Electric shocks in a single-ended system

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:



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.

Question

In the below system we are getting electric-shocks. (I tried with different PSUs same issue)

at the output of amplifiers or end of BNC cables:



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?

You are experiencing leakage current of Y capacitors between output GND and mains L and N. According to safety rules, the leakage current must be guaranteed not to exceed 0.5 mA rms which isn't dangerous.

Y capacitor are only in SMPSs right? How about using a linear supply? Would it solve this problem?

In the below system we are getting electric-shocks. (I tried with different PSUs same issue)

Click to expand...

There is some problem with the PSU and if different PSu is giving the same problem them there is a problem with the ground at the power point (plug/ socket). Please check the continuity.

The 60V RMS can be tested with a regular neon line tester; did you test with one (the reason for using a neon line tester is that it has a lower impedance compared to the DVM).

Is the PC power cable connected to the same (electrical circuit) path that uses the same ground connection?

See with a 110V lamp whether the 60V source can deliver current. There may be a serious fault (if it can).

IS the PSU a wall mounted SMPS like box?

Y capacitor are only in SMPSs right? How about using a linear supply? Would it solve this problem?

Click to expand...

Yes, a transformer isolated supply, either with linear voltage regulator or secondary switcher won't have the leakage current problems.

A SMPS with PE connection has usually no Y capacitors between L/N and secondary, but Y capacitor between PE and L/N and larger capacitors between PE and secondary. They don't expose an AC voltage and leakage current at the output.

FvM Many thanks for your answers. I couldn't find enough information in this issue besides such forums.

I have couple of problems still didn't settle in my mind:

What is this phenomenon called I mean "50Hz AC floating voltage at the SMPS terminal". Is this called common mode voltage or floating voltage or?

1-) I will try to bu a linear power supply to solve this issue. Any linear supply is "transformer isolated supply"?

2-) The DAQ side is earth grounded as in my illustration. If I also tie the SMPS's DC ground to the earth by a wire, the issue disappears. But Im not sure if it is safe. Also this might cause ground loop issues. So what I need is both to minimize or get rid of that AC floating voltage(since people are not happy to feel that voltage shock even though I say it is harmless); and secondly we don't want ground loops which can affect the measurements. I would be glad to have your suggestions.

Thanks in advance

I don't know a specific term for the ground leakage current respectively contact voltage. As said it's caused by a pair of Y capacitors that make the secondary ground float to the mid voltage between N and L potential.

I presume that all linear power supplies are using a 50/60 Hz transformer (except for a few hybrid lab supplies with primary SMPS and linear post regulator).

Ground loops can be a problem in sensitive measurement and a reason to avoid grounding DC minus at the power supply. A possible workaround is a ground connection through series resistor (e.g. 100 ohm) or antiparallel rectifier diodes that drain leakage currents but break ground loops.

FvM, Last question: You wrote "A possible workaround is a ground connection through series resistor (e.g. 100 ohm)"I want to try this. Do you mean: wiring a 100 ohm resistor between DC minus and the earth ground?

Thanks