CABLES, WIRES AND CABLE INSTALLATION PRACTICES 197the compound PTFE can be used. These compounds would tend to be used for special situations
such as control circuits and emergency power circuits where overloading could be allowed for a
limited period of time, or if exceptionally high surrounding temperatures need to be tolerated e.g.
near engines, hot vessels, hot pipes, boilers.
Note that when high voltages are used in marine and offshore installations it is usually neces-
sary to adopt the international standards that apply to onshore oil industry installations.
See sub-section 9.5 for the choice of insulations materials needed for fire survival services.9.3.5 Insulation semiconductor screen
The need for a semiconductor screen is very similar to that for a conductor screen, as described in
sub-section 9.3.3. IEC60502 and BS6622 specify the same applicable voltage limits for screens with
different compounds.
The insulation screen is important in three-core cables because it prevents the inter-core electric
stressing that would occur if the screen were not present. It maintains a radial stress pattern in each
core, which is independent of the other cores.
However, the application of the screen itself is slightly different. It is carried out in two parts.
The first part is a non-metallic semiconducting tape or polymeric cross-linked compound that is
applied over the whole surface of the insulation. This material should be capable of being removed
from the insulation without damaging its surface. This requirement is necessary for terminating and
jointing the cable during its installation. The second part requires a metallic tape or braid, usually
copper or aluminium, to be applied over the non-metallic part to make full contact with it. The
metallic part is connected to an external circuit during termination and installation of the cable. The
connection is usually only made at one end of the cable so that induced circulating currents do not
occur and damage the screen itself.
9.3.6 Inner sheath
An inner sheath, usually made of extruded polymer, is used to cover the insulation screen, and to fill
in the interstices between the cores of a multi-core cable. It is important to fill the interstitial spaces
for two reasons. Firstly to ensure good circularity and dimensional accuracy of the finished cable,
and secondly to prevent an internal passage within the cable along which flammable gases could
travel. The transmission of such gases along a cable must be eliminated by design and construction
for cables that are used in hazardous areas.
The sheathing material need not necessarily be the same as the insulation material. It is usually
more economical to use a different material such as PE, PVC, CSP, EVA, for general applications
and HOFR or ZH for situations where fire resistance and smoke emission must be considered.
The specifications of sheathing materials can be found, for example, in BS7655, BS6724
and IEC60502.
9.3.7 Lead sheathing
Lead is used as a sheathing material for protecting the cable from chemical attack whilst it is buried
directly in hostile ground conditions, e.g. in chemical and refinery plants.