Page 108 - Offshore Electrical Engineering Manual

Installation 95

INSTALLATION

1. Cables should be routed in such a way as to facilitate maintenance and the
installation of additional cables, with minimum need for expensive scaffolding.
For example, a cable route located under a deck, so that there is no immedi-
ate access and the sea is directly below it, is not recommended. The cost of a
small cabling modification in such a location would be overshadowed by the
enormous scaffolding bill.
2. Cable routes should avoid known fire risks where possible. Cables to main and
standby machinery should be run on separate routes.
3. Cables should be laid parallel on cable ladders and tray in a neat and orderly
fashion.
4. Where heavy three-phase currents are carried, cables are usually single core.
Single-core cables of the same conductor cross-section usually have a higher
fault rating than their three-phase equivalents and are easier to install, having
a lower weight per metre and a lower bending radius. To avoid eddy currents
being induced in local steelwork, such cables must be run in a trefoil configura-
tion. In some situations the cables must be run in a flat profile because of space
limitations, in which case a balanced arrangement must be adopted which still
avoids the promotion of eddy currents. Support or other steelwork must not pass
between phases, as the steel will heat up owing to hysteresis loss. Where two or
more trefoil cable groups run on the same route, they should be at the same hori-
zontal level with a clear space between the groups of at least one cable diameter.
5. Cable bends should not be tighter than the minimum bending radius specified
by the manufacturer. Drip loops should be provided at external cable termina-
tions. These will also be useful if the cable has to be reterminated at some
later date. In any case, straining of cables at cable glands should be avoided,
and cables should be perpendicular to gland plates for a minimum of 100 mm
before entering the gland.
6. Expansion loops will also be required across the expansion gaps necessary
between module walls and support steelwork.
7. Some cable insulation materials tend to become brittle with lower temperatures.
This is particularly the case with polyvinyl chloride (PVC) compounds. In low-
temperature conditions, care needs to be taken when installing most cable types.
It is desirable that cables with PVC insulation should not be installed directly at
ambient temperatures of 5°C and below. In such conditions the cable should be
stored for at least the previous 24 h at a temperature of 20°C or more.
8. Cables should be clamped or cleated to a rack or tray with suitable ties. The
usual arrangement is to use nylon 12 ties (not nylon 66, as it becomes brittle
in the offshore environment), supplemented at suitable intervals (depending on
the weight) by ethylene-vinyl acetate-coated stainless steel banding or cable
cleats of the correct diameter. Ties should be nonmagnetic for single-core
cables cleated in flat formation. The recommended cleating/banding intervals
are shown in Table 2.8.1.