When I measure the potential difference between for example a 12V DC output device's one output terminal such as a power supply's or a transducer's and the "earth ground", I see 50Hz voltage signal with around 160V amplitude in oscilloscope. Its not always 160V sometimes less depending on the transducer and the output.

An example with a 12V DC power supply:

I turn on the power supply.

I measure the voltage between its plus and ground terminals and it is 12V as expected. So far so good.

But if I measure the voltage between any one of the terminals of this power supply and the "earth ground" of a power outlet, the voltmeter shows like 99V rms.

This was peculiar so I wanted to see the signal on a scope.

Here is the oscilloscope output when the probe's tip is connected to one of the terminals of the power supply:

enter image description here

An example with a scope.

I experience the similar phenomenon with a turned off oscilloscope.

The scope is turned off in this example. When I plug the scope's power cord to mains outlet with a 2-prong AC power cord and measure the potential difference between the scope's GND and the "earth ground" of the outlet I measure like 48V rms voltage.

But if plug the scope's power cord to mains outlet with a 3-prong AC power cord and measure the potential difference between the scope's GND and the "earth ground" of the outlet this time I measure zero volt.

Floating outputs?

Am I experiencing floating outputs in both cases?

In power supply case I can see that the floating terminals with respect to the earth ground can be the reason, But I don't get why there is a potential difference such as 48V between a turned of scope's GND( whe the scope uses a cord without an earth pin) ant the "earth ground" of the outlet.


In the below system we are getting electric-shocks at the output of amplifiers or end of BNC cables:

enter image description here

I named the points to make the question clearer.

A1, A2, A3, A4 are amplifiers for some transducers(model 101 in this case). PSU is a power supply which provides DC power to the amplifiers. PSU is connected to the mains outlet via a 3-prong power cable where L is line, N is neutral and E is earth ground as shown in the illustration.

"dc" are the plus terminals and "gdc" are the ground terminals of the power supply which powers the amplifiers.

Output of the amplifiers are sent to a DAQ system via BNC cables. "out" are the inner/signal carrying wire of the BNCs, and the "gb" ground/shield of the BNCs.

DAQ system has analog inputs and they are single ended and all the grounds are connected to the earth as shown in the illustration.

In this system when the PSU is on, and when none of the the BNCs are connected to the DAQ inputs we get electrocuted. I measure sometimes like a 60V rms AC voltage between one terminal of the BNCs and the "earth ground". But when connecting any of the BNC to the DAQ system this floating voltage disappears.

1-) Is this voltage occurring because of the floating power supply PSU? Is this voltage dangerous?

2-) How can I solve this issue? Should I connect the "gdc" to the earth or "gb" to the earth?

3-) DAQ side is single ended and grounded all the way to the earth through PC's motherboard. If I make non-floating outputs by wiring PSU DC GND to earth would that create ground loop issue this time?

  • \$\begingroup\$ Nothing sounds unusual here. OK it's a rather long question weaving this way and that way so I hope I didn't miss sone subtlety but nothing to get excited about - it's all about the same nipple cringing energy you get when removing a nylon shirt (when I could only afford nylon LOL). \$\endgroup\$ – Andy aka Jan 4 '17 at 17:43
  • \$\begingroup\$ @Andyaka i couldnt get your metaphor:(. would wiring "gdc" to the earth solve the electric shock problem? how about ground loops? \$\endgroup\$ – floppy380 Jan 4 '17 at 17:50
  • \$\begingroup\$ Don from this result, can you calculate the leakage capacitance across the isolation transformer in pF? You should be able to. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 4 '17 at 17:57
  • \$\begingroup\$ @TonyStewart.EEsince'75 there is no isolation transformer. PSU is a 24V SMPS. You mean the transformer inside that? \$\endgroup\$ – floppy380 Jan 4 '17 at 18:04
  • 2
    \$\begingroup\$ THAT is the isolation transformer and it has internal capacitance across the windings and usually about 1nF from primary to secondary for EMC reasons. \$\endgroup\$ – Andy aka Jan 4 '17 at 18:13


simulate this circuit – Schematic created using CircuitLab

Then can you see what is best way to suppress leakage from schematic, without compromising ground fault safety?

Can you estimate current and pF leakage from Vin and Vout Common mode?

