Conductor bundle

Different power lines have different conductors.  Some have bundles of two conductors ("twin") and some four ("quad") - see a picture of a power line showing twin bundles.  The conductors themselves can be different sizes too.

The magnetic field depends only on the current and not on the size of the conductor carrying it.  So the conductor bundle does not affect the magnetic field.  But it does affect the electric field.  This is because, for a given voltage, the smaller the conductors, the higher the electric field at the surface of the conductor.  And the higher the electric field close to the conductors, the more of the volts are dropped there, and the less volts are left to drop near the ground.  So smaller conductors lead to lower electric field close to the ground.  Simlarly, putting two or four conductors into a bundle increases the effective size of the conductor, so the electric field close to the conductors is lower, and the electric field close to the ground is higher.

This graph shows the effect on the field at 1 m above ground for some actual bundles used in the UK:

"Zebra" is a steel-cored aluminium conductor 28.6 mm diameter.  "Araucaria" is an aluminium alloy conductor 37.3 mm diameter.

Historically, most UK transmission lines had zebra conductors, spaced 305 mm, in either a horizontal twin bundle or a square quad bundle.  More recently, other options have been used.  Some quad bundles have been enlarged to 500 mm for aerodynamic reasons.  Some twin bundles have been uprated to the larger araucaria conductors at 500 mm spacing, and there are now some triple bundles.  (There are many other conductors and bundles as well, we show just the main ones to illustrate the principles).  Finally, we illustrate the field that would be produced with a single conductor, but this is never used in practice.

The graph clearly shows how smaller bundles and conductors reduce the electric field at ground level.  But there's a limit - these smaller bundles have higher electric fields close to the conductors, and if that gets too large, it causes corona - ionisation of the air - which produces audible noise.