We give here graphs of the typical static magnetic fields produced by underground cables. More on sources of static fields and on static fields generally. For more details on how to calculate the field from a DC cable, download our tutorial: How to calculate the magnetic field DC.pdf.
These calculations are for a typical design of cable such as 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. (These are typical for underground cables. Subsea cables would typically have the same electrical characteristics but would be spaced further apart.)
This field combines with the earth's static magnetic field, which has a magnitude of about 50 µT in the UK. How the two fields combine depends on the relative direction of the fields, which in turn depends on the direction of the cable. The following graph is for the cable running north-south:
see also the effect of this field on compasses.
For this particular cable and this particular orientation relative to the earth's field (taken as exactly 50 µT) the values are:
|magnetic field in µT at horizontal distance from centreline|
|0 m||5 m||10 m||20 m|
|+/-500 kV, 2 kA||direct buried||0.5 m spacing 1 m below ground||field from cable||49.23||6.91||1.92||0.50|
|combined field from cable and earth's field||96.9||45.77||48.40||49.56|
|compass deviation / degrees||0||12.7||2.0||0.3|