Some of the advantages of helicopters are the ability to fly very slow and land in small unapproved areas. As such, they perform many jobs that increase the risk of hitting a wire. Wire strikes have happened in just about all segments of the helicopter industry.
EMS pilots landing on roads and fields have to be extremely careful, especially at night or during the day when bright sunlight produces glare. Small power lines crossing roads and fields can be very difficult to see and several accidents have occurred when a departing helicopter contacts a wire. If a helicopter is heavily loaded and has little power available the pilot needs to gain airspeed to increase lift for climb out. This raises the risk of hitting an unseen wire. Just such an accident happened earlier this year in Tennessee (NTSB Identification: ERA11IA436). If power is available, a max performance or straight-up climb can mitigate the risk of an accident. I believe NTSB data shows the probability of an engine failure is smaller than the probability of hitting a wire or obstruction.
Even pilots who operate around power lines routinely must be alert. In July of 2011 a pilot flying an aerial application flight contacted a power line that ran perpendicular to the direction of the spray run. The pilot told the NTSB he was aware of the power line, but became distracted by horses that were located near the field. Moreover, during an aerial power line observation flight, the pilot hit the static wire for the power line he was patrolling. The pilot reported to the NTSB that he never lost control of the helicopter, but landed as soon as he could in a parking lot close to where the wire strike occurred. In this case, the helicopter was equipped with wire strike protection and a 12 inch piece of the 7-strand wire was found in the wire cutter located below the main rotor mast.
Robinson Helicopter has identified wire strikes as the number one cause of fatal accidents in helicopters. The company has published a safety notice (SN-16) that provides advice like crossing power lines at the support towers, being aware of the smaller grounding wires and flying at least 500 feet AGL whenever possible.