I have a system were we wish to place a USB charger outlet. The output is placed in a bench made of metal, so even if the output is isolated and low voltage, it would be a good idea to ground the common output (V-) in case of failures of the transformer isolation or the power supply shorting in a bad way. We plan to ground the metal surface of the bench, but in case of this failure, we still have the bonding of ground between the shield or casing of USB connectors (V-/shield). So a double layer of protection.
I can ground the V (-) just fine, and the device works perfectly.
If I attach the multimeter in AC mode between building GND (AC input GND) and the V(-), I read 44V AC.
This happened with my USB charger and a couple of 12V SMPSs that I have laying around from different manufacturers. It seems like this is normal behaviour for this type of devices in my installation.
What could be the reason of this? Could this be caused by a problem with my grounding scheme? (I plan to test in other installations.)
- The USB charger its the Leviton T5632-W, but also tested an Eaton one.
- Also tested with a generic SMPS for an LED strip, two different models with 12V output.
- My ground seems to be all right, I have 120V between phasee and GND, and less than 1V between neutral and GND.
- This a USA type installation, with a grounding rod near the main panel, bonded with neutral only there.
After some reading yesterday, I came to some ill informed conclusions.
- The output of an SMPS its almost "isolated", and you can leave a floating circuit just OK. It might be safer in some cases. NEC doesn't require grounding <50V, and systems normally don't require it. Some people mention that it could be detrimental if your ground has a lot of noise or undesired voltages. But normally it is ok and it is good to have a decent 0v reference in your circuit.
- In most USB chargers (that I tested) the V(-) and shield are bonded together.
- USB chargers that I opened or tested don't have continuity between the AC input ground and the V (-) or V (+). They are effectively almost isolated (Did some reading about in reality the input / output of SMPS having a capacitive coupling, but this is still out of my understanding).
- Some people recommend attaching V(-) or reference voltage of an SMPS output to ground, some other people don't. NEC doesn't recommend anything here (I think,) so it seems like both options are ok.
PD1: Just read this:
Did you measure using DC on the multimeter? You need to measure the AC voltage that might be present between output and ground. Then you could see several tens rising to over one hundred volts AC - this of course doesn't mean it's dangerous - it's just the EMI decoupling capacitors (if any) on the output. For DC measurements your meter's input impedance will likely reduce the voltage to near zero.
So that's exactly the situation. Seems like my plan of grounding the DC output its fine and safe then.