Confused about rules for fitting new smoke alarms?

[uptown47]

Hi all,
I'm a DIY'er and I'm planning on doing the electrics in our new extension myself and then get them signed off.

Because of this I've been familiarising myself with the BS7671 regs before starting the work (starting in just over a weeks time when the screed is dry).

One thing that has confused me is the rules around smoke alarms. I am based in Scotland so, yesterday I bought two smoke alarms and a heat detector (for the kitchen) that are all interlinked and comply with the new regs coming out in Scotland in 2022.

Initially when I read the wiring regs it seemed to say that they should go on their own breaker at the CU but also seems to say that they can run off the same circuit as the lights in the room (which makes sense as a homeowner is more likely to realise their lighting circuit breaker is off than their smoke alarm breaker).

So, first question, am I okay to run the alarms from the lighting circuit in each room?

Next question is around RCD protection. They seemed to (initially) say that the alarm feed should NOT be protected by an RCD. But, then it seems to say that it CAN have an RCD as long as that RCD isn't protected by another RCD ??

Can anyone clarify this for me as the small CU I have for the kitchen/extension is protected by an RCD so, if the requirement is for NO RCD then I may have to upgrade my board

The regs I'm reading are the 17th Ed so not the latest ones so maybe something has changed in the meantime? Just want to make sure I do it right as all this will be getting inspected in the future and don't want to be worrying that there's something not done correctly.

 

[motorbiking]

I'm not sure about the RCD question as your Consumer unit will need one. But you are supposed to radial the smoke alarms back to the CU, but that it can be on the same 5A MCB as the lights.

 

[ETAF]

Just as an FYI

doing the electrics in our new extension myself and then get them signed off.

have you already found an electrician who has agreed and prepared to sign off the work? if not worth having a look at the various post here regarding this issue , and cost to self notify can be high.
As i say - JUST an FYI .....

 

[uptown47]

I'm not sure about the RCD question as your Consumer unit will need one. But you are supposed to radial the smoke alarms back to the CU, but that it can be on the same 5A MCB as the lights.

What do you mean "radial" back to the CU? Do you mean a ring circuit - out to the lights / smoke alarms and then back to the CU? Apologies for the confusion

Just as an FYI
have you already found an electrician who has agreed and prepared to sign off the work? if not worth having a look at the various post here regarding this issue , and cost to self notify can be high.
As i say - JUST an FYI .....

Yep, I've got a guy (friend of the guy who's running the extension job) who will check it all and sign it off. I'm not sure if our Local Council will want to come and check it as well yet. But, either way, I want it done "right" - especially as I can't think of many things worse than a house fire!!

 

[EFLImpudence]

What do you mean "radial" back to the CU? Do you mean a ring circuit - out to the lights / smoke alarms and then back to the CU? Apologies for the confusion

I have no idea if he is correct - especially in Scotland - but he means that although it would be on the lighting MCB (and technically be the same circuit in our world), it requires what you would likely call a separate circuit.

 

[winston1]

Yep, I've got a guy (friend of the guy who's running the extension job) who will check it all and sign it off. I'm not sure if our Local Council will want to come and check it as well yet. But, either way, I want it done "right" - especially as I can't think of many things worse than a house fire!!

In signing he says among other things that he designed and supervised the work. Did he? Or are you going to be implicated in him lying?

 

[uptown47]

I have no idea if he is correct - especially in Scotland - but he means that although it would be on the lighting MCB (and technically be the same circuit in our world), it requires what you would likely call a separate circuit.

So, fed from the same CB but it's own radial circuit? Like a spur from the CU?

In signing he says among other things that he designed and supervised the work. Did he? Or are you going to be implicated in him lying?

Yes, he'll be approving the final design. He won't be supervising the work but will be welcome to check the work thoroughly as there's no plasterboard etc on the extension at the moment so I'll be taking plenty of photos of the 1st fix and he's welcome to remove socket fronts etc after the 2nd fix. There will be no "lying" as you put it. I have no desire to have an unsafe installation in my house - hence the research that I'm doing to make sure that it's right.

 

[Taylortwocities]

So, first question, am I okay to run the alarms from the lighting circuit in each room?

No. That is not how it works.
You run power to the first alarm. Then 3-core snd earth cables interconnect all the alarms. The 3cores are the live and neutral from the first alarm plus the interconnect.

 

[uptown47]

No. That is not how it works.
You run power to the first alarm. Then 3-core snd earth cables interconnect all the alarms. The 3cores are the live and neutral from the first alarm plus the interconnect.

Thanks for that. The alarms are interconnected via RF so they only need power. In this case, am I okay to power them from the lighting cct?

 

[Taylortwocities]

Thanks for that. The alarms are interconnected via RF so they only need power. In this case, am I okay to power them from the lighting cct?

Yes, ok if RF interconnected.

 

[uptown47]

Yes, ok if RF interconnected.

Brilliant. Thanks.

 

[motorbiking]

What do you mean "radial" back to the CU? Do you mean a ring circuit - out to the lights / smoke alarms and then back to the CU? Apologies for the confusion

It can't be a off a junction box that supplies the lights, so it has to be one new cable going back to the consumer unit and then into the same RCBO/MCB as the lights

 

[uptown47]

It can't be a off a junction box that supplies the lights, so it has to be one new cable going back to the consumer unit and then into the same RCBO/MCB as the lights

Okay, that makes sense then. So it is just a spur radial from the lighting CB in the CU or on it's own CB from the CU. Thanks

 

[flameport]

It can't be a off a junction box that supplies the lights,

Why not?

 

[uptown47]

Why not?

Presumably because the cable to the alarms could then be damaged but the lights would still work so you might not notice? Only a guess though.