RM850 disarmed not coming back to life

[bordi_br]

Hi, I've recently acquired 4 units of RM850 for using with RTX 3090 to train machine learning models.

One of my setups is:

i9 10940x
prime x299-a
256gb ram
2SSDs, 2HDDs, 2nvme SSDs
corsair cpu watercooler
RTX 3090
psu RM850

On this setup, the PSU worked fine out of the box, booted up ok. I've replaced a CX750 for the RM850 because when the RTX 3090 got to max processing, in a few minutes the computer shuts down. I could turn it on again and continue to use it fine with the CX750.

When I replaced the psu for the RM850, I maxed out the RTX and in 1 hour or so, the computer shut down and did not came back to life.  Note that the CPU was practically idle when the computer shut down. Basically it was only the GPU drawing 350W. I saw at the specifications that the RM850 can hold constant 850W, so how can my 165W tdp cpu and other peripherals are drawing 500W from the PSU?

I've tried to troubleshoot, but since I had another unit of the RM850, I've replaced only the unit, using the same cables and it came back to life. I did the paperclip test on the dead RM850 and it is dead indeed. It tries to turn on but in 1sec or so it turns of. Sometimes I hear a clicking inside similar to a relay actuating. If I rapidly try to connect and disconnect the 4 and 5 pins of the 24 pin connector, it keeps turning on and off. The PSU fan spins a little, which is strange because it is supposed to spin only if a certain temperature is reached. I had AC on at 24 degrees C for ambient temp. The SN first four digits is 2112.

On another setup with a ryzen 9, the PSU was erratic, sometimes it would turn on, sometimes not. Spent 4 hours troubleshooting and nothing seems to make sense. Replaced the GPU for another low power one, tried to look for shorts on the motherboard. Nothing really worked, until it did work and it is working now magically.

Now I have a dead PSU but at least my PC is on, however I am afraid to use the RTX at 100%. Can I do it? do I need to contact RMA to replace or return it?
Can the RM850 run the setup I've posted?

Thanks