Factors affecting 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.
Current and voltage
The magnetic field depends on the current and the electric field
depends on the voltage. The largest transmission lines in use have
a rating of over 4000 A per circuit, but the average current in
a typical circuit is more like 700 A. Distribution lines typically
have currents of hundreds of A or less.
Ground clearance
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 5.5 m for low-voltage distribution lines, but it is rare for
lines to be this low, and the ground-level field falls rapidly with
the height of the line above ground. 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.
This graph shows the clearance makes more difference close to the line.
See more detail on this.
Because the maximum field depends so much on the clearance, fields expressed
as a percentage of the maximum can be misleading.
More on this.
Height above ground
The higher above ground, the closer to the conductors, and so the
higher the field. But usually, over the first couple of metres or
so, this is a relatively small effect. More detail on how this effect
varies with distance from the line.
Phasing
See also what SAGE says about phasing.
The field also depends on the relative phasing of the two circuits.
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. This produces a field which falls as the
inverse square of distance from
the line. However most lines have "transposed" phasing,
with the opposite order of the phases on one side to the other. This
introduces an extra degree of symmetry and extra cancellation between
the fields from equal currents on the two sides; the resultant field
falls more nearly as the inverse cube
of distance, producing a much lower field at large distances from
the line. This is illustrated below. See also more detail on inverse square and inverse cube variations for power lines.
Balanced and unbalanced currentsMagnetic fields also depend on whether the currents are balanced or not. This is quite a complex technical area so we give the details on a separate page.
All the calculations on this page are comparted to the base case
of a 275 or 400 kV line of a type known as L12, transposed phasing,
12m clearance, 500 A in each circuit, calculated at 1 m above ground.
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