Electronics > Repair
Weird USB oscilloscope ground issue?
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m k:
With earthed extension cord you can check what nearest GFCI has to say.

Insulated area doesn't need protective earth, assume that it is not there.

When connecting devices to earthed socket assume that metal parts are live.
Alex_Baker:
I plugged the power strip under my desk into a bathroom outlet, which does not have a GFCI itself but is connected to one elsewhere. I then repeated my earth-to-earth and earth-to-pc GND current tests from the outlet under my workbench, and I could never get the GFCI to trip. I got about 4mA going from PC earth to workbench earth, and 10ma going from PC GND to workbench earth.
magic:
I thought typical GFCI trip on AC not DC?

And they trip on mismatch between phase and neutral, not on protective earth current.
m k:

--- Quote from: Alex_Baker on June 27, 2024, 02:39:36 pm ---I plugged the power strip under my desk into a bathroom outlet, which does not have a GFCI itself but is connected to one elsewhere. I then repeated my earth-to-earth and earth-to-pc GND current tests from the outlet under my workbench, and I could never get the GFCI to trip. I got about 4mA going from PC earth to workbench earth, and 10ma going from PC GND to workbench earth.

--- End quote ---

That is good.
Now you(and me) can be much more certain that no hazardous situation is present.
You can also assume that original ground current is between secondaries and possible leak is less than dangerous.

Comparing this latest situation and earlier connections you should be able to narrow the problem down, at least some.
Alex_Baker:

--- Quote from: magic on June 27, 2024, 04:42:46 pm ---I thought typical GFCI trip on AC not DC?

And they trip on mismatch between phase and neutral, not on protective earth current.

--- End quote ---
That is what I was wondering, this house was built in 1990, so not sure what type of GFCI is installed.
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