INCREASING THE AMPACITY OF UNDERGROUND
Once there is an understanding of the possible limitations associated with each cable type, it is
necessary to consider how uprating might occur on a given circuit. This report section describes
various techniques that may be applied to investigate Ampacity limitations and then ways to
improve Ampacity, or at least have a better understanding of what is limiting the Ampacity
6.2 Route Thermal Survey
A route thermal survey is traditionally involved evaluating the entire cable route in a
detailed manner to understand Ampacity limitations. Many North American utilities adhere
to Association of Edison Illuminating (AEIC) standards regarding cable design. One of the
principles of these standards is that if the soil characteristics are not well known, the design
Ampacity should be based upon a maximum operating temperature that is 10°C below the
allowable operating temperature (e.g., values in Table 4-8).
Regardless of following the AEIC standards or not, utilities sometimes design cable circuits
without a good knowledge of the route characteristics, particularly with older circuits. The
ambient soil temperature and soil thermal resistivity were not well known, so assumed values
were often incorporated into rating calculations. Those following the AEIC guidelines obtained
some additional conservatism in the ratings by using the lower 10°C operating temperature in
the event the assumed parameters were inaccurate. However, as the circuits age and load growth
continues, many utilities are revisiting the rating assumptions to see if additional transmission
capacity is available without major investment in infrastructure.
Also, during the process of uprating a cable circuit, hot spot mitigation may require removing
existing trench backfill materials and replacing with a good quality thermal backfill.
The following subsections discuss some of the techniques employed for a route thermal survey
and describe soil and backfill characteristics that are important to consider in evaluating methods
for uprating cable systems.
"Neher-Mcgrath Calculation" "Neher-Mcgrath
442-1981 - IEEE Guide for Soil Thermal Resistivity Measurements
Description: This guide covers the measurement of soil thermal resistivity. A thorough knowledge of the thermal properties of a soil will enable the user to properly install and load underground cables. The method used is based on the theory that the rate of temperature rise of a line heat source is dependent upon the thermal constants of the medium in which it is placed. The designs for both laboratory and field thermal needles are also described. The main purpose of this guide is to provide sufficient information to enable the user to select useful commercial test equipment, or to manufacture equipment which is not readily available on the market, and to make meaningful resistivity measurements with this equipment. Measurements may be made in the field or in the laboratory on soil samples or both.
6-1 6.2.1 Thermal Property Analysis
In the equivalent thermal circuit, the earth thermal resistances are the largest component typically
representing over 50% of the total thermal resistance. They are also the least understood.
As compared with overhead lines where weather parameters (wind speed and direction, solar
radiation, temperature) may be valid for a 1-2km of line length, soil characteristics along
underground cable routes can vary over a few meters. If the cables are buried in city streets, there
exists a strong possibility of encountering "borrowed fill" instead of native soils. These "fills"
may satisfy civil/construction requirements but if topsoil, cinders or organic soils are used, the
thermal performance may be very poor. For this reason, it is very important to test the soils so
that appropriate values of thermal resistivity may be used in design calculations.
Thermal property analysis based on transient heat flow was first suggested as early as 1888
(Wiedman, 1888). During the mid-1900s, significant research and other work was conducted
in North America (Mason and Kurtz-1952, Blackwell-1954, Carslaw and Jaeger-1959).
This demonstrated the practical use of a thermal needle "line heat source" method. The Insulated
Conductors Committee, organized in 1947, performed a special project on soil thermal resistivity
in 1951. A special subcommittee (No. 14) headed by Professor H. F. Winterkorn of Princeton
University continued work in this field for 10 years and published the AIEE Committee Report
In the 1970s, EPRI-sponsored research resulted in the design and development of the
Thermal Property Analyzer. The basic approach was to develop a portable, fully automated
test instrument with standardized testing procedure that could be employed for both field and
laboratory with results that could be extended to power cable systems.
18.104.22.168 Thermal Resistivity
Thermal resistivity, sometimes call "rho", is a property of a material. In the contents of cable
installation and field measurements, the thermal resistivity is measured for a soil or trench
backfill. The most common approach to thermal resistivity measurements now is the "transient
thermal needle" method, which is based on the "line heat source theory".
Essentially, an underground cable is a long distributed heat source. The "transient thermal
needle" method takes advantage of this characteristic by using a "thermal probe" which contains
a heating coil throughout its length and a thermistor type temperature sensor at the mid-point of
the heater. The length to diameter ratio of the probe is high enough so that end effects do not
impact the measurements. An example thermal probe is shown in the following figure:
Once the probe is installed in the soil sample or in the native soil (field), the heater in the thermal
probe is energized with a constant power while the change in temperature is recorded over time
(usually 20-30 minutes). The slope of the Log time-temperature curve is proportional to the
thermal resistivity of the soil sample. A thermal property analyzer (TPA) was developed to
automate this process and is commonly used for both field and laboratory measurements.
The transient thermal probe method (e.g., IEEE Standard 442) is a relatively quick and
accurate approach to measuring soil thermal properties provided the theoretical assumptions
are understood and care is taken in the test set-up to stay within the limits of the theory.
The test assumes various conditions: The probe is an instantaneous and constant heat source (no thermal capacitance)
Heat flow is radial
Conduction is the only mechanism of heat transfer
There is no contact resistance at the soil/probe interface
There is an infinite sample boundary
The test sample is homogeneous and at moisture and thermal equilibrium
No moisture migration occurs during the test
For these assumptions to be valid, it is important that the probe insertion and testing be
performed carefully, usually by a qualified specialist, to insure that the results are valid.
Contact resistance is very important and a critical part of inserting the probe into the soil.
Also, it is important to keep the probe temperature at reasonable values to avoid drying the soil.
A drill rig with a hollow stem auger is used to drill down to the required depth for soil sampling
and to perform in situ thermal resistivity measurement tests. Sometimes a backhoe or hand
digging down to the required depth is also used to access the soil where testing will be done.
In the case where the hole is advanced using a drill rig, the thermal probe is attached to an
extension rod and then tapped into the native soil at the required depth. The testing is then
performed from the surface (see Figure 6-1). In addition to performing measurements in the field, called in situ measurements, soil samples
are collected during soil boring for detailed laboratory analysis and to evaluate parameters
such as dry-out curves (thermal resistivity as a function of moisture content under constant
dry density) or thermal stability that cannot be done effectively in the field. The samples are
collected in thin-wall Shelby tubes. If the soil is very loose or non-cohesive (granular), a split
spoon sampler or large diameter California sampler with liners is used to collect undisturbed
samples. If soil conditions (granular, very hard or rocky) are such that undisturbed tube samples
cannot be collected, either disturbed bulk samples or auger cuttings are taken. If bed-rock is
encountered, core samples of 5-8cm (2- 3 inches) diameter are collected to be tested in the
laboratory. Standard ASTM procedures should be implemented for soil boring, sampling, storage
Borehole logs, which characterize the soil types with depth, are often made so that if the cable
burial depth varies, the type of native material – and its thermal resistivity – can be known for
rating purposes. A typical bore hole log is shown in Figure 6-4. The borehole log information
may also be used for other geotechnical purposes such as designing structural loads, laying
out directional drilling route, etc. This geotechnical information is very useful for the civil
contractor to determine the type of equipment required for excavation, de-watering, backfilling,
and other activities. The ambient temperature recorded at the start of an in situ thermal resistivity
test is an important value to record for the cable designer.
In the laboratory, a soil sample is prepared to evaluate thermal dry-out characteristic– the
variation in thermal resistivity of the material as a function of moisture content. The results of
these tests (dry-out curves) are presented on charts that show thermal resistivities at the in situ
moisture content (if known), at "critical moisture content" and in totally dry condition (worst
case). Some degree of drying beyond the native moisture levels should be expected in the
presence of energized power cables, so an adjusted soil thermal resistivity that factors in
the drying should be incorporated into ampacity calculations.
Once the soil thermal resistivity results are known, they can be used in ampacity calculations.
For the case of installed cable systems, it may be necessary to do testing outside the cable trench
to get native conditions, and within the cable backfill to characterize the special thermal backfill
that may have been used around the cables. If the trench is known to have a common material
throughout, testing of the backfill material may only be needed at a few selected locations.
