206 HANDBOOK OF ELECTRICAL ENGINEERING
A typical value of 2.5 km/W is used for dry desert locations, which would cause the cable to be
derated to approximately 75% of its nominal rating.
Cables are either laid directly in the ground, in some form of buried ducting system, or in
air-filled trenches with lids. The most economical method is direct buried as far as the laying cost is
concerned. However, factors such as ground bearing pollution and corrosive substances may require
ducts or lined trenches to be used for the full route length. Direct burial will generally provide
better heat removal than a ducting system, unless the soil is very dry. Dampness in the soil assists
in the process of heat removal. Some installations such as refineries prefer concrete lined trenches
with ‘shelves’ and lids. These will tend to have ‘still air’ conditions and the concrete enclosure will
provide a thermal insulation effect. Hence a concrete trench may have a poorer heat removal property
than direct burial in the same soil. See Reference 4 Chapter 8 for tables of derating factors for soil
resistivity and grouping of cables that are buried.
9.4.2.4 Derating factor due to grouping cables together
Manufacturers quote current ratings of their cables laid in air or in the ground for a few simple
cases of grouping cables together e.g., in trefoil, 3 cables touching horizontally, 3 cables touching
vertically, cables spaced apart by a multiple of their outside diameter. The number of combinations
of groups of cables, with different spacings and surroundings, becomes far too many to tabulate.
This subject has received much attention by cable manufacturers, research establishments and the
international standards organisations, see References 8 and 9, BS7672, IEC60287, IEC60364.
For a particular project it is common practice to determine a small number of grouping cases
that will apply to most of the cable routes. Special cases such as the trenches entering a large substation
or a switch house would require a separate set of factors and derating calculations, because these
trenches could be tightly filled with cables. It is common for cables to be laid in horizontal groups
e.g., on trays or racks, and vertically e.g., one tray above another. The spacing between the vertical
groups will influence the derating factors to apply at each level.
9.4.2.5 The worst-case scenario
A cable may experience various different environments along its route. For example it may start at
a switchboard, run through the switch room in a trench with a lid or steel flooring, pass through a
duct in a wall and under a roadway, run a long way directly buried and finish on a ladder rack at
the consumer. At each of these environments the thermal resistivity and ambient temperature will
be different. The environment that causes the most derating of the rated current should be taken and
used for the whole cable.
9.4.2.6 Worked example
A 5 MVA 11,000/6900 V ONAF transformer is installed in a desert in the Middle East. Its 11,000 V
primary 3-core cable is laid in the ground in a duct at a depth of 1000 mm. Its 6900 V secondary
3-core cable is run above ground in air. The air temperature is 45◦C. The primary 3-core cable runs
in the same ducted trench as several other cables, less than 6, in horizontal spacing. The trench is
back filled with dry sand that has a thermal resistivity of 2.5 km/W. The ground temperature is 35◦C.