| Electronics > Beginners |
| Safety issue: What appliances will constitute a mains ground reference? |
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| Minor Tom:
If you measure higher voltage equipment (tube amplifiers, for example), it is sometimes important to know which of your appliances used will establish a reference to mains ground. What about the common wallwart? The older ones will use conventional transformers, while the current ones will be switching power supplies. I assume the output of both of these will have no fixed reference (floating). I assume this, because their mains connectors establish no connection to safety ground (at least the german versions), so having any reference to mains at the output will be potentially dangerous. So any equipment powered by wall warts should establish no mains reference. Correct? Traditional oscilloscopes with metal casing will be safety grounded - and so will be the ground connection of the probes, regardless whether the scope is analog or digital, I suppose. What about the modern plastic casing digital oscilloscopes? And a final question - is there an easy, reliable way to check for a mains reference with hobbyist measurement equipment? I assume a multimeter ohm measurement will tell if there is a connection from output to mains connector, but a simple capacitor could defeat this. I hope this bunch of questions is not to overwhelming - and I will be thankful even for partial answers. |
| CatalinaWOW:
While this may not meet everyone's standards, I find it sufficient to measure connections to the power cord with an ohm-meter. |
| Minor Tom:
Thanks for your answer! I just modified my post and recognized your answer only afterwards, so this was not in response to it :) |
| madires:
SMPSU based wall warts usually have an EMI suppression cap between primary and secondary which passes a low current. So a SMPSU isn't really floating. In modern scopes/DSOs the probe's ground is still connected to PE/earth. But you can get scopes/DSOs with isolated inputs or simply buy differential probes. A DMM is fine. You can detect a cap by measuring the voltage between the output (or signal input) and PE/earth when the device is switched on. Check signal ground and the signal line. In most cases the cap is connected to ground, but you can't assume anything until checked ,) |
| Shock:
Obviously if you have a low resistance that is the first warning sign. Some older equipment service manuals (Sony CRT TVs comes to mind) tell you how to test for leakage current. They either recommend a proper leakage tester, perform a current measurement, or use a resistor and capacitor in parallel to earth and measure AC across them. The reason to measure current (in one way or another) is voltage alone may show as ghost voltage. An alternative is comparing a DVM to a multimeter that has a LoZ function, or try a voltmeter with a lower impedance than a DVM. These will slightly load the measurement and if you see decent drop in voltage you can get a rough current calculation (just don't do this on sensitive circuits). When deliberately shunting an unknown current to earth or loading down a mains measurement with your test equipment keep in mind you can blow fuses and trip RCD type devices. Always treat damaged or unknown equipment like it has exposed mains and the potential for high current. Read up about appliance classes and PAT testing as well. |
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