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You are here: Home / Sources / Overhead power lines / Factors affecting the field from a power line / Phasing / How phasing and clearance interact

How phasing and clearance interact

How the effect of the phasing of a power line varies with clearance of the conductors above ground

In our main page on the phasing of a double-circuit power line, we explain how an arrangement called transposed phasing produces lower fields to the sides of the power line than the alternative untransposed phasing.  This is because the magnetic field produced by each circuit is, roughly, a dipole, and with transposed phasing the two dipoles are in opposite directions, leading to greater cancellation of the fields.  (See some drawings of the actual field lines)

This effect varies with the ground clearance of the conductors (see also how the ground clearance affects the field directly).  Well to the sides of the power line, it is always true that transposed phasing gives lower fields, whatever the clearance.  But between the two circuits, a different effect comes into play.

Each circuit on its own produces a dipole field, shown in simplified form here:

diagram of field lines from separate circuits

As you pass from one side to the other of the circuit in question, the field lines change direction - the vertical component of the field reverses direction.

Now consider the fields from both circuits simultaneously:

diagram showing field lines from both circuits together

As you go from outside one circuit to inside that circuit, the vertical component of the field from that circuit has reversed direction - but the field from the other circuit has not.  So if the two fields were cancelling each other outside the circuits - the situation with transposed phasing - now, inside the circuits, they are not cancelling so well - the horizontal components may still partially cancel, but the vertical components are adding.

This is more significant at lower clearances because the vertical component is more significant.  So at the lowest clearances, transposed phasing actually gives higher fields than untransposed at ground level between the two circuits.

Similarly, with untransposed phasing, to the sides of the line the two  dipoles add, but between the conductors, they partially cancel.  This leads to a "dip" in the field on the centreline, half way between the two conductors.

(Actually, whatever the phasing, if the clearance is low enough this dip will appear.  That is because if you are close enough to the actual conductors of one circuit, the field you experience basically comes just from that circuit.)

These effects are illustrated in the following graph.

  • At a relatively high clearance, 20 m, the transposed phasing is lower than the untransposed everywhere
  • At a relatively low clearance, 8 m, the untransposed phasing dips between the two circuits and the transposed phasing gives the higher field in this region
  • At intermediate clearances, 12 m, typical of actual power lines, the two are about the same between the circuits
  • To the sides of the line, transposed phasing is always lower.

 graph showing how phasing and clearance interact

Note: these graphs are calculated for L12 lines with 500 A in each circuit.  In real power lines, the currents are not always equal both between circuits and within a circuit, and these reduce the effectiveness of the transposed phasing.  See more detail on exactly how fast fields fall with distance under these different conditions.

See also:

  • the main page on phasing

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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