The invention supports and promotes testing of Substation Automation (SA) functions involving intermediary Logical Nodes (LN) in SA systems of operating substations, specifically with minimized impact on the substation and/or the SA system and without having to completely deactivate or isolate entire parts of the substation. A test is set up by defining at least one input LN where the test scenario input signals will be applied, and by determining output LNs where expected output values shall be observed. Based on the data flow between LNs as defined in the System Configuration Description (SCD) file a test preparing tool automatically determines all intermediary LNs logically arranged between these input and output LNs. The intermediary LNs and the IEDs that are affected by the function test are identified and subsequently marked for “test” or “simulation” mode.