11095-eng-01-a checking ground electrode impedance for commercial, industrial and residential...

8/13/2019 11095-Eng-01-A Checking Ground Electrode Impedance for Commercial, Industrial and Residential Buildings

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Checking ground

electrode impedance for

commercial, industrial

and residential buildings

Most facilities have grounded electrical systems, so that in the event of a lightning strike

or utility overvoltage, current will find a safe path to earth. A ground electrode provides the

contact between the electrical system and the earth. To ensure a reliable connection to

earth, electrical codes, engineering standards, and local standards often specify a minimum

impedance for the ground electrode. The International Electrical Testing Association specifies

ground electrode testing every three years for a system in good condition with average

up-time requirements. This application note explains earth/ground principles and safety in

more depth and then describes the principle testing methods: 3 and 4 pole Fall-of-Potential

testing, selective testing, stakeless testing and 2 pole testing.

Why Ground?

The US National Electrical Code(NEC) gives two principle reasonsfor grounding a facility. Stabilize the voltage to earth

during normal operation. Limit the voltage rise created

by lightning, line surges or

unintentional contact withhigher-voltage lines.Current will always find and

travel the least-resistance pathback to its source, be that a utilitytransformer, a transformer withinthe facility or a generator. Light-ning, meanwhile, will always finda way to get to the earth.

In the event of a lighting strikeon utility lines or anywhere inthe vicinity of a building, a low-impedance ground electrode willhelp carry the energy into theearth. The grounding and bonding

systems connect the earth nearthe building with the electricalsystem and building steel. In alightning strike, the facility willbe at approximately the samepotential. By keeping the potentialgradient low, damage is minimized.

If a medium voltage utility line(over 1000 V) comes in contact

with a low voltage line, a drasticovervoltage could occur fornearby facilities. A low impedanceelectrode will help limit the

voltage increase at the facility.A low impedance ground canalso provide a return path forutility-generated transients.Figure 1 shows a groundingsystem for a commercial building.

Ground ElectrodeImpedance

The impedance from thegrounding electrode to theearth varies depending ontwo factors: the resistivityof the surrounding earthand the structure of theelectrode.

Resistivity is a propertyof any material and itdefines the materialsability to conduct current.The resistivity of earth iscomplicated, because it:

Figure 1: A grounding system combining reinforcing steeland a rod electrode

Application Note

Depends on composition of thesoil (e.g. clay, gravel and sand)

Can vary even over smalldistances due to the mix ofdifferent materials

Depends on mineral (e.g. salt)content

Varies with compression andcan vary with time due to

settling Changes with temperature,freezing (and thus time of year).Resistivity increaseswith decreasing temperature.

Can be affected by buried metaltanks, pipes, re-bar, etc.

Varies with depthSince resistivity may decrease

with depth, one way to reduceearth impedance is to drive anelectrode deeper. Using an arrayof rods, a conductive ring or agrid are other common ways ofincreasing the effective area of

an electrode. Multiple rods shouldbe outside of each others areasof influence to be most effective(see Figure 2). The rule of thumb isto separate the elements by morethan their length. For example:2.5 meter rods should be spacedmore than 2.5 meters apart to bemost effective.


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15A

TRUERMS

HOLD

OFF

mA

A

Ground Loop Tester

The NEC specifies 25 ohms asan acceptable limit for electrodeimpedance. The IEEE Standard142 Recommended Practicefor Grounding of Industrial andCommercial Power Systems(Green Book) suggests a resis-tance between the main groundingelectrode and earth of 1 to 5 ohmsfor large commercial or industrialsystems.

Local authorities including theauthority having jurisdiction (AHJ)and plant managers are respon-

sible for determining acceptablelimits for ground electrode imped-ance.Note: Power distribution systemsdeliver alternating current andground testers use alternatingcurrent for testing. So, youd thinkwe would talk about impedance,not resistance. However, at powerline frequencies, the resistivecomponent of the earth impedanceis usually much bigger than thereactive component, so you will seethe terms impedance and resistanceused almost interchangeably.

How do groundresistance testers work?

There are two types of groundresistance testers. Three and fourpoint ground testers and clamp-onground testers (see Figure 3).Both types apply a voltage onthe electrode and measure theresulting current.

A three or four-pole groundtester combines a current sourceand voltage measurement in alunch box or multimeter-style

package. They use multiple stakesand/or clamps.

