Domestic plugged iMac 2012 into multiple socket gives a little shock My family have recently moved to a new place, sort of rural area (in the mountains), I did found a power socket in the room, connected a multiple socket into it and plugged in iMac + wifi router, there was no problem at the beginning, but sometimes I feel sort of electrified field over the aluminium case (when trying to plug USB cabels). 

The real case :)
Last night it was cold to work, so I decided plug 600W heater and after 1h I did touch the iMac and was "kindly" suggested to leave the party :) (very noticable electric shock). 

I know it's more an engenieering question, but maybe someone had experienced similiar situation and can suggest what is the possible work around?

P.S: I used the same multiple socket in the previous house and had no problems, the new character here is the 600W heater that I've found recently from previous owners (looks like it's from early 2000')
The short answer here is that the outlet isn't grounded/earthed.  It could be that socket or it could be the entire structure (at a minimum).  Also I would get an electrical socket tester and verify your wiring.  If anything is at fault, you run the risk of damaging your equipment to injury or death.

Your best bet is to get an electrician to come in and properly ground your electrical system.

Short of that, I would get a UPS (battery backup) and isolate your iMac from the outlet - this will at least give you a little bit of protection, though it won't do a single thing to protect your iMac in the event of a fault (and this is where it will get expensive).