Clearance

How the clearance 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.

Both electric and magnetic fields depend on the clearance of the line. The minimum ground clearance of a 400 kV line is 7.6 m, dropping to 5.2 m for low-voltage distribution lines.  See a full listing of minimum clearances for high-voltage lines under different circumstances. But in reality it is rare for lines to be this low, and the ground-level field falls rapidly with the height of the line above ground.

Magnetic field

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.

graph showing how magnetic field depends on clearance

 

This graph shows the clearance makes more difference close to the line.  Because the maximum field depends so much on the clearance, fields expressed as a percentage of the maximum can be misleading - see more details below.

Electric field

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.

graph showing how electric field varies with line type 

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.)

 graph showing how electric field depends on bundle

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.

Clearances required for field to be below 9 kV/m for 400 kV lines

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).

 

towerconductor bundlephasing
number of conductorsconductor typespacingTU
L13tripleAraucaria500 mm8.4 m9.3 m
L12twinAraucaria500 mm7.8 m8.8 m
Redwood500 mm7.9 m8.9 m
triple (pointing sideways)Araucaria500 mm8.3 m9.7 m
L8twinRubus400 mm7.5 m8.5 m
Sorbus400 mm7.6 m8.5 m
Matthew400 mm7.5 m8.5 m
L6(M)quadZebra305 mm square8.5 m9.2 m
500 mm horizontal
300 mm vertical
8.7 m9.5 m
500 mm square (L6(M), BICC and BB)8.9 m9.8 m
500 mm square (BK and JLE)8.8 m9.9 m
triple (pointing sideways)Araucaria500 mm8.4 m9.1 m
triple (pointing down)Araucaria500 mm8.3 m9.1 m
twinAraucaria500 mm8.0 m8.5 m
Redwood500 mm8.1 m8.6 m
L2twinZebra305 mm7.2 m8.3 m
Rubus500 mm7.5 m8.8 m
Rubus400 mm7.4 m8.6 m
Sorbus400 mm7.4 m8.6 m
Matthew400 mm7.4 m8.6 m

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).

Different ways of presenting the effect of clearance

This graph shows the way the field falls with distance for two different clearances of the line above ground. As expected, close to the line, the lower clearance produces the higher field.graph showing effect of clearance

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.

graph showing effect of clearance as percentage 

Fields presented as percentages of the maximum have sometimes been used to suggest the calculations are unreliable because the field falls too rapidly, but this comparison is not valid unless the clearances are the same in each case.

See also: