Abstract:
Underrating of underground cables could result in the drying out of the soil around the cable. This
results in inferior cable performance and has potential negative environmental impacts. IEC 60287 is
used for cable rating ampacity calculations. One of the parameters used in IEC 60287 is the thermal
resistance per unit length between the cable surface and the surrounding medium (T4), or T4 can be
defined as the thermal resistance to the outside installation environment of the cable.
The soil parameters may be unknown at the initial design phase for final cable rating calculations. Due
to the unknown installation conditions; the calculated uncertainty for cable system designs may lead to
either under-utilization of the cable system, or underrating of the cable system.
With increasing demand for underground cable systems in South Africa; this increases the probability
of soil drying out due to unknown installation conditions, hence the need for improved cable rating
design and calculations which will eliminate soil drying out. Cable rating solution that prevents soil
drying out for South African (native) soil has been investigated. It was found that the following
parameters should be taken into consideration when performing current rating that prevents soil drying
out: the soil thermal resistivity at zero percent moisture content, sampling rate for measuring the soil
thermal resistivity should be less than 60 m and that the soil ambient temperature at cable laying depth
should be measured.
