Step 01
Select test locations
Points are chosen along the cable route at the depth of the proposed trench, sampling each distinct material along the way.
Thermal Conductivity Test
Buried power cables are de rated by the soil around them. This test measures how readily that soil carries heat away, which is what decides the cable size you can get away with.
Thermal Conductivity Test: what it actually tells you
A cable carrying current heats up, and the only way that heat leaves is through the surrounding soil. If the soil is a poor conductor, the cable runs hotter, and the current it can carry safely drops. Cable ampacity calculations therefore take soil thermal resistivity as a direct input, and a wrong assumption there is expensive in both directions.
The measurement uses a line source. A needle probe containing a heater and a temperature sensor is inserted into the soil, a constant power is applied, and the rate at which temperature rises is recorded. The slope of temperature against the logarithm of time gives thermal conductivity, and its reciprocal is thermal resistivity.
Moisture dominates the result, so the number matters most at its worst. Soil dries out around a hot cable over time, and thermal resistivity can double or worse as it does. We test at field moisture and, where the design needs it, at the dried condition too, so the rating is based on the case that actually governs.
At a glance
- MethodTransient line source needle probe
- Typical range0.2 to 2.5 W/mK
- ConditionsField moisture and dried worst case
- OutputConductivity, resistivity, moisture, density
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Insert the probe
The needle probe is inserted into undisturbed soil, or into a remoulded specimen compacted to the specified density, with full contact and no air gap.
Step 03
Apply constant power
A steady heat input is applied and the temperature rise is logged against time until the response is clearly linear on a logarithmic scale.
Step 04
Compute
Conductivity is taken from the slope of the linear portion, and resistivity reported as its reciprocal in kelvin metres per watt.
Step 05
Test the dry condition
Where required, the specimen is dried and re tested to establish the worst case value for cable de rating.
What you receive
- Thermal conductivity in watts per metre kelvin
- Thermal resistivity in kelvin metres per watt, at field and dried moisture
- Moisture content and density at the time of test
- Design values for cable ampacity calculation
- Recommendations on thermal backfill where native soil is poor
When this test is the right one
- Buried high voltage cable route design
- Substation and industrial plant cable sizing
- Ground source heat pump and borehole heat exchanger design
- Thermal backfill specification and acceptance
- Buried pipeline heat loss assessment
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
IEEE 442 guide for soil thermal resistivity measurementsASTM D5334 thermal conductivity of soil and soft rock by thermal needle probeIEC 60287 electric cables, calculation of the current rating
Common questions
What engineers most often ask us about thermal conductivity test.
Because that is the condition that governs. Heat from the cable drives moisture away from the trench over months, and thermal resistivity rises as it goes. Rating a cable on wet season soil and then running it through a dry summer is how cables overheat.
The usual answer is a thermal backfill, a well graded sand or a fluidised thermal backfill placed around the cable, which holds a low resistivity even when dry. We test the proposed backfill so the specification can be written against a measured value.
No, though the two are often confused because both are called resistivity and both depend on moisture. Electrical resistivity is about current flow and corrosion. Thermal resistivity is about heat flow and cable rating.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
Electrical Resistivity Test
A non intrusive way to read the ground between boreholes, and the measurement earthing engineers and pipeline designers...
Read more
Soil & Rock Testing
The laboratory is where samples become design parameters.
Read more
Chemical Testing
Ground chemistry decides how long your concrete and reinforcement last.
Read moreNeed thermal conductivity test on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.