Statistics of power line fields

Overall summary

 This table summarises the fields at various distances from typical power lines in the UK.  We then give more detail below.

  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

Average fields at various distances from high-voltage power lines

The various graphs and tables of fields from power lines in this section show calculations for “typical” values of current. An alternative is to take actual loads where these are available.

The following table gives more detail on the average magnetic field at various distances from National Grid lines. These figures were calculated from one year’s worth of recorded load data and are the average for a representative sample of 43 different lines (a mixture of 275 kV and 400 kV).

Distance m
0
50
100
200
300
Average Field µT
4.005
0.520
0.136
0.034
0.015

Because these fields are calculated from actual load data, they reflect the actual degree of unbalance between the two circuits of a transmission line. More on the effect of unbalanced currents.

Distance for field to fall to various values

 The usual way of expressing the field from a power line is to show how the field falls with distance. Sometimes it is more helpful to express the distance for the field to fall to a certain value of field.

In the UK

thumbnail flagThis table presents this information for the UK. The ranges reflect the variety of different lines that exist, and particular the variety of different loads they carry.

VoltageDistance (m) for typical field to fall to:
10 µT1 µT 0.4 µT 0.1 µT 0.01 µT
400/275 kV*30-4050-6090-110200+
132 kV*0-1010-3030-6090+
33/11 kV*0-50-2010-4050+
400 V***1020+

* typical fields from this line do not reach this value anywhere

Other countries

thumbnail flagA report from the Netherlands* calculated the equivalent distance for fields from overhead lines at 50 kV up to 380 kV to fall the various values.  (These distances were calculated as the single distance from all power lines that included the same number of homes as the actual number of homes exposed at the given level - this is a way of averaging over lines where the range of the fields is greater or lesser.)

field  /  µT0.20.30.40.5

distance  /  m

90

75

65

60

* Kelfkens et al, Costs and benefits of the reduction of magnetic fields due to overhead power lines

See also:

  • graphs comparing the fields from different voltage power lines
  • graphs showing the fields from specific power lines