Connect two lab bench power supplies in series safely

Question

Lets assume in European Union, that I have two power supplies, that each have 3 terminals. One marked +, second - and third GND. I assume that GND is connected to the protective earth of AC power cord and that PE can be disconnected from - on one of the power supplies. Is it possible to connect them both with safety against electric shock maintained?

I imagine that I leave GND and - connected on the first supply, and I'll disconnect GND from - on the second supply, then connect GND of both of them to prevent shock from touching the chasis of the second supply. Next I connect + of the first supply to - of the second supply and take output voltage from + of the second supply. Lastly I plug them to the AC outlet and power them on.

EDIT: Could someone please elaborate on how does connecting GND of my circuit on bare PCB to protective earth protect me from electric shock?

Answer 1

Could someone please elaborate on how does connecting GND of my circuit on bare PCB to protective earth protect me from electric shock?

So this question is actually about electrical device isolation and earthing in case of fault.

It doesn't afford any protection in the case of a bare PCB. Connecting PCB GND to earth doesn't stop you touching the hot terminal on your IEC input connector if you have one. Some guys build nixie clocks that run off 200V generated by pumping a 12V supply. All rules and safety precautions are thrown out of the window when you have the raw PCB exposed. "No user serviceable parts inside" comes to mind. European CE marking will save you from being totally vaporised if the encased circuit behaves as designed, and in a limited number of fault situations.

I often use two Korad three terminal 30V supplies in series, and there's no way I can think of to zap myself other than sticking 60V into my mouth. That wakes you up. As soon as you introduce another problematic device to your circuit, anything might happen.

The main point of earthing is to enclose the whole device in shield of metal. If a wire comes loose then it shorts against the earthed case, resulting in either the internal /external fuse blowing, or the earth leakage circuit breaker triggering. Simply connecting the PCB GND to earth is more of a noise reduction strategy. I asked about the merits of metal enclosures in Are plastic enclosures safe for hobby mains voltage projects?.

Answer 2

The plus and minus terminals float, that is, have no internal connection to the earth ground.

The ground connection is normally an output. As such, no matter how you connect the plus and minus outputs together both chassis will still be connected to ground.

Normally, if you need an earth ground on your test circuit, you would connect it to one or other ground output on the power supply. Both ground outputs "should" already be connected together via the power line.

EDIT: As Glen points out, depending on the nature of the power supply there may be a maximum voltage you can float the outputs to. You should check the manuals if you have them.

Answer 3

Could someone please elaborate on how does connecting GND of my circuit on bare PCB to protective earth protect me from electric shock?

Most of the time, it is fine to leave low-voltage circuits floating. However there are situations in which grounding your circuit can make sense.

To steal an example from another post, lets say you have a 200V boost converter to power some nixie tubes. You know that the 200V is hazardous and you shouldn't touch it.

Now lets say that 200V rail inadvertently gets connected to mains ground, if your 0V rail is also grounded then the boost converter trips out from over-current, no big deal.

If your 0V rail is not grounded then suddenly your entire circuit is at about -200V relative to mains ground.