They are insulated, by 10 or 20 feet of air, and by glass insulators at every support point.
They don’t use conventional PVC or thermoplastic insulation like low voltage wiring because
- up in the air they are protected from accidental contact
- thermoplastics will break down from UV exposure and have to be replaced periodically - a high maintenance cost. Air is free, no-maintenance, and lasts for ever
- ultra HV will require hugely thick and costly insulation to insulate it.
- insulation adds cost not only for the insulation but also because the wires become heavier and need to be stronger to support the weight, and the towers then need to be closer together, etc.
- cladding Insulation degrades the thermal performance. Bare wire has the highest heat dissipation, clad wire is usually thermally insulated and the I-R drop heats the wire increasing resistance and lowering power carrying capacity; bare wire has the highest thermal performance.
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