Buried Power Cables
Buried Power Cables
(OP)
I am in the geotechnical field and was curious as to where electrical engineers are finding testing for the thermal resistivity of soil, and how they found their lab of choice...search engine, etc.
Thanks much
Thanks much






RE: Buried Power Cables
IEEE Standard 81 is the IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System
RE: Buried Power Cables
The electrical resistivity of soil is often tested, however.
RE: Buried Power Cables
I am curious as to where the typical values come from. Are they based on soil classification, atterberg limit tests on the soil, density, watercontent? If not field tests, is it common practice to take specimens from the field and send to a lab for actual numbers?
Again, thanks much for your input
RE: Buried Power Cables
The technical source for all of this comes from one paper - AIEE Paper 57-660 (aka Neher-McGrath).
So basically, everyone defaults to rho = 90 in soil.
(This is based on the 2002 NEC - I haven't checked the 2005 version on this)
RE: Buried Power Cables
RE: Buried Power Cables
The NEC assumption of average thermal resistivity of 90°C-cm/W in 90% of the USA soil is based on a soil studies performed in the 1950s. The NEC also provide typical values of rho of 120 for very dry soil (rocky & sandy) and rho=60 for damp soil (coastal areas & high water table).
The soil thermal resistivity average values in the NEC appear to be satisfactory to size low voltage and MV feeders and most applications is less expensive to over design the feeder than justify economically a thermal resistivity study.
However, for HV transmission line other driving factors require to determine the soil thermal resistivity as accurate as possible an often use backfill other than native soil to maximize the heat dissipation capability of the cable to improve the line ampacity. Utilities and TL owner often spec one soil sample location every 1000 ft or less at each depth the cable(s).
Some of standard and reference guide typically used in the USA are:
IEEE Std 442- IEEE Guide for Soil Thermal Resistivity Measurement
EPRI Report TR-108919: Soil Thermal Properties Manual for Underground Power Transmission: Soil Thermal Property Measurements, Soil Thermal Stability, and the Use of Corrective Thermal Backfills
NOTE: There is in progress a draft standard in progress “IEEE P1254- Guide for Soil Thermal Stability Measurements & Data Evaluation”
RE: Buried Power Cables
Please add red dye to the concrete. The concrete should extend up at least a foot above the top duct so that if a backhoe operator should try to dig through he would have to take multiple bites to reach a duct.
RE: Buried Power Cables
I concur with you that concrete is a good material with low thermal resistivity around 55 °C-cm/W. However, if the rest of the backfill has poor rho this will provide limited help.
Direct buried with low thermal resistivity medium is a lower cost option with superior performance in term of maximizing the cable ampacity.
RE: Buried Power Cables
Although some materials other than native backfill do have poorer thermal performance, you are sometimes required to replace with native soil for political/environmental reasons.
Using concrete (and I like the 'red die' suggestion) will at least increase the effective contact area with the native soil to a much larger area, aiding in heat dissipation.
RE: Buried Power Cables
RE: Buried Power Cables
There was a paper published about 20 years ago on thermal conductivity by a fellow at SCE. He used a device that formed a cylinder of soil with a heater in the middle. Both ends of the device were well insulated so all the heat essentially traveled out raidially through the soil. He measure temperature at the inside and outside of the soil cylinder. He developed the test because there was not good data and accurately calculated the rho value. The soils were he was working were unknow and the utility was experiencing cable failures.
It's a very good paper and one should be able to find it by searching the IEEE archives. I am traveling at the moment and cannot get a copy I have at the home 40.
Soils also change seasonially. I know of one case where a duct run in a wet climate was backfilled with pea gravel. The installation never had any trouble untill on dry year in August cable started poping. The duct had been essentially under water untill the water tabel dropped. Then it was in dry gravel, a very good insulator.
RE: Buried Power Cables
Also, placing a red ribbon 1 or 2 feet above the duct bank does not prevent dig ins. Excavating contractors never ever see those things and keep right on digging. Of course, we charge them through the nose for repairs. About the only way to stop a dig in is to stand around with a Thompson or an Uzi.
RE: Buried Power Cables
Since until a few years ago when the cable was buried it was forgotten as there was no reliable equipment to monitor its temperature where and when necessary. Because of this, the only factor that could prevent cable failure caused by overheating was to thoroughly measure the thermal resistivity of the soil surrounding u/g cable circuits. Applying the distributed temperature measuring system can “correct” inadequacy of the project initial phase.
NOTE: a draft standard in progress “IEEE P1254- Guide for Soil Thermal Stability Measurements & Data Evaluation” - Do you know when it will be finished?
RE: Buried Power Cables
As you pointed out, the correct unit of the thermal resistivity (rho) in SI units is K.m/W (Degree Kelvin. meter per Watts). In US often the rho units are in oC.cm/W also called as “thermal-ohm-feet” in some literature and computer program such as AmpCalc. (Please notice that K.cm/W = oC.cm/W = K.m/100W.)
It is also truth the thermal resistivity has significant changes in value not only with moisture but also with soil density (compaction), surface conditions, etc. Typically the worst likely scenario considered in the conventional underground design.
This lead to roughly estimate the cable temperature during the operation of the underground line. As the technology progress, there is available more accurate temperature monitoring system some of them based in the property of fiber optic reflection with the temperature generated from the power cable. That will help the engineer to minimize the risk especially in hot summer peak demand.
Regarding your question about the status of the draft standard P1254 I understand that may be subjected to changes. I do not have any better info in this matter. You may check the IEEE standards administrator, Jodi Haasz at j.haasz@ieee.org
RE: Buried Power Cables
Under residential property I use schedule 80 PVC conduit which is a lot stronger than schedule 40. Concrete encasement is not practical in this application and invites use of a digging bar.
Direct burial is a poor economy. By the time that I remove rocks from the trench and backfill, level and compact the bottom of the trench, and so forth I can stick in a conduit and be done with it.
Another issue is that direct burial cables need to conform to the bottom of the trench necessitating a cable length 1.5 to 2 times the trench length. For a 3 pair telephone cable this is easy but for power cables a bit hard to achieve.
RE: Buried Power Cables
K2ofKeyLargo