Definition
This idiom is used to describe a situation where someone is fortunate enough to avoid an unpleasant or embarrassing situation due to a sudden interruption or ending, usually when they were about to have to do or say something undesirable.
The fire alarm went off just as I was about to explain why my report was late; I was saved by the bell.
The power went out during the heat of the debate; it was a case of being saved by the bell for me.
I was about to answer a very difficult question when the phone rang; I was definitely saved by the bell.