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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.)

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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.

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  • \$\begingroup\$ To be clear I isolate / float the equipment under test... Other wise touching the ground of the scope probe to equipment the wrong way could result in a scope case at mains voltage (Scope probe ground is scope ground)... feel that tingle as you rest your hand on the scope to adjust the settings.. :-( . \$\endgroup\$ – Spoon Feb 3 '15 at 12:41
  • \$\begingroup\$ @Spoon There is no wrong way. Both leads out of the secondary of the transformer are isolated from earth ground. Connecting one to the ground of the oscilloscope is in fact necessary to make a measurement. No tingle. \$\endgroup\$ – tcrosley Feb 3 '15 at 15:48
  • \$\begingroup\$ It depends on a few "and If"s. If the scope is floating and If the device is connected to the mains and If you connect the probe "ground" to a Live/Hot part of the circuit (More likely on a broken or newly constructed or Work in progress device) then there is a safety issue... Having had a couple of "interesting moments" myself and inadvertently electrocuted a friend and a work colleague 25 years ago (separate incidents) you could say I'm passionate about not getting others or myself dead. \$\endgroup\$ – Spoon Feb 3 '15 at 18:00
  • \$\begingroup\$ @Spoon I was assuming the circuit under test was connected to the isolation transformer, not the scope. Sorry to hear about your past experience, that must have been terrible. \$\endgroup\$ – tcrosley Feb 3 '15 at 18:02
  • \$\begingroup\$ None were very serious ... but any of them so easily could have been. \$\endgroup\$ – Spoon Feb 3 '15 at 21:01
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There is a couple of things to consider here.

First, the Earth terminal on the scope is primarily intended to protect against a short from the AC mains Hot conductor to the frame of the scope INSIDE the scope. In other words, it's standard safety stuff.

You CAN use a GFCI (RCD) to protect you from harm should that happen. Do note that the only thing that should be plugged into that GFCI is the scope itself.

Use another GFCI powering your AC Mains-powered device to protect you from harm should there be a short from it's AC Hot to frame.

If I'm going to be working directly with AC Mains voltages in my project, I use an isolation transformer to power that project. This allows both the Hot and Neutral conductors to float with respect to earth ground. Not only does this protect you should you accidentally come in contact with the Live conductor, it also allows you to safely probe either the Live or Neutral conductor with your scope.

I mentioned at the start that there were a couple of things to consider regarding the scope's Earth connection.

The other consideration is that if the scope's chassis is NOT connected to Earth ground, it is possible to raise the CASE of the scope to a dangerous level should you connect the probe's ground lead to a high voltage (AC Mains Live conductor, for example). Again, the fix for that situation is to use an isolation transformer to power the device you are working on.

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