Electricians reading this might be as puzzled as I was when I first took the cover of this old reported as faulty domestic smoke alarm/detector. Yes the Red and Black cables which look to be the feed are the live and neutral from the nearby lighting circuit. I did wonder if this detector had actually ever worked wired like this. It was supposed to be paired with another one. I changed them both for new maintenance free types and briefed the home owners how to test and look after them. Job done.