33 kV underground cables are usually a single cable: the three
cores are twisted round each other in a single outer sheath.
Because the cores are so close together and twisted, the fields
they produce directly are very small. Instead, the field comes
from any net current in the sheath. This is very variable and
cannot be predicted accurately.
The following graph shows typical fields from the net current
in a 33 kV cable.
Occasionally a 33 kV cable may have separate cores. Then the
field would be more like a 132 kV cable with separate
cores.
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 |
|
33 kV |
single cable |
0.5 m depth |
typical |
1.00 |
0.29 |
0.15 |
0.07 |
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. These calculations are for a single, isolated cable.
This is rare in practice. Other nearby cables would modify
the field.
