Perhaps I got a dud spool, but I had a spool of 50lb power pro which repeatedly broke at between 8 and 10kg (tested by me using some scales).
I tried it in different parts of the spool, and the line was brand new. After one trip out, I tested it, then pulled it all off the reel.
My testing was not as scientific as the link that Jamie posted, but I used the same test with the same knot on some 30lb braid made by another manufacturer and it was breaking at around 13kg.
As mentioned, may have been a dud spool, but I would never use it again!