Underground cables: man and the environment

Underground cables also involve more than just technical challenges. They lead to new concerns and worries for landowners, in particular, and they are impossible to lay or operate without having an impact on nature.

When it comes to their respective impact on the ground and soil, overhead lines and underground cables differ quite fundamentally. While the construction of a new overhead line necessitates underground construction activities only at those locations where the pylons are to be erected, the construction of underground cable systems involves much more extensive excavation work. That’s why independent experts draw up comprehensive environmental studies for each of our underground cable projects. These studies serve as the basis for the statutory environmental impact assessment.

If permanent negative effects are unavoidable, the responsible authority stipulates on the basis of the expert opinion the compensatory measures that must be taken. Once construction of the underground cable system has been completed, the areas above the cables can in most cases be used for agricultural purposes, like they were before, with the exception of a few minor restrictions within the area of the protective strip.

Protective strip and land use

In order to ensure that the power link operates without any problems, no buildings, deep-rooting trees or shrubs are allowed to stand on a defined protective strip above and to either side of the underground cable trench. There are no restrictions on animals grazing and fields being tilled. Farmers can work their land as normal. The protective strip is marked by signposts along the route.

Warming of the soil and crop yields

Many famers ask us what impact the heat dissipated by the cables could have on their crop yields. Scientific studies have revealed that the heat from the underground cables has no negative impact.

Before our pilot project in Raesfeld began, we, in collaboration with the University of Freiburg, conducted in-depth investigations into potential effects of underground cables on the thermal and water balance of the soil – also with a view to later agricultural output. To simulate a cable as realistically as possible and by simple means, an initial field test carried out in 2005 saw us bury pipes in a sand bed and fill them continuously with hot water. In 2011, we then studied an experimental cable route set up in a substation near Düsseldorf. This involved trialling various bedding materials, including fluidised soil, as used in Raesfeld after we had assessed the results of all the trials. Over a period of four years, we then grew a range of crops above and next to the cable system: potatoes, maize, winter wheat, spring barley and winter oilseed rape.

What we found was that the cables lie so deep in the ground that heat in the soil above them dissipates quickly as it rises and the temperatures in the top layers of the soil are similar to those measured in the reference field located next to the cable system. The seasonal and weather-related fluctuations in temperature have a big influence on the soil layers. Any influence exerted by the underground cable is scarcely detectable.

Monitoring in Raesfeld (since 2016)

In the meantime, we have implemented the first underground cable section of our Diele–Niederrhein (EnLAG Project No. 5) power line construction project in Raesfeld/Münsterland and launched trial operation. In this pilot project, and working hand in hand with soil and agricultural experts, we have also implemented an intensive monitoring programme designed to verify previously obtained results under real-world operating conditions.

700 sensors continuously measure the level of warming and the water balance of the soil. Furthermore, the Chamber of Agriculture of North Rhine-Westphalia will be conducting and documenting trials to see how the soil develops during the course of recultivation.

Electric and magnetic fields

Wherever current flows, magnetic and electric fields arise: constant fields in the case of DC voltage (also known as static fields or DC fields) and pulsating, periodically changing fields in the case of AC voltage (alternating fields).

An electric field is caused by the voltage present between two points. The higher the voltage, the larger the electric field. Electric fields arise wherever electrical appliances are connected to the power supply. When an electrical appliance, such as a coffee machine, a TV or a PC, is connected to a power socket, an electric field is generated – even when the appliance is not switched on. In other words, all household appliances that are constantly plugged into the wall socket by means of their power cable are surrounded by an electric field – even when they’re not in use (coffee machine, microwave oven, electric bread cutter, radio, TV, PC, etc.). EHV cables do not “emit” an electric field: their wire screen keeps it entirely in the cable.

A magnetic field is caused by the flow of a current. Whenever you switch on your hairdryer, electric iron, TV, PC or light, a magnetic field is generated in addition to the electric field. This magnetic field surrounds the appliance and the conductor through which the current is flowing – for example, the power cable of the hairdryer, iron, TV, PC or lamp.

The 26th Ordinance on the Implementation of the German Federal Immission Control Act (26. BImSchV) stipulates ceilings for the strength of electric and magnetic fields generated by electrical installations. For the magnetic DC fields of DC voltage systems, 26. BImSchV stipulates compliance with a limit of 500 microteslas (µT) for places where people reside permanently. The ceiling for the magnetic field strength with 50-hertz AC systems is 100 µT. The fields generated by our underground cable systems will comply with these requirements.