I'm adding a couple of mains fed smoke alarms to my lighting circuit.
The first one is simple enough as there is a run of cable above the
location in the hallway. I'm intending to "loop in" the smoke alarm as
you would with a ceiling rose - allbeit without a switch

I'm not sure how to build the second alarm (upstairs) into the circuit.
Can I simply spur it off the downstairs smoke alarm using three+earth
(for the interconnect)? Or do I need to "loop in" the second alarm as
well? I've tried to illustrate both. I can't see why the first method
would be a problem, just wanted to check.

Spurred: ( = T&E, ||| 3&E Interconnect)

Rose ==== Smoke Alarm 1 ==== Rose
|||
|||
Smoke Alarm 2

Full Loop In: ( = T&E, =~= 3&E Interconnect)

Rose ==== Smoke Alarm 1 =~=~= Smoke Alarm 2 ==== Rose

"thankyousam" wrote in message
oups.com...
I'm adding a couple of mains fed smoke alarms to my lighting circuit.
The first one is simple enough as there is a run of cable above the
location in the hallway. I'm intending to "loop in" the smoke alarm as
you would with a ceiling rose - allbeit without a switch

I'm not sure how to build the second alarm (upstairs) into the circuit.
Can I simply spur it off the downstairs smoke alarm using three+earth
(for the interconnect)? Or do I need to "loop in" the second alarm as
well? I've tried to illustrate both. I can't see why the first method
would be a problem, just wanted to check.

Spurred: ( = T&E, ||| 3&E Interconnect)

Rose ==== Smoke Alarm 1 ==== Rose
|||
|||
Smoke Alarm 2

Full Loop In: ( = T&E, =~= 3&E Interconnect)

Rose ==== Smoke Alarm 1 =~=~= Smoke Alarm 2 ==== Rose


You can just spur it off the first alarm.

"rrh" wrote in message
. uk...

"thankyousam" wrote in message
oups.com...
I'm adding a couple of mains fed smoke alarms to my lighting circuit.
The first one is simple enough as there is a run of cable above the
location in the hallway. I'm intending to "loop in" the smoke alarm as
you would with a ceiling rose - allbeit without a switch

I'm not sure how to build the second alarm (upstairs) into the circuit.
Can I simply spur it off the downstairs smoke alarm using three+earth
(for the interconnect)? Or do I need to "loop in" the second alarm as
well? I've tried to illustrate both. I can't see why the first method
would be a problem, just wanted to check.

Spurred: ( = T&E, ||| 3&E Interconnect)

Rose ==== Smoke Alarm 1 ==== Rose
|||
|||
Smoke Alarm 2

Full Loop In: ( = T&E, =~= 3&E Interconnect)

Rose ==== Smoke Alarm 1 =~=~= Smoke Alarm 2 ==== Rose


You can just spur it off the first alarm.

That's a bit dodgy innit? If your lighting is set up as one circuit to cover
the whole house, it may be OK but if the downstairs lights are on one fuse
and the upstairs lights a different fuse, someone could pull the fuse for
the upstairs and expect the smoke alarm to be safe to work on when it's not.

Steve.

I must admit that my instinct would always be to supply any sort of
alarm in a seperate manner to anything else. Disaster scenarios are
always "well thats's unlikely !" - but that's why you do a risk
analysis.

Let's say a fault develops in the lights circuit the OP is talking
about - it might generate adequate smoke for the alarm to sound before
the fuse blows, but what if it was the other way round - OK the fuse
has protected the system but the heat generated might by then have
ignited something else which will now not be detected by the alarm -
because it / has / no power !!

I woudl suggest if you can that you supply them both off a seperate
circuit from the CU.
And if you want to be really secure, you have a couple of battery ones
as well and exercise a bit of discipline in changing the batteries
regularly.
Rob

"Steve" wrote in message
...

"rrh" wrote in message
. uk...

"thankyousam" wrote in message
oups.com...
I'm adding a couple of mains fed smoke alarms to my lighting circuit.
The first one is simple enough as there is a run of cable above the
location in the hallway. I'm intending to "loop in" the smoke alarm as
you would with a ceiling rose - allbeit without a switch

I'm not sure how to build the second alarm (upstairs) into the circuit.
Can I simply spur it off the downstairs smoke alarm using three+earth
(for the interconnect)? Or do I need to "loop in" the second alarm as
well? I've tried to illustrate both. I can't see why the first method
would be a problem, just wanted to check.

Spurred: ( = T&E, ||| 3&E Interconnect)

Rose ==== Smoke Alarm 1 ==== Rose
|||
|||
Smoke Alarm 2

Full Loop In: ( = T&E, =~= 3&E Interconnect)

Rose ==== Smoke Alarm 1 =~=~= Smoke Alarm 2 ==== Rose


You can just spur it off the first alarm.

That's a bit dodgy innit? If your lighting is set up as one circuit to
cover the whole house, it may be OK but if the downstairs lights are on
one fuse and the upstairs lights a different fuse, someone could pull the
fuse for the upstairs and expect the smoke alarm to be safe to work on
when it's not.

Steve.


Can't lay my hands on the regs just now but they insist that interconnected
alarms are wired to one circuit only. Mains-powered alarms without battery
backup ("Grade E") should have their own dedicated (and preferably
non-RCD-protected) circuit from the consumer unit; mains-powered alarms with
battery backup ("Grade D") may be wired either that way or to a regularly
used local lighting circuit that is separately electrically protected.

That's a bit dodgy innit? If your lighting is set up as one circuit to
cover
the whole house, it may be OK but if the downstairs lights are on one fuse
and the upstairs lights a different fuse, someone could pull the fuse for
the upstairs and expect the smoke alarm to be safe to work on when it's
not.

They shouldn't expect any such thing, as to connect them up in that manner
would be against the regulations, which requires all smoke alarms to be
interconnected and on the same circuit (either a regularly used lighting
circuit, or a dedicated alarm circuit).

If they are expecting smoke alarms to be distributed across multiple alarm
circuits, then they aren't competent to work on an electrical installation.
They are also not competent to work on electrical installations if they
think that flicking an SP MCB is enough to consider a circuit to be
deenergised, let alone isolated.

Christian.