Info please on a mains powered smoke alarm

[pirate]

Not come across this before, so would like some help, please.
I went to install smokes and heat alarm (to new Scottish regs) in a fairly modern flat last week. The flat is on the top floor of three.
Noted that the flat had one smoke in the hall. Assessed minimum requirement as smoke in hall, smoke in Living room, heat in kitchen, just the usual for this type of property. CO alarm, modern one, already installed in kitchen, as required.
Fitted Aico heat in kitchen and smoke in living room, and went to install matching RF smoke in hall.
Client asked if I could put new one over the old one, meaning take out old one and put new one over the hole in the ceiling.
Took existing smoke off its base, discovered it was hard wired mains item and testing revealed it was "live". Thought maybe put the plug-in connector in a suitable MF enclosure, and mount new one in its place. Up my ladder, I said to the client, can you check the CU and see if there's a designated breaker for this? The smoke was off the ceiling and disconnected at this point. Yes, she said, there's a breaker marked "smoke alarm" in there.
OK, switch that off, I said, meaning to isolate it as mentioned above. She switched the breaker off and all hell broke out in the common entrance/stairwell with a very loud siren that would wake the dead! Actually...no-one appeared to ask what the noise was all about. She switched the breaker on, and all was quiet. I reconnected the smoke, it kept beeping, but after a few presses of the reset button, all was quiet.
We chose another location for the new smoke, house-coded the new units and tested all good.
By the time I got home, she had transferred all my invoice to my bank, which was nice!
Sorry, a bit long-winded! My query is:
Was there a time when all flats in a block had one smoke each, but setting one off, or in my case, interrupting the power, caused a central alarm to sound to warn other residents? If so, it surely means that that circuit in the CU was interlinked elsewhere, so was in fact a circuit independent of the flat's supply?
If so, it means that the smokes were on a dedicated circuit, terminated in each flat, and run off the communal supply for the block, with the breaker in each flat's CU powered from elsewhere.
My client called the factor (the instructions for the mains smoke were of no help) and the factor simply said they had no idea.
I'm happy that my client is adequately protected to regs now, but am curious as to why the originally installed smoke acted that way.
Every day is a school day...
Many thanks for any replies to explain this system. I expect to encounter a few more in the months to come.

 

[littlespark]

I’m not sure, but I had noticed this in my daughters flat in Aberdeen.

I take it at sometime, the fire regs required detectors in the communal stair areas and a sounder in each flat. This may have been done by a proper fire alarm panel, or maybe older interlinked aico detectors for example.

When she moved in, I did the EICR, and added the detectors as required within the flat for her landlord… (her flatmates parents) but left this old one in the flat hallway…. It matched the one out on the landing, and no one else in the block really knew about it.

I can only assume the communal detectors should be off the communal lighting supply and not from a private flat.

 

[pirate]

Thanks for that info @littlespark
The odd thing is that there don't appear to be any detectors in the communal hall/stairway, but there certainly is a very loud sounder (which I didn't trace at the time).
I'm wondering if maybe the system you describe has communal detectors which sound the ones in each flat, whereas the one I encountered looks like each flat has a smoke which is connected to a communal sounder so that a fire in any flat alerts the others.
Either way, I agree that the smokes/sirens, while individually on breakers in each flat, must be served by the communal supply for lighting, so all these breakers are on the same circuit and not"connected" to an individual flat's supply. I'd need to go back to check the way it is wired in the CU, expecting to see a "rogue" circuit which is, for convenience, housed in the CU though not really part of the flat's wiring installation. The puzzling thing is that it would mean that if I switched off the main switch in the CU for other purposes, the alarm circuit must remain activated, ie "live", which isn't ideal. Either that, or there is some clever trickery going on of which I am ignorant.
I'm not going back specially to check for that, but will investigate next time I am there. Meantime I am happy that the client is protected to the new regs, and if/when I do a smokes installation in any of the other flats in the development I will be aware of this anomaly.