Ground testers have thefollwing characteristics:

AC test current. Earthdoes not conduct dc

very well. Test frequency that is

close to, but distin-guishable from thepower frequency andits harmonics. Thisprevents stray currentsfrom interferring withground impedancemeasurements.

Separate source andmeasure leads tocompensate for thelong leads used in thismeasurement.

Input filtering designed to pickup its own signal and screenout all others.

Clamp-on ground testers arevery different because clamp-onground testers have both a sourcetransformer and a measurementtransformer. The source trans-former imposes a voltage on theloop under test and the measure-ment transformer measures theresulting current. The clamp-onground tester uses advancedfiltering to recognize its ownsignal and screen out all others.

Ground Testing SafetyAlways use insulated gloves, eyeprotection and other appropriatepersonal protective equipmentwhen making connections. It isnot safe to assume that a groundelectrode has zero voltage or zeroamps, for reasons given below.

To perform a basic ground test(called Fall-of-Potential) on anelectrode, the electrode must bedisconnected from the build-ing. New selective methods allow

accurate testing with the electrodestill connected. See SelectiveMeasurements.

A ground fault in the systemmight cause significant current toflow through the ground conduc-tor. You should use a clampmeter to check for current beforeperforming any impedance testing.If you measure above 1 amp (ruleof thumb) you should investigatethe source of the current beforeproceeding.

If you must disconnect anelectrode from an electrical

system, try to do so during amaintenance shutdown when

you can de-energize the system.Otherwise, consider temporarilyconnecting a backup electrode tothe electrical system during yourtest.

Never disconnect a groundelectrode if there is a chance oflightning or other system failure.

A ground fault in the vicinity cancause voltage rises in the earth.The source of the ground fault maynot even be in the facility you aretesting, but could cause voltagebetween the test electrodes. Thiscan be especially dangerous nearutility substations or transmissionlines where significant groundcurrents can occur. (Testinggrounding systems of transmissiontowers or substations requires the

use of special Live Earth proce-dures and is not covered in thisapp note.)

Ground impedance testers usemuch higher energy than yourstandard multimeter. They canoutput up to 250 mA. Make sureeveryone in the area of the testis aware of this and warn themnot to touch the probes with theinstrument activated

Figure 2: Ground electrodes have areas of influence thatsurround them

2 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.

Figure 3


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I

V

EP2 C2

Source

Measure

Measure

ElectrodeUnder test

PotentialSpike

CurrentSpike

MeasuredResistance

Electrode/EarthImpedance

Distance of P2 from E

E

valid resistance

Figure 4: Schematic for Fall-of-Potential

Figure 5: A plot of measured impedances versus position of the potentialstake allows us to see the earth impedance

The connections are made to: E/C1 the ground electrode

being tested S/P2 A voltage (potential)

measurement stake driven intothe earth some distance awayfrom the electrode. Sometimescalled the potential auxiliaryelectrode

H/C2 A current stakedriven into the earth a furtherdistance away. Sometimescalled the current auxiliaryelectrode

Figure 4 shows this sche-matically and Figure 5 shows thethree connections made using atypical ground tester.

The ground tester injects analternating current into the earth

between the electrode under test(E) and the current stake (C2).The ground tester measures thevoltage drop between the P2stake and E. It then uses ohmslaw to calculate theresistance between P2 and E.

To perform the test you positionthe C2 stake at some distancefrom the electrode under test.Then, keeping the C2 stake fixed,you move the P2 stake along theline between E and C2, measuringthe impedance along the way.

The tricky part comes in

determining where to drive thestakes to get a true reading of theresistance between the electrodeand the earth.At what point does the dirtsurrounding the electrode stopbeing a contributor of resistanceand become the vast earth?Remember that we are not inter-ested in the resistance betweenthe electrode and our stakes. Weare trying to measure the resis-tance that a fault current wouldsee as it passes through the massof the earth.

Checking ConnectionResistance Leading Up tothe Electrode

Before testing the electrode, startby checking its connection tothe facility bonding system. MostFall-of-Potential testers have theability to measure 2-pole, lowohms and are perfect for the job.You should see less than 1 ohm: At the main bonding jumper Between the main bonding

jumper and the ground elec-trode conductor

Between the ground electrodeconductor and the groundelectrode

Along any other intermediateconnection between the main

bonding jumper and the groundelectrode

The Fall-of-PotentialMethod

The Fall-of-Potential method isthe traditional method for testingelectrode resistance. The procedureis specified in the IEEE-81 standardGuide for Measuring Earth Resis-tivity, Ground Impedance and EarthSurface Potentials of a GroundSystem and in numerous other

national standards.In its basic form, it works well forsmall electrode systems like oneor two ground rods. We will alsodescribe the Tagg Slope Techniquewhich can help you draw accurateconclusions about larger systems.

