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Magnetic field calculations
On underground cable systems
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Local authorities may set limits to the intensity of magnetic fields at the soil surface. While reduction of the emitted magnetic field strength is possible, it will result in reduced circuit ampacity. For this reason, a balanced approach must be followed during the design stage of the project. In some cases additional measures may be necessary to further reduce the magnetic field strength to the specified level.
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Installation configuration and magnetic fields at the soil surface
The installation configuration of buried cable circuits determines the intensity of the magnetic fields at the soil surface. The cable depths, the separation between conductors, flat or trefoil configurations are important factors. The closer the conductors are to each other, the larger the reduction of the magnetic field. However, the current rating will decrease because of the increased thermal influence between the conductors and the soil surface. If additional measures are required to further reduce the magnetic field strength, the effects these measures have on the ampacity of the cable circuit must also be determined.
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The maximum magnetic field above a cross-bonded circuit
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Calculations of magnetic fields and of ampacity
All of the factors that influence the magnetic field strength
also influence the circuit ampacity. The effects of the various
factors can be calculated and computer models have been developed
which incorporate these calculations. The models can also calculate
the effects of additional measures that can be implemented to
further reduce the magnetic field strength. The ampacity
calculations used in the model can be applied to static, cyclic or
dynamic loading situations.
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Steady state maximum current carrying capacity of the cross-bonded
circuit under consideration.
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Results and experiences
In one example, a 20 times reduction in magnetic field strength at the soil surface was obtained by reducing the conductor spacing and increasing the burying depth to 2m. However, the circuit ampacity in this case decreased by a factor of 1.4. Crossbonding will increase the ampacity and reliability in emergency situations, but will also increase the magnetic field strength. The lowest magnetic fields in cross-bonding situations are found in circuits having a tight trefoil configuration that are buried deep.
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