Wisely (I think) after connecting my DUT (Tektronix 2710 Spectrum Analyzer) to an isolation transformer, I measured AC voltage between the DUT chassis ground and my scope (Tektronix 2440) ground lead. I see around 19VAC as measured by a fluke multimeter. Therefore, I did NOT try to attach the scope ground to the DUT.
The 2710's service manual says to use an isolation transformer as its PSU is switched mode (and apparently not isolated), which can be a big problem during service.
It should be noted that the DUT GND connection in relation to the isolation transformer is floating. There's no "ground" on the isolation transformer and therefore none to the DUT. As I understand the concept, connecting an otherwise isolated device to mains (earth) GND would defeat the purpose of the isolation.
My question is: where is this 19V coming from? Would a leakage path be possible through the scope's input EMI filter? Would connecting the grounds together draw a huge amount of current? Or, is this perfectly normal?
Never been here or done this... and never discovered it either as I've happily connected scope GND to instruments (not on an isolation transformer, however), with no issues in the past.