| Electronics > Repair |
| Weird USB oscilloscope ground issue? |
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| shakalnokturn:
If neither the PC's case-PSU- ground continuity is suspect, it isn't the PC's PSU leaking massively and it isn't a DC voltage drop linked to the Picoscope power (as I understand that has been ruled-0ut ?). Could it be a power device, likely inverter-based (solar power, aircon...) sharing the same line but further down that's getting away with putting a fair deal of DC leakage current through the PE wire on that line due to lack of (adequate) GFCI? I'm not familiar with the American NEC, here in France the MCB's allow up to 500mA of ground fault before tripping. |
| Alex_Baker:
--- Quote from: shakalnokturn on June 27, 2024, 10:11:53 pm ---If neither the PC's case-PSU- ground continuity is suspect, it isn't the PC's PSU leaking massively and it isn't a DC voltage drop linked to the Picoscope power (as I understand that has been ruled-0ut ?). Could it be a power device, likely inverter-based (solar power, aircon...) sharing the same line but further down that's getting away with putting a fair deal of DC leakage current through the PE wire on that line due to lack of (adequate) GFCI? I'm not familiar with the American NEC, here in France the MCB's allow up to 500mA of ground fault before tripping. --- End quote --- I will try to follow up on this at some point, I want to try and unplugging things in the vicinity until it is only the pc plugged in, perhaps in that entire circuit. There are no large power devices around me, only lots of little ones. |
| m k:
You can also measure PE resistances. Bathroom and workbench PE sockets are pretty close, desk socket seems to be something else. Use the extension cord to bring wall sockets side by side. Probe tip can have better connection than its side. You may get lower shorted resistance by measuring few millis of conducting material. If probe is too short you must tilt it to get a PE connection. Then the connection can be less than optimal and if it is missing completely you may need to go back to previous situation to get a confirmation. Keep also in mind that wall socket can be less than optimal, connection can be loose or broken. So uncertain connection is not necessary your meter. In case of uncertainty I would be "watta..." and open the socket. Don't do that if you are not sure. |
| Alex_Baker:
--- Quote from: m k on June 29, 2024, 09:50:39 am ---You can also measure PE resistances. Bathroom and workbench PE sockets are pretty close, desk socket seems to be something else. Use the extension cord to bring wall sockets side by side. Probe tip can have better connection than its side. You may get lower shorted resistance by measuring few millis of conducting material. If probe is too short you must tilt it to get a PE connection. Then the connection can be less than optimal and if it is missing completely you may need to go back to previous situation to get a confirmation. Keep also in mind that wall socket can be less than optimal, connection can be loose or broken. So uncertain connection is not necessary your meter. In case of uncertainty I would be "watta..." and open the socket. Don't do that if you are not sure. --- End quote --- The only problem with this is that there are small voltages between earth sockets, which makes it impossible for my multimeter to do a resistance reading. I know there shouldn't be voltages on the PE wire, but nonetheless there is. |
| m k:
What is small? Digital multimeter can create AC volts from thin air. You can demonstrate that by moving hanging probes apart. For checks you can also use line voltages. But then you must be sure that your meter and leads are correctly protected. If PE is connected one side measures a line voltage contra it. I guess you can quite easily lose near 5% of line voltage for bad connections. If that stray voltage is real you can also use it and create a distance map. If there is a DC component you can also create a direction map. But don't take that result granted, situation can change depending how surrounding machines are on and off. It's also time consuming since you must move extension cord around and trust that its contact is equal enough. One overall downside with wall sockets is that some are extremely difficult to reach. And that doesn't even include exclusive ones, like fridge and washer. |
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