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Application Note

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

Earth ground testingfor 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 connec-

tion to earth, electrical codes,

engineering standards, and

local standards often specify

a minimum resistance for the

ground electrodes. This appli-

cation note looks at how one of

the worlds leading mining and

exploration companies usesFlukes earth ground testers to

maintain a safe working site.

The Pilbara region

Rio Tinto has been at the fore-front of sustained innovationand growth in the Pilbara regionfor more than forty years.

Rio Tintos iron ore operationsin the Pilbara have expandedto an annual capacity of 220million tonnes, with advancedplans to further expand. With anetwork of 12 mines, three ship-ping terminals and the largestprivately owned heavy freightrail network in Australia, thecompanys Pilbara operationsmake up a major part of iron oreactivities globally. Its opera-tions began in 1966 and have

Rio Tinto is one of the worldsleading mining and explora-tion companies. They find, mineand process the earths mineralresourcesmetals and mineralsessential for making thousandsof everyday products that meetsocietys needs and contribute toimproved living standards.

Products include aluminium,copper, diamonds, energy prod-ucts, gold, industrial mineralsand iron ore.

The companys activities spanthe world but are strongly rep-resented in Australia and NorthAmerica with significant busi-nesses in South America, Asia,Europe and southern Africa.

expanded to meet the growingneeds of the worlds iron andsteel industry.

Rio Tinto is a world-classasset manager, operating andmaintaining all mining, rail,power and port facilities inthe Pilbara on behalf of assetowners Hamersley Iron andRobe River.

Grounding systems

Regular earth testing is a veryimportant practice for miningcompanies such as Rio Tinto.The purpose of a ground,besides the protection of peopleand equipment, is to provide a

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safe path for the dissipation offault currents, lightning strikes,

transient over-voltages, staticdischarges, EMI and RFI signalsand interference.

A ground is a conducting con-nection, whether intentional oraccidental between an electri-cal circuit or equipment and theearth, or to some conductingbody that serves in place of theearth. It helps by stabilising thevoltage to earth during normaloperation, and limits the voltagerise created by lightning, linesurges or unintentional contact

with higher-voltage lines.In the event of an electricalfault or lightning strike any-where in the vicinity of themining area, a low-impedanceground electrode will help carrythe energy into the earth. Bykeeping the potential gradientlow, damage is minimised.

Without an effective ground-ing system, workers could beexposed to the risk of electricshock, not to mention instru-mentation errors, harmonic

distortion issues, power factorproblems and a host of pos-sible intermittent dilemmas.If fault currents have no pathto the ground through a prop-erly designed and maintainedgrounding system, they will findunintended paths that couldinclude people.

Poor grounding not onlycontributes to unnecessarydowntime, but a lack of good

grounding is very dangerousand increases the risk of equip-ment failure. A good groundingsystem will improve the reli-ability of equipment and reduce

the likelihood of damage due tolightning or fault currents.

The need for earthtesting

Over time, corrosive soils withhigh moisture content, high saltcontent, and high temperaturescan degrade ground rods andtheir connections. Previousfaults may have melted connec-tions that arent easily visible.So although the ground systemwhen initially installed had low

earth ground resistance values,the resistance of the ground-ing system can increase if theground rods are eaten awayfrom causes due to corrosion ofground rods, breaks in intercon-necting wires and water tablechanges.

Earth resistance is the resis-tance of the earth to the passageof electric current, and testresults show the resistanceoffered by the earthing rodswith the connection leads,

which should be less than 1Ohms in the case of Rio Tintosinstallation. Resistivity definesa materials ability to conductcurrent, which is a complexproperty to measure withinthe earth as it is affected byseveral factors, including soilcomposition, mineral content,temperature and depth.

Intermittent electrical prob-lems at a mine site could beattributed to poor grounding orpoor power quality.

That is why it is highly recom-mended that all grounds andground connections be checkedupon installation and on anannual basis. During theseperiodic checks, if an increasein resistance of more than 20per cent is detected, the tech-nician should investigate thesource of the problem, andlower the resistance by replac-ing or adding ground rods to theground system.

Rio Tinto conducts regularearth ground tests to ensure theoperational safety of a mine.This is the first line of defense.The ability to detect and monitor

ground connections can providevaluable data for undertakingappropriate risk assessments formining operations.

