What is a "corridor"?
A "corridor" in UK parlance is a strip of land along a high-voltage power line where development is restricted. So, for example, a 60 m corridor might mean that new homes and schools were not allowed within 60 m of the power line and new power lines were not allowed within 60 m of existing homes and schools.
Other countries have a similar concept called a "right of way" or an "easement".
The UK policy on corridors
The current UK policy is that we do not have corridors. This issue was looked at throroughly as part of the SAGE process. SAGE performed a cost-benefit analysis, based on the evidence of a possible link between magnetic fields and childhood leukaemia, which concluded that corridors were not justified. The Government response noted this and concluded that corridors were not proportionate, and that it would not be introducing them. So, as long as the high-voltage safety clearance distances are maintained, there is no restriction in the UK on building homes close to power lines.
But if we did have corridors?
Corridors have been suggested many times. And people have suggested different widths. We try to explain here where these different figures come from.
60 m/30 m
SAGE itself said that, if corridors were introduced, they should be set at the width where, on average, the magnetic field falls to 0.4 µT. This is because 0.4 µT is the field at which it is often said the statistical association between magnetic fields and childhood leukaemia appears. (In fact the true situation is more complicated than this, but 0.4 µT is probably as good a figure as any other to use given all the uncertainties.)
SAGE calculated that this distance is 60 m for 275 kV and 400 kV power lines and 30 m for 132 kV power lines. It used the same data as are presented on this site - see here for how the field typically falls with distance, here for data on a sample of actual lines, and here for the distance for the field to fall to various different values. The distance is measured from the centreline (so 120 m total width, or alternatively about 50 m from the outer conductors, given that the crossarms of 400 kV pylons are nearly 10 m wide).
This is of course only an average - for some lines the field falls to 0.4 µT closer, and for some it extends farther. SAGE calculated that 70% of lines fall to 0.4 µT at or within 60 m, and that just 15% of homes would still have a field greater than 0.4 µT at distances greater than 60 m (See Sage First Interim Assessment, Supporting papers, page 98).
200 m
The 200 m figure is usually derived from the Draper study of childhood leukaemia. That study looked at leukaemia rates all the way out to 600 m from the power lines. But for convenience it analysed the variation separately in 100 m bands, and for convenience again, for some analyses it grouped those into just two bands, 0-200 m and 200-600 m. The way the results came out, there was a higher rate in the 0-200 m band and a lower rate in the 200-600 m band. This has led some people (e.g. the Cross-Party Inquiry in 2007) to suggest that corridors should be set at 200 m from the powerline (400 m total width). But whilst the risk in Draper certainly falls with distance, the 200 m figure itself is more based on the fairly arbitrary choice of cutpoints for analysis in the Draper study than on the actual results.
600 m
600 m is the farthest distance the Draper study considered, and as there was still an increased leukaemia rate at this distance, some people have suggested any corridors should be 600 m. But 600 m is just the farthest distance Draper considered. They could have chosen a smaller distance, or if they looked farther, we don't know if they would still have found an increase.
(The choice of 600 m in Draper was based on a calculation that the farthest possible distance to get a field of 0.1 µT, the reference category, was 400 m. So 400 m would have been sufficient distance to ensure the study captured every possible instance of fields above 0.1 µT. But the Draper study added a margin of 200 m to be absolutely sure.)
Effects of these different choices for distance
You can see the numbers of homes within these various distances of power lines in the UK here.