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  • \$\begingroup\$ I have no idea how to measure that and cannot find any material or tutorial. Do you mean in a nutshell I should replace that cap to suppress the leakage? Do you mean I should open the SMPS and replace the cap between the windings?? Btw I tried with different power supplies issue is still there. Or is that C_leak a representative capacitance? \$\endgroup\$ – floppy380 Jan 4 '17 at 21:31
  • \$\begingroup\$ leakage cap is stray interwinding capacitance. The 10M probe sees 320Vpp out of 750Vpp ac input . That is less than half the 750Vpp of 230Vac(rms). So you need to use a 1M probe or add a resistor for a 2nd test to verify the impedance of this 320Vpp signal such as 50 Ohms. you should be able to generate a couple hundred volts on your ungrounded scope probe with just your ungrounded finger because your body is an antenna to stray line E fields which are high impedance. report back your results. Then try 50 Ohms across probe. Then try 1mH. For your education. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 5 '17 at 2:28
  • \$\begingroup\$ I was planning to try with a linear supply. But very strangely one day after the issue was not there. This was the 2nd SMPS and yesterday it was also causing this issue. But today that floating voltage disappeared to zero. So strange that it disappeared itself. \$\endgroup\$ – floppy380 Jan 5 '17 at 10:18
  • \$\begingroup\$ Considering you haven't tried to do or indicate you understand what I have said so far. it's not surprising. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 5 '17 at 14:19

1-) Is this voltage occurring because of the floating power supply PSU? Is this voltage dangerous?
Yes, the DC provided by PSU is floating - it has no direct connection to earth. The AC voltages you measure are capacitively-coupled via small capacitance. Currents are very small, so are not dangerous to you. Unless you have huge AC sources nearby. Almost any environment where AC power exists will yield such results as you have measured.
2-) How can I solve this issue? Should I connect the "gdc" to the earth or "gb" to the earth?
Grounding the PSU will mostly eliminate the AC voltage. However it will establish a major ground loop, forming an inductive (magnetic coupling) antenna. AC currents will flow around this loop, through the coax ground shield - not a good plan, because these currents might influence the voltages that your DAQ system is measuring.
In any case, you have multiple "ground" loops in your DAQ measuring system - unavoidable when they're all fed from the same PSU. One loop is shown in red. Two methods reduce inductive pick-up: keep the loops small by using short coax cables, and reduce their area by tying coax cables together.


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What you are saying is leaving the system like this is not dangerous? So I will not try to tie the gdc to the earth because of ground loops. Others who use this are complaining and scared about this shocks. How can I demonstrate them this 60V rms AC voltage is harmless just a capacitive low current phenomenon? \$\endgroup\$ – floppy380 Jan 4 '17 at 19:02
  • \$\begingroup\$ I am assuming that your PSU is working properly - that there is no short across its internal isolating transformer. A faulty PSU is possible, but you have tried a few. An AC ammeter might be able to measure current, but you might be measuring ground-loop currents, not the capacitive-coupled currents. Best demo is to grasp PSU -ve terminal, and simultaneously ground yourself. I'd do the measurement first, to ensure currents are small. \$\endgroup\$ – glen_geek Jan 4 '17 at 19:19
  • \$\begingroup\$ If users can actually feel shocks, those currents are worth worrying about, even if your PSU measures OK. Your 4-channel DAQ has a DC power source too. Could it be faulty? \$\endgroup\$ – glen_geek Jan 4 '17 at 19:26
  • \$\begingroup\$ This happens when the amplifier transducer system(on the right side in illustration) is disconnected totally from the DAQ system. It is independent from the DAQ system. Its not faulty either. Its a floating power supply issue. But I have a question here: The floating ends wrt to earht are high. But DAQ sees the difference of them which is not harmful. But how about the CMRR. Would that be harmful for the DAQ? HERE IS THE DAQ: mccdaq.com/pdfs/manuals/PCI-DAS6034-35-36.pdf \$\endgroup\$ – floppy380 Jan 4 '17 at 20:13
  • \$\begingroup\$ it says CMRR at 60Hz is 85dB for +-10V range. What does that mean? \$\endgroup\$ – floppy380 Jan 4 '17 at 20:14

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