Thermal resistivity: Real RHO values for the professional power engineer

Thermal resistivity: Real RHO values for the professional power engineer

Understanding a soil’s thermal stability can help power engineers more accurately design power distribution systems to prevent thermal runaway.


Thermal stability

Heat transfer in a porous medium like soil can be a complex process.  Heat is conducted through soil solids and water but is also transported as latent heat in the soil pores.  This makes the modeling of heat flow in soil both interesting and complex since it involves thermal and hydraulic processes.

Vapor movement across pores can carry substantial amounts of latent heat, but if the soil around the heat source isn’t wet enough for the water to move back and evaporate again, the soil at the heat source will dry out.  Soil drying around a heat source like a power cable can create the potential for thermal runaway which can lead to cable failure. Understanding a soil’s thermal stability can help power engineers more accurately design power distribution systems to prevent thermal runaway.

Accurate ampacity calculations depend on measured rho values

Appendix B of the National Electrical Code (B.310.15(B)(2)) states, “Typical values of thermal resistivity (rho) are as follows:

  • Average soil (90 percent of USA) = 90
  • Damp soil (coastal areas, high water table) = 60
  • Very dry soil (rocky or sandy) = 120.”

However, as many engineers who have used “90” as a safe and typical rho value have discovered, the NEC is simply wrong. These numbers are essentially meaningless because there is no “average soil”, wet or dry.

90 is not the magic number

Forty years of soil thermal research shows that:

  • Soil and rock rho values actually vary from 10  to 1000 ℃ cm/W.
  • There is no “typical” value for 90% of soil types.
  • Thermal resistivity of porous materials like soil, rock, and concrete are not constants.
  • Resistivity changes with density, water content, and temperature of the soil or concrete.

Measure, don’t guess

Even in a well-designed underground cable system, the soil may account for half or more of the total thermal resistance. Soil and backfill thermal properties should not be assumed. These properties are relatively easy to measure in the field and in the laboratory. A safe, professional installation requires actual measurement and evaluation of thermal rho.

A rho value reported in a void can be misleading

If a soil thermal resistivity report only reads “Soil X has a thermal resistivity of XXX °C-cm/W”, seek clarification. What was the moisture content? How densely was it packed? Are there organics in the soil? Soil moisture, density, and soil makeup are critical factors in determining a soil’s thermal resistivity. Any reporting of thermal resistivity for the purpose of design should include moisture content and density data (see example). A physical description of the soil should also be included. The thermal dryout curve is the most comprehensive way to report soil thermal resistivity. Thermal dryout curves can be generated automatically with the VARIOS lab instrument.

TEMPOS compliance with ASTM and IEEE standards

See how the TEMPOS field thermal properties analyzer complies with ASTM and IEEE standards here.

Methods of soil analysis: part 4

Chapter five of the Soil Science Society of America (SSSA) Methods of Soil Analysis Part 4 addresses soil heat. The TEMPOS and VARIOS probe needle sizes, heating times, accuracy specifications, and internal data analysis meet or exceed recommendations outlined in the SSSA methods.

Thermal properties testing

The TEMPOS is a fully-portable field thermal properties analyzer. The VARIOS lab instrument measures thermal resistivity as a function of water content and generates easy, automated thermal dryout curves. Both use the transient line heat source method, which reduces water movement for higher accuracy and speeds up measurement time.  Sophisticated data analysis is based on 40+ years of research experience on heat and mass transfer in soils and other porous materials.

Equipment rental

Want to do thermal properties testing but not quite ready to make the full investment? Consider renting the TEMPOS to get the data you need.  Contact METER for pricing, availability, and rent-to-own details.

Lab services

Accurately measuring material thermal properties is easy with the TEMPOS, but establishing an effective measurement protocol and carefully controlling important factors that affect thermal properties can be challenging and time consuming.

METER scientists have over 40 years of experience making high-quality thermal properties measurements.  We offer convenient thermal properties lab services. If you don’t have time or aren’t completely comfortable making the thermal properties measurements, our services could be perfect for you.

Contact METER for information on lab services.

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