Underground
High-voltage underground cables
4% of the high-voltage electricity network in England and Wales
is underground, mainly in urban areas or areas of great scenic beauty.
With underground cables the individual conductors, being insulated,
can be closer together, leading to greater cancellation and lower
fields. However, unless they are buried very deeply, they can also
be approached more closely, leading to higher fields. Overall, ground-level
magnetic fields from underground cables fall much more rapidly with
distance than those from a corresponding overhead line, but can
actually be higher at small distances from the cable.
The graph shows an illustration of this for one particular underground
cable and the equivalent overhead line.
See also:
National Grid’s booklet on underground transmission
lines (temporarily withdrawn for updating)- graphs and tables of fields
produced by underground cables at different voltages
Fields from underground cables are very sensitive to the height
above ground. more detail
Occasionally, instead of being buried directly in the ground, an
underground cable is placed in a tunnel, which can be ten or more
metres below ground. In this instance, the conductors cannot be
approached closely by members of the public, and the magnetic field
at the surface is much reduced, lower than an equivalent overhead
line and often lower than background fields from other sources.
Underground cables, whether directly buried or in a tunnel, produce
no external electric field.
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