Direct and Alternating Earth Currents

7/29/2019 Direct and Alternating Earth Currents

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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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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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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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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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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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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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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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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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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

Documents

Direct and Alternating Earth Currents