400 kV underground cables: magnetic fields

• Trough: the separate cores of the cable are in a concrete trough, typically only 0.3 m or less below ground, but also only 0.15 m apart
• Direct buried: the separate cores of the cable are laid directly in the ground, typically 1 m below ground and 0.3-0.5 m apart
• Tunnel: the cable is carried in a tunnel typically 20 m below ground

See photos and diagrams of these different types of cable.

Cables in tunnels are so far below ground that the fields at ground level are usually below background levels.  Maximum fields from typical examples of the other two types are shown in this graph.  This is for 1 m above ground.  At this height, the direct-buried cable produces the higher field.  Closer to the ground, the trough produces the higher field. Fields from underground cables are very sensitive to the height above ground. more detail

Typical fields are lower than the maximum field because the loads are usually lower.  Typical fields are shown in the following graph.

Underground cables do not produce any external electric fields.

This table gives some actual field values for the same conditions.

Notes

1. All fields calculated at 1 m above ground level

2. All fields are given to the same resolution for simplicity of presentation (0.01 µT = 10 nT) but are not accurate to better than a few percent.

3. Calculations ignore zero-sequence current.  This means values at larger distances are probably underestimates, but this is unlikely to amount to more than a few percent.

4. Cable designs are not standardised to the same extent as overhead lines and the examples given here are representative.

5. The trough calculation is for a double circuit and the direct buried is for a single circuit, but in practice there may be other nearby circuits which affect the field.