The wires in high-voltage power lines are not encased in rubber insulation, they are simply attached to poles with insulators and thus electrically touch only the current source and consumer.
However, it is not uncommon to see birds sitting on these wires. It turns out that the birds are grasping the bare wire through which a huge current is flowing. So why do not they suffer?
The thing is that when the bird sits on the wire, it creates a parallel connection of conductors. One conductor is the bird itself, and the other is the section of wire under the bird’s feet. The resistance of the bird is many, many times greater than the resistance of the wire, so a negligible current flows through it which cannot damage it (in a parallel connection, the total current is distributed between the parallel sections of the circuit inversely proportional to the resistance).
However, a bird can still die if it mishandles high-voltage power lines. All it has to do is sit on the wire and touch the metal part of one of the poles that hold the wires in place. These poles are obviously grounded because they are installed on the ground. In addition, now the resistance of the bird is much less than the resistance of the air (with which it creates a parallel connection in this case), so the current that will go through the bird will be enormous. A current of such great strength will literally incinerate the bird almost instantly.
These rare birds touching the wire and the pole at the same time are the only victims of the fact that the wires in high-voltage lines are not encased in insulation, but only insulated from the pole. However, bird deaths do not affect, damage or disrupt the transmission process in any way. That is why wires still remain without insulation, because it would be very expensive and difficult to insulate them.
In addition, it should be noted that only high-voltage power lines are not insulated from the outside environment or the air. They are suspended on huge poles very high up.
But those wires that run from substations to people’s homes, to street lamps, and so on, on poles of low height, are already insulated along their entire length (at least, this is provided by modern electrification technology). In these wires, located at a much lower height, by the way, not so much current flows, and the voltage on them is less. And because they are completely encased in insulation, they are no longer a threat to birds. Although the reason for such insulation of these wires is, first of all, the safety of people, who, though with difficulty, can also come into casual contact with them. After all, these wires are much more common, and they are much closer.