At 2000 feet, during a navigation exercise, the instructor initiated a practice forced landing. When the instructor was satisfied with the student's performance, the student initiated a go-around at about 200 feet above the ground. At about the same time the front of the helicopter contacted a powerline. The helicopter pitched up severely and lost speed rapidly. The single wire snapped after damaging the main rotor mast cover and bending control rods to the main rotor. The instructor took over and immediately carried out an autorotative landing. The wire had a span of approximately 600 metres between two hills. It was difficult to see. This accident was not the subject of an on-scene investigation.