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MREC 44th Annual Rural Energy Conference February 16, 2006
Earth Currents Direct, Alternating & Transient
Michael F. StringfellowPowerCET Corporation
Santa Clara, CA, Scottsdale, AZ & Chicago, IL
Earth Currents
Earth currents are usually far too small toaffect animal or human health
Conclusion of earth current effects usuallyresults from bad science
Preconceived ideas
Defense of existing installations
Support of lawsuit
Bad measurements
Improper analysis of measurements
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Good Science Application of the Scientific Method
Experiment to test a hypothesis
Carefully run to eliminate observer errors andinstrumentation effects Double blind trials
Application of Sanity Check to data
Check with known physical laws New findings showing departure from existing knowledge
require more rigorous proof May meet resistance from establishment, even when
correct
Bad Science
Use of selected data to support preconceivedtheories Jumping to conclusions
Instrument errors and misuse Poor training
Poor understanding of fundamentals
Failure to Understand or apply physical laws
Understand the proper use of statistical methods
Use Sanity Check
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MREC 44th Annual Rural Energy Conference February 16, 2006
Earth Currents &
Stray Voltage Earth Currents invoked to explain
continuation of ascribed stray voltagesymptoms in animals
Three main categories
Alternating Currents
Power frequency and harmonics
Direct Currents
Transient Currents Radio frequency, usually short duration
Step and Touch VoltageLevels of Concern - 1
Power Frequency Alternating and DirectVoltages
Threshold of perception (dairy cows)
0.5 V RMS 1 V peak (conservative)
Threshold of current density
10 mV Based on theoretical comparison of current
magnitude with 10 kV/m electric-field exposure
No experimental data to support this low level
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MREC 44th Annual Rural Energy Conference February 16, 2006
Step and Touch Voltage
Levels of Concern - 2
Transientvoltages
Stronglydependent oninverse offrequency andduration
Animal Perception of Electrical Transients
Experimental Data
0.001
0.01
0.1
1
10
100
0.001 0.01 0.1 1 10 100 1000
Duration in Milliseconds
Peak to Peak
Current in Amperes
Low ThresholdHigh Threshold
AneshansleyReinemannGustafsonCurrence
Alternating EarthCurrents
Source
Alternating currentsfrom electric utility
Mostly currents fromdistribution lineshaving multi-
grounded neutral
Possible Paths
Through the earth
Directly via theanimal
Via groundingelectrodes on farm
Via concrete floors,especiallyequipotential planes
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MREC 44th Annual Rural Energy Conference February 16, 2006
AC Earth Currents &
Step Potentials Measured by earth (step) potential
Calculated with knowledge of soil resistivity
Typical magnitudes 10 A/m2 on farms Compares to < 1 A/m2 at remote locations
Step potentials a few mV per meter 100 times too small to exceed perception threshold for
animals
Often exceeds 10 mV but rarely >50 mV
Source may be identified by waveshape
Wrongly inferred from currents in rings of life Ampere magnitude induced currents
Induction versusConduction
Currents in a 1- mile circular ground conductor(Ring of Life) identified as earth currents fromutility system
Waveshapes however are very different
Current identical at all points on ring Not what one would expect from earth currents
Earth current density too low to account for ring
current magnitude observed Induction by magnetic field however is of correct
ampere-level magnitude Also explains waveshape difference
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MREC 44th Annual Rural Energy Conference February 16, 2006
Comparison of
WaveshapesDistribution Line Neutral/Ground Current
Ring of Life Current
Direct Earth Currents
Source
Naturally occurringgeomagneticallyinduced currents
Cathodic protectionsystems Pipelines
AC to DC powerconverters Milking equipment
controllers
Galvanic action betweendissimilar metals
Possible Paths
Through the earth
Directly via the animal
Via grounding electrodes
Via concrete floors &especially equipotentialplanes
Via farm metalwork
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Geomagnetic Currents Induced by magnetic fields of solar
originVery slow alternating quasi direct
Significant only during geomagnetic storms ~100 times quiescent currents
Induce potentials of up to 50 v/km at highlatitudes Step potentials up to 50 mV/m Surface currents up to 100A/m2
Cathodic ProtectionSystems
Maintains pipeline at negative direct voltage withrespect to earth
Uses either passive electrodes or rectifiers togenerate voltage Maintained at typically 1 to 10 V
Typical magnitudes 10 A/m2 near pipeline
Step potentials a few mV per meter 100 times too small to exceed perception threshold for
animals
Sometimes exceeds 10 mV but rarely >50 mV
Measurable at distances up to 1,000 feet from pipeline
Easily identified from harmonics of unsmoothed rectifier
