Handbook of Electrical Installation Practice - BeKnowledge

510 Handbook of Electrical Installation Practice
Impregnated paper-insulated cables with lead sheathing for l.v.
and m.v. applications
Paper-insulated cables to BS 6480 have a continuous sheathing of lead or lead alloy
applied over the assembled cores. Lead alloys have their compositions specified in
BS 801, Lead and lead alloy sheaths of electric cables, and for unarmoured cables
Alloy E or Alloy B are generally used. Both alloys are more resistant than lead to
the effects of vibration. Alloy B is used in cases where severe conditions of vibration
are to be experienced. It may be noted that lead can develop a crystalline structure
when subjected to vibration, and cracks can then develop through the wall of
the sheath on the cable with the ultimate result that moisture or water can enter
and cause electrical breakdown of the insulation. Armoured cables may have lead,
Alloy E or Alloy B sheaths.
The lead or alloy sheathing is generally provided with some form of outer protection.
Single-core and unarmoured multicore cables are generally provided with
an oversheath of PVC or medium density polythene (MDPe) but the majority of
multicore cables are of the armoured type. Armour, for mechanical protection of
the lead sheath, can be in the form of double steel tape armour or wire armour.
Steel tape armour provides a certain degree of mechanical protection against sharp
objects penetrating the cable from outside and consists of two helical lappings of
tape, applied with gaps between convolutions, the outer tape covering centrally the
gaps in the underlying tape. The tapes are applied with gaps to allow bending of the
cable.
For protection giving longitudinal mechanical strength and a certain degree of
protection from penetrating objects, single-wire armouring is used, consisting of
a helical layer of galvanised steel wires. For additional longitudinal strength,
for example for cables which need to be suspended vertically, double-wire armouring,
consisting of two layers of galvanised steel-wires, is applied, the outer layer of
wires being applied in the reverse direction from the inner layer. A separator of
waterproof compounded textile tape is applied between the two layers of wires to
allow the wires a certain free movement in relation to one another when the cable
is bent.
Armoured cables have a bedding applied first to the lead or lead alloy sheath
and this may consist of lappings of compounded paper and textile tapes, all adequately
coated with waterproof compound, or if the cable is to have an overall
extruded PVC on MDPe sheath the bedding may also consist of an extruded
covering.
Unless cables are to be left with the wire armour in the bright condition, it is usual
for the armouring to have an overall protection in the form of serving applied in
the form of compounded textile tapes or as an extruded sheathing.
If single-core lead sheathed cables are required to be armoured for mechanical
reasons, the armouring must be composed of wires of non-magnetic material, such
as aluminium alloy, otherwise considerable electrical losses and heating effects occur
due to magnetic hysteresis in steel wires with alternating current loads of more than
about 50A.
Cables to BS 6480 have been largely superseded by extruded insulation types,
particularly for l.v. applications.