PURPOSE: To specify a contact accident of mixed different frequency power sources and to protect a power source by detecting a current or a voltage supplied to a trolley wire, and generating an operation output when its detected signal frequency is a specific frequency.

CONSTITUTION: Trolley wires 3 are connected between a first AC power source 1 having a frequency of 50Hz and a second AC power source 2 having a frequency of 60Hz, switching section 4 for switching trolley wire 3 section to an intermediate point of the wires 3 is provided, and AC power is supplied to an electric railcar 7 which travels under the wires 3 from the power sources 1, 2. In the case, a detector (transformer) detects a current or a voltage supplied to the wire, and a relay generates an operation output (e.g. a trip signal of breakers 6a, 6b of the section 4) when a detection signal frequency of the detector is 55Hz or 55H ± α (α is a small frequency). Thus, a different frequency mixed contact accident of a rare accident can be specified, and the power sources 1, 2 can be effectively protected.