This paper presents the results of a test campaign meant to determine coupling paths into a laboratory test setup of typical power grid substation electronics. The devices were tested against conducted threats in a bulk current injection (BCI) setup and radiated threats inside a transverse electromagnetic (TEM) waveguide as well as with a near-field TEM horn antenna. The various excitation methods and different test setups offer the possibility to trace back the observed effects to coupling paths into the tested system. For frequencies below 300 MHz the dominating coupling path to the system under test (SUT) was determined to be the Ethernet cable connected to the telecontrol device. Above 800 MHz direct radiated coupling into the test devices was identified to be dominating. The conducted coupling via a predefined wiring harness with supply cabling was less relevant for both selected substation devices.