Today I witnessed burying 10 kilovolt power cable in the ground. I noted the cable mark and Googled it. The description is rather interesting. It's a single conductor cable – it takes three such cables for a three-phase line. The specification (it's in Russian, so I might do the translation wrong) lists the following components:
- a set of aluminum wires with total cross-section of 240 square millimeters
- several layers of plastic surrounding that set of wires
- a "separating layer" made of "water blocking conductive tape"
- a shield of copper wires with total cross-section of at least 25 square millimeters
- a "separating layer"
- one more layer of plastic
Now I understand that at 10 kilovolts insulation is not trivial – a layer of duct tape won't do and that explains why there are so many layers of plastic. Also the cable must be not trivial to damage so there's need for the outer layer.
But what's the reason for the copper wire shielding? What purpose does it serve?