Page 202 - Handbook of Electrical Engineering
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186    HANDBOOK OF ELECTRICAL ENGINEERING

                              Table 9.3.  Electrical and physical properties of galvanised steel wire

                            Property                          Mild steel   Carbon steel
                                                     −8
                                        ◦
                            Resistivity at 20 C (ohm-m × 10 )  13.2        15.9
                            Temperature coefficient of electrical  0.0045   0.0045
                                         ◦
                              resistance at 20 C, in per unit of
                              constant mass
                            Coefficient of linear expansion per  12.2       12.2
                              unit per C × 10 −6
                                    ◦
                            Thermal conductivity W/mK        59.4          51.9
                            Density kg/m 3                   7.86 × 10 −3  7.86 × 10 −3
              power cables. For submarine cables the wire diameter can be up to 6.0 mm and for some deep ocean
              applications two layers of armour wires are used. Table 9.3 shows some of the electrical and physical
              properties of mild steel and carbon steel, see also volume 1 of Reference 1.

                    Steel wire armour as opposed to steel wire braid has lower electrical impedance for a given
              length of cable. This is an important benefit in networks that are solidly earthed at their power source.
              Some special applications that require as low an impedance as is practical to achieve in the cable
              have some of the armour wires replaced by copper wires. Hence the parallel circuit consisting of the
              steel and copper wires has a lower total impedance than the steel wires on their own. The impedance
              of the armouring, with or without the copper wires, is predominantly resistive and so the inductive
              reactance at the power frequency can therefore be ignored.


              9.1.5 Lead

              Metallic lead is occasionally used as an extruded sheath to provide protection against chemical
              corrosion where it is necessary to bury cables in polluted or aggressive soils. Examples of these are
              found in chemical plants, refineries, storage tank farms and areas that have high water tables.
                    The lead is often alloyed with small amounts of tin and antimony to improve its ability to
              withstand mechanical fatigue, such as fatigue experienced in long distance transportation to site. Lead
              sheathed cables should not be installed where regular cyclic physical movement will be experienced.
              Table 9.4 shows some of the electrical properties of metallic lead used for sheathing cables.

                                  Table 9.4.  Electrical and physical properties of metallic
                                  lead sheathing
                                  Property                         Lead sheathing
                                                           −8
                                              ◦
                                  Resistivity at 20 C (ohm-m × 10 )  20.6
                                  Temperature coefficient of electrical  0.00336
                                               ◦
                                    resistance at 20 C, in per unit of
                                    constant mass
                                  Coefficient of linear expansion per  29.0
                                          ◦
                                    unit per C × 10 −6
                                  Thermal conductivity W/mK        34.3
                                  Density kg/m 3                   11.3 × 10 −3
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