On-line ground electrode calculations and grounding network calculations
Grounding safety from step and touch potential hazards
system modeling and fault studies
Pipeline interference effects
Electrical Forensic Studies
We mostly measure soil resistivity using
the Wenner method. Other methods that we have used generally infer the soil resistivity
from ground electrode measurements. For example, we have measured the ground resistance of
steel pilings, for which the size and depth was known and have determined the average soil
resistivity by modeling the configuration in the computer.
The Wenner method involves inserting four
evenly-spaced test probes into the soil, in a straight line. A test current is applied to
the outer two probes. This produces a potential difference between the inner two probes.
The apparent resistance (obtained by dividing the potential difference by the test
current) is used to determine the apparent soil resistivity for that probe spacing. We
usually take readings for a series of probe spacings of between 1 and 36 metres. This
enables us to determine an average layered soil resistivity structure for the area.
We use a digital electronic four-terminal
grounding tester for the measurements. The instrument provides a direct resistance reading
in ohms. Sometimes, and particularly in mountainous areas, the surface soil resistivity is
too high for this instrument. It is then necessary to use a basic current and voltage
method with a higher voltage source to circulate sufficient current. We have used a
variable frequency source and tuned voltmeter to minimize the effects of stray currents
We have developed software to resolve the
soil resistivity test data into the equivalent layered soil structure. We use two methods
for this. Both of them develop a perfect layered soil structure that would produce as
close as possible to the measurement results obtained in the field.
Decent Method This method is described in Appendix A of the IEEE
Guide 81-1983 titled "IEEE Guide for Measuring Earth Resistivity, ground Impedance
and Earth Surface Potentials of a Ground System". This standard has not been updated
since 1983. Unfortunately there are errors in the text of Appendix A, which are corrected
in our software.
The steepest decent method is best used for
the analysis of soil resistivity data that fits a two-layered model. It converges more
slowly that the Filter Function Method, but does not modify the original input data.
Function Method The Filter Function Method is based on a 1979
technical paper by P.A. Davis, Faculty of the Graduate School of the University of
Minnesota, which is titled: "Development and Application of Resistivity Sounding
Inversion for Several Field Arrays". The paper was submitted by Mr. Davis in partial
fulfillment of the requirements for the degree of Master of Science, which was
The method processes the field data through
a filter function that rapidly converges on a multi-layered soil structure that best fits
the measurement data.
The method relies on the input of
logarithmically spaced data, which means that the readings must be taken with probe
spacing increased logarithmically for each reading. Readings are usually not taken
with this spacing pattern. The software incorporates a curve fitting algorithm to develop
logarithmic spaced data from the non-logarithmic field data. In so doing, some information
is lost and for this reason it is best to use this method only for resolving the soil into
more than two layers of resistivity.
We usually use the fall-of-potential (FOP)
method for measuring ground impedance. The FOP method consists of creating an artificial
current return electrode some distance from the object being measured and injecting a test
current into the object, using this electrode for current return. The test current will
cause a voltage rise on the object which is measured using another reference electrode
also located some distance from the object being tested.
Many people use small portable, battery
powered three and four terminal grounding testers to measure the ground resistance of
objects. These instruments work well for small ground electrodes or ground grids
that have an impedance of 1 ohm or more. They will still give
readings for larger ground systems with lower impedances, but the readings may contain
Our experience enables us to determine when
other methods should be used, in particular the variable frequency method:
Around 1980, David Bensted pioneered the use
of the variable frequency method of measuring ground impedance. The method consists of
using a power source with a frequency other than 60 Hz (in areas where the normal power
frequency is 60 Hz) and a narrow-band tuned voltmeter, to measure ground impedance. It
enables measurements to be undertaken without shutting down equipment, even though there
may be relatively high levels of 60 Hz "noise" present. We usually use a
portable generator to provide the power source for these measurements and operate at 50 or
70 Hz. The tuned voltmeter has a 3 Hz bandwidth and can effectively filter out 60 Hz noise
that has a voltage level 10 times higher than the test signal.
