Switching PSU problem

[The Master]

The ground of my PCBs (0V negative from the PSU) and the PSU case are both connected to the mains earth.
They are housed in an ABS box with screws visible on the outside so the whole lot needed to be earthed.

There is another one of these blue ceramic caps inside the PSU that goes from the negative out to case ground. I'm guessing this is the MOV you mentioned.
This is essentially bypassed by the earth wire I have running between case ground and PCB ground.

The "0V" is the negative output of the PSU.

I've got some 0.25W 1MOhm resistors that I could string together as a test.

Someone I work with has suggested the blue cap between the rectified mains and negative output may be failing.
This PSU has been on 24/7 for months without a problem but I know caps will fail eventually.
I've got several other working supplies and some that have never been turned on yet so I can try swapping out small components like that.

[richard]

The ground of my PCBs (0V negative from the PSU) and the PSU case are both connected to the mains earth.

yet you claim a 2kv potential difference .

one more try

it is impossible to get a dc voltage that high between two points connected by a low impedance unless a massive current is flowing , circuit theory 101

[The Master]

My multimeter says 2KV DC. I'm not sure I can trust it.

I understand that Ohm's law doesn't allow this condition. 2KV across even 1Ohm would give 2,000A of current flowing through the PSU case and some wires that only handle around 4A.

I'm sure all of the ground points I've checked are connected by low impedance.
There's no excess heat or melted wires etc.
My components haven't fried despite apparently having hundreds of volts difference between their grounds.

The only conclusion can be that there isn't really 2KV. There has to be something though as my circuits keep resetting and the multimeter is showing such high voltages.

Is there anything I can do to help narrow it down? I will try the 1MOhm resistor idea to see if it makes any difference.

[Dave]

First thing I would have done when I saw such LARGE potential is to, Get another meter and double check it...

[richard]

My multimeter says 2KV DC. I'm not sure I can trust it.

I don't doubt the possibility that the voltage is real, as a tv serviceman I have experienced those sorts of voltages between supposedly earthed TV's and properly grounded antennas many times.

maybe a photo of the psu indicating where the measurement is being made could shed some light

ps its not a meter auto_scaling measurement error

[The Master]

I've attached some pictures which I hope will help. The earth pin on the IEC socket has 2 green wires coming from it. (I've attached an old picture so you can see what's going on behind the PCB. There was only 1 wire back then.)
One wire goes to the PSU and then the front panel. The other goes to the screws holding the IEC socket then splits to the 10 metal sockets on the back and the ethernet module (little PCB top-left).
All 3 screws on the ethernet PCB and the 2 screws on the front panel are connected to PCB ground.

The 12V supply goes into the PCB at the back. The negative side connects to this PCBs ground plane. That ground plane is then connected to the ground plane on the ethernet module through the short wires between them (Red: 7V out, Black: ground, Green/Blue: data).

The ribbon cable to the front panel links the ground planes on the ethernet module and the front panel. It also has 5V/3.3V supplies and some data lines.

So all 3 PCBs have their ground planes connected but the ethernet module and front panel also have their ground planes connected to mains earth (and the PSU case). The PCB at the back is the only one without a separate earth connection as it's only contact with the outside world are the 4 pins on each mini-XLR socket. It doesn't even make contact with the metal cases of these sockets.

While the PCB was resetting I connected the multimeter to the 12V screw terminals on the PSU. This is what stops the problem until I take the probs off.
The high voltage was measured with 1 probe touching the top of the PSU case and the other touching the 3 screws on the ethernet module. The screw where the earth wire connects has the lowest voltage. I also put the probe on the back PCB where the ground wire connects to it from the ethernet module and this had the highest voltage.

There were 4 of these that were all turned on several months ago. The other 3 are still running fine now. This one was sat on top of 1 of the others so both would have experienced the same ambient temperatures and electrical noise etc.
There are another 4 of these which are still running after a year.

I opened up every PSU before powering up for the first time to check for solder balls, bad joints, lose components etc. Can never be too careful with cheap Chinese electronics

I'm trying to source another multimeter to double check the reading.

[pedja089]

PCB in top right corner with metal port isn't grounded. Look at ground plane. And solder mask...
Also secondary of PSU isn't grounded. Remove blue capacitor looking thing, and place short. Then measure static build up.
PIC probably resets when spark jumps from screw to GND.
Multiple path to for ground can act as antenna. Move green wire from all PCB's to - terminal of PSU. Then try to test it.