Why are overhead transmission lines not insulated?

Overhead transmission lines are not insulated primarily due to three reasons: first and foremost is to reduce the weight of the lines, secondly to save cost and the last is they are at sufficient height to avoid direct contacts.

The thickness and material of insulation required for the transmission lines depend on the transmission voltage.

Higher the voltage thicker shall be the insulation. Hence insulating the lines adds up extra loads to it.

Apart from this, storm and accumulation of ice over the line can add up the load and affect the stability of the line.

To avoid this the lines are left uninsulated. Ultimately, by not insulating the lines the power transmission companies can save a lot of money.

As far as there is adequate gap between the lines and the ground and also if there sufficient distance between the phases transmission lines do not need any insulation.

The ambient air around it can provide sufficient insulation to the conductor.