Abstract: A low level fault condition in a mine trolley system is distinguished from a normal system load by detection circuitry connected to the trolley system between a main circuit breaker and the trolley wire. The detection circuitry monitors the current through the trolley wire and samples the current at preselected intervals of time to detect a rise in current through the trolley wire. The rate of current rise is compared to an adjustable reference signal. If the current rise exceeds a preselected rate, then activation circuitry of the main circuit breaker is latched to determine if the net current change is above a preset limit. If other available protective devices, such as fuses, in the trolley system fail to isolate the faulted circuitry within a preselected period of time after an excessive current rise is detected, then the activation circuitry is tripped to open the normally closed contacts of the main circuit breaker. In this manner the entire flow of current through the trolley wire is interrupted.

Abstract: A circuit for the arcless de-energization of a direct current load receiving direct current from a rectified alternating current source. The circuit includes a means for rectifying alternating current from an alternating current source to provide direct current to the direct current load. The means for rectifying has a current flow controller, such as a silicon controlled rectifier, connected with each phase of the alternating current source. The circuit also has means for de-energizing the then conducting current flow controller in said means for rectifying, the means for de-energizing having a current flow controller, such as a silicon controlled rectifier. The circuit further includes a voltage detection device for detecting a predetermined voltage drop in the circuit; and a time delay device for regulating the activation of the means for de-energizing.