The maximum fields that are produced by a line occur directly underneath the line, underneath the lowest point of the conductors, which is usually towards the middle of each span. Actual conductor clearances above ground would generally be higher than this (and therefore the fields produced near ground level would be lower) for two main reasons. Firstly, for most of the length of a span, the conductor clearance is higher than it is at the lowest point. Secondly, the actual ground clearance of the conductors depends on their temperature. For the vast majority of the time they operate at less than their rated maximum temperature and therefore sag less, resulting in higher ground clearances.

The maximum fields that are produced by a line occur directly underneath the line, underneath the lowest point of the conductors, which is usually towards the middle of each span. Actual conductor clearances above ground would generally be higher than this (and therefore the fields produced near ground level would be lower) for two main reasons. Firstly, for most of the length of a span, the conductor clearance is higher than it is at the lowest point. Secondly, the actual ground clearance of the conductors depends on their temperature. For the vast majority of the time they operate at less than their rated maximum temperature and therefore sag less, resulting in higher ground clearances.

We give two graphs here to illustrate the effect.  Both are for 400 kV lines as these produce the highest electric fields.  The first shows the electric field for three representative designs of UK power line with their respective standard conductor bundle.  The lower group of solid lines are for the common transposed phasing, the upper group of dotted lines for the much rarer untransposed phasing, which produces higher fields.

The second graph takes just one of these line types, the L6, and shows the effect of different conductor bundles. (The field from the smaller of the two quad bundles is almost identical to that from the triple bundle so the lower of the two red lines and the green line are practically superimposed on each other.)

These graphs can be used, for example, to read off what clearance is required to achieve compliance with electric-field exposure limits. We also provide a table giving the result below.

The details:

Quad bundles: zebra conductors.  L6 twin and triple: araucaria conductors.  L2 twin: zebra conductors.  The field from triple bundles may depend on the orientation of the triangle.  L6 lines were mostly built in the 1960s with a quad bundle spaced at 305 mm, but many have subsequently changed to a larger spacing of quad (400 mm or the 500 mm we illustrate here), or to twin, or, more recently, triple.  New lines are likely to be triple conductors on a tower design similar to the L12.

We discuss above the effect of the clearance of the conductors above ground on the electric field.  The higher the clearance, the lower the field.  Because the electric field exposure limit for the general public in populated areas in the UK is 9 kV/m, we list here the clearances required for the field to be below 9 kV/m.  If the clearance is greater than the stated value, the electric field will be below 9 kV/m.

The field depends on several factors as shown in the table: the tower design, the conductor bundle, and the phasing (transposed, T, or untransposed, U - other phasings lie between these values).

All calculations for 400 kV, 1 m above ground level, and using the infinite-straight-line approximation in accordance with the Code of Practice on demonstrating compliance.  Clearances are at 0.1 m resolution (the first 0.1 m increment of extra clearance needed to reduce the field below 9 kV/m, not the nearest 0.1 m).

This graph shows exactly the same data but presented as percentage of the maximum field for each case rather than as the actual field value. This makes it look as if one field is falling much more rapidly with distance than the other, but this is just a consequence of plotting as a percentage rather than as the actual value.