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index/glossary | EMFs At A Glance | EMF The Facts (pdf)
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    • Terminology – an introduction
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        • Voltage
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        • Phasing
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      • Statistics of power line fields
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You are here: Home / Sources / Overhead power lines / Factors affecting the field from a power line / Phasing / Phasing of National Grid system

Phasing of National Grid system

"Phasing" refers to the order of the phases (the separate conductor bundles) of the two circuits of a double-circuit power line.  See a full explanation with diagrams of how "transposed phasing" - one circuit reversed with respect to the other - produces lower magnetic fields to the side of the line (and sometimes underneath it as well), and how, when you consider the direction of current flow as well, you get to the concept of Optimum and Non Optimum phasing.

On this page we give the best available data on the extent of phasing in the UK.

The UK as a whole

As part of the UK policy of Optimum Phasing, the electricity industry agrees to report on the prevalence of optimum phasing at three-year intervals.  The first report was in 2013, and the summary results table is reproduced here:

 table showing extent of phasing in UK

This shows that, across all lines at 132 kV and above (and also including 110 kV in Northern ireland), 68% of double-circuit lines have optimum phasing.  The prevalence of optimum hasing is lower (50%) on 132 kV lines and higher (86%) on 275 and 400 kV lines.

More detail on National Grid system

The National Grid system in England and Wales was built from the 1960s onwards with a policy of transposed phasing where possible.   The following table gives detailed statistics for National Grid in England and Wales.

 

Phasing category Sub-category and explanation length of line
km %
Optimum Transposed and current flows known or assumed to be in same direction

6191

 
Untransposed and current flows generally in opposite directions

99

 
total

6290

87

Non Optimum Single power line

338

 
Three power lines meeting at "T" point - not geometrically possible to have complete transposed phasing

358

 
total

696

10

Not classifiable

Direction of currents varies so not possible to classify.

2

0.03

Not applicable - single circuit True single circuit - either built as such or second circuit not connected

138

 
Single National Grid circuit but other circuit is used by DNO

64

 
total

202

3

Overall total  

7190

100

Lengths are for "routes" - that is, the physical length of power line.  The length of "circuits" would be twice as big.

How are these expected to change?

The UK has adopted a policy of Optimal Phasing.  The details are spelled out in a Code of Practice.

According to this, electricity companies will consider changing lines which do not have optimum phasing when work is being undertaken on those lines that involves replacing the conductors.  Lines will be changed when this is "reasonable", which generally means when it does not require extra structures to be built.  So:

  • Non Optimum lines that result from three lines meeting at a "T" point will often not be optimum because of the geometrical constraint, changing this would involve building a special phase transposition tower, this is not considered "reasonable", so these lines are unlikely to change.
  • Non Optimum lines that don't involve a "T" point will be considered for changing.  But changing them requires the order of the downleads at the ends to be changed, and there may of may not be space to achieve this without building extra structures.
  • New lines will continue to be built with Transposed phasing where possible.

So we should expect to see the fraction of the system which has Optimum phasing increase over time, but maybe not by very much.

See also:

  • UK policy on EMFs
  • more on phasing

Latest news

  • New publication on cancer incidence from the UK electricity industry Cohort Study August 27, 2019
  • How has the reported risk for childhood leukaemia changed over time? February 11, 2019
  • Media stories about microshocks in children’s playground September 10, 2018
  • New studies on leukaemia and distance from power lines June 1, 2018
older news

Contact Us

To contact the electricity industry’s EMF Unit Public Information Line (UK only):
telephone 0845 7023270 or email [email protected].

See Contact us for more contact details including our privacy policy.

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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