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www.libertyconsulting.com.au Copyright © LCS 2016
Soil Resistivity Testing Services
Liberty Consulting Services (LCS) has conducted extensive electrical soil resistivity testing for many years, in some of
the most difficult soil conditions Australia has to offer. During this time, LCS has developed reliable techniques to
provide accurate test data so that a low margin of error is able to be obtained for earth system designs using the
provided test measurements. The use of adequately powered test equipment to suit the conditions is crucial to
performing meaningful electrical soil resistivity testing.
Why soil resistivity testing is so important
Soil resistivity is the basis upon which earth grids are
designed so that electrical parameters are determined
and modelled (e.g. grid resistance, and mesh voltages
due to injected fault currents). This is a crucial step in
designing and assessing any earthing system.
Soil is rarely homogeneous, and is commonly made up
of horizontal layers which need to be identified so
that the correct placement of earthing conductors can
be determined during the design process. The use of
uniform or “averaged” tested resistivity values can
lead to significant errors in earthing designs.
Qualified LCS engineers perform testing so that
difficult conditions encountered in the field can be
handled on site, and re‐testing suspect data points is
done as required. This ensures the provision of
accurate test data to the designing engineer(s).
These Standards detail soil resistivity test methods:
Soil Resistivity Testing Standards ENA EG‐0 Power System Earthing Guide
(Section 7 – especially 7.2.3)
ENA EG‐1 Substation Earthing Guide (Section 5.2 – especially 5.2.2)
AS/NZS 1768 Lightning protection (Appendix C.10 – especially C10.1)
IEEE Std 80 IEEE Guide for Safety in AC Substation Grounding (Sections 13.3 & 13.4)
IEEE Std 81 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System (Section 7)
Soil electrical characteristics
The key determinant of error in the earthing design
process is the inaccuracy or variation of the soil
resistivity input to the design.
Therefore it is important to understand electric
current flow in non‐uniform, multi‐layered soils, and
how to accurately test soil resistivity in a diverse
range of conditions.
www.libertyconsulting.com.au
Tel: 0434 005 393
www.libertyconsulting.com.au Copyright © LCS 2016
There are four basic parameters that influence soil
resistivity values:
1. Chemical content (soil type, constituents);
2. Moisture content;
3. Temperature; and
4. Particle size and distribution.
The use of geotechnical data from the proposed
testing area will often provide good background
information and sanity check of test results.
Accurate soil resistivity for power system earthing
The Wenner or “four pin equal spacing” method is the
preferred test regime used by LCS. Other methods
include the Schlumberger or Driven Rod, and these
have advantages in certain specific conditions.
In the Wenner method, it is important to understand
how far apart test probes need to be in successive
tests to provide an accurate soil profile as an input to
the design software.
The largest spacing (see “a” below) needs to be at
least equal to the “zone of influence” of the earthing
system being designed. A sample test instruction is
provided with recommended spacings.
The meter measures “apparent resistivity”, not the
“actual” resistivity. The value obtained is a “weighted
average” of the actual resistivity down to the depth
(spacing) being tested. This raw data must be
interpreted by software (e.g. CDEGS RESAP module) in
order to determine the actual soil resistivity.
How many test points are required in a traverse set?
Experience has shown that the greater number of test
points in a Wenner traverse, a higher accuracy soil
model (multi‐layer) will be developed by software. We
recommend at least 10 data points in each traverse.
Software computations are more accurate if the data
recorded does not have too many “gaps” between the
spacings. The preferred ratio between spacings is 1.5.
A reading taken at 5 metres, would need to be
followed up by a 7.5 metre spacing, and preceded by
at least 3.5 metres, in order to adhere to this 1.5 rule.
Soil resistivity test results and analysis
The use of sophisticated modelling software such as
CDEGS’ RESAP module is able to provide accurate
multi‐layer soil profiles. A % error is provided with
each software model.
Multi‐layer software analysis is critical to providing
accurate inputs to the design process. High/Low
versus Low/High soil models give quite different
results for an injected earth fault current.
Large variations in soil resistivity near the surface
influence step and touch voltage performance.
Whereas large variation deeper below the ground
surface affect the transfer of earth potential rise to
adjacent metalwork and services.
Using equivalent uniform soil resistivity and empirical
equations from IEEE80 give large errors. Therefore
simply “averaging” the raw test data results is not
advisable.