smoke/burglar alarm question

[panny300]

been reading threads about smoke/burglar alarms,but still a bit puzzled.
if you fit the new type of consumer unit with a main switch and two rcd's,how would a smoke/burglar alarms connect to it,as i understand it the alarms have not to be on the same rcd as sockets,but would'nt there be a ring circuit on both rcd's(up and downstairs).
and i understand everyone is not fitting the type of consumer that has the 2 spare ways for the rcbo's.
so how would the alarms be connected.
do smoke alarms have to be on their own circuit.

 

[securespark]

If you surface-mount the wiring for them and fit a high integrity split board, you can put them on breakers, or RCBO's if not.

 

[panny300]

thanks for the reply,i would assume no one would want cables surface mounted up and down their walls.
i read various forums and most popular cu seems to be just the double rcd type.so are sparks just putting the smoke/burglar alarms on let's say the rcd with upstairs sockets,
just trying to understand the practical problems rather than just what regs say.
if someone has had this type of board fitted and later decide they want alarms how would they then fit them without a new board,

 

[Spark123]

I don't believe you are meant to put them on the same RCD as any socket circuit. This leaves a couple of options, use an RCBO in a CU with a high integrity way (has at least one way protected by just the main switch), or fit a second CU.

http://www.tlc-direct.co.uk/Technical/DataSheets/Hager/Hager_17.pdf

 

[Chri5]

The point is simple, if you have a 1 or 2 x RCD without a direct rail (no RCD) if and when the RCD pops it pops all circuits on the service.

So if a fire melted cable and popped the RCD the smoke detection circuit would be dead, and unless it's a battery back up you wouldn't have any smoke detection working with a fire burning.

Hence why it is recommended that a smoke detection circuit is direct with it's own RCBO or MCB if the rules allow (<50mm in wall).

Same sort of thing with an alarm panel, the lad in the strippy jumper carrying a bag with the words swag on it, see's you go on holiday.
He shorts an external light, the RCD trips and power fails. A few hours later (once the bell battery discharges) he then breaks in and there's no alarm system to worry about.

 

[panny300]

thanks all,so if the alarm has a battery back up,can it be put on one of the socket rcd's,as the alarm would still be working even if the rcd tripped?
and would you have to tell a customer that they would have to fork out for a second cu just to have a couple of smoke alarms fitted.
sorry for the questions,but as i said just trying to understand the real world problems of the regs.

 

[Spark123]

The smoke alarm issue is sort of covered in the document I linked to above, the link is a bit tempramental - try this one: http://download.hager.com/hagergrou...rmation/ConsumerUnitGuideToThe17thEdition.pdf

 

[panny300]

thanks spark,that's ok if you are doing a rewire and you can design it from scratch,what i was asking is how you sparks are coping putting these regs in existing installations.
where you have the old wylex boxes,or split load cu's,
can you also answer the question about battery backed alarms,if they can be put on the same rcd as sockets,as they will still function if the sockets trip the rcd,
sorry but it also brings up the question of how do you cope with nuisance tripping now you cant take them off the rcd side?

 

[Spark123]

If you are replacing a CU then you have the option of putting in a consumer unit with multiple RCDs which is suitable.
Not 100% sure on the battery backed ones issue, I don't have access at the mo to the BS for these. I was still under the impression these need to be on a separate RCD to sockets, possibly on a regularly used lighting circuit. Maybe someone else can confirm.

 

[Bonce]

I was reading this post with interest after finding the following document.

http://www.voltimum.co.uk/news/9234//Designing-to-the-17th-Edition.html

Near the end it has a section entitled 'Supplying the fire detection and alarm circuit' .

It seems to suggest with battery back up that smoke alarms can be placed under RCD protection. I don't know if its right or not as regards the regs but just thought it may help.

It would seem pretty sensible to keep them seperate under an RCBO if it can be done.

 

[Taylortwocities]

From a Building Regs viewpoint there is no requirement for existing, extended or refurbished houses to have smoke alarms. The exception is where a loft has been converted to a habitable space.

For the loft conversion issue & for new builds then wired, linked, battery backed smoke alarms are a requirement.

Power supplying the alarms can come from either the separate RCBO (in a high-integrity board) or from a regularly-used lighting circuit.

The logic being that you would notice that a lighting circuit had tripped.

PS You might like to get a free copy of The Elecrtical Contractors Handybook from alarm specialist Aico. See http://www.aico.co.uk/handybook.htm