1 earth testing 2010
TRANSCRIPT
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Earth Testing
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Earth Testing Pioneer
Dr George Tagg pioneered earth testing at Megger
Designing, manufacturing and selling for well over
50 years
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Earthing System ResistancePurpose : Measure resistance of earthing system to Earth - ascertain thatprospective fault current can be conducted safely to Earth and thus limittouch voltage.
Methods : 3-pole: Fall of Potential 2-pole: Direct measurement
3-pole:Slope Method 3-pole with clamp Stakeless measurements: Earth Clamp.
Note:
The earth electrode under test must be disconnected from the system itis earthing (apart from clamp & stakeless measurements)
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Step and touch voltages
V V
Step Touch
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Resistance and GPR from earth electrode
0.0
0.4
0.8
1.2
1.6
2.0
R e s i s t a n c e ( O h m s )
0
200
400
600
800
1000
V o l t a g e ( V )
Resistance GPR
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Earth sphere of influence
Resistance of an earth electrode 1 Resistance of the electrode and the
connections to it (low)
2 Contact resistance of the surroundingsoil to the electrode (should be low)
3 Resistance of the surrounding body ofearth around the electrode these can be
thought of as shells and create a sphere ofinfluence (varies)
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2-pole: Direct measurementMeasure coupling between two earth points; measureresistance of earth electrode to Earth .
C1 (E)C2 (H)P1 (ES)P2 (S)Imeas
Emeas
Earth electrodeunder test
Second earth electrode or other
low resistance, conductiveconnection to Earth.
Measures resistance ofthe two Earth electrodesin series.
R = E meas /Imeas
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2-pole Method Advantages
Quick and easy
Better than a continuity measurement with amultimeter because of noise rejection
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2-pole Method Disadvantages
Result is a loop resistance, not just the electrode wewant to measure but the return path as well
Only OK if we have a good known low value returnpath and a high resistance electrode under testInterfering spheres of influence can give incorrect
results
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3-pole: Fall of Potential (full method)Classic method for measuring resistance of a singleearthing electrode, or of a system of electrodes to
Earth.
A
B
C1 (E)
C2 (H)P1 (ES) P2 (S)
Imeas
Emeas
Earth
electrodeunder test
Auxiliary test
electrodes
R = E meas /Imeas
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Fall of Potential - Full MethodVary location of P2 (Potential) spike by regular steps
along a straight line between the electrode under test and
the C2 (Current) electrode.
Take resistance R = E meas /Imeas at each step and plot agraph of R versus distance.
Resistance of system taken where slope is flat.
NB The C spike must be outside the sphere of influence
to achieve a viable reading
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Fall of Potential Method Advantages
Can always make an accurate measurement of earthresistance
Measures the overall system resistance at this point,to calculate earth fault currentTest leads & contact resistance are not included in
the measurement (may want to use 4 terminals)Can check by testing in a different direction ordistance
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Fall of Potential Method Disadvantages
Extremely time consuming and labour intensive.- Temporary probes must be placed.
- Cables must be run to make connections.Space constraints can make it hard to place remoteprobes.Must disconnect individual ground electrodes tomeasure them.
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Fall of potential - Current Probe Sphere of Influence
AuxiliaryCurrent
Probe (C)
AuxiliaryPotentialProbe (P)
GroundElectrode
UnderTest (X)
P probe must be outside of both
spheres of influence for correctmeasurement
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Positions
GroundElectrodeUnder
Test (X)
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Position
GroundElectrodeUnder
Test (X)
True systemresistancemeasuredhere
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Fall of potential test and result
CurrentProbe
Position
Distance of Potential Probe from X (d p)Ground
ElectrodePosition
X C
R e s
i s t a n c e
i n O h m s
CurrentProbe (C)
PotentialProbe (P)Position
GroundElectrodeUnder
Test (X)
True systemresistancemeasuredhere
Usually approx 62%of X to C distance
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For deeper spikes -Readings are taken further apart
Guidelines for setting test probes are based on the depth of the electrode under test.
2m depth p spike 15.5m c spike 25m
3m depth p spike 18.5m c spike 30m
6m depth p spike 25m c spike 40m
10m depth p spike 31m c spike 50m
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3-pole: Fall of Potential (short method)
Site P2 (Potential) spike at 62% of B and take resistancemeasurement R a[R = E meas /Imeas ].
Locate P2 0.1B around the 62% point and take additionalresistance readings, R b and R c .
If the three readings are within an agreed accuracy limit, thesystem resistance is the average.
