I've had chickens and ducks for 11 years and our enclosure has always been fox proof. We've 4' equestrian netting with a top rail plus mains electric wire on the outside at 6 inches from ground level, fox nose height, top rail height and then on top of the 4'4" fence taking it up to about 4'7" in height.
We found a beheaded duck (during the day) two weeks ago, we lost a chicken yesterday and I caught the fox today again. It bolted towards the fence, cleared the entire lot in one swoop, grabbed a duck (she's touch and go) before dropping her and bolting out. I was INSIDE the coop the WHOLE time in broad daylight! I saw the whole bloody thing. The fox saw me, came in anyway, panicked as I ran at it screaming, leapt out and then turned and sat there looking at me. Electric is working well as a goat came to see what the fuss was about in the next paddock and got zapped.
Has anyone else had this? I'm thinking this may be one we have to trap and relocate as I've never seen anything like it. Completely ignored the electric and took such a running leap to clear it I'm not sure putting any more electric out will work.