33 kV overhead lines: magnetic fields

33 kV lines are either carried on small lattice steel pylons or on wood poles.  There are many different variants of lines design but they all produce roughly similar fields.

The maximum field shown here is produced by a lattice pylon design when the ground clearance is the minimum allowed – 5.5 m – and the loads are the highest allowed – not usually above 1 kA in each circuit but there may be exceptions.  The field also depends on the phasing. 33 kV lines usually have Untransposed (U) phasing.

Typical fields are lower than the maximum field because the clearance is usually higher and the loads are usually lower.  The two curves shown here are for typical loads, the normal U phasing, and two different line designs: a lattice steel pylon (the higher field), and a wood-pole design (the lower field).

33 kV double-circuit line:	33 single-circuit wood pole:

This table gives some actual field values for the same conditions.

Note:

1. All fields calculated at 1 m above ground level.

2. All fields are given to the same resolution for simplicity of presentation (1 nT = 0.001 µT) but are not accurate to better than a few percent.

3. Calculations ignore zero-sequence current.  This means values at larger distances are probably underestimates, but this is unlikely to amount to more than a few percent and less closer to the line.

4. The “maximum field under the line” is the largest field, which is not necessarily on the route centreline; it is often under one of the conductor bundles.

5. Sometimes, a 33 kV circuit could be carried on a line designed for a higher voltage.  Then the magnetic fields could be larger than shown here.