Fluke 1625-2 Site Application

Earth ground testing

for mine sites with

Fluke 1625

All mine sites have grounded
electrical systems, so that in
the event of a lightning strike
or utility overvoltage or ground
fault, the current would be able
to find a safe path to earth or
the neutral of the transformer.
To ensure a reliable connec­tion to earth, electrical codes,
engineering standards, and
local standards often specify
a minimum resistance for the
ground electrodes. This appli­cation note looks at how one of
the world’s leading mining and
exploration companies uses
Fluke’s earth ground testers to
maintain a safe working site.

Application Note

Rio Tinto is one of the world’s
leading mining and explora­tion companies. They find, mine
and process the earth’s mineral
resources—metals and minerals
essential for making thousands
of everyday products that meet
society’s needs and contribute to
improved living standards.

Products include aluminium,
copper, diamonds, energy prod­ucts, gold, industrial minerals
and iron ore.

The company’s activities span
the world but are strongly rep­resented in Australia and North
America with significant busi­nesses in South America, Asia,
Europe and southern Africa.

The Pilbara region

Rio Tinto has been at the fore­front of sustained innovation
and growth in the Pilbara region
for more than forty years.

Rio Tinto’s iron ore operations
in the Pilbara have expanded
to an annual capacity of 220
million tonnes, with advanced
plans to further expand. With a
network of 12 mines, three ship­ping terminals and the largest
privately owned heavy freight
rail network in Australia, the
company’s Pilbara operations
make up a major part of iron ore
activities globally. Its opera­tions began in 1966 and have

expanded to meet the growing
needs of the world’s iron and
steel industry.

Rio Tinto is a world-class
asset manager, operating and
maintaining all mining, rail,
power and port facilities in
the Pilbara on behalf of asset
owners Hamersley Iron and
Robe River.

Grounding systems

Regular earth testing is a very
important practice for mining
companies such as Rio Tinto.
The purpose of a ground,
besides the protection of people
and equipment, is to provide a

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

safe path for the dissipation of
fault currents, lightning strikes,
transient over-voltages, static
discharges, EMI and RFI signals
and interference.

A ground is a conducting con­nection, whether intentional or
accidental between an electri­cal circuit or equipment and the
earth, or to some conducting
body that serves in place of the
earth. It helps by stabilising the
voltage to earth during normal
operation, and limits the voltage
rise created by lightning, line
surges or unintentional contact
with higher-voltage lines.

In the event of an electrical
fault or lightning strike any­where in the vicinity of the
mining area, a low-impedance
ground electrode will help carry
the energy into the earth. By
keeping the potential gradient
low, damage is minimised.

Without an effective ground­ing system, workers could be
exposed to the risk of electric
shock, not to mention instru­mentation errors, harmonic
distortion issues, power factor
problems and a host of pos­sible intermittent dilemmas.
If fault currents have no path
to the ground through a prop­erly designed and maintained
grounding system, they will find
unintended paths that could
include people.

Poor grounding not only
contributes to unnecessary
downtime, but a lack of good

grounding is very dangerous
and increases the risk of equip­ment failure. A good grounding
system will improve the reli­ability of equipment and reduce
the likelihood of damage due to
lightning or fault currents.

The need for earth
testing

Over time, corrosive soils with
high moisture content, high salt
content, and high temperatures
can degrade ground rods and
their connections. Previous
faults may have melted connec­tions that aren’t easily visible.
So although the ground system
when initially installed had low
earth ground resistance values,
the resistance of the ground­ing system can increase if the
ground rods are eaten away
from causes due to corrosion of
ground rods, breaks in intercon­necting wires and water table
changes.

Earth resistance is the resis­tance of the earth to the passage
of electric current, and test
results show the resistance
offered by the earthing rods
with the connection leads,
which should be less than 1
Ohms in the case of Rio Tinto’s
installation. Resistivity defines
a material’s ability to conduct
current, which is a complex
property to measure within
the earth as it is affected by
several factors, including soil
composition, mineral content,
temperature and depth.

Intermittent electrical prob­lems at a mine site could be
attributed to poor grounding or
poor power quality.

That is why it is highly recom­mended that all grounds and
ground connections be checked
upon installation and on an
annual basis. During these
periodic checks, if an increase
in resistance of more than 20
per cent is detected, the tech­nician should investigate the
source of the problem, and
lower the resistance by replac­ing or adding ground rods to the
ground system.

Rio Tinto conducts regular
earth ground tests to ensure the
operational safety of a mine.
This is the first line of defense.
The ability to detect and monitor
ground connections can provide
valuable data for undertaking
appropriate risk assessments for
mining operations.