Remember: for this method, theground electrode must be discon-nected from the building electricalservice.

How it worksThe Fall-of-Potential methodconnects to the earth at three

places. It is often called thethree-pole method. You maywant to use a fourth lead forprecise measurements on low-impedance electrodes, but for ourinitial discussions we will considerthree leads.

Checking ground electrode impedance for commercial, industrial and residental buildings. Fluke Corporation 3


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Table 1: Approximate Distance to Auxiliary Stakes usingthe 62 % Rule (in meters)

Depth of Electrodeunder Test (E)

Distance from E toPotential Stake (P2)

Distance from E toCurrent Stake (C2)

2 15 25

3 19 30

6 25 40

9 31 50

Table 2: Approximate Distance to Auxiliary Stakes forElectrode Arrays (in meters)

Widest Dimension(Diagonal, diameteror Straight-line) of

Electrode Array underTest (E)

Distance from E toPotential Stake (P2)

Distance from E toCurrent Stake (C2)

20 30 50

25 50 80

30 70 100

50 100 17070 130 200

Measure-

ment Tips

Bring a good, long tapemeasure.

Finding the horizontal

part of the curve will

require at least

5 measurements.

Its a good idea to take

three of your resistance

readings with the P2

stake at 20%, 40% and

60% of the distance

between E and C2. This

will allow you to use the

Tagg Slope Technique.

When placing the stakes

make sure the current

stake, the potential

stake and the electrode

under test form

a straight line.

If you get a very high

impedance measure-

ment or over-range,

try pouring some water

around the test stakes

to improve their contactto earth. This isnt

cheating since our inten-

tion is not to measure

the resistance of our

stakes, but to measure

the resistance of the

electrode.

Keep the potential and

current leads separated

to avoid signal coupling

between the two.

At a new constructionsite, you may want to

take multiple sets of

measurements. Resis-

tance may drop over

time as the earth settles

The current probe generates avoltage between itself and theelectrode under test. Close to theelectrode, the voltage is low and

becomes zero when the P stakeand electrode are in contact. Closeto the electrode, the potential

probe is said to be within theinfluence of the electrode. Closeto the current probe the voltageis almost the full voltage outputby the tester. But somewhere inthe middle, something interestinghappens.As we move from the influence ofthe electrodes and into the massof the earth, the test current nolonger causes significant changein potential. If you plot a seriesof measurements, moving thepotential stake away from theelectrode under test, and towardsthe current stake you will noticea flattening of the curve. An idealcurve is shown in Figure 5 (seeprevious page). The flattest partof the curve is where we read

the earth resistance. In reality,the curve never goes entirely flatbut reaches a very gentle slopewhere changes in resistance aresmall.

The extent of the influence of theelectrode depends on its depth andit area. Deeper electrodes requirethat the current stake be drivenfarther away (see Table 1). Forlarge ground rings, grids or arraysthe influence of the electrode mayextend for hundreds of meters.Table 2 gives suggested startingpoints for current and potential

stake placement. Because of thepossibility of interaction betweenan electrode rings, grids or arrays,

and the measurement stakes youshould not take shortcuts plot theFall-of-Potential graph to be sureyou are getting accurate results.

In testing a bonded array ofelectrodes the combined resistanceof the array will be less than thelowest reading you measure forany individual electrode. If, forexample, you have two 2.5 meterrods spaced more than 2.5 meterapart you can be confident thatthe combined resistance will besubstantially less for the combinedsystem.

The three-wire measurement willdeliver good results if you use ashort C1 lead, or if you dont mindhaving a fraction of an ohm of leadresistance in your reading. For

ground resistance measurementsover 10 ohms, the effect of the resis-tance of the C1 lead will be small.But for very precise measurements,especially at low resistances, a four-wire tester allows you add a fourthlead to eliminate the contribution ofthe C1 lead. By running a separatepotential lead (P1) to the electrodeunder test you can take the dropalong the C1 current lead out of themeasurement.



