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The following figure illustrates that the ground-level magnetic fields of underground cables decrease much more quickly with distance than those of a corresponding overhead power line; however, they can actually be higher directly above the underground cable.The range of values is shown in which the maximum expected magnetic flux densities vary under maximum operating conditions. The illustration is based on data from the ECOLOG institute which conducted measurements on several power lines and underground cables and extrapolated the measurement data using simulations to the maximum operating conditions at request of a research project funded by the BfS . More information in final report.
Gas Insulated Lines (GIL) are used in high voltage systems. GIL systems consist of a central aluminum conductor that is fitted into an aluminum tube (see figure). The tube is filled with a non-inflammable insulating gas mixture. GIL systems are mainly installed in areas with restricted spaces such as tunnels or in cases of sensitive environmental conditions (like fire protection requirements). Several GIL systems can be laid more closely than underground cables due to lower interference and less heat dissipation in the soil. GIL systems produce very weak magnetic fields and no electric fields in their surroundings in contrast to overhead power lines and underground cables.