Safety control apparatus of substantially improved reliability and sensitivity is provided for detecting faults and automatically deenergizing electrically powered mobile or portable equipment, such as mining machinery, which is energized in the field by means of relatively long cables often carrying high voltage electrical power from a sub-station, in the event of either a short circuit between a sensing conductor and ground or any failure of the ground connection included in the system to protect the operators of the equipment and the equipment itself. The improved safety apparatus is further characterized by its ability to respond to detected faults so quickly as to virtually eliminate any chance for operator injury or damage to equipment, even when very high voltage power is being used. Although the apparatus responds to both ground interruption (or high resistance to ground) and the mentioned type of short faults in a highly sensitive manner, it is essentially immune to the effects of induced transients such as have plagued the reliability of prior safety systems for the same general purpose.