, while alarm means 48 sounds an alarm that derrick 3 has approached a power line. Should sensors 7 through 7n provide such a signal and the operator in his judgment can see that he can safely raise derrick 3 in the presence of the power line with safety, he may then utilize override contol means 25 to provide an override signal to maximum signal level sensing circuit 23 which then is deactivated to allow actuator means 32 to continue to raise derrick 3. The operator would also use the override signal when lowering derrick 3 after the derrick has entered a strong electric field. 
     Although the system of the present invention has been shown as having multiple sensors, a single sensor appropriately placed on derrick 3 may also be utilized in which case multiplexing unit 14 would not be necessary and the signal from the single sensor may be applied directly to narrow band filter and amplifier 17. 
     FIGS. 3A, 3B, 3C and 3D show different types of configurations for sensors 7, although the specific configuration of sensor 7 is not restricted to any one of four types shown, but is restricted to a sensor which will produce a signal in the presence of an electric field that is representative of the strength of the electric field. 
     The present invention as hereinbefore described is a safety system for use with an aerial truck which utilizes a derrick or a conductive structure which is raised from one position to another position during the course of operation and which may come into contact with electric power lines and cause injury and death to the operators of such apparatus.

Summary:
A safety system for use with apparatus having a conductive member and where the apparatus changes the attitude and/or altitude of the conductive member during its operation includes at least one sensor which senses an electric field and provides a signal representative of the strength of the electric field. A safety device responsive to the signal from the sensor prevents the further changing of the conductive member&#39;s attitude and/or altitude when the electric field strength is greater than a predetermined safe value.