Thermal Diffusivity Although thermal diffusivity is not commonly recorded (typical transient needle TPA equipment
can measure this parameter) for most applications, its application is in "transient" calculations.
In simple terms it can be treated as the "inertia" in the heat and mass transfer equation. The 3
terms – resistivity (?), diffusivity (a) and heat capacity (C) are related by the equation:
Thermal Stability Thermal stability is a system driven parameter and is a soil characteristic that describes how well
a soil maintains a constant thermal resistivity when exposed to cable heating. The main issue is
to consider if the heat leaving a cable would result in the soil being below its critical moisture
content, in which case the soil would experience net drying and an increase in thermal resistivity.
Smaller diameter cables with direct contact to the soil are more likely to result in thermal
instability because of a larger heat flux (temperature gradient) at the cable-soil interface.
A classic example of a thermally instable material is modeling clay. The clay can be dried at
room temperature over time. If the dried sample is then placed in water, it does not readily
reabsorb water to return to a malleable substance. Some soils – including soils with high clay and
silt contents – have these characteristics. A common situation where this may be an issue for
power cables is the use of bentonite as a grout material either in trenchless casing installations or
for cable conduits; pure bentonite has high thermal resistance and is prone to drying. Bentonite is
prone to shrinkage and cracking (leaving voids) if drying does occur. A better solution is to use
as much sand as possible while minimizing the bentonite content, and to seal the ends of the
casing or ducts so that the grout cannot dry.
Moisture Migration in the Soil For any given soil or backfill the major influence on the thermal resistivity is the moisture
content. In a dry state, the pore spaces between soil particles are filled with air (thermal
resistivity of about 45°C-m/W). As water (thermal resistivity of about 1.65°C-m/W) replaces air,
the soil resistivity decreases substantially by as much as 3 to 7 times, as the good heat conduction
paths are expanded (additional thermal "thermal bridges"). This is illustrated by the "thermal
dry-out curve" (thermal resistivity vs. soil moisture content) shown in Figure 6-3. A soil that is
better able to retain its moisture, as well as being able to efficiently re-wet when dried, will have
better thermal performance characteristics. The soil water content is expressed as a percentage of
the weight of water to the dry weight of soil solids, as determined by oven drying at 105°C.
The heat generated by energized cable tends to cause soil moisture to migrate away from the
cable/backfill interface. In unstable backfills or soils, this drying increases the resistivity
substantially, inducing further heating of the cable and thus more drying of the soil. Eventually
this cycle may create a totally dry zone of the backfill around the cable, resulting in excessive
heating and potential thermal runaway. In a stable backfill the heat induced drying raises the
resistivity marginally thus minimizing the potential for thermal runaway.
Thermal stability is best illustrated by means of thermal dry-out curves (Figure 6-3). The
"critical moisture" is defined as the moisture content below which the relatively flat nature of
the thermal dry-out curve gives way to a disproportionate increase in the thermal resistivity.
Above the critical moisture a soil will resist thermal drying (by means of capillary suction),
whereas below this value, thermal runaway is inevitable (unless soil moisture is externally
replenished, (i.e., rain).
Although some native soils at high moisture content (10-25%) may exhibit fairly low thermal
resistivity (0.4 to 0.6°C-m/W), this value may increase a few fold when dry. Well-graded sands
and stone-dust containing 10-15% fines (-200 Sieve size material) make good corrective thermal
backfills. Cable Route Soil Test Spacing.
The soil testing and sampling frequency for thermal resistivity testing along the cable route can
vary depending on the area and the length of the route. In rural areas where the use of fills is
minimal and historical construction has not been significant, sampling and in-situ testing every
500m might be done. In urban areas or locations where fill materials have been used, sampling
might be done 200-500 m. Known variations in geology or other conditions might affect how
often along the route testing and sampling are done. The goal of testing is to capture test results
for any unique soils and potential hot spots along the cable route while categorizing where each
soil type is found.
Factors Affecting Soil Thermal Resistivity
The soil composition is an important characteristic affecting soil thermal resistivity. Soils are
typically a conglomerate of various materials, and the ratio of these materials within a soil affects
the thermal resistivity. The following table summarizes the dry thermal resistivity values of
Thermal Resistivities of Soil Components
Component Dry Thermal Resistivity °C-m/Watt
Shale (sound) 0.60
Shale (highly friable) 2.00
Organics (peat, etc.) ~5.00
Petroleum Oil ~8.00
Because the soil components are so important in affecting the thermal resistivity, a good
understanding of the geology along a cable circuit is valuable to assessing where soil testing
should be performed and how much variation might be expected along a given cable route.
It is important to note from Table 6-1 and Figure 6-3 that dry soils have a much higher thermal
resistance than moist soils because the thermal resistivity of water (1.65 °C-m/Watt) is much
lower than that of air (~45 °C-m/Watt). In addition to the air having higher thermal resistivity,
heat transfer takes place by radiation instead of conduction that is much less efficient.
Soil Texture and Dry Density
The soil texture is also critically important to thermal resistivity. The grain size distribution and
grain shape are evaluated by a sieve analysis (e.g., ASTM D422, etc.) to determine the variation
in particles both in backfill materials and native soils. Figure 6-5 shows a sieve analysis for 4
materials and a band of "good" granular thermal backfill.
Water Content and Ground Water Level
As is seen in Figure 6-3, soils with higher moisture content generally speaking have better
thermal resistivity. Some soils naturally retain water better than others. Certain soils may not
retain water well – e.g., they have a high hydraulic porosity – but are below the water table so
they remain saturated even though the dry density is low and dry thermal resistivity would
otherwise be high.
The dry density of a soil determines its ideal ability to conduct heat away from the cables.
Factors that influence the dry density are porosity, solids content, interparticle contacts and pore
size distribution. Having a well-graded material with a range of particle sizes improves the dry
density and minimizes pores and voids in the material.
Other Subsurface Characteristics
Concerns for solutes and hysterisis apply only in areas where significant fluctuation in the water
table may "wash out" fines from the backfill, making it thermally poor. For most applications,
this is not a concern for cable system uprating and, in any case, would be found during soil
thermal resistivity testing.
Surface Characteristics and Vegetation
Surface conditions have an impact on soil thermal resistivity. For example, soils below asphalt
roadways generally will not gain or loose moisture readily under normal conditions. However,
in the presence of cables, the drying that does occur may not be mitigated by heavy rains since
the water will not be easily reabsorbed.
Surface vegetation can be significant factor affecting soil thermal resistivity. The root systems
on large trees and plants will draw moisture out of the soil, drying it. Also, the decaying
components of plants and their root systems will tend to increase the organic component of
soils, which tends to increase the soil thermal resistivity (see Table 6-1).
Surface cover has strong influence over earth ambient temperatures, especially at shallower
depths. A difference of 4-5°C has been measured between grasses versus asphalt cover over
Engineered Thermal Backfills
The general goal of engineered thermal backfills (ETB) is to enhance the removal of heat away
from buried cables. In most cases, the native soil materials have a higher thermal resistivity than
good quality backfills and, in any case, are difficult to reconstitute in the trench with the same
density as the native soil. For this reason, special backfill materials are often designed for use in
a cable trench. These include well-graded sands, stone screenings, concrete or Fluidized Thermal
Backfill. In addition to having excellent thermal properties, they are engineered to meet civil
requirements (strength and ease of voids-free installation) that are associated with the particular
application. The criteria considered for these ETB are:
Low thermal resistivity over the expected range of operating conditions
Low critical moisture content and high thermal stability limits
No adverse affects on materials used for cable conduits, cable jackets or pipe coatings
Easy to install
Inexpensive and locally available to the location where the materials will be used
Types of engineered thermal backfills are discussed in the following sections.22.214.171.124 Granular Backfill Materials
These materials should be composed of hard, well-graded, natural or crushed mineral
aggregate (limestone, granite, quartz or other similar rock). The material should be sound
(porosity less than 2%) and be free of any organic material (peat, root matter, topsoil, vegetation)
and foreign matter (wood, rubble, cinders). The sieve analysis should match closely to that given
in Figure 6-5. The maximum particle size should be no larger than 1/4" sieve size with a fines
content (material finer than #200 sieve size) of 12% to 18%.
During supply and installation of this material, quality assurance is very important. Sieve
analysis on the delivered materials should be performed periodically to check and verify its
compliance with the above characteristics.
