The first thing you need to know about farms, especially older ones is that they are like no other installation you'll ever work on. In my experience, and I've had loads with farms, anyone who can fit a plug is classed as an electrician! Point 2, there is generally no such thing as a division of circuits, if you connect it and it works, then that's fine. I've seen sockets on light circuits, lights on socket circuits, I even know of one with a bathroom heater on the lights, it blows the fuse, so stick a bigger one in!
Seriously it could be any one of a number of things causing the problem, and don't take it as red that the place has been rewired, "it was rewired about 5 years ago mate" one farmer told me when I went to investigate a persistantly blowing fuse, the only thing it didn't have was varnished cambric! there was some lead sheathed, some rubber sheathed, and some pvc sheathed, "honestly mate, it had a full rewire" insisted the farmer.
I've seen bits wired in cable used in mines, because the pit electrician was a friend of the farmer, rcd sockets with no earths attached, you name it and I've probably seen it, there is an old saying in the police force and it applies equally well in our job, A B C, Accept nothing, Believe nobody and Challenge everyone, remember that and you'll not go far wrong.