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Electric and magnetic fields and health

index/glossary | EMFs At A Glance | EMF The Facts (pdf)
  • What are EMFs
    • Terminology – an introduction
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      • “EMF Commercial”
    • Adding fields together
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        • 400 kV
        • 400 kV – specific cases
        • 275 kV
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        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
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        • Height above ground
        • Conductor bundle
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      • Calculating and measuring fields from power lines
        • Geometries of power lines
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        • On-line calculator
      • Fields from power lines – more detail on the physics
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        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
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      • Fields greater than 0.2 or 0.4 µT
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You are here: Home / Sources / Overhead power lines / Calculating and measuring fields from power lines / Height above ground

Height above ground

Fields are assessed at 1 m above ground

The preferred height above ground for assessing fields, particularly electric fields, under power lines is 1 m.

This is partly because where standards specify a height they specify 1 m, e.g. the ANSI/IEEE standard.  Some other standards, for example the IEC standard, do not specify a height.  But 1 m has become a widespread practical standard.

However, there is also a scientific reason why 1 m is the right height when you are considering induced currents and compliance with exposure limits.

The reason why

The derivation of an external electric field from the basic restriction (the induced current density) is performed by numerical modelling.  This can be performed for any field required, but for convenience, is usually performed for uniform fields.  Specifically, the value of 9 kV/m which is the limit in the UK is derived from modelling by Dimbylow that used uniform fields.

Under power lines, the field is not uniform, increasing by typically 10% or so over the height of a person.  The question is therefore, at what height does this non-uniform field induce the same current as a uniform field of 9 kV/m?

The answer is, to a first approximation, at the half-way point up the body, making 1 m a reasonable approximation.  The reason is as follows.  The electric field meets the surface of the body all over the body.  At each point of the surface of the body, a current is induced.  The total current induced, built up from all over the surface, flows through the body and distributes itself amongst the various organs according to their conductivity.  So what matters is the total current induced.  To a first approximation, this is proportional to the space potential at the top of the head.  The space potential at the top of the head is the integral of the field over a vertical line from ground to that point, and for slowly varying fields, this is given by the equivalent uniform field at the midpoint.

This approach needs modifying in detail, in particular to allow for the central nervous system - the part of the body the limits apply to - not being in the path of total current flow.  But to a first approximation, the field at the centre of the body is a more reasonable approximation to the equivalent uniform field than the field at the top of the body.

For magnetic fields, currents are induced mainly in the head and trunk, so a similar argument might lead to a slightly higher height, perhaps the centre of the chest.  But we usually standardise on 1 m for magnetic fields too.

Counter arguments

There are two counter arguments, both are which are based on misunderstandings.

One counter argument is that the highest field (which, underneath a power line, will be at the top of the body) should be assessed.  This would be valid if the electric field were itself the limit; then it would clearly be correct to find the highest field and compare it to the limit.  But the electric field is not the limit.  The limit is the basic restriction, and the electric field is relevant only in relation to the size induced current it produces.  As discussed above, this depends more on the average field than the maximum field.

The other counter argument is that if the organ of most interest is the neck, the field should be assessed at the neck.  This is wrong because the current flowing through any given organ is not produced by the field at that point.  The current is produced by the field over the surface of the body, which then flows through the inside of the body and the organs in it.

Clearly, 1 m won't be exactly the right height for all circumstances.  Children are shorter than adults so the best height may be smaller - but that in turn means that calculating at 1 m for a child is usually erring on the safe side.  For a person riding on a horse the height might be greater.  But in terms of compliance with exposure limits in the UK, compliance with the public limits is required when exposure is for a "significant period of time".  A person is unlikely to spend a significant period of time on horseback directly underneath a power line.

See also:

  • How the field from a power line varies with height

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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