Fluidized Thermal Backfill (FTB)
One of the difficulties with any granular backfill is that it must be installed properly,
regardless of its ideal thermal properties. Granular backfills should be installed in shallow lifts
15cm (6 inches) at a time and well compacted to give good density. This is labor intensive and
great care must be used when working close to directly-buried cables or conduits so as not to
Leading up to and during installation, FTB delivered to the project site should conform to the
respective mix design and performance specifications of low strength and/or high strength FTB.
This should be checked with samples collected during the project. When installed by pouring
into the cable trench, the material should be free flowing and without any segregation. This will
help insure that the material completely surrounds the cables, conduits or pipes. The amount of
water in the FTB mix may be adjusted to increase or decrease the flow (slump) as directed by the
field engineer. If lower slump FTB is required for a particular area, it is generally better to adjust
the water content at the batch plant rather than as the material goes into the trench. Air content
(natural trapped) should not be higher than 2%. Mixing at the batch plant and transportation to
the project site should be done in accordance with ASTM or American Concrete Institute (ACI)
If trench shoring and sheathing is being used; these should be removed immediately after the
installation of FTB, unless otherwise required by the field engineer. If FTB is installed in cold
conditions, care should be taken to protect the installed FTB from freezing. This applies to both
low strength and especially the high strength FTB. Follow ASTM or ACI specifications for such
installations. Sampling and testing for quality control/assurance should be performed on FTB
samples taken every 250 feet along the cable trench, or every 100 cubic yards of material
installed, or as directed by the field engineer.
Component materials from an FTB mix design are shown in Figure 6-6. 126.96.36.199 Grouts for Cable Conduits and Trenchless Casings
For extruded or self-contained cables in ducts or the space between inner-ducts and trenchless
(directional drilling, pipe jacking, etc.) casings, the air space between the cable and conduit or
conduit and casing is often filled with air, which is a poor thermal conductor. Filling the duct
with a thermally conductive material improves the cable ampacity by 5-10%, depending on the
configuration and the type of filler. The annular space is generally small, and utilities generally
want to retain the ability to remove the cables from the conduits later in the event of a failure or
for upgrading. Therefore, it is not practical to fill the annular space with a solid filler material (or
one that becomes solid over time), so a pumpable material that will not hard set is ideal.
IEC-60287 allows that cables with grouted conduits may be treated as direct-buried cables for
the purposes of ampacity. Various materials that have been considered for conduit grouts are:
Bentonite and sand/bentonite slurry
Grease and viscous oil, along with other compounds
Factors that must be considered when selecting a grout are the total length that must be filled and
the amount of annular space. For trenchless installations, the potential softening of a plastic duct
at elevated temperatures – including potentially the heat generated as cement-based grouts cure –
could soften ducts and cause partial collapse. The safe pumping pressure for the grout material
must therefore be considered when a grout is pumped on the outside of air-filled cable conduits.
The grout material typically will have a thermal resistivity of 0.4 to 1.4°C-m/Watt, which is
much lower than air (45°C-m/Watt) at the set moisture content. A sand-bentonite slurry backfill
with a thermal resistivity of approximately 0.7°C-m/Watt is easy to formulate and generally easy
to install. Varying the amount of sand, bentonite and water affects the pumpability of the grout.
Bentonite tends to absorb a lot of water, so this must be factored into the mix. Mixing
the sand/bentonite slurry also requires special equipment (i.e., colloidal mixer). The thermal
resistivities of these components are as follows:
Sand– 0.12-0.20°C-m/Watt – optimizes the thermal resistivity but negatively affects
Water– 1.65°C-m/Watt – optimizes the flowability but negatively affects shrinkage
Bentonite– 3.50°C-m/Watt – optimizes the pumpability but negatively affects thermal
These materials are combined by a soil specialist for use by the contractor or utility
6.3 Review Circuit Plan and Profile
A classical approach to performing uprating on underground cable circuits is to review the circuit
plan and profile drawings, preferably the "as-built" versions which may show additional details
about the locations of the buried power cables, as well as better illustrate the locations of other
underground utilities that may impact cable ratings.
The plan drawings will show a variety of factors that may be relevant to determining the cable
ampacity and possible locations where uprating could be considered:
Phase and circuit or pipe spacing among the cables being studied, which would impact
mutual heating effects.
The locations of other utilities that cross the cables, especially other transmission or
distribution cable circuits that could produce mutual heating effects. Also, steam lines
may be present.
Sections of the route that parallel other utilities, including power cables. Parallel cables
within a certain range may produce sufficient mutual heating to cause de-rating. A general
guideline is if the horizontal spacing is within 25% of the depth, mutual heating may be a
factor (e.g., if the cables being studied are at 4m depth, parallel cables or other heat sources
within 1m horizontal spacing should be examined for mutual heating effects).
Topographical profiles may show areas where overburden has accumulated above the
Profile drawings mainly indicate the cable circuit's depth of cover below grade and usually the
locations of other utilities that cross the cable circuit. Areas that are important to note on the
profile drawing are:
Entry/exit to manholes since cables frequently dip to enter a manhole
Road crossings where the cable burial depth may be increased to accommodate the
required road bedding materials
Directional drilling locations where the burial depth is significantly greater than
6.4 Evaluate Daily, Seasonal or Other Periodic Load Patterns
Load shape is generally not that important for most transmission equipment, particularly
overhead lines where the thermal time constant is relatively short. With underground
transmission cables, the long thermal time constant – 35-150 hours – can significantly impact
loading patterns for both normal and emergency ratings, particularly for short-duration
In typical normal ampacity ratings on cables, daily load cycles are modeled by rating techniques
through the application of a load factor or loss factor. The load factor relates the average
daily load to the peak load. As mentioned earlier, the loss factor (or load factor of the losses)
considers the average daily heat output relative to the peak heat output. Consider the following
figure that shows several load shapes that all have the same peak current but substantially
different load and loss factors:
All of the curves in Figure 6-7 have the same peak current (1000A), but substantially different
loss factors. On a daily basis, the different loads shown will release different amounts of energy
into the surrounding soil. This has a significant impact on conductor sizing for a desired rating or
on the available current for a given conductor size. Note that the loss factor is also the per-unit
power delivered on a daily basis.
If the cable construction and installation conditions are held constant and the loss factor is varied,
the cable ratings will vary substantially.
0% 20% 40% 60% 80% 100%
Loss Factor (%)
Ampacity as a Function of Loss Factor
From the standpoint of uprating, increases in loss factor over time mean that the ampacity will
tend to decrease. For example, on a recent uprating study for a New England utility, the loss
factor in 1959 when the circuit was built was 57% but had grown to 83% in 2001. While the
utility was able to increase capacity on the circuit with some extraordinary methods, the normal
book rating actually decreased with respect to time because of the increasing loss factor.
Load shape may also play an important role from the standpoint of emergency ratings. If the
daily load cycle is such that the load during portions of the day (typically at night) is lower than
at other times of the day (typically mid-afternoon), short duration emergencies can vary greatly.
This is illustrated in Figure 6-9 where the normal ampacity (1.0 per unit), A, is determined for a
peak temperature of 90°C, and two 4-hour emergency ratings are determined:
Emergency Rating B– The peak temperature is 105°C with a rating of 2.6 per-unit (as
compared to the normal rating). This emergency starts going into a low-load period, so the
pre-emergency load temperature is about 73°C.
Emergency Rating C– The peak temperature is also 105°C with a rating of only 1.3 per-unit
(as compared to the normal rating). This emergency starts going into a peak load period, so
the pre-emergency temperature is about 85°C.
The above example illustrates that considering the load shape for emergency ratings is important.
Dynamic ratings (discussed later) is a main benefit for this type of analysis in optimizing – and
generally increasing – the current carrying capacity of an underground cable circuit.
Temperature measurements are an important part of verifying assumptions when calculating
ampacity and studying ways to improve ratings. Ideally, one would want to measure the cable
conductor temperature – the hottest location in the cable system – to be sure that insulation
temperature limits are not exceeded. However, because the conductor is energized, this is
typically difficult to do. Instead, it is common to measure the temperature on the outer surface
of the cable either on the pipe coating of a pipe-type cable or the jacket of the other cable types.