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Distance From Electrode Under Test

Fall-of-Potential Plot

P2 Distance from Electrode Under Test (meters)

R(Ohms)

Resistance

(m) (m) (ohms)

100100100

100100100100100100

102030

405060708090

15 30 45 60 75 90

d

62 % d

E

P2 C2

ElectrodeUnder test

PotentialSpike

CurrentSpike

Figure 7: Earth impedance can be found from this curve by usingthe Tagg Slope Technique

The 62 % Rule

You may be able to use a shortcutif your test meets the followingcriteria: You are testing a simple elec-

trode (not a large grid or plate) You can place the current stake30 meters or more from theelectrode under test

The soil is uniformUnder these conditions you can

place the current stake 30 metersor more from the electrode undertest. Place the potential stake at62 % of the distance between thecurrent stake and the electrodeunder test and take a measure-ment. As a check, take two moremeasurements: one with thepotential probe 1 meter closer to

the electrode under test, and one1 meter farther away (see Figure6). If you are on the flat portion ofthe fall-of-potential curve then thereadings should be roughly thesame and you can record the firstreading as your resistance.

The Tagg Slope Technique

Large electrodes or groundingsystems require some specialconsideration. If youve plot-ted resistance readings for nine

different P2 locations and there isno clear flattening on your graph,then the Tagg Slope Technique(also called the slope method) canhelp establish the earth imped-ance. Figure 7 shows an exampledataset for which there is noobvious flat section. This curve ischaracteristic of a test in whichthe current and potential probesnever get outside the influenceof the electrode under test. Therecan be a number of reasons for acurve like this: For electrode systems that cover

large areas it may be difficult toplace stakes far enough away You may not be able to place

the C1 stake at the center ofthe electrode

The area you have to placestakes may be limited

If you have resistance read-ings at the 20 %, 40 % and 60 %points between E and C2, thenyou can apply the procedure tothe data youve already taken.

Calculate the slope coefficient ()using three resistance measure-ments from 20 %, 40 % and60 % of the distance from theelectrode under test to the C2current stake.

Then go to the table in the back ofthis application note and look upthe P2/C2 ratio that correspondsto your . This will tell you whereto look on your graph to ascertainthe earth resistance. For thesample data in Figure 7:

If we go to the table, for = 0.71the corresponding P2/C2 percent-age is 59.6 %. So the approximateearth resistance would bemeasured at (59.6 % X 100 m), orat 59.6 m. This is very close to our60 % point at 60 m, where weread 6.8 ohms. So it would be safeto say the earth resistance for theelectrode under test is roughly6,8 ohms.

Figure 6: Stake positions for the 62 % rule.

( R60 %

R40 %

)

( R40 %

R20 %

) =

( 6.8 5.8

)

( 5.8 4.4

)

= = 0.71



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S

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42

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GroundResistance300k

ACResistance300k

SATURNGEO

NORMAEarth / Ground Tester

C2

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S

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GroundResistance300k

ACResistance300k

SATURNGEO


Probe Aux earth electrode

Electrode

under test

More than 65 ft More than 65 ft

Grounding

Electrode

Conductor

>10 cm

Voltage

Current

Equivalent Circuit

ACVoltageSource

Rn-1Rx R1 R2 R3RnR2R1Rx Rn-1

U

I

Figure 9: Connections for selective Electrode Impedance Mesaurement Figure 10: Connecting the Saturn GEO X for a stakeless measurement

The Selective Method

The Selective Method is avariation of the Fall-of-Potentialmethod, available on high-endground testers like theFluke 1625. Testers with thiscapability can measure theground impedance of a specificground electrode without discon-necting it from an array or fromthe electrical system. This meansyou dont have to wait for a shut-down to test or risk the safetyhazards of disconnecting theelectrode from a live system.

The same rules for current stakeand potential stake placementapply as with Fall-of-Potential. Ifthe conditions are met for the 62 %

rule (see previous page) then itcan help reduce the number ofmeasurements. Otherwise its agood idea to build a completeFall of-Potential plot. You can usethe Tagg Slope Technique if yourcurve does not flatten out.Both the Fall-of-Potential methodand the Selective Method usestakes to inject current and

measure voltage drop. The bigdifference is that selective testingcan accurately measure the testcurrent in the electrode under test.

