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.
Obtaining a higher rating with an underground cable can involve installing multiple groups of conductors - see how this affects the magnetic field.
This table gives some actual field values for the same conditions.
| |
|
|
|
magnetic field in µT at distance from centreline |
|
0 m |
5 m |
10 m |
20 m |
|
275 kV |
trough |
0.13 m spacing 0.3 m depth |
maximum |
83.30 |
7.01 |
1.82 |
0.46 |
|
typical |
20.83 |
1.75 |
0.46 |
0.12 |
|
direct buried |
0.5 m spacing 0.9 m depth |
maximum |
96.17 |
13.05 |
3.58 |
0.92 |
|
typical |
24.06 |
3.26 |
0.90 |
0.23 |
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.