For conduit installations, temperatures might be measured in the conduits.
To perform these measurements, thermocouples are often used. Thermocouples are temperature
sensors based on the principle that when two dissimilar metals are joined, a predictable voltage
will be generated that relates to the difference in temperature between the measuring junction
and the reference junction (connection to the measuring device). The types of metals that are
used depend on the application (temperature range, location, cost, etc.). There are varieties of
thermocouple types (T, F, N, J, etc.). For cable related measurements, "Type-T" thermocouples
are most often used because they have a temperature range most closely matched to typical cable
operating temperatures. The Type-T thermocouple has a blue outer jacket in the U.S., France and
U.K. (up until 1993) or dark brown outer jacket in Britian (since 1993) and Germany. Inside, the
thermocouple wire consists of a copper electrode (positive, +) and a constantan electrode
(negative, -). Each electrode has an insulating coating that varies in color depending on the
country of origin (U.S. is blue on the positive and red on the negative; Britian is white negative; France is yellow on the positive and blue on the negative; and German is red on the
positive and brown on the negative). It is important that when connecting thermocouple wire to a
meter, data logger or other measuring device to verify that the polarity is correct. Otherwise, the
schematic will essentially create 3 thermocouple junctions in series (rather than one), which
could provide misleading results. Also, the thermocouple wire or extension grade thermocouple
wire must also be run from the measurement location all the way to the test instrument.
A thermocouple junction is created by joining the copper and constantan wires together as shown
in Figure 6-10. The junction can be left bare which minimizes thermal capacitance and increases
temperature measurement response. However, depending on the environment, the junction
may need to be coated or soldered to protect the thermocouple junction from corrosion, etc.
Laboratory-grade thermocouples are typically welded together. A thermocouple has an accuracy
of typically less than 1°C.
A key advantage to thermocouple temperature measurement is that the wire itself and the
equipment to measure thermocouple temperatures are both relatively inexpensive and minimal
training is required to use the technology. Several companies including Telog Instruments,
Omega, and Fluke make data loggers that cost less than U.S. $1,000 to read and possibly record
thermocouple temperatures. Battery-powered recorders can log data for 6-18 months, recording
temperatures every 15 minutes for an extended period. Once suspected or known cable circuit
hot spots are identified, low-cost thermocouples and data loggers may be placed at these
locations and checked periodically, particularly during periods of high load.
Thermocouple Wires (Copper and Constantan with U.S. Color Scheme – Left)
and Completed Junction (Right)
By comparing measured temperatures with those predicted using load history and the equivalent
thermal circuit from ampacity calculations, it is possible to evaluate the assumptions used in
ampacity calculations. From an operations standpoint, monitoring the cable temperatures gives
some assurance that the cables are not exceeding their allowable temperature during typical load
cycles However, this equipment suffers from both lower spatial resolution (4-10m) and lower accuracy
(2-3°C). All fiber test loops are limited by the losses in the system, so fusion splicing is the
preferred method for joining fibers.
Fiber used for DTS measurements is typically installed in a parallel conduit or directly buried
alongside an existing cable or pipe. Retrofitting a fiber on a direct-buried system is impractical
unless there is a conduit (communications or power) within a meter or so of the energized cables
in which the fiber may be installed. An example of fiber that might be installed directly buried or
in a conduit is shown in Figure 6-12.
The fiber optic cable typically consists of 4-6 fibers 50 x 125µm fibers, each with a 900 µm tight
buffer (only 1 or 2 fibers are needed, but some may be damaged during installation so spares are
desirable), Kevlar strength members to improve pulling strength (usually only 3000N, 675 lbs.),
and a fire-retardant PVC jacket.
Some XLPE cable manufacturers are embedding optical fibers under the jacket of the cable to
facilitate temperature measurements. Since this is physically closer to the conductor – ultimately
where we would like to know the temperature – this has some advantages.
Optical Fiber Cable used for DTS Measurements
A disadvantage of DTS equipment is the cost of the electronics to measurement the fiber
temperature, which may be upwards of U.S.$60k. Also, the equipment is not well suited for
operation in the field on an extended period of time, which makes "spot" measurements or
extended measurements in manholes or stations difficult. Most of the equipment must be
operated from 10-35°C, which is fairly limited in particularly warm or cool climates 6.6 Ampacity Audit
The "ampacity audit" is the concept of investigating ampacity for a cable circuit by applying the
various techniques described in this report. The basic concepts include performing a soil thermal
survey to determine soil characteristics and ambient temperatures, evaluating load history, and
then calculating ampacity. If following AEIC guidelines on a circuit that previously had used
assumed soil parameters, the 10°C increase in conductor temperature by itself will generally
allow a 20% increase in ampacity.
The ampacity audit is geared towards verifying the ampacity by whatever means are available
and assessing which locations along the route limit the overall circuit ampacity. Possibly this
might include obtaining a DFOTS temperature trace for the route to find hot spots, or looking at
route plan and profiles to find limiting installation conditions. These "hot spots" would then be
investigated further to see how they might be mitigated.
6.7 Remediation of "Hot Spots"
Remediation of hot spots is sometimes possible if the scope of the hot spot is limited.
If the hot spot is the result of overburden, or increased burial depth, it might be possible to
remove some of the overburden above the cables. This reduces the thermal resistance to heat
leaving the cable and may improve ampacity.
Poor soil thermal resistivity can often lead to hot spots, particularly if a low quality thermal
backfill was used – or not backfill at all. A hot spot may be eliminated or partially mitigated by
excavating around the cables and installing a good quality thermal backfill. This will improve the
heat transfer characteristics away from the cable, lowering the operating temperature for a given
In extreme cases, usually where one circuit experiences a hot spot from mutual heating of
another circuit, the installation of heat pipes can help. The heat pipe is a passive device that takes
advantage of the heat of vaporization to remove heat from a location. A heat pipe is constructed
using an alcohol-water or ammonia-water mixture in a partially filled copper tube. A partial
vacuum is drawn on the tube to adjust the vapor pressure to the operating temperature range for
the particular application. The heat pipe is then installed at an angle with the low point installed
near the heat source (cable, steam main, etc.). Heat from the source is absorbed by the liquid
alcohol-water or ammonia-water solution, causing a phase change to vapor which rises, carrying
the heat away. The gaseous vapor then condenses back to a liquid away from the hot spot and
then drains back to the hot spot location. This continuous process removes heat from
the location. The number and installation geometry of the heat pipes is typically designed by a
An example of a heat pipe installation to mitigate the effects of crossing cable circuits is shown
in the following figure.
Heat Pipes being Installed to Mitigate a Hot Spot where a Steam Main Crosses
a Pipe-Type Cable
The following methods are mostly applicable to pipe-type cables, although there are applications
that could extend to extruded or self-contained cables.
Gas-filled pipe-type cables may be uprated slightly by replacing the dry nitrogen gas with a
dielectric liquid such as polybutene or alkylbenzene. Since a liquid is a more efficient heat
transfer media than a gas, fluid-filling alone provides a small ampacity improvement (~2-3%).
However, this allows for additional uprating methods to be applied.
Fluid circulation is a relevant uprating technique when a short section, relative to the overall
circuit length, is limiting the pipe-cable rating. By circulating the dielectric liquid within the
cable pipe, the heat generated in the hot section will be transferred to other portions of the route,
mitigating the hot temperatures at that location. One requirement for this to be implemented is
to have a fluid return pipe or a parallel cable pipe that will permit a continuous circulation
path. Flow rates may be up to 800 liter/minute (200 gpm), but slow circulation with only
20 liter/minute (5 gpm) may be used for small hot spots or where the fluid viscosity limits
the flow rate.
If no fluid return pipe or parallel cable pipe is present, fluid oscillation may be used. In this
configuration, fluid is moved through the pipe at 4-40 liter/minute (1-10 gpm) and cycled
between the 4,000-11,000 liter (1000-3000 gallon) fluid reservoirs at either end of the pipe
A major consideration for fluid circulation is the free area in the pipe and the viscosity of the
dielectric liquid used in the pipe. As a result, the pressure rise when pumping dielectric fluid
through a pipe could be too excessive for practical uses. If the flow rate is limited to a value
below what is necessary to mitigate a hot spot, circulation may not be possible to mitigate a
hot spot; the utility might consider changing to a lower viscosity dielectric liquid to re-examining
pressure rise limitations.
