<P> Medium and high - voltage power cables, in circuits over 2000 volts, usually have a shield layer of copper or aluminium tape or conducting polymer . If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated at the contact point, resulting in corona discharge, and eventual destruction of the insulation . Leakage current and capacitive current through the insulation presents a danger of electrical shock . The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground . Stress relief cones should be applied at the shield ends, especially for cables operating at more than 2 kV to earth . </P> <P> Shields on power cables may be connected to earth ground at each shield end and at splices for redundancy to prevent shock even though induced current will flow in the shield . This current will produce losses and heating and will reduce the maximum current rating of the circuit . Tests show that having a bare grounding conductor adjacent to the insulated wires will conduct the fault current to earth more quickly . On high - current circuits the shields might be connected only at one end . On very long high - voltage circuits, the shield may be broken into several sections since a long shield run may rise to dangerous voltages during a circuit fault . There is a risk of shock hazard from having only one end of the shield grounded . The maximum recommended shield potential rise is 25 volts . IEEE 422 and 525 lists the cable lengths that would limit shield potential to 25 volts for a single point ground application . </P> <Table> <Tr> <Th> Size Conductor </Th> <Th> One Cable per Duct (ft) </Th> <Th> Three Cables per Duct (ft) </Th> </Tr> <Tr> <Td> 1 / 0 AWG </Td> <Td> 1250 </Td> <Td> 4500 </Td> </Tr> <Tr> <Td> 2 / 0 AWG </Td> <Td> 1110 </Td> <Td> 3970 </Td> </Tr> <Tr> <Td> 4 / 0 AWG </Td> <Td> 865 </Td> <Td> 3000 </Td> </Tr> <Tr> <Td> 250 kcmil </Td> <Td> 815 </Td> <Td> 2730 </Td> </Tr> <Tr> <Td> 350 kcmil </Td> <Td> 710 </Td> <Td> 2260 </Td> </Tr> <Tr> <Td> 400 kcmil </Td> <Td> 655 </Td> <Td> 2100 </Td> </Tr> <Tr> <Td> 500 kcmil </Td> <Td> 580 </Td> <Td> 1870 </Td> </Tr> <Tr> <Td> 750 kcmil </Td> <Td> 510 </Td> <Td> 1500 </Td> </Tr> <Tr> <Td> 1000 kcmil </Td> <Td> 450 </Td> <Td> - </Td> </Tr> <Tr> <Td> 2000 kcmil </Td> <Td> 340 </Td> <Td> - </Td> </Tr> </Table> <Tr> <Th> Size Conductor </Th> <Th> One Cable per Duct (ft) </Th> <Th> Three Cables per Duct (ft) </Th> </Tr>

What is a drain wire in shielded cable