RCD with a TT system

[La Poste]

Hi.
I was just reading the useful information to Sparkys thread and I came across this which confused me a little.

Fire detection circuits must be supplied independently of other circuits and not protected by an Rcd protecting multiple circuits. 560.7.1

A single RCD protecting a TT systems must be installed at the origin of the installation unless the part of the system between the origin and the RCD fulfils the requirements for protection by Class 11 equipment or equivalent insulation 531.4.1

This seems like a bit of a contradiction to me, any comments from anyone?
I mean if you were working on a domestic TT system and the owner wanted mains supplied smoke alarms how would you go about it?
Thanks.

 

[jeremy]

The don't rcd smokes bit is about having them on the same rcd as sockets,etc as if something trips the rcd at 30mA then the smokes aren't powered. If there is a 100mA rcd protecting the whole lot then everything but essential supplies, smokes, freezers (certain circumstances) is covered by a 30mA rcd the 100mA should only trip if there is an earth fault with the essential equipment (possible but unlikely). If you were to drill through the smoke wiring and trip the 100mA chances are that you're awake and will notice that the power has gone off! Hope this explains it. If not please say and I'm sure there's someone here who can clarify it

 

[La Poste]

Ok Thanks.
I thought the logic behind the rule was that if one circuit trips an RCD then because the smoke alarms are on their own indpendent circuit the smoke alarms would allways function as they would still be supplied by current. I read it as a health and safety issue.
Another instance, if you wire a smoke alarm into a lighting circuit via a three plate in the ceiling then if late at night your lights trip that means your smoke alarms will be not working also. I understand that smoke alarms have battery backup but I thought the logic behind the rule was that whatever happens to other circuits in your installation you will allways have fire cover.
Wiring into a lighting circuit that is RCD protected at 30ma or a TT system with 100ma protection would not give you this protection because a fault in any of these circuits would also knock out your smoke alarms.
That's how I read the rule.
Does anyone see it differently?
Thanks.

 

[antimatter]

Hi.
I was just reading the useful information to Sparkys thread and I came across this which confused me a little.

This seems like a bit of a contradiction to me, any comments from anyone?
I mean if you were working on a domestic TT system and the owner wanted mains supplied smoke alarms how would you go about it?
Thanks.

Main switch > 6a 30ma RCBO - FDC 100ma RCD (TD) > all other circuits

 

[La Poste]

Thanks mate that's how I see it but have read here in other threads people wiring smoke alarms into lighting circuits via the three plate in the ceiling.
Surely this is wrong.

 

[antimatter]

....all circuits should be divided.......etc etc

i know it is naughty but smoke detectors, should be on their own circuit, and interlinked, installed at every level in a dwelling with battery back up to comply with the regs.

 

[jeremy]

Better to have smokes in non rcd rewireable fuse protected rubber wired lighting circuits than none at all!! at least when it all catches fire you'll get a bit of a warning!! But this would only apply really to a retro fit system. If you're starting from scratch then you have the opportunity to give the smokes their own circuitry and abide by the regs

 

[sjm]

Aico suggest wiring their smoke detectors from a local regularly used lighting circuit as being the best option, wired directly from the ceiling 3 plate.
The OSG states "where all circuits are protected by RCDs there is an advantage in supplying fire detectors and alarms from regularly used lighting circuits.
The electrical safety council recommend supplying smoke detectors from a a seperately electrically protected, regularly used lighting circuit and say that in their opinion "seperately electrically protected" does not preclude sharing an RCD with several other circuits where this is permitted by BS7671. Check out www.esc.org.uk/forum new or rewired domestic and similar installations NDQ17 and NDA17.

 

[La Poste]

Thanks mate that's a great link.
I prefer the option of wiring into the lighting circuit as it will become obvious when the supply is broken just wasn't sure of the regs.

 

[Ponty Massive]

The bleep noise the smokes make when the power goes off is enough to tell you there is a fault.

 

[trebor]

Hi.
I was just reading the useful information to Sparkys thread and I came across this which confused me a little.

This seems like a bit of a contradiction to me, any comments from anyone?
I mean if you were working on a domestic TT system and the owner wanted mains supplied smoke alarms how would you go about it?
Thanks.

560.7.1 is reference to fire detection systems circuits not domestic mains powered which are covered by the Building Regs 2002, the building regs preference is for the smokes detectors to be supplied from a regularly used ligting circuit with seperation isolation for the lightingr, it does not make a distinction between any earthing system as a lost supply for whatever reason is normally noticed quickly