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A USB device is connected to a Laptop PC. Laptop PC has a ground earthed power supply. Oscilloscope's negative lead is connected to ground via its own power supply. Oscilloscope's negative lead is touching the USB's positive shield.

In this scenario it will be a short circuit as in the figure.

My questions are. In case the laptop power supply has only 2 pins is there any danger? I have a very old oscilloscope which has only 2 pins (I think neutral and line). What happens if power supply is ground earthed and the oscilloscope is not?

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    \$\begingroup\$ See EEVBlog \$\endgroup\$ Commented Nov 22, 2013 at 13:47
  • \$\begingroup\$ What are you trying to measure with your oscilloscope? The obvious answer here is don't let the laptop's +5VUSB touch your Osc's GND lead and you'll never find out, and I have a feeling that's not what you're looking for. \$\endgroup\$
    – Mels
    Commented Nov 22, 2013 at 15:34
  • \$\begingroup\$ no the figure is the dangerous one people agree on. my setup is a bit different on the right side. my oscilloscope is from 70s analog 10Mhz. it has 2 power pins on its power wall connector(not 3 as typical ones have). so i think they are line and neutral. it seems my oscilloscope is not earth grounded or it is somehow? i couldn't figure out if there is still a safety issue. \$\endgroup\$
    – user16307
    Commented Nov 22, 2013 at 15:39

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Your setup as you show it is not ideal but should work. Are you really sure the laptop power supply connects its negative output with the power line ground thru 1 kΩ? That resistance sounds very low. Have you actually measured this with a ohmmeter (while everything is disconnected, of course)? If it is around a MΩ instead, you can ignore this whole issue since 5 µA is irrelevant to a USB power supply.

There are several obvious solutions:

  1. Run the laptop off of its batteries while doing the tests.

  2. Use a isolation transformer between the laptop supply and the power line.

  3. Connect the oscilloscope ground to the USB ground, not to USB power. It's hard to imagine why you would connect it to the 5 V power in the first place. The difference is only 5 V, so should still leave plenty of usable common mode range from the scope, even if the signals you want to see are referenced to 5 V. What exactly are you trying to see on the scope?

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  • \$\begingroup\$ Thats not my setup in the figure. That setup is believed to be the dangerous one. Thts why I drew it. My question is if either oscilloscope or laptop power supply DC gnd isnt connected to earth ground is there still a danger? I have an analog oscilloscope from 1970s and it has only 2 pins(line and neutral). It has a chassis earthing point (I guess they were doing earthing with an extra cable in the old days?). In my case my oscilloscope doesnt have earth. Its power plug has 2 pins. But laptop is connected to earth gnd. My question is what happens in that case?( \$\endgroup\$
    – user16307
    Commented Nov 22, 2013 at 15:05

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