Mains smoke alarm green light only works when the light switch is on

[Michael 65]

I just replaced an old mains smoke alarm with a BRK 660 MBX. Now, the old one had a green light on all the time, this new one i needed to change the baseplate as well, as the connectors were different, so i pulled the wires down through the hole from the ceiling, so i had three bare wires, brown, blue and white, screwed the baseplate to the ceiling and loosened the small screws so i could put each wire into the connection, blue wire opposite the blue wire in the baseplate, brown opposite brown wire and white opposite white wire. and tightened the screws, now, this white wire from the ceiling had fallen out of the ceiling with some yellow and green tape around the end of it that disappears in the ceiling, the tape wrapped around a plastic thing, and on one of the rafters was another plastic thing screwed to a wood rafter, so i pushed the taped end back into the plastic yoke on the rafter, guessing this was where it was in the first place. the smoke alarm is a push fit with a battery back up. i turned the fuse back on at the fusebox, the red light blinked every 45 seconds for the battery, but the green light only came on when the light switch was on. Does anyone know if why this is happening, is it something to do with the white wire. the old one worked perfectly, the green light was always on, even when the light switch was off.

[AnthonyB]

Sounds like its been connected to a switchable live and not a permanent live. Some domestic lighting circuits are wired so there is only a switchable live from the on/off switch to the light fitting.

[Michael 65]

So as this worked fine with the last smoke alarm fitted, why does it only work now with the light switch on, and does it have anything to do with the white wire in the ceiling recess.

[green-foam]

As AnthonyB has said, you have connected it to the switched live not the permanent live.

[Michael 65]

So why did the old one work without the light switch being on, i cant see anything else up their to connect the white wire to. the wire itself is only about 8 inches long, and that yoke on the rafter is the only thing i could see that it could possibly be connected to. as its plastic, could it mean that the two pieces of plastic, ie one on the rafter, and the other at the end of the white wire with the tape around it, maybe i put it back in such a way that the two pieces are not touching each other, the tape might be in the way, as you can see guys, i do not know much about this subject, if i can get help to sort this out it would be appreciated, 

[Michael 65]

These mains wired smoke alarms should be banned, i have come to that conclusion because it gives people a false sense of security, as even the manufacturers say the sensors in them only last 10 years, as you can now buy these alarms with 10 year battery life, they will start beeping after or near that time to tell you the battery is low, meaning, the whole unit needs updating. Mains wired with a battery back up, mean just change the battery every couple of years, and as its connected to the mains, it could be on the ceiling well past its 10 year sensor life. So i think i will not bother at all with these mains wired ones.

[AnthonyB]

If you were going to ban any it would be the battery only ones, where there have been multiple deaths over the years due to flat or missing batteries - this is why the law has, for many years, required the mains ones in new or refurbished premises.

If you don't know what you are doing, get an electrician to fit them.