Thanks for the answer. Actually I read about the Antminer S3 in another thead. I just thought someone with his r-pi controlled batch of U2 might have stumbled over a block too even though the math doesn't leave much room for this to happen.
But even for an Antminer S3 the odds are so small these days. That's why I rather got an Antminer U3. At least I have some work to do to get an keep this little bugger running