District heating safety issues: Interactions between grounding systems and thermal installations

The structure of a district heating network lends itself to interactions among the grounding system of the generating plant, and those of the users, with the possibility that hazardous induced voltages may occur on the latter. Hazards are possible due to the bonding of the heating system pipes to customers grounding systems, as well as to possible high values of fault currents. These electrical safety issues due to grounding electrode interactions have been so far poorly treated in technical literature. District heating pipes are generally dielectrically separated in segments by means of insulating joints. A deeper analysis of such joints shows that the hot water circulating in the pipes may bypass their insulation, rendering the effectiveness of the dielectric separation strongly dependent on the joint geometry, and on the water electrical conductivity. The authors propose possible schemes for the hot water pipes connections, and discuss their efficiency based upon the electrical configuration of the generation plant. An experimental verification procedure to assess the electrical safety of the district heating system is also proposed.