Abstract: A system for automatically preventing the discharge of spilled oil from electrical distribution substation equipment into streams or ground water. The system utilizes a containment basin, sump, electric operated water drain valve and a thermal dispersion type oil/water interface detector. The oil/water interface probe is placed at a specified height in the sump. It is capable of determining if the probe is in oil, water or air. The interface detector along with other relays will allow water to be draped from the containment basin but prevent oil from being discharged. The system will also initiate an alarm which may locally or remotely warn that a spill has occurred.