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.
| |
|
|
|
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.
