No claims protection works as the discount applied on the premium. If you have an at fault claim, then the premium will go up. The NCD is then applied. So although you keep your NCD intact, the premium will probably increase. As has been said, without it the premium will increase AND the NCD applied will be less.
I have recently renewed and my GF had a non fault claim in Sept last year. Imagine my surprise when I told them this, and the premium went DOWN by £1.40! Explain that!?