Example Pipe Cable Dielectric Fluid Circulation Loop with Heat Exchangers
The basic principle of fluid circulation is based on work done by CIGRE and discussed in
Electra (see references). The approach is to evaluate sections of the fluid circulation route that
have basically the same characteristics and then use boundary conditions to match the flow rate
6-22 from one section to the next. Dielectric liquid (or water in parallel circulation/cooling tubes)
is relatively non-compressible, although the density will vary with temperature around the
circulation loops. To consider this, the dielectric fluid characteristics – density and specific heat
– are adjusted for each section that is being modeled. As fluid circulates through the pipes, the
temperature of the fluid leaving one section is assumed to be the temperature entering the next
section, satisfying the boundary conditions.
To model each section, the "un-cooled" (temperature that would result absent of any cooling or
circulation movement in the pipes) temperature is calculated based on circuit loading, the cable
construction and installation conditions at each section. Then, heat absorbed or removed would
cause increases or decreases in the dielectric fluid temperature as it moves through the pipes.
The temperature change with respect to distance is of the form:
T(x) TUN COOLED K A exp(P x)
In the equation, "x" is the distance, "K" is a value proportional to the maximum change in
temperature possible for a section of infinitively long length, "A" is a value relating the mutual
heating affects among the pipes (either cabled or fluid return), and "P" is a value characterizing
the rate of temperature change as air moves along the pipe section. There is one exponential term
for each pipe in which fluid is circulating.
The "un-cooled" temperature is the steady-state temperature of the dielectric liquid inside the
cable pipe that would result if the loading remained fixed on the energized cables and no fluid
circulation was in place. As the flow rate is reduced, the value of K approaches zero. As the flow
rate increases, the value of K approaches the difference between the inlet temperature of
the dielectric liquid and the un-cooled temperature for a given section.
Changes in dielectric liquid temperature as it passes through the cable pipes provide an
indication of the heat being removed in each section based on the mass flow through the
section as defined by the following equation:
Where is the mass flow rate in kg/sec., is the density in kg/m3, is the free area within the
pipe in m2, and is the velocity in m/sec. The heat removed, Watts, in a given section can then
be found from the following equation:
Where is the specific heat in kJ/kg-°C, and TOUT and TIN represent the outlet and inlet
temperatures, respectively, of the dielectric liquid in a given section. By knowing the net heat
absorbed or lost in a given section and the length of that section, it is possible to evaluate the net
heat removed (or gained) in a given section. For a forced-cooled system (described next), the net
heat gained by the system will assist with sizing the forced-cooling plant and heat exchangers. Fluid circulation could be applied to extruded dielectric or self-contained liquid filled cables by
installing parallel water cooling pipes next to the cables and then circulating water through those
pipes. Although technically feasible, this is not often done. In addition, some utilities such as
National Grid in the U.K. circulate the dielectric liquid in the fluid channel of self-contained
liquid-filled cables. Again, this is relatively rare.
6.8.3 Forced Cooling (Water or Oil)
Forced cooling is an extension of fluid circulation. The main difference is that rather than just
moving heat around from "hot spots" to "cold spots" in the cable route, the dielectric liquid (or
water in the case of water cooling of extruded or self-contained cables) is diverted from the cable
pipe, passed through a heat exchanger to remove heat, and then re-introduced to the cable pipe.
This has the potential of increasing the ampacity by 50-70%, although the cost and maintenance
of these active systems can be high.
Considerations for Active Uprating
With fluid circulation or forced cooling in pipe-type cables, there are some cautions associated
with using these uprating methods. Pressures along the hydraulic loop may become excessive as
a result of hydrostatic head pressure, fluid flow restrictions near joints, and cross-over plumbing
between feeders and fluid return pipes. The high pressures could cause the termination housing
to fracture, potentially resulting in a cable failure, dielectric fluid leak and fire.
Pressure drop along long circulation loops must be considered. The degree of snaking of the
cable phases within the pipe can affect the fluid flow and pressure drop, potentially limiting the
flow rate. The pressure drop as a function of length can found by evaluating the Darcy-Weisbach
equation, as follows: heating, Neher-Mcgrath, duckbank, heating, neher-mcgrath, underground cables, thermal, RHO
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f is the friction factor, empirically determined based upon the Reynolds Number
cable-to-pipe inner diameter ratio
? is the density, kg/m3
V is the flow velocity, m/sec.
Dh is the hydrostatic diameter, meters
The pumping plant, in particular the fluid circulation pump, must be able to accommodate the
circulation pressure. The density and viscosity of the dielectric liquid will impact the allowable
pressure drop, in addition to the free area within the pipe and the degree of cable snaking. On
long circulation loops, multiple loops may be needed with intermediate fluid circulation stations
to limit the pressure drop. Complex control systems, particularly in the event of a cable failure,
must also be developed to manage the various cooling loops and stop fluid circulation in the
event of a fault. Fluid circulation is often considered for the buried pipe sections. However, when forced cooling
is used, the riser sections – lengths of pipe between the trifurcator and termination – may become
limiting and could require specialized plumbing to allow circulation in these areas. Diffusion
chambers may also be necessary to avoid damaging the outer layers of insulation. A factor to
consider for uprating older pipe circuits where the riser section may be limiting is that
installation of a diffusion chamber on a riser that is not so equipped can be difficult because
working "close in" on the riser pipe would be difficult with the cable already in place, and
removing and installing a new termination to put in the diffusion chamber may be impractical.
6.9 Shield/Sheath Bonding Scheme
As discussed to some extent in Section 4, there are three methods for grounding the shield/sheath
on single-conductor (extruded dielectric, self-contained fluid-filled) cable systems; multi-point
bonding, single-point bonding and cross-bonding. Multi-point bonding involves tying the
shield/sheath connections together and to local ground at both terminals and usually at
intermediate manholes, resulting in a path for induced circulating currents but with minimal
induced voltages. Single-point bonding involves grounding the shield/sheaths at only one
location along a given section, preventing circulating currents but leaving the other end ungrounded
where a standing voltage will appear. Cross-bonding involves dividing the cable
sections into groups of three minor sections that are close to the same length and transposing
sheath connections at the 1/3 and 2/3 locations, thereby eliminating net circulating currents
and minimizing induced voltages.
Generally, single conductor transmission cables are designed with cross-bonding or single-point
bonding to minimize shield/sheath circulating currents in the presence of relatively high phase
currents. The one exception to this common practice is submarine cable installations where
multi-point bonding of the sheath is almost mandatory because of the long installation lengths
and typically wide phase spacing. Contrary to transmission practice, most distribution circuits are
multi-point bonded where the utility transformer and customer service panel are both grounded
for safety and so the neutral can carry imbalance currents. The circulating currents in multi-point
bonded systems generate additional I2R losses (heat) in the shield/sheath that impacts ampacity.
Multi-point bonding systems generally have about 20-30% lower ampacity than single-point
bonded or cross-bonded systems constructed with similar cable sizes. If the ampacity audit
reveals that a circuit has lower ampacity than desired and happens to be multi-point bonded,
the shield/sheath connections might be reconfigured for sectionalized single point bonding or
cross-bonding to eliminate the circulating current and gain a significant improvement in
ampacity. The reconfiguration may require changing out some or all joints since the joints must
have shield interrupts to provide for single-point bonding or to facilitate transposing the sheath
connections for cross-bonding. If a system that was previously multi-point bonded is being
reconfigured for single-point bonding, a ground continuity conductor should be installed to
provide a low impedance path for fault current; the shield breaks will otherwise block the flow of
fault current. Also, single phase or three-phase link boxes or cross-bonding boxes will be needed.
If uprating using a reconfigured sheath bonding scheme is being considered and utility practice
is typically with multi-point bonding systems, care should be used to clearly mark all manholes
where standing voltages may appear. A shield that is not grounded locally but is connected to
6-25 ground at the adjacent manhole may experience a significant voltage rise with respect to local
ground during near-by system fault conditions from a combination of induced voltages and
system potential shifts. This is not a phenomenon peculiar to single-point grounded
arrangements, as any shielded cable is susceptible when the shield is connected to a remote
ground yet remains ungrounded locally. The remedy for this situation is to provide secure,
temporary shield grounding as appropriate. This has always been a recommended practice.
