400 V 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
average is about 4 A. more
on net currents.
The following graph shows typical fields from the net current
in a 400 V cable.
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 |
|
11 kV |
single cable |
0.5
m depth |
typical |
0.75 |
0.22 |
0.11 |
0.06 |
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.
