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You are here: Home / Sources / Overhead power lines / Factors affecting the field from a power line / Conductor bundle

Conductor bundle

How the field from a power line varies with the type of conductors

Different power lines have different conductors.  Some have bundles of two conductors ("twin") and some four ("quad") - see a picture of a power line showing twin bundles.  The conductors themselves can be different sizes too.

The magnetic field depends only on the current and not on the size of the conductor carrying it.  So the conductor bundle does not affect the magnetic field.  But it does affect the electric field.  This is because, for a given voltage, the smaller the conductors, the higher the electric field at the surface of the conductor.  And the higher the electric field close to the conductors, the more of the volts are dropped there, and the less volts are left to drop near the ground.  So smaller conductors lead to lower electric field close to the ground.  Simlarly, putting two or four conductors into a bundle increases the effective size of the conductor, so the electric field close to the conductors is lower, and the electric field close to the ground is higher.

This graph shows the effect on the field at 1 m above ground for some actual bundles used in the UK:

graph showing how field depends on conductor bundle

"Zebra" is a steel-cored aluminium conductor 28.6 mm diameter.  "Araucaria" is an aluminium alloy conductor 37.3 mm diameter.

Historically, most UK transmission lines had zebra conductors, spaced 305 mm, in either a horizontal twin bundle or a square quad bundle.  More recently, other options have been used.  Some quad bundles have been enlarged to 500 mm for aerodynamic reasons.  Some twin bundles have been uprated to the larger araucaria conductors at 500 mm spacing, and there are now some triple bundles.  (There are many other conductors and bundles as well, we show just the main ones to illustrate the principles).  Finally, we illustrate the field that would be produced with a single conductor, but this is never used in practice.

The graph clearly shows how smaller bundles and conductors reduce the electric field at ground level.  But there's a limit - these smaller bundles have higher electric fields close to the conductors, and if that gets too large, it causes corona - ionisation of the air - which produces audible noise.

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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