Phasing

The actual field produced by an overhead line depends on several factors. This page illustrates this for one standard line, a 400 kV L12 transmission line with typical loads. Our detailed calculations of fields all specify the conditions they were calculated for.

The field also depends on the relative phasing of the two circuits. There are two main types of phasing:

A few transmission lines (and many distribution lines) have "untransposed" phasing, with the phases in the same order from top to bottom on the two sides of the towers. This produces a field which falls as the inverse square of distance from the line. However most lines have "transposed" phasing, with the opposite order of the phases on one side to the other. This introduces an extra degree of symmetry and extra cancellation between the fields from equal currents on the two sides; the resultant field falls more nearly as the inverse cube of distance, producing a much lower field at large distances from the line. This is illustrated below. See also more detail on power law variations for power lines, where we explain how the extent of the reduction depends on the balance of the currents.

Most of the National Grid system in the UK has transposed phasing but it is not always possible for every line to be transposed.

The graph above shows that, close to the centreline, something different is happening - the "transposed" field goes higher than the "untransposed".  See more information on this effect which depends on the clearance of the line.

Phasing in UK policy

SAGE, the stakeholder group in the UK that considered precautionary measures for EMFs, considered phasing: see what SAGE says about phasing. This led to the Government adopting a policy of "optimal phasing".  This usually means transposed phasing.  The details of how this policy is applied are given in a Code of Practice.