This is particularly important when working on a de-energized single-point bonded circuit that
parallels an energized circuit since the parallel circuit can induce a voltage.
Also, single-point bonded or cross-bonded cable systems require periodic maintenance to check
the jacket integrity and insure there are no unexpected current circulation paths. Fault location
efforts may also be complicated by single-point bonded or cross-bonded sheaths, possibly
requiring that the bonding connections be reconfigured during cable fault location. Jacket
fault location in duct bank installations is difficult unless the ducts are under the water table.
Underground Cables Need a Proper Burial
Apr 1, 2003 12:00 PM
By Deepak Parmar and Jan Steinmanis, Geotherm Inc.
Overhead systems are out in the open, so it is easy to detect and fix design and installation problems. Underground problems, however, are out of sight and out of mind, at least until cables start failing. Although utilities design their underground circuits for a 30-year life, improper installations often can lead to premature field failures.
Unless you lay your cables to rest properly, they may come back to haunt you. Here's a brief example. A wind-generating farm was installed with underground cables tied directly to a main feeder cable. Unfortunately, the cables were simply placed in a trench using native soil backfill with minimal soil compaction. Ampacity calculations were performed using typical soil values, but thermal properties were not measured. Since wind turbines operate almost continuously, the feeder cable often ran at maximum capacity. The heat generated from the feeder cable dried out the surrounding soil completely. Because the native soil was poorly compacted fine silt, it acted like an insulating blanket and the cable failed prematurely.
A significant source of potential problems with underground circuits is the improper selection and installation of thermal backfill materials. To prevent premature failures, you must ensure you place cable systems in a hospitable environment.
Too few utilities have stringent specifications or quality-assurance programs for installing cable-trench backfill; this often leaves the decision up to the civil contractor. The effects of poorly installed thermal backfills and soils may not be evident for many years, until cable loads increase and temperatures rise beyond allowable levels, resulting in cable failures. The remedial cost of removing and replacing poor backfills is high, especially under paved roads. The loss of revenues from derating a system may be even higher. Installing a new circuit may be the only, albeit expensive, option.
Importance of Soil and Backfill
All the heat generated by an underground power cable must be dissipated through the soil. This is quantified by the soil thermal resistivity (or thermal rho, °C-cm/W), which can vary from 30 to 500°C-cm/W. Electrical engineers understand the performance of the cable quite well, but to most, the soil behavior is a mystery, usually handled by using a thermal backfill with a supposedly "safe" thermal rho.
The ability of the surrounding soil to transfer the heat determines whether an operating cable remains cool or overheats. Improving the external thermal environment and accurately defining the soil and backfill thermal rho commonly results in a 10% to 15% increase in cable ampacity, with 30% improvements noted in some cases.
You can address potential problems by measuring the native soil's thermal properties and by using properly designed and installed corrective thermal backfills in the cable trench. In recent years, we've learned that using thermal probes connected to a Thermal Property Analyzer (EPRI EL-2128) can accurately measure the thermal rho in the field and laboratory.
The use of a soil thermal rho of 90°C-cm/W has become ingrained in cable engineering practices. Soil studies performed in the 1950s found this was a "safe" value for most moist soils. This value is commonly used for distribution cables, where cable loads are usually low and the native soil is used as the backfill. For transmission cables, it is assumed that the "thermal backfill" placed around the cables will be much better than the native soil and that it will have a thermal rho of less than 90°C-cm/W.
Most moist soils (with the exception of organic clays and silts, volcanic soils, peat and fills with ash and slag) have a rho of less than 90°C-cm/W. Moist sands, which are commonly placed around transmission cables, may even have a rho of less than 50°C-cm/W.
The critical word is "moist." Many soils, especially uniform sands, can dry substantially when subjected to heat from the cables. The thermal rho of a dry soil would exceed 150°C-cm/W, and possibly approach 300°C-cm/W for a dry uniform sand. (The dry thermal rho of a properly designed and installed thermal backfill should be less than 100°C-cm/W and possibly as low as 75°C-cm/W).
In fact, a contractor, if left to his or her own devices, most likely would use readily available fine sand or concrete sand as the backfill. From a construction viewpoint, this sand makes an inexpensive and excellent bedding material, but thermally, it is very poor because it dries out easily under high cable loads.
Unfortunately, over the years utilities have used many unsuitable sands or "thermal backfills" because of ease of installation and availability. Several route thermal surveys of existing circuits installed before 1980 confirm this practice. Almost any sand, when moist, will give a reasonably low thermal rho. The crucial aspect is how easily it dries when subjected to cable heat loads.
Soils in semi-arid climates are naturally quite dry, so the assumption of a moist soil is not valid. It doesn't take much to dry these soils completely. In many parts of the country, the soil mineral and consistency is such that there is a high intrinsic thermal rho. Soil that is not properly compacted in the cable trench will be less dense and have a substantially higher thermal rho. Even distribution or low-voltage cables that are continuously under full load may dry the soil.
Cables that are near other heat sources, such as steam mains, will experience higher ambient temperatures, and if in the vicinity of other cables, will experience mutual heating and run hotter.
The thermal rho is important not only for transmission cables but also in any situation resulting in high heat generation. The assumption of a soil and backfill thermal rho of 90°C-cm/W may be erroneous, possibly leading to long-term problems when the cable is heavily loaded.
Poorly compacted trench backfill is a major problem. Not only is the thermal rho of uncompacted soil significantly higher, but the loose soil will dry more easily, which increases the possibility of thermal runaway.
Corrective Thermal Backfills
Generally, native soils do not make good thermal backfills because their thermal rho values are poor, or they are difficult to properly re-compact in a cable trench. There are also problems associated with stockpiling, screening of debris, and contamination of good soil with organic topsoil. In the long run, the operational reliability gained by placing a classified thermal backfill around the cable has advantages over the variability and uncertainty of recompacted native soil.
Compacted granular backfills can have good thermal properties. Since most of the heat conduction is through the soil mineral particles and their contacts, one must ensure a high-density soil mixture to maximize these contacts. A well-graded sand to fine gravel can be a good thermal backfill when compacted to its maximum density as determined by a laboratory standard Proctor test (ASTM D698). The total cost of a compacted backfill must include material and transportation costs, as well as installation labor and quality-assurance costs.
The one often-neglected factor about compacted backfills is the need for quality assurance during installation. If the gradation of the backfill is not correct (sieve analysis ASTM D422), or it is not at the optimum moisture content (ASTM D698), or not enough compaction effort is applied, or the backfill lifts are too thick, then the maximum density will not be achieved and the thermal capability degraded.
Cement stabilized sand frequently has been used as a cable trench backfill in many countries. A typical mix design consists of 14 parts sand to one part cement, mixed with about 8% water. If the correct sand is used and properly installed, this material can have acceptable thermal performance. However, this backfill is quite strong and thus would be difficult to excavate. Quality control is required during mixing and installation, otherwise the thermal performance cannot be assured.
Many North American utilities have been using stone dust or crushed stone screenings as thermal backfill. If well graded and of the right mineral type, it provides a low and stable thermal resistivity when compacted at optimum moisture content and density. It does require thorough testing to establish density, moisture and thermal performance, and a good quality-control program to ensure proper installation.
With compacted soils, maximum soil density is needed in the restricted trench areas near cables or around cable pipe groups where proper compaction is difficult. Yet, it is precisely in these zones adjacent to the cables, where the heat flux is highest, that suitable compaction is most important to ensure maximum heat dissipation from the cables.
Fluidized Thermal Backfills
Over the past 10 to 15 years, we've seen great acceptance of fluidized thermal backfills (FTB™), which are formulated to meet thermal resistivity, thermal stability, strength and flow criteria. This free-flowing, controlled-density fill is ideal for hard-to-access areas, such as narrow trenches, small diameter tunnels or areas congested with many underground services — basically where mechanical compaction is not feasible or practical. While the material cost of FTB may be higher, it should be considered for general usage because of its assured quality and quick installation, thus speeding up construction and decreasing overall costs, which are important factors when working in busy city streets.
FTB is a slurry backfill consisting of medium aggregate, sand, a small amount of cement, water and a fluidizing agent. FTBs can be formulated using locally available aggregates. The component proportions are chosen by laboratory testing of trial mixes to minimize thermal resistivity and maximize flow without segregating the components.
