Overhead
Overhead lines
Electricity is carried round the country on overhead lines at various
voltages. All overhead lines produce electric and magnetic fields.
The field is highest directly under the line and falls to the sides.
see also a similar graph
for electric fields
Typical ground-level
UK field levels from overhead power lines
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|
Magnetic Field
(microteslas) |
Electric Field
(volts per metre) |
The largest steel pylons
(275 kV and 400 kV) |
Maximum field (under line)
Typical field (under line)
Typical field (25 m to side) |
100
5-10
1-2 |
11,000
3000-5000
200-500 |
Smaller steel pylons
(132 kV) |
Maximum field (under line)
Typical field (under line)
Typical field (25 m to side) |
40
0.5 – 2
0.05-0.2 |
4,000
1000-2000
100-200 |
Wooden poles
(11 kV and 33 kV) |
Maximum field (under line)
Typical field (under line)
Typical field (25 m to side) |
7
0.2-0.5
0.01-0.05 |
700
200
10-20 |
more detail
on fields from overhead lines including a list of all the different
voltages in common use in the UK and the fields they produce.
What is a transmission
line?
A transmission line is a high-voltage overhead power line - the lines
operated by National Grid for long-distance transmission of electricity.
In England and Wales there are 7000 km of overhead transmission lines
at 275 kV and 400 kV. Virtually all of these lines carry two separate
circuits, one each side of the towers, each with three wires or bundles
of wires.
more detail
on what a transmission line looks like and what its parts are.
more detail on the fields
they produce.
drawings of the actual
field lines produced by transmission lines
What is a distribution line?
Distribution lines are at lower voltages than transmission and are
used by Distribution Network Operators for distributing electricity
round an area. Some are still on steel pylons but many are wood
poles.
more detail on the fields
they produce
How do overhead lines produce fields?
The magnetic field produced by a current in a conductor falls with
distance from the conductor. Where there is more than one current
forming part of one or more electrical circuits, there is also partial
cancellation between the magnetic fields produced by the individual
currents, and that cancellation generally becomes better at greater
distances. Overall, the magnetic field is highest at the point of
closest approach to the conductors and falls quite rapidly with
distance. Similarly, there is partial cancellation between the electric
fields produced by the voltages on individual conductors, and the
electric field is usually highest at the point of closest approach
to the conductors and falls quite rapidly with distance.
Therefore overhead lines produce a magnetic field which peaks
underneath the conductors and falls rapidly with distance either
side.
More detail on how fields fall
with distance
More detail on the factors
that affect how much field a line produces
How accurate are calculated fields?
Magnetic fields can be calculated with considerable accuracy if
the currents are known.
More detail including
a direct comparison of calculations and measurements
How many people live near high-voltage
power lines?
Although people living near high-voltage power lines are a group whose
exposure is high and can often be calculated reasonably well, they
are a small group. In the UK, 0.07% of homes are within 50 m of transmission
lines and 0.21% within 100 m. Percentages in other countries seem
to be comparable (USA 1.1% within 40 m; Denmark 0.43% within 75 m),
with higher percentages partly reflecting a broader definition of
“transmission”. More detail
on numbers of homes near power lines
Averaged over the population, high-voltage power lines contribute
only a small fraction of collective average exposure to EMFs, because
so few people are exposed to them. The best estimate possible from
the UK is that high-voltage power lines contribute 5% of total average
population exposure.
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