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  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
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    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
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      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
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    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
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      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
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    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
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      • ICNIRP 1998
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      • Power lines and property – UK
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    • Static fields – the expert view
You are here: Home / Sources / Overhead power lines / Factors affecting the field from a power line / Ground resistivity

Ground resistivity

How does the field from a power line vary with the resistivity of the ground?

The nature of the ground that a power line passes over - how wet it is, what the mineral content is, how conducting it is - makes very little difference to the fields the power line produces.  Where it makes a difference at all, it is only significant a long way away from the power line, where the field is already small.

When we calculate fields from overhead power lines, we normally assume that the ground is a good conductor for electric fields and not a conductor at all for magnetic fields.  That may sound perverse but it gives the right answers!  We explain this further here and what effect it has if we change those assumptions.

Magnetic fields - the summary

As soil gets wetter, or has a higher content of certain minerals, it becomes more conducting.  In theory, this makes a difference to the magnetic field from the power line.  But in practice, the difference is too small to be significant (or even detectable) until you are a long way away from the power line - hundreds of metres - and if you are that far away, the field is tiny anyway.  Even then, it is only exceptionally conducting soils (perhaps because they are exceptionally wet) that will make any discernable difference.

Close to the power line, where the fields are high, the electrical properties of the ground make no practical difference at all.

Magnetic fields - more detail

The ground - any ground - is always slightly conducting because of the moisture content.  The greater the moisture content, and the greater the concentration of certain minerals, the more conducting it becomes.

That means that the magnetic fields from an overhead power line can induce currents in the ground.

The currents that are induced are actually dispersed throughout the ground.  But we can model their effect as if they are concentrated just at one depth.  Those currents are called "image currents" and we can work out their depth if we know the resistivity of the ground.  The less conducting - the more resistive - the ground, the deeper the image currents are.

For typical ground resistivities, the depth of the image currents is many hundreds of metres.  That means that, when you are under the line or close to it, these image currents are too deep in the ground to have any detectable effect on the magnetic field.  Only when you get several hundred metres away from the power line yourself do these image currents start making any noticeable difference. (And when you're that far away from the power line, other factors come into play anyway, such as any unbalance between the currents in the conductors.)

You can see this from this graph.  It shows the field calculated, including the image currents, for four different ground resistivities - infinite (i.e. the ground not conducting at all), 100, 10 and 1 Ω m.

graph showing effect of ground resistivity on fields

Most soils are in the range 10-1000 Ω m or even higher.  You can see for the graph that even for the less resistive - more conductive - end of this range, this makes only a small difference to the field out to 300 m or so, when the field is already below 10 nT or 0.01 μT.  For the more resistive - less conductive - soils, you can't really see a difference even at 500 m.  We didn't even calculate for 1000 Ω m, even though that's still a typical soil value, because the line would have been indistinguishable from the infinite resitivity line.

So that is why it is perfectly valid to assume, as we usually do, that the ground is not conducting at all for magnetic-field calculations.

Note on the calculations: this graph comes from our detailed test of compared calculated and measured fields, so is specific to one set of conditions, but the principles apply to all power lines.  There are different ways of modelling the image currents but they all give broadly similar answers.

Underground cables

We've talkled here about overhead power lines, where they produce image currents below the ground.  With underground cables, which are already below the ground, the physics is a bit different, but the result is the same - the effect of soil conductivity is, for practical purposes, negligible.

 

Magnetic rocks and soils

What we've been talking about so far is the effect of the ground resistivity (or, viewed the other way round, conductivity) on the magnetic field.  That's principally determined by the moisture content.

But rocks and soils can also contain magnetic material - iron, or magnetite, etc.  In principle this can affect the magnetic field too.  It is mainly these magnetic rocks and soils that produce what are called "magnetic anomalies" - local variations of the earth's magnetic field from place to place. Magnetic anomalies are measured (see e.g. the British Geological Survey) and they rarely amount to more than 1% - usually even less.  So it's a safe bet that the effect of magnetic rocks or soils on the magnetic field from a power line is likewise only a fraction of a percent.

A bit more detail: the larger anomalies happen where the rocks have a permanent magnetisation, or remnant magnetisation, enduring from when they were laid down.  There are rare places where this is big enough to interfere with a compass - the Cuillin on the Isle of Skye are notorious for this, for example.  But permanent magnetisation is a DC phenomenon, and has no relevance for the magnetic fields from AC power lines.  What matters for AC power lines and the AC magnetic fields they produce is the "magnetic permeability" of the rocks - the property by which magnetic materials passively distort the field lines passing nearer them.  That is only a fraction of the magnetic anomaly, which is already usually less than a percent of the field - hence the confidence that magnetic rocks have a negligible effect on fields from power lines or cables.

Electric fields

In summary: as far as electric fields are concerned, all soils or other ground materials are already conducting, and exactly how conducting they are makes no practical difference.

In more detail:

For electric fields, it is the other way round to magnetic fields.  The voltages on the power line conductors induce a set of image charges (rather than the image currents you get for magnetic fields) in the ground, but these are essentially a mirror image - they are at the same depth below ground as the real conductors are above ground.  When we calculate electric fields, we always include these image charges.

There is no soil or rock that has a large enough resistivity for this model to break down, and so for practical purposes, the ground resistivity makes no difference - we can assume the ground is a perfect conductor and calculate fields on that basis.

Even more detail: how good is this approximation?

There's a quantity called the "relaxation time" - it's a measure of how quickly the charges induced in the ground can adjust to the changing field, and it's simply the resistivity times the permittivity.  For a pretty wet soil, that comes out at a nanosecond or so, and for the driest, least conducting soils or rocks imaginable, it's still only a microsecond or so.  That has to be compared to the period of oscillation of a 50 Hz AC electric field - 20 milliseconds.  The relaxation time is so much less than the period of oscillation that the approximation is valid for the whole range of soil and rock conductivities found in practice.

This is, incidentally, a way of explaining why electric field line meet the ground at right angles:

drawing of electric field lines

You can imagine the mirror image of this continuing below the ground with the ground as a symmetry line - it can only be symmetrical if the field lines cross the ground at right angles.

 

Resistivity and conductivity

We can describe the electrical properties of the ground (or anything else for that matter) in two ways: how good a resistor it is (resistivity, usually denoted by the Greek letter ρ) or how good a conductor it is (conductivity, usually denoted by the Greek letter σ). ρ=1/σ and σ=1/ρ.

Resistivity is measured in "ohm metres" (Ω m) and conductivity in siemens per metre (S/m).

 

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Navigation
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view