Be wary of commonly available "controlled density fills," "flowable fills" or "slurry backfills," which use large volumes of fly ash or sand. These may meet the mechanical and flow requirements for trench backfilling, but too often they provide totally unsuitable thermal performance. Fluidized thermal backfills should be formulated and tested only by soil thermal specialists who understand the tricks of the trade in making thermal measurements.
Fluidized thermal backfills do not have to be compacted; they flow in a fashion similar to concrete. In fact, FTB is typically supplied from concrete trucks, and may be poured or pumped, and seldom requires any special shoring or bulkheading. It solidifies to a uniform density by consolidation, with excess water seeping to the top. Regular FTB can be pumped up to 150 m (500 ft) using conventional concrete pumping equipment and greater distances with special modifications. It hardens quickly so that the ground surface may be reinstated the next day, but the low strength (100 to 250 psi [0.7 to 1.8 MPa]) allows it to be broken up with a backhoe if required. If a higher strength is required, the cement content can be increased and the water adjusted without degrading the thermal performance.
FTB will flow readily to fill all the spaces, without vibration, yet harden quickly. Future settlements are negligible. It also affords mechanical protection for the cables or cable pipes and provides support for underground and surface facilities (road pavement). FTB has good heat dissipation properties even when totally dry. Depending on the mix design, typical thermal rhos are 35 to 40°C-cm/W wet, and 70 to 100°C-cm/W dry, with excellent thermal stability. The FTB can be formulated for use in both flat and hilly terrain. Thicker, slower flowing mixes can be formulated when addressing an area with a significant slope.
Backfills … The Right Way
The use of a well-designed thermal backfill can enhance the heat dissipation and increase the allowable ampacity of an underground power cable, as well as alleviating thermal instability concerns. The corrective backfill will reduce the heat flux experienced by the native soil so that it will not dry out; therefore, the stability of the native soil is no longer a concern. A good backfill should be better able to resist total drying and also have a low dry thermal rho if it is completely dried. It should be available at a reasonable cost, and be easy to install and easy to remove if required. The thermal backfill must be laboratory evaluated and include specifications for mineral quality, gradation (sieve analysis), thermal dryout curve and optimum density. Typically, the entire trench width is filled with thermal backfill to a minimum height of 300 mm (12 inches) above the cables. For poor native soil conditions or heavily loaded cables, the thickness of the backfill can be increased to maintain a low composite thermal rho. A fluidized thermal backfill is the ideal way of providing a high-quality cable backfill.
Deepak Parmar is president of Geotherm Inc. From 1960 to 1978, he worked on various civil engineering (soil and rock mechanics) projects in the United Kingdom and Canada. Since forming Geotherm Inc. in 1978, Parmar has worked solely on underground and submarine power cable projects. He received the BS degree in civil engineering from Woolwich Polytechnic, United Kingdom, in 1966, and the Diploma in Management Studies (DMS) from Slough, United Kingdom, in 1972. He is a member of the Engineering Institute of Canada, Canadian Society for Civil Engineers, Canadian Geotechnical Society, Canadian Society for Electrical and Computer Engineers, Tunneling Association of Canada, IEEE/PES/ICC, Canadian Electrical Association and CIGR….
Jan Steinmanis is vice president of Geotherm Inc. He received a B.A.Sc. degree in civil engineering from the University of Toronto, Canada, in 1975. From 1976 to 1982, Steinmanis worked as a research engineer with Ontario Hydro, where he worked on several civil engineering projects and on the Electric Power Research Institute (EPRI)-funded projects for the Development of Thermal Property Analyzer. He also conducted several research projects, including the soil geotechnical-thermal properties database for Canada (a Canadian government-funded project). He is a registered professional engineer.
Elements of a Cable Route Thermal Survey
• Perform in-situ thermal rho testing and sampling of the native soils. This may be done in conjunction with any required geotechnical testing, such as for manholes. Review any available soils information so test locations cover all the soil types.
• In the laboratory, perform thermal dryout tests (thermal rho vs. soil moisture) on select samples. This will define the thermal rho for drier soil conditions.
• Source and design the fluidized thermal backfill (or compacted granular backfill) based on locally available materials. This also will include a thermal dryout curve.
• Choose the design thermal rho values for the native soil and thermal backfill based on the lowest expected soil moistures.
• Use a computer cable design program to optimize configuration of cables, trench size and thermal backfill envelope.
Description Thermal Resistivity Dry (°C-cm/W)
Shale (sound) 60
Shale (highly friable) 200
Oil (petroleum) 800
Thermal stability describes the ability of a moist soil to maintain a relatively constant thermal rho when subjected to a cable heat load, thus preventing a power cable from exceeding its safe operating temperature. Thermal instability (or "thermal runaway") occurs when a soil is unable to sustain the heat from a cable. The soil progressively dries, resulting in a substantial increase in the thermal rho and attendant increase in the cable-operating temperature. If soil moisture is not replenished or current reduced, the ultimate result may be a totally dry thermal rho and cable failure caused by overheating.
Visually, thermal runaway can be described on a thermal dryout curve. At high moistures, the curve is relatively flat, so any minor drying of the soil will not change the thermal rho very much (thermally stable). Excessive cable heat will dry the soil below the knee of the curve (critical moisture), and the thermal rho will increase significantly. This will cause the cable to get hotter, thus drying the soil more. The thermal rho will "walk" up the thermal dryout curve as the soil dries, eventually giving a totally dry thermal rho near the cable.
When Not to Worry About Thermal Stability?
Thermal instability concerns can be minimized by always using a fluidized thermal backfill around the cable. The thermal dryout curve of a good backfill has a sharp knee at a low critical moisture content and the totally dry thermal rho is quite low. For these backfills the thermal stability may be treated as a binary concept, that is, if the lowest expected moisture is above the critical moisture content then the backfill is stable for normal heat rates and the moist thermal rho may be used in ampacity calculations. If the lowest expected moisture is below the critical moisture then the backfill is unstable and the totally dry thermal rho must be used for the design. For FTB, the totally dry thermal rho is usually less than 90°C-cm/W, so it is still quite acceptable. By using a sufficiently large thermal backfill envelope, the heat flux through the native soil will be quite low; therefore, the native soil will not dry out, and the stability of the native soil is not a concern.
Underground Power Cable Installations: Soil Thermal Resistivity
Gaylon S. Campbell
Decagon Devices Inc.
Pullman, WA 99163
Keith L. Bristow
CSIRO Land and Water
PMB PO Aitkenvale
Townsville QLD 4814
Who would have thought that an electrical power engineer would need to be an expert at
soil physics as well. But, increasingly, such knowledge is becoming critical in the design
and implementation of underground power transmission and distribution systems. The
issues are simple enough. Electricity flowing in a conductor generates heat. A resistance
to heat flow between the cable and the ambient environment causes the cable temperature
to rise. Moderate increases in temperature are within the range for which the cable was
designed, but temperatures above the design temperature shorten cable life. Catastrophic
failure occurs when cable temperatures become too high, as was the case in Auckland,
NZ in 1998. Since the soil is in the heat flow path between the cable and the ambient
environment, and therefore forms part of the thermal resistance, soil thermal properties
are an important part of the overall design.
The detailed calculations needed to correctly design an underground cable system
have been known for over 60 years. The procedures typically used are outlined in Neher
and McGrath (1957), and, more recently by the International Electrotechnical
Commission (1982). These calculations can be done by hand, but most engineers now
use either commercial or home-brew computer programs. The calculations are quite
detailed, and are generally based on sound physics or good empiricism, until one gets to
the soil. Then the numbers chosen often are almost a shot in the dark. Since, even in a
well-designed system, the soil may account for half or more of the total thermal
resistance, engineers need to treat that part with as much respect as they do the cables and
Thermal Resistivity of Soil
Good theories describing thermal resistivity of soil have been around for a long
time (de Vries, 1963; Campbell and Norman, 1998). These models are based on
dielectric mixing models, and treat the overall resistivity as a weighted parallel
combination of the constituent resistivities. Five constituents are important in
determining the thermal resistivity of soil. These are quartz, other soil minerals, water,
organic matter, and air, in order of increasing resistivity. The actual values for these
materials are 0.1, 0.4, 1.7, 4.0, and 40 m C/W. Without knowing anything about the
weighting factors for these in an actual soil or fill material, four things should be clear: 1)
Air is bad. Fill must be tightly packed to minimize air space, in order to achieve
acceptably low thermal resistances. 2) Replacing air with water helps a lot, but water is
still not a very good conductor. 3) Organic matter, no matter how wet, will still have a
very high resistivity. 4) Fill materials high in quartz will have the lowest resistivity,
other things being equal. We will illustrate some of these points with examples.
