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In DCS, we are using external Weidmuller/Phoenix 230VAC to 24VDC PSU as an interrogation supply for Digital Inputs. + from 24VDC PSU is extended to the field limit switch contacts, and return supply is connected to the DI module having -24VDC from the PSU as reference. Please advise the best industrial practice either (-) of the 24VDC needs to be grounded or floating.