The utility neutral, buildingsteel and ground electrode areall bonded and grounded. Whenyou inject a current into thissystem of parallel ground connec-tions the current will divide. Ina traditional Fall-of-Potentialtest you have no way of know-ing how much current is flowingbetween any particular electrodeand the C2 current stake. Selec-tive testing uses an integrated,high sensitivity clamp-on currenttransformer to measure the testcurrent in the electrode undertest. Figure 8 shows how the

current transformer fits into thetest circuit. The selective groundtester digitally filters the currentmeasurement to minimize theeffects of stray currents. Beingable to accurately measure thecurrent in the electrode undertest effectively isolates theelectrode and allows us to test itwithout disconnecting it from thesystem or from other electrodes.

Stakeless or Clamp-on Method

The stakeless or clamp-on method allows you tomeasure the impedanceof a series loop of groundelectrodes. The test issimple and it may beperformed on an elec-trode that is connected toa working electric service.To make the measure-ment the tester usesa special transformer

to generate a voltage on theground conductor at a uniquetest frequency. It uses a secondtransformer to distinguish the testfrequency and measure the result-ing current through the circuit,which is determined by the loopresistance.

This method is available in someFall-of-Potential testers (like theFluke 1625 or Fluke 1623) or ina single clamp on unit like theFluke GEO 30. Figure 10 showsthe connection of the source andmeasure clamps of the Saturn GEO X.

Figure 9 (see next page) showsthe equivalent test circuit for thestakeless method. When you testa building ground electrode usingthis method, you are actually test-

ing a loop including: Electrode under test Ground electrode conductor The main bonding jumper The service neutral Utility neutral-to-ground bond Utility ground conductors

(between poles) Utility pole grounds

Because this method uses theservice as part of the circuit, itmay be used only after the servicehas been completely wired, thatis, it cannot be used prior to hook-up to the utility. In this method

the clamp checks the continuity ofthe interconnections of all of thecomponents above. An abnor-mally high reading or an opencircuit indication on the instru-ment points to a poor connectionbetween two or more of the afore-mentioned critical components.The ground electrode of most facil-ities is in parallel with numerousutility ground electrodes. Theseelectrodes can be pole electrodes,

Figure 8: Connections for selective ground electrodemeasurement



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C2

H

P2

P1

C1

S

E

ext

START

DISPLAY

CHANGE

OFF 3RA

R42

SELECT

p o l e p o l e

pole

Ground

Resistance300k

ACResistance

300k

SATURNGEO


OFF

ON

OFF

ON

OFF

ON

OFF

ON

OFF

ON

OFF

ON

OFF

ON

OFF

ON

OFF

ON

C2

H

P2

P1

C1

S

E

ext

START

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OFF3

RAR

4

2

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pole pole

pole

Ground

Resistance300k

ACResistance300k

SATURNGEO


ElectrodeUnder test

Source

Measure

Current Paths

Figure 11: Test current paths in the stakeless method

pole butt plates or un-insulatedneutral conductors. The impedanceof the utility ground electrodesusually combines into a very lowimpedance.

Lets take an example. Say youhave 40 pole electrodes of roughly20 each, and these electrodesare connected together by a low-impedance ground wire from poleto pole. The equivalent resistanceof the 40 electrodes in parallel is:

Since half an ohm is smallcompared to the resistance weexpect for our electrode under test,we can assume that most of the

measured resistance is due to theearth resistance of the electrodeunder test.There are some potential pitfallsfor this method: If you measure in the wrong

place in the system, youmight get a hard-wired loopresistance, for example on aground ring or on a bondedlightning protection system.If you were intending to readearth resistance, measuringthe conductive loop would giveunexpectedly low resistance

readings. You may get low readings dueto the interaction of two veryclose, bonded electrodes, likeburied conduit, water pipes, etc.

The quality of the measurementdepends on the availability ofparallel paths. If a building issolely supplied by a generatoror transformer that has only asingle electrode, the assump-tion of multiple paths wontwork and the measurement

will indicate the earth resis-tance of both electrodes.Thismethod will not measure earthresistance.

A problem with the utilitygrounding system might inter-fere with readings.

In general, if you get readingsbelow 1 ohm, double-check tomake sure you are not measuringa hard-wired conductive loopinstead of the earth loop.

Two-pole MethodThe two-pole method uses anauxiliary electrode such as awater pipe. Figure 12 shows theconnections. The tester measuresthe combined earth resistance of

the electrode under test, the earthresistance of the auxiliary elec-trode, and the resistance of themeasurement leads. The assump-tion is that the earth resistanceof the auxiliary electrode is verylow, which would probably betrue for metal pipe without plasticsegments or insulated joints.The effect of the measurementleads may be removed by measur-ing with the leads shortedtogether and subtracting thisreading from the final measure-ment.

