The helicopter pilot reported that he was flying in support of a utility company crew that was installing line guards for a conductor damper on an energized 161 kilovolts (Kv) electrical transmission line. The transmission line had three phases (power lines) that were separated vertically and staggered horizontally from one another and ran parallel to the ground. The lower phase power line was about 9.5 feet (horizontally) further out from the middle phase power line. During approach to the lines, the main rotor blade clearance from the middle phase line was about 2.5 feet, which resulted in an electrical arc to the main rotor. The pilot landed the helicopter away from the transmission line without further incident. A postaccident examination revealed substantial damage to the main rotor hub and two main rotor blades. The pilot reported no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation. According to the Occupational Safety & Health Administration (OSHA) 1910.269(I)(10) the Minimum Approach Distances (MAD) is to "ensure that workers do not approach or take any conductive object closer to the energized parts as set forth in the table below (excerpt). " Nominal voltage in kilovolts Distance: Phase to ground exposure (feet) 161 to 169 4.00625 The MAD for this operation was a distance equal to or greater than 4.00625 feet.