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index/glossary | EMFs At A Glance | EMF The Facts (pdf)
  • 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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      • 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
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      • 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
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      • Bees and microshocks
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      • Survival from childhood leukaemia
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      • The “contact current” hypothesis
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      • IARC
    • Electric fields and ions
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      • ICNIRP 1998
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You are here: Home / Sources / Underground power cables / Underground cables with multiple conductors

Underground cables with multiple conductors

 As the capacity required from an underground cable increases, you can only go so far in meeting that by putting in bigger conductors.  After a certain point, you have to install a whole extra set of conductors instead.  How does this affect the magnetic field?  On this page we build up the field from successively more sets of conductors.

This is the geometry of conductors we will use to illustrate the principles.  In practice, where space permits, each group of three conductors would probably be spaced more widely apart, and we give examples of this at the end of the page.

diagram showing dimensions of cables with muliple groups

Consider first of all a single current of 500 A in one group of conductors only (the first group of three on the left-hand side).  This will be our reference case that we compare all the subsequent calculations to.

graph of field from single current 

Now suppose we add a second circuit, also carrying 500 A, this time in the last group of conductors on the right-hand side.  The field depends on whether the phasing is transposed (T) or untransposed (U) (the concept of phasing applies just as much to underground cables as to overhead lines):

 graph showing field from two currents

There is a peak of field over each separate group of conductors.  To the sides, the field is either larger or smaller than the single current depending on the phasing (untransposed - fields reinforce - resultant field is larger; transposed - fields partially cancel - resultant field is smaller.  But note that the phasing that cancels to the sides actually reinforces over the cables themselves and vice versa).

Now suppose we have 500 A in each of the four conductor groups (two groups per circuit).  We'll assume the two groups in each circuit are transposed with respect to each other.  The two circuits could then also be transposed with respect to each other as well.  But with four groups - four phasing orders - to play with, it turns out you can get an even lower resultant field by an alternative phasing, and we show both.

graph showing field from four currents

The two groups in each circuit are close enough together that they produce a single peak in field, but the field dips between the two circuits.

A note on currents

Because we've been adding groups of conductors each with 500 A in them, we now have a total of 2000 A compared to the 500 A we considered in the single group of conductors.  Although the only reason for using two groups per circuit would be to get increased capacity, for comparison purposes, we can look at the fields when we keep the total current the same, 500 A, in each case.

 graph showing field from four currents 500 A

Alternatively, we could look at the maximum possible field.  A typical rating for one of these cables might be 2000 A, and the field with 2000 A in each of the two groups of conductors in each of the two circuits looks like this:

 graph showing field from four currents

Cables with wider spacings

If space permits, underground cables might often be laid out with each conductor group more widely spaced from the next, and space for a haul road down the middle, like this:

diagram of cable dimensions wider spacing 

The magnetic field follows the same principles as before, but with a distinct peak from each group of conductors and dips between them:

 graph of field from four separated currents

And three groups of conductors per circuit?

If the required rating is high enough, it may be necessary to go to three groups of conductors per circuit, and here is an example of one possible such arrangement and the field it produces:

 diagram of cable with six groups

 graph showing field from six currents

Note that the phasing gets even more complicated with a total of six conductor groups.  This graph is for an arrangement that produces a high degree of cancellation to the sides, but not necessarily the optimum.

Notes

All calculations are for 1 m above ground level.

National Grid cables are often rated at 1600 A or 2000 A.  We chose 500 A for these comparisons because this is a typical load for National Grid circuits (and is what we use for many of our graphs for overhead lines as well).  The results can be scaled to any chosen current.

These calculations ignore zero-sequence currents, and assume the currents in each circuit are identical, which results in greater reduction in the field from transposed phasing than would usually be found in practice (this is discussed in detail for overhead lines)..

See also:

  • more on the fields from underground cables in general
  • different types of underground cable

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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