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The Problem of Limited Distance/Space
Distance of Potential Probe from X (d p)
R e s i s t a n c e
i n O h m s
CurrentProbe (C)PotentialProbe (P)
GroundElectrodeUnder
Test (X)
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The Slope Method
Based on the theory behind the Fall of Potential method:- for complex grounding systems and/or- situations where lead lengths prohibitive
This method allows measurement of the earth systemresistance without finding the flat portion of the curve,reducing the test distances
Use three measurements in calculation; can take more
Advantage: Provides an approach for dealing with complexsystems
Disadvantage: Less accurate; requires more maths
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3-pole: Slope Method
Tables of values for the co-efficient of slopeagainst actual P spike distance is published in the
instrument user guide
Take calculated value of and look up idealdistance of the voltage spike (P2) for measuring theelectrode resistance
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3-pole: Slope Method
Vary location of P2 (Potential) spike by regularsteps along a straight line between the electrode
under test and the C2 (Current) electrodeMeasure resistance at each step and plot a graph
of R versus distance.Measure resistance at 0.2B, 0.4B and 0.6B: R1,
R2 and R3.Slope coefficient, =(R3-R2)/(R2-R1) relatesdistance B and ideal distance of the voltage spike
(P2) for measuring the resistance.
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3-pole: Slope Method
Measure R1 at 20% distance to C2
C2 (H)
Earthelectrode
under test
B
C1 (E)
P1 (ES)
Imeas
Emeas
0.2B
R1
C2 (H)
=(R 3-R2)/(R2-R1)
= (R 3-R2) / (R 2 9.3 )
R1= 9.3 ohm
R = E meas /Imeas
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3-pole: Slope Method
Measure R2 at 40% distance to C2
C2 (H)
Earthelectrode
under test
B
C1 (E)
P1 (ES)
Imeas
Emeas
0.4BR2
C2 (H)
=(R 3-R2)/(R2-R1)
= (R 3 16 ) / (16 9.3 )
R1= 9.3 ohmR
2= 16 ohm
R = E meas /Imeas
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3-pole: Slope Method
Measure R3 at 60% distance to C2
C2 (H)
Earthelectrode
under test
B
C1 (E)
P1 (ES)
Imeas
Emeas
R30.6B
C2 (H)
=(R3-R2)/(R2-R1)
= (19.2 16 ) / (16 9.3 )
R1= 9.3 ohmR
2= 16 ohm
R3= 19.2 ohm
R = E meas /Imeas
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3-pole: Slope Method
Calculate value of
C2 (H)
Earthelectrode
under testB
C1 (E)
P1 (ES)
P2 (S)
Imeas
Emeas
0.4B
0.6B
0.2B
Auxiliary testelectrodesR3R2R1
C2 (H)C2 (H)
=(R 3-R2)/(R2-R1)
= (19.2 16) / (16 9.3)
=0.478
R = E meas /Imeas
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3-pole: Slope Method
=0.478
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The Slope Method
R1 = 0.2 x D C; R 2 = 0.4 x D C; R 3 = 0.6 x D CCalculate the value for using the following equation:
R3 - R2R2 - R1
The value for corresponds to a value for P T /C.- PT: potential probe position where True Resistance found.- % of distance P T is of D C.- Table of values in Getting Down to EarthMeasure the resistance at P T.Repeat the process for a larger value of C.
Factors Affecting Resistance Variation
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Type of ground composition beneath the site
Moisture content of soil (tides, rainfall, season)
Quantity and type of electrolytes added to the soil
Adjacent conductors if any
Temperature of the earth
Electrode depth
Electrode diameter
Electrode(s) spacing
See 62% rule
Twice diameter = -20%
- 40% (x2 indepth)
+ 20% for - 10 o(ICE is very high)
?
< 10 x lower
60%
Large 10/1
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Soil Resistivity/ Wenner Method
Purpose: Survey a site for the lowest resistanceconnections for Earth.
A A A
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Purpose of this test: Find lowest possible resistance in a location Obtain the values needed to design the earth system
Factors affecting soil resistivity Soil composition Moisture in the ground Temperature
Consider Resistivity will vary through the year Moisture more constant at water table
Stable temperature below the frost line
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Soil Resistivity typical values (Ohm-cm)
Surface soils, loam etc. 100 5000Clay 200 10000
Sand & gravel 5000 100000Surface limestone 10000 1000000Shales 500 10000
Sandstone 2000 200000Granites, basalts etc 100000Slates etc 1000 10000
What Can Be Done to Improve Earth Resistance?
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What Can Be Done to Improve Earth Resistance?
Lengthen earth electrode
Increase the rods diameter
Use multiple electrodes bonded together
Treat the surrounding soil chemically
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Application of ART
Potential Probe (P) CurrentProbe (C)
GroundElectrodes
Building earthconnection/s
I Total
I System
Ie1
Ie 2Ie 3Ie test Test
Ie Test > I Total20
XConnection
l k l h d l
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Clamp-On / Stakeless Methodology
GroundElectrode
UnderTest
Building earthconnection/s
ICLAMP
VCLAMP
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Stakeless Measurements for earthsystem resistance (DET10/20C)
A method that does not needthe earth electrode to bedisconnected
R =Emeas /Imeas
Rx
R1
R2
Rn
EmeasImeas
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Clamp-On Method
Includes the bonding and overall connectionresistance (not available with Fall of Potential).
