Telecom guys here are very used to getting belted by phone lines. There are a lot of older areas where the copper wiring went past capacity, and they used multichannel extenders to get either 2, 4, 8 or 16 POTS lines running off a single twisted pair with the required power being line derived. Thus you find a single pair in a trunk cable or inside a building having either 160, 200 or 250VDC on it, and no marking that it is high voltage. The paper cabling definitely gets unhappy with this with time, as often you will find a fault where the joint end has burned back about 2m from the insulation failing if moisture got in. I had to put in a new cable conduit, as the existing cable was both failing and refused to come out of the steel conduit in the concrete. Had the Telco dig up the street to the joint, then they laid the new underground cable and I provided the new external conduit up a floor to get the cable to a wall riser. then it was terminated and went back down again to get to the original MDF.