Earthing Systems Connected via Metallic Screens of the 20 Kv Underground Cables in Non-urban Areas

Secondary substations’ earthing systems are connected to each other via metallic screens of the 20 kV underground cables also in suburban and rural areas nowadays. Topology is different from the earthing systems in city centers, where earthings are connected via multiple mesh connections forming a solid ground level. The standards EN 61936-1, EN 50522 and the Finnish SFS 6001 (High voltage installations) do not clearly consider the case of connected earthings. In 2015 studies were launched to investigate this issue. According to results of the studies, the connected earthings should be evaluated as a whole, and not separately as in the overhead networks. There is a need for renewing earthing network design principles because at the moment, the connections between the secondary substations are not systematically taken into account in the non-urban areas’ earthing design. Results show that the resulting impedance was typically 50-80% lower than the secondary substations’ individual earthing resistances. It means that there is great potential for savings in the earthing network without risking the safety. Furthermore, there is a need to develop earthing impedance measuring methods. Methods that are used for overhead network earthing measurements are not often suitable for cable network, where the earthings are connected. This paper brings out recommendations how the design and measurement principles could be developed and which possible changes in the relevant standards should be considered.