Density and Thermal Resistivity
Figure 1 shows how important compaction is for achieving acceptably low
thermal resistivity in backfill materials. A value often assumed for thermal resistivity of
soil in buried cable calculations is 0.9 m C/W. None of the curves in Fig. 1 ever get that
low, even at very high density. Typical density for a field soil that can sustain plant
growth is around 1.5 Mg/m3. At this density, even the quartz soil has a resistivity more
than 4 times the assumed value. Three important observations can be made from Fig. 1.
First, organic material is never suitable for dissipating heat from buried cable, no matter
1.2 1.4 1.6 1.8 2 2.2
Bulk Density (Mg/m3)
Thermal Resistivity (m C/W)
Figure 1. The thermal resistivity of a dry, porous material is strongly dependent on
Second, thermal resistivity of dry, granular materials, even when they are compacted to
extreme density, is not ideal for cable backfill. Third, the air spaces control the flow of
heat, so, even though quartz minerals have 4 times lower resistivity than the loam
minerals, the overall resistivity of the two are similar at similar density.
It is worth mentioning that arbitrarily high densities are not attainable just by
compaction. Uniform sized particles pack to a given maximum density. To attain
densities beyond that, without crushing particles, smaller particles are added to the voids
between the larger particles. Highest densities are therefore attained by using well-graded
Water Content and Thermal Resistivity
Even though water resistivity is higher than that of soil minerals, it is still much
lower than air. If the pore spaces in the soil are filled with water, rather than air, the
resistivity decreases. Figure 2 shows the effect of water. The density is around 1.6
Mg/m3, much lower than the highest values in Fig. 1, but with a little water the
resistivities are well below 1 m C/W. Now, with more water in the pores, the effect of
the quartz is more pronounced. The resistivity of organic soil, though better than when
dry, is still much too high to provide reasonable heat dissipation for buried cable.
0 0.1 0.2 0.3 0.4
Water Content (m3/m3)
Thermal Resistivity (m C/W
Figure 2. Adding water to a porous material drastically decreases its thermal
Water content in the field
Since thermal resistivity varies so much with water content, and water content in
soil is so variable, it is reasonable to ask what water contents to expect in field soils.
Below, and even slightly above a water table the soil is saturated (all pores filled with
water). In these situations, one can be certain that resistivities will remain at the lowest
values possible for that soil density. Minimum water content in the root zone of growing
plants typically ranges from 0.05 m3/m3 in sands to 0.1 or 0.15 m3/m3 for finer texture
soils. These water contents correspond, roughly, to the water contents in Fig. 2 at which
resistivity begins to increase dramatically. This is sometimes called the critical water
content, and is the water content below which thermally driven vapor flow in a
temperature gradient will not be re-supplied by liquid return flow through soil pores.
This point is very significant in buried cable design, because, when the soil around the
cable becomes this dry, the cable heat will drive the moisture away, drying the soil around
the cable and increasing its resistivity. This results in additional heating, which drives
away additional moisture. A thermal runaway condition can ensue.
Lower dry resistivities than those shown in Fig. 1 can be achieved using especially
designed backfill materials. A Fluidized Thermal BackfillTM (FTBTM) can be poured in
place. It has a dry resistivity of around 0.75 m C/W, decreasing to below 0.5 m C/w
While it is possible to compute thermal properties of soil from physical properties,
it is usually easier to measure them directly than to do the computations. Methods are
given by ASTM(2000) and IEEE(1992). The accepted method uses a line heat source.
Typically a heating wire and a temperature sensor are placed inside a small bore
hypodermic needle tube with length around 30 times its diameter. Temperature is
monitored while the needle is heated. In this radial heat flow system a steady state is
quickly established, and one can plot temperature vs. log time to obtain a straight line
relationship. The thermal resistivity is directly proportional to the slope of the line.
Several companies offer instruments suitable for either field or laboratory measurements
of thermal resistivity, and probes can be left in place to monitor thermal properties after
the cable is installed and in use.
In addition to the issues discussed above there are also several site-specific issues
that need to be taken into account when designing and implementing underground power
cable installations. These include trade-off analysis between depth of installation, cost of
installation, and thermal stabilisation. The deeper one buries the cables the more stable
the thermal environment, especially if shallow water tables and capillary upflow result in
relatively moist conditions around the cables. Surface conditions will also impact on the
water and energy exchange between the soil and atmosphere and hence the thermal
environment around the cables. In cities the surface will more than likely be covered by
roads, buildings, parks or gardens, while in rural areas bare soil or vegetative cover will
be most common. It is important that surface condition and its impact on the underlying
thermal environment be taken into account, and especially any change in surface
condition that could result in unwanted consequences. Adding vegetation for example
could result in significant soil drying, with potential consequences as discussed earlier.
Clay soils in particular can crack on drying, resulting in development of air gaps around
cables, and every effort must be made to avoid this happening. Potential 'hot spots' along
the cable route (such as zones of well drained sandy soils or vegetated areas that could
lead to significant soil drying) should receive particular attention to ensure long-term
success of any installation.
There are five important points that the electrical power engineer should take from
this short discussion. First, soil and backfill thermal properties must be known for a safe
and successful underground power cable installation. One can't safely assume a value of
0.9 m C/W. Second, density and water content play important roles in determining what
the thermal resistivity will be. Specify the density of a backfill material, and assure,
through design and appropriate management that water content can't get below the
critical level. Third, natural soils which support plant growth will always have much
higher resistivities than engineered materials because of their lower density and variable,
but sometimes low water content. Fourth, engineered backfill materials are available
which can assure adequate thermal performance under all conditions. Fifth, measurement
of thermal conductivity, both in the field and in the laboratory, is relatively
straightforward, and should be part of any cable design and installation project. Finally,
there are several site-specific issues such as depth of cable placement, vegetation and soil
water management, and avoidance of excessive drying and soil cracking that could lead to
air gaps, all of which need to be taken into account when designing and implementing
underground power cable installations.
ASTM (2000) Standard test method for determination of thermal conductivity of soil and
soft rock by thermal needle probe procedure. ASTM 5334-00
Campbell, G. S. and J. M. Norman (1998) An Introduction to Environmental Biophysics.
Springer Verlag, New York.
DeVries, D. A. (1963) Thermal properties of soils. in W. R. van Wijk, Physics of Plant
Environment John Wiley, New York
IEEE (1992) Guide for soil thermal resistivity measurements. Inst. of Electrical and
Electronics Engineers, Inc. New York.
International Electrotechnical Commission (1982) Calculation of continuous current
ratings of cables. Publication 287, 2nd ed.
Neher, J. H. and M. H. McGrath. (1957) The calculation of temperature rise and load
capability of cable systems. AIEE Transactions on Power Apparatus and Systems.
The Neher-McGrath Calculations provide a method for calculating underground cable temperatures or ampacity ratings and are derived from the following technical paper:
J. H. Neher and M. H. McGrath,"The Calculation of the Temperature Rise and Load Capability of Cable Systems", AIEE Transactions, Part III, Volume 76, pp 752-772, October, 1957.
This paper considers the complicated heat transfer issues associated with the determination of underground system ampacities. The paper cites the following basic equation for calculation of a cable ampacity:
However, this single equation masks the great complexity involved in these procedures. There are scores of complicated equations involved in developing the terms in this equation and those required for temperature calculations. (The paper defines over 80 variables and contains in excess of 70 formulas excluding appendices.) To solve for unique ampacities or temperatures at each cable position, a multiple set of equations must be developed to take into account interference heating from every position in the system, and a matrix solution technique for simultaneous equations utilized.
AmpCalc handles all the complexity and allows the user to quickly and easily determine ampacities for virtually any underground ductbank or direct burial arrangement.