How the phasing affects the field produced by an overhead line
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.
"Phasing" is the way the two circuits on the two sides of a pylon are wired relative to each other.
Transposed and Untransposed phasing
The field 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. Each side of the tower - each circuit - produces a magnetic field that oscillates backwards and forwards (in mathematical terms, it's a dipole). Because the order of the phases is the same, the two magnetic fields are always in the same direction, so they add up. The resultant field to the side of the line (shown in red here) is the sum of the two (with a scaling factor to allow for how fields fall with distance anyway).
However most lines have "transposed" phasing, with the opposite order of the phases on one side to the other. Now the magnetic fields from each circuit are going in opposite directions. There's an extra degree of cancellation between the fields . The cancellation isn't exactly perfect because you are closer to one circuit than the other, so the field from that circuit is stronger than the field from the other circuit, but as can be seen, the resultant field to the side of the line is smaller:
In mathematical terms, untransposed phasing - two dipoles in the same direction - is still overall a dipole, and produces a field that falls as the inverse square of distance from the line. Transposed phasing, two dipoles in opposite directions, is a quadrupole, and produces a resultant field that falls more nearly as the inverse cube of distance, producing a much lower field at large distances from the line. This is illustrated below.
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.
Optimum and Non-optimum phasing
The discussion of transposed and untransposed phasing above assumed that the currents were in the same direction in both circuits. Occasionally, the currents may be in opposite directions in the two circuits along a stretch of line. then the situation reverses. The lowest magnetic field to the sides of the line is produced by Untransposed phasing.
To describe this we use the terminology "Optimum" and "Non Optimum" phasing. "Optimum" is normally "Transposed" but the full definition is shown in this table:
|Current flow in the two circuits||Physical phasing|
|Transposed||Any intermediate arrangement||Untransposed|
can never be Optimum
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 "optimum phasing". This usually means transposed phasing. The details of how this policy is applied are given in a Code of Practice.
Single Circuit Lines
"Phasing" is not a concept that applies to single-circuit lines. There is no expectation or requirement in the UK policy or the Code of Practice to build a line as a double-circuit, when it could otherwise be single-circuit, solely to take advantage of the phasing.
Optimum phasing is less effective with pylons that have the conductors in a triangular array, such as the T-pylon. The Code of Practice does not explicitly address this, but see our discussion of the issues.
Phasing on the UK system
The original 132 kV grid system was not built with a specific policy for phasing, and only 50% of double-circuit overhead lines currently have optimum phasing.
The 275 and 400 kV National Grid system in the UK was built from the 1960s onwards with a policy of transposed phasing where possible. So most of it is indeed transposed, with only about 10% of National Grid lines classed as "Non Optimum" phasing
See full details of both these statistics.
Phasing for underground cables
Some underground cables have multiple groups of conductors, and for these, the phasing of each group relative to the others has a similar effect to the phasing of overhead lines.
How much does phasing matter?
Optimum phasing is clearly best to minimise the fields to the side of the overhead line. But all fields fall rapidly with distance. Intermediate phasing arrangements fall faster than untransposed phasing, and transposed (or optimum) the fastest, but it is not always possible to achieve optimum phasing because of system or other engineering considerations, and all overhead lines, be they transposed, untransposed or one of the many other combinations of phasing, comply with the Government guidelines.