At least one conductor is disposed in sealed vessel, which is filled with insulating gas, an hand hole is provided to a part of sealed vessel and is closed by a sealed cover, and optical fibers are disposed that form a closed loop surrounding conductor in sealed vessel. Optical fibers are hermetically led out of sealed vessel from sealed cover and are provided with at least a light source unit and an optical signal processor unit at the lead-out end. An airtight hollow tube disposed looping around leaving a predefined distance of separation is provided to conductor in sealed vessel, the end of tube is hermetically connected to sealed cover by sealing/bonding means, and optical fibers can be inserted in sealing/bonding means.