The conventional FOP test configuration is to
measure the voltage rise of the ground system with reference to a test probe that is
progressively moved closer to the current return electrode, along a straight line between
the two. The test results are plotted as a curve of apparent measured resistance against
the distance along the measurement traverse. If the current return electrode is
sufficiently far away, there should be a flat portion of the curve where the test probe
was in an area of zero soil voltage. The ground resistance is given by the resistance
measured in that part of the curve. For an ideal measurement where the ground electrode
being measured is small compared with the distance to the current return electrode and the
soil resistivity is completely uniform, the true resistance can be shown to be indicated
at 61.8% of the distance along the traverse towards the current return
electrode. We have noticed that this is often incorrectly used as a criteria, when the
plotted curve has no flat portion. The real reason for such a curve may be that the
ground system is interconnected with other objects and is much larger than was thought.
The conventional method often fails to work
properly, particularly in the case of large ground systems with a low ground resistance.
If the measured resistance is in the order of 1 ohm or less,
significant error will be introduced by inductive coupling between the test lead that
carries current to the remote current return electrode and the test lead to the voltage
probe. The coupling can be reduced by spacing the leads wider apart, but the reduction is
not sufficient to eliminate the error. An alternative is to run the voltage traverse
at 90 degrees to the direction of the current return electrode.
There are a number of other pitfalls in
carrying out the fall-of-potential test. Attempts have been made to minimize these
problems by developing "smart" testers and special test configurations. Closer
examination shows that these devices and methods cannot produce as accurate a result as a
well-configured fundamental fall-of-potential test.
These tests are used to evaluate existing bonding systems. We use a
high current, four terminal resistance measurement method to test the bonding between
different areas of a plant. Resistances as low as a few milli-W can be measured over several 100 metres in this way.
We have carried these tests out at many different kinds of
industrial plants. The tests provide confidence in the existing grounding system. Here are
At a gas or oil plant, the readings will show whether there is any
chance of sparking or heating when ground faults occur.
At a high voltage substation, tests will reveal if there is poor
bonding at disconnect handles that may be hazardous for an operator.
We have carried out the CSA Code tests for patient
care areas in hospitals and extended care facilities. These tests require measuring the
ground potential rise of the outlets during simulated faults and voltage drop under rated
load as well as noise levels on the electrical grounds.
Computer and engineering calculations can
be carried out using software and methods that we have developed and updated over the last
electrode calculations and on-line grounding network calculations.
We are presently revising this process and intend
to offer on-line service in a different way. The procedure described in
green text below is no longer available, but many of the advantages and
disadvantages apply in a similar way:
You prepare the data as a text file,
using our instruction manual and send it to us by e-mail. We will do the calculations and
return the results to you by e-mail. There are advantages
and disadvantages to working this way. The costs
of doing analyses this way are more reasonable than buying software to do
grids. We can calculate the resistance of ground grids located in soil with
non-uniform resistivity. If the grid is large enough that there is a voltage drop across
it or it has a reactive component in the impedance, these effects can be modeled by the
software. Separate objects, either floating or electrically interconnected by impedances
can also be modeled.
from step and touch potential hazards. We can calculate potentials that
develop in the soil around a ground grid and from that, the step and touch potentials at
the soil surface and can compare these against the tolerable levels as defined by the
IEEE. We have developed a rational method for calculating step potentials from soil
system modeling and fault studies. We can model complex single or multiphase power systems and
determine the split of current during normal operation and ground faults. This can be used
to calculate the ground component of ground fault current for applying to ground grid
analyses in determining step and touch potential safety.
We can model very complex
power systems using TACLINK. A problem that
often arises is how to determine the fraction of ground fault current at a
substation that flows into the ground and the fraction that flows in
transmission line ground wires that are connected to the substation.
Various approximate methods have been developed over the years such as
using shielding factors. With TACLINK, the
split of current can be calculated properly, allowing for effects like
inductive coupling, tower footing resistances.
interference effects. We can model and calculate the inductive coupling
effects between power lines and pipelines. These effects become more serious each year as
utilities seek new rights-of-way for pipelines and are forced to place more pipelines
along existing power line rights-of-way. The fault level on power lines is also increasing
with system capacity and expansion.
Electrical Forensic Studies
Other than providing opinions on electrical
problems that are grounding related, we have completed several studies into electrical
fires. These studies included an electrical fire on a 50 ft. power boat and a home fire.
The boat fire was initially thought to be due to poor wiring. We were able to show that
there was a deficiency in the marine quality power connector that had been used for the
shore power connection. The home fire was thought to have been caused by failure of an
electric baseboard heater. We were able to prove that this was not the cause of the fire.
In both cases, we devised electrical tests that proved our conclusions were correct.
If you would like more
information on our Consulting Engineering Services, please contact us.