CABLES, WIRES AND CABLE INSTALLATION PRACTICES 185Tin is also used as a coating for copper armour wires or armour braiding where rubber
compounds are used in the inner or outer sheathing.
9.1.3 Phosphor bronze
Phosphor bronze is an alloy of mainly copper, 5 to 10% tin and approximately 0.1% phosphorous.
The alloy has superior mechanical strength when compared with copper. It is also very resistant to
corrosion, particularly in the presence of water.
Phosphor bronze is used as wire in the armouring of cables to provide moderate protection
against mechanical damage. It is formed into a tightly woven braid to form a non-magnetic, highly
conductive, armoring and electromagnetic screen.
It is a practical alternative to tinned copper braid in many applications where steel wire should
not be used.
The alloy is also used in the form of a thin tape for the purpose of repelling insects e.g.,
ants, termites and marine teredo worms. The tapes are placed underneath the main armouring and
on top of the inner sheathing. Table 9.2 shows some of the electrical and physical properties of
phosphor bronze.
9.1.4 Galvanised steel
Galvanised steel is used for the cable armour where a high degree of mechanical protection is required,
and where high pulling forces are experienced during the installation of the cable, especially in the
laying of submarine cables. The armour wires are formed into a helical cage to give the highest
protection or as a braid when flexibility is required during the installation and a lesser level of
protection can be accepted.
Mild steel is used for the armouring of cables laid on land. For submarine cables the material
can be specified as carbon steel, which has a higher tensile strength.
The depth of the galvanising is specified in the international standards. The cross-sectional
area of each armour wire (in a helix and not in a braid) varies from typically 0.9 mm for small
power and control cables e.g. 1.5 and 2.5 mm^2 conductors, to 2.5 mm for 400 mm^2 low voltage high
Table 9.2. Electrical and physical properties of phosphor
bronze
Property Phosphor bronze
Resistivity at 20◦C ohm-m 9. 50 × 10 −^8
Temperature coefficient of electrical
resistance at 20◦C, in per unit of
constant massSimilar to copperCoefficient of linear expansion per
unit per◦C18. 0 × 10 −^6Thermal conductivity W/mK 75.0
Density kg/m^38. 92 × 10 −^3