Hokuriku Shinkansen is electrified by a 30 kV a.c. commercial frequency AT feeding system. As for the commercial frequency, Japan is divided into two zones, 60 Hz zone in the western part and 50 Hz zone in the eastern part. Two different power frequencies exist in Hokuriku Shinkansen which passes the power frequency border between two different electric power supply companies. In case of a contact fault between two different power frequencies, a beat-formed large fault current will flow, which should be immediately detected and cut off. A protective relay for contact faults between the two different frequencies has been developed and has been put into practical use. The protective relay operates by detecting the 55 Hz component specific to the current caused by the contact fault and also detecting power frequency of the opponent traction substation. They have been installed at a frequency-bordered sectioning post and at traction substations which sandwich the sectioning post.