Overhead lines are suspended from strings of insulators which are in turn supported by, in this case, steel towers. The surrounding air acts as an insulating medium (at atmospheric pressure, air breaks down at 3kV/mm). Each phase in this line consists of a bundle of two conductors. The conductors are of type ACSR (see below). Bundles of conductors are used instead of a single conductor to reduce the effects of corona, the partial ionisation of the air close to the conductor surface. Bundling also reduces the inductive reactance of the line. For protection against lightning, ground wires (also called shield wires) are fitted above the phase conductors and are electrically connected to the towers and via the towers to ground.
Source: Transgrid.
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