Handbook of Electrical Installation Practice - BeKnowledge

96 Handbook of Electrical Installation Practice
As has been discussed, it is usually the communications cables that represent the
greatest cable density in the built environment. The types of cable can be grouped
as those for voice, those for digital electrical data and those for optically encoded
data. The first group includes telephone cables and although the use of such cables
is rapidly being superseded by the advent of wireless telephony within commercial
premises, there is still a large requirement for multipair cabling of this type. In such
systems spare pairs must be installed at the time of the original work, and up to
300% spare capacity is not considered excessive. Electrical digital data has replaced
analogue data transmission and utilises specialised cable types. The equipment specialist
supplier will specify the cable types required, and within many local area networks
(LANs) in a building there will be a range of cable types required, all with
specific EMC limitations. Fibre optic cables are employed for the optically encoded
signals, and these require special boxes both to terminate the core(s) and to retain
the cable sheath securely. The cables must be provided with space to set an appropriate
radius as they enter termination points, and unless an underfloor void is available,
this is a potentially difficult area to design. However, more cables of this type
are now ‘blown fibre optic’ types, where the cores are transported down conduits
on cushions of air, and no difficulty results from onerous conditions for sheathing
restraints or radii.
A feature of large data cable installations is that many of the cables will originate
from positions other than the origin of the electrical installation (i.e. telephone
frame rooms, computer suites, etc.), and this involves a new concept. The cable
distribution system for data now has a unique layout, different from that for the
power cables, and instead of multicompartment systems, two different systems
are required. Where the systems pass over each other EMC considerations become
important, as does access to the overlaid system.
SEGREGATION
The need for segregation has been discussed and defined already. Designers must
consult the Building Regulations and BS 7671 for details of safety requirements as
far as circuits of different voltages and categories are concerned, and must also be
aware of any specific requirements for physical separation of any data or control
circuits in the building. It may be necessary to consult the designers and suppliers
of any LAN installation, or of any other data highway cable system being installed.
It is necessary to remember that the building management system (BMS) is likely
to be an IT based network rather than an electromechanical system, and the BMS
itself may well have EMC and segregation specifications to be met. The general
requirements to avoid EMC problems are set out in publications from respective
trade advisory bodies and learned institutions (such as the IEE) and these should
be regarded as a minimum basic requirement. However, good engineering practice
must also be followed and potential problems can often be avoided by simple measures;
for example, the running of data cables down lift-shafts is unlikely to lead to
a good installation and may well have other problems of its own, not least those of
maintaining fire breaks and of maintainability.