Ok, seems cause is confirmed. Thanks everyone. Now couple words about solutions below.
I don't think you can get around this "positive-negative" mismatch without DC-DC converter.
Unless you're willing to mount RB so that metallic parts of its chassis don't touch metallic parts of rack and other devices (i.e. have its chasis galvanically isolated from the rest of your DC). You could put it on a non-conductive shelf (plastic or wooden) but you would still risk shortcuts when somebody would be working around your RB.
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It's possible to use isolated DC-DC converter here or try to reverse polarity on the plug.
Thank You both. It looks lilke isolated DC-DC converter or separate AC-DC PSU would be the best option for me (perhaps second one would be easier to work with). For other ppl googling around about this. Puting the device in isolated environment electricaly does the job but is quite unreliable too. First of all even if You isolate some parts of the case Your're creating 53V potential between the device (mikrotik) and every other metalic part in server room. It's like children's game "not to touch the lava". Every time You touch it can shock You, every time You touch it with something metalic there will be a lot of sparks, braker trips and voltage on DC line drops for a while rebooting every device powered from this line. Not a good idea in proffesional use.
Reversing polarity is a no go too. And probably it WILL NOT work at all, becasue grounded (+) is not the same as reversed polarity. In grounded (+) circuit polarity is still the same, voltage is counted backwards but (+) is still (+) and (-) is still (-), so in normally connected circuit charge is still suppose to flow the same direction. When You reverse polarity You're also reverse charge flow direction which in best way does not work at all, in the worst it will create some smoke in the device which is not protected from "reverse polarity error". Don't do it
Thanks a lot again everyone
Best regards
w0jtas