Ground beds provide safety
grounding of electrical equip­ments in mines. The goal in
ground resistance is to achieve
the lowest ground resistance
value possible that makes sense
economically and physically.
Ideally a ground should be of
zero ohms resistance.

David Oxley, electrical super­visor at the reliability assurance
department at Rio Tinto’s
Cape Lambert site in the Pil­bara region, oversees these
inspections.

David said, “We need to
ensure that earth grounding
at all substations is at its peak
performance, and that we are
comply with statutory rules
onsite.”

David and his team perform
checkups at their substations
regularly.

The process

Special instruments make it
simple and easy for maintenance
and safety teams to measure
earth-resistance and trouble­shoot problems.

For Rio Tinto, the Fluke 1625
is used to measure the capabil­ity of the earth ground system at
the substations.

Earth resistance
measurement: the
Fall-of-Potential
measurement

The classic Fall-of-Potential test
method is used at Rio Tinto’s
Cape Lambert facility to measure
the ability of an earth ground
system or an individual elec­trode to dissipate energy from a
site.

2 Fluke Corporation Earth ground testing for mine sites with Fluke 1625

Typically the earth electrode
of interest must first be discon­nected from its connection to
the site. Second, the tester is
connected to the earth elec­trode. Then, for the 3-pole
Fall-of-Potential test, two earth
stakes are placed in the soil in
a direct line—away from the
earth electrode, normally with a
spacing of around 20 meters. By
using the Fluke 1625 to gener­ate a current through the two
outer ground stakes (the auxil­iary earth stake and the earth
electrode), the drop in voltage
potential is measured between
the two inner ground stakes.
Using Ohm’s Law (V=IR), the
Fluke tester automatically calcu­lates the earth resistance.

Earth resistance measure­ments are often corrupted by the
existence of ground currents and
their harmonics. To prevent this
from occurring, the Fluke 1625
uses an Automatic Frequency
Control (AFC) system. This
automatically selects the testing
frequency with the least amount
of noise enabling you to get an
accurate earth ground value.

Saving time with
selective testing

The Fluke 1625 is a distinc­tive earth ground tester. Not
only does it measure ground
resistance using the classic
fall of potential test but also
enable time saving testing using

the selective method. Selec­tive testing does not require
the electrode under test to be
disconnected during the mea­surement, thus increasing safety.

The selective method allows
testers to measure the ground
resistance of a specific ground
electrode without disconnect­ing it from an array or from a
structure’s distribution system.
This means that safety hazards
are minimised as there is no risk
from disconnecting the electrode
from a live system.

With selective measurement,
two earth stakes are placed in
the soil in a direct line, away
from the earth electrode, of
around 20 metres. The Fluke
1625 is then connected to the
earth electrode of interest, with
the advantage that the connec­tion to the site doesn’t need to
be disconnected.

Instead, a special clamp is
placed around the earth elec­trode, which eliminates the
effects of parallel resistances in
the grounded system, so only
the earth electrode of interest
is measured. A known current
is generated by the Fluke 1625
between the outer stake and the
earth electrode, while the drop
in voltage potential is measured
between the inner earth strake
and the earth electrode. Only
the current flowing through the
earth electrode is measured
using the clamp. The generated
current will also flow through

other parallel resistances, but
only the current through the
clamp is used to calculate
resistance.

“The Fluke 1625 also allowed
us to perform testing without the
need to shut down any of our
20 substations. This capability
minimised downtime and gave
us the freedom to conduct test­ing for all substations at any one
time.”

Steve Hood, managing direc­tor at Fluke Australia said,
“The Fluke 1625 is intended
for electrical utilities or other
high energy environments as
it has extra versatility for more
demanding applications.

“Under these circumstances,
the selective method is a much
safer and easier method for
earth ground testing for Rio
Tinto, as there is no need to
disconnect or shut down any
of the substations in order to
perform this test. The earth
testing conducted to ensures a
safe environment for all workers,
and at the same time, maximises
profit to investors by operating
responsibly, with no downtime.”

David said, “The team learned
how to use the Fluke 1625 tool
almost immediately. There was a
lot of training and support from
Fluke to ensure that our require­ments and needs were met. It is
now an indispensable tool for us
at the mines.”

Fluke. Keeping your world
up and running.

®

3 Fluke Corporation Earth ground testing for mine sites with Fluke 1625

Fluke Corporation

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©2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 10/2010 3931841A A-EN-N

Modification of this document is not permitted
without written permission from Fluke Corporation.