Although its convenient, bevery careful using the two-polemethod: A water pipe may have PVC

components, which couldgreatly increase its earthresistance. In this case thetwo-point method would givean excessively high reading.

The auxiliary electrode maynot be outside the influenceof the electrode under test. Inthis case the reading might belower than reality.

Because of the unknowns involvedin this technique, it is recom-mended only when the groundingsystem and auxiliary electrode arewell known.

Figure 12: Equivalent circuit for two-point

measurement

1

40 x 1/20R

eq= = 1/2



8/8

Summary of Ground Electrode Test Methods

Advantages Drawbacks

Fall-of-Potential Widely accepted When you see the characteristic

curve you know youve got a good

measurement.

You have to disconnect the ground The stakes may not be easy to

drive

There may not be space around

the ground electrode to drive the

stakes

Selective Method Dont have to disconnect electrode

Widely accepted

When you see the characteristic

curve you know youve got a good

measurement.

The stakes may not be easy to

drive

There may not be space around

the ground electrode to drive the

stakes

Stakeless Method Convenient

Saves time

No disconnections No stakes

Assumes a low-impedance paral-

lel path

Possible to get very low readingsby mistakenly measuring on a

hard-wired loop

Two-pole Method Convenient

Saves time

No disconnections

No stakes

Impossible to judge the integrity

of the auxiliary electrode.

Cant be sure you are outside the

area of influence

Table for the Tagg Slope Technique(for 2 decimal places)

Fluke CorporationPO Box 9090, Everett, WA USA 98206

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 40) 2 675 200 orFax (31 40) 2 675 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

2005 Fluke Corporation. All rights reserved.Printed in The Netherlands 06/20062550440 A-EN-N Rev APub_ID: 11095-eng

0.40 64.3

0.41 64.2

0.42 64.0

0.43 63.9

0.44 63.7

0.45 63.6

0.46 63.5

0.47 63.3

0.48 63.2

0.49 63.0

0.50 62.90.51 62.7

0.52 62.6

0.53 62.4

0.54 62.3

0.55 62.1

0.56 62.0

0.57 61.8

0.58 61.7

0.59 61.5

0.60 61.4

0.61 61.2

0.62 61.0

0.63 60.9

0.64 60.7

0.65 60.6

0.66 60.4

0.67 60.2

0.68 60.1

0.69 59.9

0.70 59.7

0.71 59.6

0.72 59.4

0.73 59.2

0.74 59.1

0.75 58.90.76 58.7

0.77 58.5

0.78 58.4

0.79 58.2

0.80 58.0

0.81 57.9

0.82 57.7

0.83 57.5

0.84 57.3

0.85 57.1

0.86 56.9

0.87 56.7

0.88 56.6

0.89 56.4

0.90 56.2

0.91 56.0

0.92 55.8

0.93 55.6

0.94 55.4

0.95 55.2

0.96 55.0

0.97 54.8

0.98 54.6

0.99 54.4

1.00 54.21.01 53.9

1.02 53.7

1.03 53.5

1.04 53.3

1.05 53.1

1.06 52.8

1.07 52.6

1.08 52.4

1.09 52.2

1.10 51.9

1.11 51.7

1.12 51.4

1.13 51.2

1.14 50.9

1.15 50.7

1.16 50.4

1.17 50.2

1.18 49.9

1.19 49.7

1.20 49.4

1.21 49.1

1.22 48.8

1.23 48.6

1.24 48.3

1.25 48.01.26 47.7

1.27 47.4

1.28 47.1

1.29 46.8

1.30 46.5

1.31 46.2

1.32 45.8

1.33 45.5

1.34 45.2

1.35 44.8

1.36 44.5

1.37 44.1

1.38 43.8

1.39 43.4

1.40 43.1

1.41 42.7

1.42 42.3

1.43 41.8

1.44 41.4

1.45 41.0

1.46 40.6

1.47 40.1

1.48 39.7

1.49 39.3

1.50 38.91.51 38.4

1.52 37.9

1.53 37.4

1.54 36.9

1.55 36.4

1.56 35.8

1.57 35.2

1.58 34.7

1.59 34.1

P2/C2

%

P2/C2

%

P2/C2

%

P2/C2

%

P2/C2

%

Fluke. Keeping your worldup and running.TM


11095-eng-01-a checking ground electrode impedance for commercial, industrial and residential...

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