Ground beds provide safetygrounding of electrical equip-ments in mines. The goal inground resistance is to achievethe lowest ground resistancevalue possible that makes senseeconomically and physically.Ideally a ground should be ofzero ohms resistance.

David Oxley, electrical super-

visor at the reliability assurancedepartment at Rio TintosCape Lambert site in the Pil-bara region, oversees theseinspections.

David said, We need toensure that earth groundingat all substations is at its peakperformance, and that we arecomply with statutory rulesonsite.

David and his team performcheckups at their substationsregularly.

The process

Special instruments make itsimple and easy for maintenanceand safety teams to measureearth-resistance and trouble-shoot problems.

For Rio Tinto, the Fluke 1625is used to measure the capabil-ity of the earth ground system atthe substations.

Earth resistancemeasurement: the

Fall-of-Potentialmeasurement

The classic Fall-of-Potential testmethod is used at Rio TintosCape Lambert facility to measurethe ability of an earth groundsystem or an individual elec-trode to dissipate energy from asite.

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Typically the earth electrodeof interest must first be discon-nected from its connection tothe site. Second, the tester isconnected to the earth elec-

trode. Then, for the 3-poleFall-of-Potential test, two earthstakes are placed in the soil ina direct lineaway from theearth electrode, normally with aspacing of around 20 meters. Byusing the Fluke 1625 to gener-ate a current through the twoouter ground stakes (the auxil-iary earth stake and the earthelectrode), the drop in voltagepotential is measured betweenthe two inner ground stakes.Using Ohms Law (V=IR), the

Fluke tester automatically calcu-lates the earth resistance.Earth resistance measure-

ments are often corrupted by theexistence of ground currents andtheir harmonics. To prevent thisfrom occurring, the Fluke 1625uses an Automatic FrequencyControl (AFC) system. Thisautomatically selects the testingfrequency with the least amountof noise enabling you to get anaccurate earth ground value.

Saving time withselective testing

The Fluke 1625 is a distinc-tive earth ground tester. Notonly does it measure groundresistance using the classicfall of potential test but alsoenable time saving testing using

the selective method. Selec-tive testing does not requirethe electrode under test to bedisconnected during the mea-surement, thus increasing safety.

The selective method allowstesters to measure the groundresistance of a specific groundelectrode without disconnect-ing it from an array or from astructures distribution system.This means that safety hazardsare minimised as there is no riskfrom disconnecting the electrodefrom a live system.

With selective measurement,two earth stakes are placed inthe soil in a direct line, awayfrom the earth electrode, of

around 20 metres. The Fluke1625 is then connected to theearth electrode of interest, withthe advantage that the connec-tion to the site doesnt need tobe disconnected.

Instead, a special clamp isplaced around the earth elec-trode, which eliminates theeffects of parallel resistances inthe grounded system, so onlythe earth electrode of interestis measured. A known currentis generated by the Fluke 1625

between the outer stake and theearth electrode, while the dropin voltage potential is measuredbetween the inner earth strakeand the earth electrode. Onlythe current flowing through theearth electrode is measuredusing the clamp. The generatedcurrent will also flow through

other parallel resistances, butonly the current through theclamp is used to calculateresistance.

The Fluke 1625 also allowed

us to perform testing without theneed to shut down any of our20 substations. This capabilityminimised downtime and gaveus the freedom to conduct test-ing for all substations at any onetime.

Steve Hood, managing direc-tor at Fluke Australia said,The Fluke 1625 is intendedfor electrical utilities or otherhigh energy environments asit has extra versatility for moredemanding applications.

Under these circumstances,the selective method is a muchsafer and easier method forearth ground testing for RioTinto, as there is no need todisconnect or shut down anyof the substations in order toperform this test. The earthtesting conducted to ensures asafe environment for all workers,and at the same time, maximisesprofit to investors by operatingresponsibly, with no downtime.

David said, The team learned

how to use the Fluke 1625 toolalmost immediately. There was alot of training and support fromFluke to ensure that our require-ments and needs were met. It isnow an indispensable tool for usat the mines.

Fluke CorporationPO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa +31 (0) 40 2675 200 orFax +31 (0) 40 2675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com

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 permittedwithout written permission from Fluke Corporation.

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