Background: I have only one outlet in my house that remotely approximates a proper equipment grounding conductor (it's bonded to the mains ground point via the armor of 50s vintage AC, which is better than the rest of the house, which was wired with 50s vintage NM which lacks an equipment grounding conductor), and it is the outlet for the garbage disposal in the kitchen. I can use it on a short-term basis by setting my gear up on the kitchen table and using a grounded extension cord to reach that outlet; however, that's not an acceptable situation long-term, and running a new home-run to feed my test instruments is not happening any time soon.
Is plugging my oscilloscope into a non-grounded outlet ever going to be safe, or is a GFCI insufficient protection against shock when you have an oscilloscope plugged into a circuit that lacks an equipment grounding conductor? (We plan to install GFCI protection throughout the house in the near future to bring it up to the NEC 2014 provisions for a house which lacks EGCs in the wiring.)
Best Answer
Obviously the problem is only going to be if you are using the oscilloscope to probe a circuit that is directly connected to the mains. I would use an isolation transformer, like this one, to isolate the mains voltage from the circuit under test.
Because there is no conductive connection between either of the wires of the transformer secondary and ground, there is no danger touching a live part of the circuit.