Why is measuring soil thermal resitivity (TR) important?

In the summer of 1962 there were two failures on 132kV cable feeders in London. Both feeders had been operating at high load and the soil immediately surrounding the cables had become hot and dry. As the soil dried-out its thermal insulating characteristics increased causing the cables to overheat and finally fail due to thermal runaway.

Cable ratings (ampacity calculations), such as those performed to IEC 60287, require values for the soil thermal resistivity. The use of low TR specialised backfills for cable surrounds is expensive and may be unnecessary if it can be shown that the native soil has a low value. Other areas may have a high TR, such as old industrial areas where fly ash or slag deposits exist. In these areas it may be necessary to either increase the conductor size or to import low thermal resistivity material.

Equipment to measure the TR of existing soils will enable the cable system designer to identify areas of both high and low TR thus ensuring that the cable system does not overheat or become unnecessarily costly to install.