Abstract: DC power control system for use in an electric vehicular system has DC power supply and power conductors. A meter relay in a control unit enclosure displayably measures the DC current in the conductors and simultaneously establishes an overcurrent limit for tripping out the conductors when the limit is exceeded. Meter calibration and tripping function are testable by a test switch for substituting an adjustable DC test source in place of a conductor meter shunt to the meter relay. Application of DC to shorted power conductors is prevented by preliminary low power DC isolated AC relaying across the conductors. The test switch substitutes a low resistance for the power conductors to also test the AC tripping action. If the power conductors are already energized from a remote power supply, the local power supply is prevented from access by DC responsive relaying during a preliminary time period. After a problem is detected, the control system locks out so as to demand operator attention if the problem persists.

Abstract: A mine safety system for use with a direct current power supply delivering power via a pair of conductors to the direct current load components of mining apparatus which includes a conductive frame electrically isolated from ground and at least one of the conductors. In the safety system, an alternating current generator transmits a predetermined frequency signal over the pair of conductors to the mining apparatus. A filter which passes the predetermined frequency signal is disposed at the mining apparatus in a shunt circuit with the load components, the filter thereby being connected to the generator.