How are you powering it?
Do you have a multimeter? if so, measure AC/DC from RB951 ethernet metallic frame to ground, could be a defective power unit/strip causing an electric shunt, there's nothing on the 951 per se that could damage another unit, less on its ether1 (PoE-in) port, unless its internally damaged.
This unit case is easy to open
, open it and have a visual inspection (or post good resolution photos from both sides here)
Which kind of cables are you using? are they shielded? do they have drain wire?
thanks for your prompt replay, i opened the case and the board looks in good condition, I will try with a multimeter once I will be back home and I will update you, I ordered a new one because my network is under pressure now but I hope I could find a solution.
I suppose it could be internally damaged as you said, I used a normal cat5e cable not shielded. Actually I had several problem with this port because it was used to don't get the dhcp address so easily and the led on the board kept on even if the cable us unplugged.
Going on "cable test" with winbox it says connected even when there is no cable connected on it!
Maybe I have just to throw it!