6.5.8 Earthing systems in buildings with transformer stations

Earthing systems in transformer stations are key components of electrical safety: they establish the reference potential, conduct fault currents into the ground and limit touch and step voltages to safe levels. When dimensioning, electrical variables such as expected fault currents and their temporal progression must be combined with geotechnical parameters such as soil conductivity and layer structure, as these have a decisive influence on earth resistance and voltage distribution. In addition, material resistance and design details are important in order to limit corrosion and maintain functionality throughout the service life of the installation.
Normative requirements and protection concepts specify the relevant target values; at the same time, local constraints often require compromises between ideal electrical performance, financial cost and structural limitations. Proper planning therefore includes on-site measurements, analytical assessments, mathematical simulations and, if necessary, measurement verifications after completion. Only through this combination of physical understanding, standard-compliant design and technical implementation can the protection of people and equipment in transformer stations be reliably ensured.

Normative references
The requirements of high-voltage and low-voltage systems typically go hand in hand in a transformer station. Normative references can be found in:

• IEC 61936-1: Power installations exceeding 1 kV a.c.
• EN 50522: Earthing of power installations exceeding 1 kV a.c.
• IEC 62271-202: AC prefabricated substations for rated voltages above 1 kV and up to and including 52 kV
• IEC 60364-4-41: Low-voltage electrical installations – Part 4-41: Protection for safety – Protection against electric shock
• IEC 60364-5-54: Low-voltage electrical installations – Part 5-54: Selection and erection of electrical equipment – Earthing arrangements and protective conductors
• IEC 62305-3: Protection against lightning – Physical damage to structures and life hazard
• DIN 18014: Foundation earth electrode – General planning principles (German standard)
• DIN VDE 0151: Material and minimum dimensions of earth electrodes with respect to corrosion (German standard)

Important terms and definitions
Earth electrodes are defined as non-insulated conductors in contact with earth. A locally restricted system of conductively interconnected earth electrodes is termed earth-termination system. This earth-termination system is connected to the part of an installation to be earthed via the earthing conductor and the main earthing busbar. Depending on the earth resistivity ρ, the dissipation resistance R_E is obtained in relation to remote earth.