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MREC 44th Annual Rural Energy Conference February 16, 2006
AC to DC Power
Converters Used for electrical control systems
Milking equipment
Mechanical gate controls
Typically single-phase rectifier
Easily identified from harmonics of unsmoothed rectifier
Sometimes use multigrounded output that sharesreturn current with steelwork
Few data on current magnitudes or step potentials
Electrochemical(Galvanic) DC Voltages
Always present betweendissimilar metals
0.4 to 3 V
Often present (but smaller)between similar metals indifferent electrolyte
0.1 V
Step & touch potentials 10to 100 times higher thanother sources
+0.34Copper
-0.13Lead
-0.44Iron/steel
-0.76Zinc
-1.66Aluminum
-2.37Magnesium
VoltageMetal
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Direct Current
Measurement
DCMilliammeter
CuPlate
CuCoil
Concrete Floor
Copper/Steel BatteryHypothesis
Source
Voltage
WaterResistance
0.7 V DC
Copper/steelbattery
1.6 k
400Ohms Law0.35 mA
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Sanity Check Measured source voltage: 0.7 V DC
Measured current: 0.1 to 0.4 mA DC
Cow resistance: ~ 400
Water resistance: ~ 1 to 3 k
Calculated current: 0.2 to 0.5 mA
Conclusion: Calculated current supports hypothesis of local
copper/steel battery driving current through circuitand not earth current source
Transient EarthCurrents
Usually inferred from erroneousmeasurements
Dipole antenna used for step or contactpotential measurement
Induction in connection leads
Single-ended measurements onlyreliable method
Current transformer with coaxial cable
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Transient Earth
CurrentsSource
Electromagneticradiation
Lightning
Fencers
Arcs & sparks
Possible Paths
Space or air
Earth is very lossy
Misuse of High-Frequency Instrument
Oscilloscope with high-frequency digitizerconnected with long wire leads (~100-foot)
Recorded simultaneous transient signals onall channels
Interpreted as earth currents of utility origin
Usually radio signals from distant lightning(< 100 miles away)
Also man-made electromagnetic signals
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MREC 44th Annual Rural Energy Conference February 16, 2006
Transient Measurement
Problem Long wires act like antennas
Pick up radiated (radio) signals from longdistances
Electromagnetic signals from nearby sources
Affects all channels wired this way
Proper measurement requires adequateshielding or balancing of connection leads Always will be some antenna effect on two-point
voltage measurements Even those ~5 feet apart
Measurement ofTransient Source - 1
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Measurement of
Transient Source - 2
Measurement ofTransient Source - 3
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Typical On-Farm
Transient Source
CH110 mA/div ac
1 us/div
CH250 mA/div ac
1 us/div
CH350 mA/div ac
1 us/div
CH450 mA/div ac
1 us/div
Cow contact
Parlor panel ground
Secondary ground (meter)
Primary ground (transformer)
Lanphear Cow Contact Date Stamp: 4/7/99 Time Stamp: 5:19:02 PM
Step Potentials onConcrete
Concrete has lower or equal resistivity compared tosurrounding soil, typically 30 90 ohm.m
Concrete is hygroscopic
Reinforcing steel or other conductors reduce thismuch further
Earth current through concrete depends on relative
resistivity compared to soil Higher current will flow through lower resistance of concrete
But not more than proportional reduction in resistivity
Current limited by source resistance of earth
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MREC 44th Annual Rural Energy Conference February 16, 2006
Rebar in Concrete Assume a grid of 3/8 rebar
Resistance of each four-foot length is about 0.004ohms
Voltage drop of 4 mV per amp of current
Very low step potentials even under power system faultconditions
Requires implausible levels of current flow to beproblematic to animals
Levels that could never arise from small earthcurrents from remote sources
Equipotential Plane
Resistivity significantly lower than soil orconcrete
Rebar typically 0.004 ohms per meter length
Step potential from earth currents will alwaysbe lower on equipotential plane than on earth
or concrete alone Equipotential planes always reduce step
potentials
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MREC 44th Annual Rural Energy Conference February 16, 2006
Summary Step voltages from earth currents are well below 0.5
to 1 volt Most widely accepted levels of concern based on scientific
research
May exceed 10 mV Level at which conduction current density in animals is
calculated to equal that in electric field exposure studies But never shown to be problematic in any scientific study
Equal to natural exposure of DC potentials
Equipotential planes will reduce all step potentialssignificantly Whatever the current source
Conclusions
Many natural and man-made sources of earthcurrents
Both AC and DC earth currents produce steppotentials in the 10 millivolt range
Transient earth currents mostly erroneousresult of bad measurement technique
Equipotential planes reduce step potentialsfrom all earth currents
Unlikely to be problematic for animals