11 kV underground cables: magnetic fields

11 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 11 kV cable.

graph showing tyical fields

Occasionally an 11 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

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.

For more information on fields from underground cables in general click here:

link to page for underground cables

For more fields at this voltage and fields at other specific voltages see this index:

link to page of specific power lines