Underground cables: variation of field with height

As you get closer to the ground, you get closer to the conductors, so the field from an underground cable goes up. Because a cable may only be 1 m below ground (as opposed to an overhead line which may be 12 m above ground) this effect is more pronounced than for the overhead line. It is most extreme for the shallowest cables, which are 400 kV and 275 kV “trough” cables, typically 0.3 m below ground. The effect for one of these cables is shown here. For other cables, the difference between the field at ground level and 1 m above ground may only be a factor of 2.

compare this to the variation with height for an overhead line
more detail on fields from underground cables

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