Sources of static fields

This site concerns the 50 and 60 Hz fields that are produced by AC power systems.  In the UK up to now, DC has been used only really for the undersea interconnector to France.  But in the next few years it is quite likely that we will be building some DC connections within the UK.  See the Energy Networks Strategy Group report for more information.

This page deals with how static fields are produced.  See also the evidence on whether they have health effects.

Natural static fields

The earth's static magnetic field varies over the surface of the globe and is about 50 µT in the UK.

The earth's static electric field is normally around 100 V/m but can vary greatly in different weather conditions, for example in thunderstorms.

Static fields from power systems

Overhead lines

A DC overhead line produces both electric and magnetic fields.  The electric field close to the ground varies, as it is affected by the corona ions that are typically produced on the surface of the conductors, and they depend on weather conditions.

There are currently no DC overhead lines in the UK.  If one is proposed, we will include graphs showing the fields produced here.

Underground cables

Underground cables always include a metal sheath which screens the electric field, so an underground cable produces magnetic fields only.  So far in the UK, all DC connections have been underground, to connect the point where a subsea cable comes ashore to the convertor station where it connects to the main AC power network.

For a typical design of underground cable, the maximum field it could produce at 1 m above ground is about 50 µT and this falls rapidly to the sides of the cable.  This DC field from the cable can either add to or subtract from the earth’s field depending on the relative directions.  So, close to the cable route, the DC magnetic field that would be experienced would be altered by the cable.  At distances beyond about 15 m, the field from the cable would be below 1 µT and would make little difference to the earth’s field.  See graphs of the field from a typical cable.  This change to the earth's magnetic field can affect compasses.

Convertor stations

The convertor stations at the ends of the DC cable, where it connects to the main AC transmission system, produce both DC and AC fields.  But these are very short range; normally, the field beyond the perimeter fence is dominated by the overhead lines or underground cables entering the convertor station, not by the equipment within it.