Handbook of Electrical Engineering

186 HANDBOOK OF ELECTRICAL ENGINEERING

Table 9.3. Electrical and physical properties of galvanised steel wire

Property Mild steel Carbon steel

Resistivity at 20◦C (ohm-m× 10 −^8 ) 13.2 15.9

Temperature coefficient of electrical

resistance at 20◦C, in per unit of

constant mass

0.0045 0.0045

Coefficient of linear expansion per

unit per◦C× 10 −^6

12.2 12.2

Thermal conductivity W/mK 59.4 51.9

Density kg/m^37. 86 × 10 −^37. 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

Resistivity at 20◦C (ohm-m× 10 −^8 ) 20.6

Temperature coefficient of electrical

resistance at 20◦C, in per unit of

constant mass

0.00336

Coefficient of linear expansion per

unit per◦C× 10 −^6

29.0

Thermal conductivity W/mK 34.3

Density kg/m^311. 3 × 10 −^3