Can measure the leakage current flowing throughthe system.Effective only in situations with multiple grounds inparallel (pole grounds).Cannot be used on isolated grounds (no returnpath)Cannot be used if an alternative lower resistance
return exists not involving the soil
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Clamp-On/Stakeless Methodology
In a multiple ground system the circuit can beconsidered a loop comprising:- The individual ground electrode.
- A return path via all other electrodes.- The mass of earth.
The single electrode will have a higher resistancethan the remainder of grounds connected in parallel.
Inject a voltage and measure the resultant currentproduced in a single turn ground loop.
Clamp On/Stakeless Methodology
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R 6R 5R 4R 3R 2R 1
VI
Clamp-OnGroundTester
Clamp-On/Stakeless Methodology
R loop = V loop / I loop
Cl O /S k l M h d l
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Clamp-On/Stakeless Methodology
R loop = R 6 + (1/(1/R 1 + 1/R 2 + 1/R 3 + 1/R 4 + 1/R 5))For 6 similar electrodes with a resistance of 10 :R loop = 10 + 2 = 12
If one electrode has a resistance of 100 :R loop = 100 + 2 = 102 (measuring the bad electrode)R loop = 10 + 2.4 = 12.4 (on the other electrodes)
For 60 similar electrodes with a resistance of 10 :R loop = 10 + 0.17 = 10.17
If one electrode has a resistance of 100 :R
loop= 100 + 0.17 = 100.2 (measuring the bad electrode)
R loop = 10 + 0.17 = 10.17 (on the other electrodes)
Cl O M h d Ad
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Clamp-On Method - Advantages
Test is quick and easy:- No disconnecting the ground rod from the system.- No probes need to be driven/cables connected.
Includes the bonding and overall connection resistance(not available with Fall of Potential).Can measure the leakage current flowing through the
system..
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Clamp-On Method - Disadvantages
Effective only in situations with multiple grounds in parallel (polegrounds).Cannot be used on isolated grounds (no return path):- Not applicable for installation checks/commissioning new sitesCannot be used if an alternate lower resistance return exists notinvolving the soil:- Cellular towers- Substations
Subject to influence if another part of the ground system is inresistance area:- Result will be lower than true resistance.Test is carried out at a high frequency (enables the transformers
to be small):
- Less representative of a fault at power frequency but easier to filter outnoise
Cl O M h d Di d
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Requires a good return path:- Poor return path may give high readings.
Connection must be on the correct part of the loop for
the electrode under test:- Requires thorough understanding of the system.- Wrong connection can give a faulty result.
Susceptible to noise from nearby substations and
transformers (no reading).No basis for the test in standards no objective
reference for the test resultsLess effective for very low grounds:
- Extraneous elements in reading become comparatively large.
Clamp-On Method - Disadvantages
Cl O M h d R d i
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Should not use as the only test instrument unless theearth system architecture is known and understood
Is an important part of the ground testing tool kit.- Should also have a Fall of Potential tester.
Use the Clamp-On to identify potential problems
quickly; confirm with a Fall of Potential tester:- Saves time.- Ensures accuracy- No disturbance of connections or re-connection risks
Clamp-On Method - Recommendations
A li i l G d
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Grounding
Conductor
GroundRod
ButtPlate
UtilityPole
Applications Pole Grounds
A li i S i E /M
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Applications Service Entrance/Meter
GroundRods
ServiceBox
ServiceMeter
Pole-MountedTransformer
A li ti S i E t /M t
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Applications Service Entrance/Meter
WaterPipe
ServiceBox
ServiceMeter
Pole-MountedTransformer
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Applications Pad Mount Transformer
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Applications Pad Mount Transformer
EnclosureOpen
Door
Open
Door
Underground Service
ConcentricNeutral
Buss
GroundRod(s)
Service
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Applications Transmission Towers
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Applications Transmission Towers
Ground
Rod
Applications Telephone Cabinets
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pp p
GroundRod
TelephoneCable orConduit
GroundWire
AC Panel Board
Power MeterL-N/L-L Vac
Power Service
BondingIntegrity
GroundResistance
Misuses Cellular Sites
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Clamp-OnGroundTester
TestCurrent
Misuses Cellular Sites
Misuses Substations
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SubstationGroundedPerimeter
Fence
E
Clamp-OnGroundTester
SubstationGroundSystem
TestCurrent
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ERR
OR
OFFEN
STA
CK
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