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

For a typical design of underground cable, the maximum field it could produce at 1 m above ground is approximately 50 µT and this decreases 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 approximately 15 m, the field from the cable would be below 1 µT and would make little difference to the Earth’s field.  

The typical static magnetic fields produced by underground cables are shown in the graph. These calculations are for a typical design of cable which would be used in the UK to connect an undersea cable to the convertor station. This design has two cores, spaced 0.5 m, 1 m deep, carrying 2000 A at +/- 500 kV. 

The bottom blue line is the magnetic field from the cable alone. The top yellow line is how the magnetic field combines with the Earth’s geomagnetic field, adding directly on top and making the magnetic field smaller to the sides, until it reaches a background level of approximately 50 µT (demonstrated by the black dashed line). This change to the Earth's magnetic field can affect compasses if you are very close to the cables, which is discussed below.