smoke alarm

Two years ago I had a smoke alarm installed that was properly 'wired in' by an electrician because I didn't want the hassle of having to replace batteries.
Now the 'wired in' smoke alarm is carrying on like it has flat batteries and started 'beeping' loudly this morning (Sunday) before 6AM, and now I'm wide awake and can't phone an electrician because it is Sunday. Is it normal for an electrically wired-in smoke alarm to beep like it would do only if it had flat batteries? The way it is going now, I won't be able to get to sleep tonight unless I turn off the electricity at the fuse box. That will turn off the fridge and freezer. I can't see any other alternative until Monday when I can get in touch with an electrician unless someone else has a suggestion for me. Any advice is always appreciated. V.