Three electrostatic field sensing proximity warning devices were tested using full-scale cranes and powerline systems, a scaled model facility, environmental testing facilities, and theoretical- numerical analysis. The design objective of each device is to assist in safe crane operation near overhead powerlines. Several design deficiencies were noted, including inadequate overlap in sensitivity ranges, utilization of vacuum tubes and electromechanical parts, temperature sensitivity, and inaudible alarms. In addition, the testing showed major device limitations including sensitivity to boom orientations and length, polarization- dependent responses, and unreliable operation in the presence of multiple powerline circuits. The multiple powerline circuit problem is intrinsic to electrostatic sensing devices. The remaining deficiencies and limitations are correctable within a state-of-the- art design.