The FS-4 Float system is intended for the detection of petroleum leaks and petroleum products in trap or protection reservoirs. It consists of two guiding rods and a float on which CPS-24Xi-C-RO sensor and NSSU-811 spark resistant assessment unit are installed. The float has been designed so that the sensor face can submerge under medium surfaces. If the sensor face is in contact with water or with the bottom steel prism (when the reservoir is empty), then the sensor is activated. If the sensor face is in contact with petroleum or petroleum products, then the sensor is deactivated, i.e. put in an emergency status. The capacitive sensor will be electrically connected to a specially adapted NSSU-811 evaluation unit with a relay output. It is possible to select a unit type with power supply voltage either 230 V AC or 24 V DC.